Note: Descriptions are shown in the official language in which they were submitted.
CA 02157898 2002-O1-04
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TAPERED MULTILAYER LUMINAIRE DEVICES
The present invention is concerned generally with a luminaire device for
providing
selected illumination. More particularly, the invention is concerned with
tapered
luminaires, such as a wedge or disc shape, for backlighting and control of
angular range of
illumination and light concentration generally.
A variety of applications exist for luminair devices, such as, for liquid
crystal
displays. For flat panel liquid crystal displays, it is important to provide
adequate
backlighting while maintaining a compact lighting source. It is known to use
wedge
shaped optical devices for general illumination purposes. Light is input to
such devices at
the larger end; and light is then internally reflected off the wedge surfaces
until the critical
angle of the reflecting interface is reached, after which light is output from
the wedge
device. Such devices, however, have only been used to generally deliver an
uncollimated
lighting output and often have undesirable spatial and angular output
distributions. For
example, some of these devices use white painted layers as diffuse reflectors
to generate
uncollimated output light.
Accordingly, the invention seeks to provide an improved optical device and
method
of manufacture.
Further, the invention seeks to provide a novel three dimensional luminaire.
Still further, the invention seeks to provide an improved multilayer tapered
luminaire for optical purposes, such as for controlled angular output
backlighting.
Further still, the invention seeks to provide a novel tapered luminaire device
for
controlled transmission or concentration of light.
Additionally, the invention seeks to provide a novel optical device for
providing
collimated illumination from the device.
Yet further, the invention seeks to provide an improved tapered luminaire
having
an intervening air gap layer.
Still further, the invention seeks to provide a novel luminaire allowing
controlled
and focused output illumination, or controlled angular input for
concentration.
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Further still, the invention seeks to provide an improved illumination system
wherein a light source, such as a compound parabolic concentrator, a
fluorescent tubular
light source, or variable parametric functional source is coupled to a
multilayer optical
device for generating an output.
Also the invention seeks to provide a novel luminaire optical device having a
variable index of refraction over the spatial parameters of a luminaire.
Yet further, the invention seeks to provide an improved luminaire wedge device
having nonlinear thickness variation and variable wedge angle ~ along selected
spatial
parameters enabling compensation for light output irregularities.
The invention in one broad aspect provides an optical device for collecting
light,
including a substantially uncollimated, incoherent light from a source and
outputting the
light to a viewer, comprising layer means for providing a uniform light output
at a front
surface, and light redirecting means overlying the front surface and in
optical
communication with the layer means, the light redirecting means including a
faceted
portion having facet face angles systemically varied as a function of position
to provide
at least one of uniformly diffused light output over viewer angle and
adjustable variable
overlapping illumination output.
More particularly, the invention in one broad aspect pertains to a tapered,
multilayer luminaire for collecting light, including a substantially
uncollimated,
incoherent light from a source and outputting the light, comprising a first
layer having
a wedge shaped cross sectional area with an optical index of refraction ns,
the layer
having a back surface and a converging top and bottom surface with at least
one angle
of convergence ~p. A second layer with an optical index of refraction n2,
wherein light
inputted through the back surface of the first layer enters the second layer
when the light
being reflected in the first layer decreases its angle of reflection relative
to a normal to
the bottom surface to achieve an angle of reflection less than the critical
angle. of
incidence 8c relative to the normal with the bottom layer surface,
characteristic of the
interface between the bottom layer surface and the second layer. Third layer
means is
disposed adjacent the second layer which acts cooperatively with the first and
second
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layer to control angular distribution of the light of the luminaire and far
outputting the
light from the top layer surface, the third layer means including portions of
varying
curvature to output light over a preferred viewing area.
The invention also comprehends a tapered, multilayer luminaire for collecting
light from a light source and for selectively outputting light, comprising a
wedge layer
capable of receiving light from the source and having an optical index of
refraction nl
and top and bottom surface layers converging to define at least one angle of
inclination
~, the wedge layer including a transparent back surface spanning the top and
bottom
surface layers. An air gap or transparent dielectric layer extends along the
top, or the
bottom, surface layer of the wedge layer and having an optical index of
refraction n2 for
allowing transmission of light received from the wedge layer. Light
redirecting
transparent refractive means covers the air gap or transparent dielectric
layer allowing
transmission of light across the thickness of the light redirecting
transparent refractive
means during selectively redirecting light output from the air gap or
transparent dielectric
layer, and wherein the light redirecting transparent refractive means has a
refractive
index n3 where n2 < nl <_ n3, and a plurality of facets arranged such that
light incident
upon the facets, and thereafter output therefrom, has an angular and spatial
distribution
determined by the facets.
Still further, the invention provides a tapered, multilayer luminaire capable
of
collecting light, including a substantially uncollimated, incoherent light
from a source and
selectively outputting the light, comprising a first layer having an optical
index of
refraction nl, the first layer having a back surface and converging top and
bottom layer
surfaces positioned adjacent a gas containing gap with an optical index of
refraction n2
and light input through the back surface of the first layer entering the gap
when the light
being reflected in the first layer decreases its angle of reflection relative
to a normal to
the bottom surface to achieve an angle of reflection less than the critical
angle of
incidence 9~, relative to the normal, characteristic of the interface between
the bottom
layer surface and the gas containing gap, the first layer further includes a
nonlinearly
varying thickness near the back surface for at least one of controlling the
uniformity of
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light output into ambient from the luminaire and for optically compensating
for the
nonuniformity of the light input to the first layer, and light redirecting
means underlying
the gas containing gap, the light redirecting means for selectively reflecting
light output
from the first layer and outputting the light from the top layer surface, the
light
redirecting means further adapted to include means for focusing light output
from the
first layer.
Other aspects, features and advantages of the present invention will be
readily
apparent from the following description of the preferred embodiments thereof,
taken in
conjunction with the accompanying drawings described below.
Brief Description of the Drawings
FIGURE 1 shows a prior art wedge shaped device;
FIGURE 2A illustrates a multilayer tapered luminaire device constructed in
accordance with the invention; FIG. 2B is a magnified partial view of the
junction of the
wedge layer, the first layer and the second faceted layer; FIG. 2C is an
exaggerated form
of FIG. 2A showing a greatly enlarged second faceted layer; FIG. 2D is a
partial view of
the junction of the three layers illustrating the geometry for brightness
determinations;
FIG. 2E is a multilayer wedge device with a light redirecting internally
transmitting layer
on the bottom; FIG. 2F shows a wedge device with a lower surface translucent
layer; FIG.
2G shows a wedge layer with a lower surface refracting faceted layer; FIG. 2H
shows a
wedge layer with a lower surface refracting layer and curved facets thereon;
FIG. 2I shows
a wedge layer with a refracting layer of facets having variable facet angles;
FIG. 2J shows
a single refracting prism coupled to a wedge layer; FIG. 2K shows a single
refracting
prism coupled to a wedge layer and with an integral lens; FIG. 2L shows a
reflecting
faceted layer coupled to a wedge device; FIG. 2M shows a reflecting faceted
layer with
curved facet angles and coupled to a wedge device; FIG. 2N shows a flat
reflecting
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facet on a wedge layer and FIG. 20 shows a curved reflecting facet on
a wedge layer;
FIGURE 3A illustrates a multilayer wedge device with curved
facets on the ambient side of the second layer and FIG. 3B shows a
magnified partial view of the junction of the various layers of the
device;
FIGURE 4A shcws calculated brightness performance over
angle for an asymmetric range of angles of illumination; FIG. 4B
shows calculated brightness distribution performance over angle for a
more symmetric angle range; FIG. 4C illustrates calculated brightness
performance over angle for the symmetry of FIG. 4B and adding an
external diffuser element; FIG. 4D illustrates an output using flat
reflecting facets, no parallel diffuser; full-width at half maximum
brightness (FVVHM) = 7 degrees; FIG. 4E illustrates an example of
nearly symmetrical output distribution; measured using flat facets with
parallel lenticular diffuser; FWHM = 34 degrees; FIG. 4F illustrates
an example of asymmetrical output distribution, measured using
curved facets; FWHM = 32 degrees; FIG. 4G illustrates an example
asymmetrical output distribution, measured using curved facets;
FVVHM = 26 degrees; FIG. 4H illustrates an example of a bimodal
output distribution, measured using one faceted reflecting layer and
one faceted refractive layer; and FIG.4I illustrates an example of an
output distribution with large "tails", measured using a diffuse
reflective bottom redirecting layer and a refracting/internally-
reflecting top redirecting layer;
FIGURE SA shows a top view of a disc shaped light guide and
FIG. SB illustrates a cross section taken along SB-SB in FIG. SA;
FIGURE 6A shows a cross sectional view of a multilayer
tapered luminaire device with an air gap layer included; FIG. 6B
shows another tapered luminaire in cross section with a compound
parabolic light source/ccncentrator; FIG. 6C illustrates another tapered
luminaire in cross section with a variable parametric profile light
source and a lenticular diffuser; and FIG. 6D shows another tapered
luminaire in cross section with non-monotonic wedge layer thickness;
FIGURE 7 illustrates a reflective element disposed
concentrically about a light source;
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FIGURE 8 illustrates a reflective element disposed about a
light source with maximum displacement between the reflector center
of curvature and the center of the light source;
FIGURIr 9A illustrates use of a redirecting layer to provide a
substantially similar angular distribution emanating from all portions
of the device and FIG. 9B illustrates use of a redirecting layer to vary
angular distribution emanating from different portions of the device,
and specifically to focus the various angular distributions to enhance
their overlap at a selected target distance;
FIGURE 10 illustrates one form of pair of lenticular arrays of a
luminaire; and
FIGURE 11 illustrates a lenticular diffuser array and curved
facet layer of a luminaire;
FIGURE 12A Slustrates a wedge shaped luminaire having a
pair of diffraction gratings or hologram layers; FIG. 12B shows a
wedge shaped luminaire with a pair of refracting facet layers and
diffusers; FIG. 12C illustrates a wedge shaped luminaire with a pair of
faceted layers; FIG. 12D shows a wedge shaped luminaire with two
refracting single facet layers; FIG. 12E illustrates a wedge shaped
luminaire with a refracting single facet layer and a bottom surface
redirecting layer; FIG. 12F shows a luminaire with a top surface
redirecting layer ofa refracting faceted layer an~i a bottom surface
refracting and internally reflecting layer; FIG. 12G illustrates a
luminaire with a top surface refracting/internally reflecting faceted
layer and a bottom surface refracting/iniernally reflecting faceted
layer; FIG. 12H shows a luminaire with a top surface refracting
faceted layer and a bottom surface refracting/i~iternally reflecting
faceted layer; FIG. 12I illustrates a luminaire with a bottom surface
specular reflector and a top layer transmission diffraction grating or
transmission hologram; FIG. 12J shows a luminaire with a bottom
surface specular reflector and a top surface refracting faceted layer
and diffuser; FIG. 12K illustrates a luminaire with a bottom layer
specular reflector and a top layer refracting/internally reflecting
faceted layer; FIG. 12L shows a luminaire with a bottom specular
reflector and a top layer refracting/internally reflecting faceted layer;
FIG. 12M illustrates a luminaire with an initial reflector section
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including an integral lenticular diffuser; FIG. 12N shows a luminaire
with a roughened initial reflector section of a layer; FIG. 120
illustrates a luminaire with an eccentric light coupler and converging
to the wedge shaped section; FIG. 12P shows a luminaire with an
eccentric light coupler and a diffuser and roughened or lenticular
reflector; FIG. 12Q illustrates a luminaire with a bottom specular or
diffusely reflecting layer and a top refracting layer and FIG. 12R
shows a luminaire for generating a "bat wing" light output;
FIGURE 13 illustrates a combination of two wedge shaped
sections formed integrally and using two light sources;
FIGURE 14 shows a tapered disk luminaire including a faceted
redirecting layer;
FIGURE 15 illustrates a luminaire operating to provide a
collimated light output distribution;
FIGURE 16A shows a prior art ambient mode LCD and FIG.
16B illustrates a prior art transflective LCD unit;
FIGURE 17 shows a luminaire operative in ambient and active
modes with a faceted redirecting layer and a lenticular diffuser; and
FIGURE 18 illustrates a luminaire with an array of micro-
prisms for a faceted surface disposed over a diffuse backlight.
Detailed Description of Preferred Embodiments
A multilayer luminaire device constructed in accordance with
one form of the invention is illustrated in FIG. 2 and indicated
generally at 10. A prior art wedge 11 is shown generally in FIG. 1. In
this wedge 11 the light rays within the wedge 11 reflect from the
surfaces until the angle of incidence is less than the critical angle
(sin-11/n) where n is the index of refraction of the wedge 11. The
light can exit equally from both top and bottom surfaces of the wedge
11, as well as exiting at grazing angles.
The multilayer luminaire device 10 (hereinafter "device 10")
shown in FIG. 2A includes a wedge layer 12 which has a
characteristic optical index of refraction of n 1. The term "wedge
layer" shall be used herein to include all geometries having converging .
top and bottom surfaces with wedge shaped cross sectional areas. The
x, y and z axes are indicated within FIGS. 2A and 2C with the "y" axis
perpendicular to the paper. Typical useful materials for the wedge
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layer 12 include alinost any transparent material, such as glass,
polymethyl methacrylate, polystyrene, polycarbonate, polyvinyl
chloride, methyl methacrylate/styrene copolymer (NAS) and
sytrene/acrylonitrile. The wedge layer 12 in FIG. 2A further includes
a top surface 14, a bottom surface 16, side surfaces 18, edge 26 and a
back surface 20 of thickness to spanning the top, bottom and side
surfaces. A light source, such as a tubular fluorescent light 22, injects
light 24 through the back surface 20 into the wedge layer 12. The
light 24 is internally reflected from the various wedge layer surfaces
and is directed along the wedge layer 12 toward the edge 26. Otber
possible light sources can be used and will be described hereinafter.
Generally, conventional light sources provide substantially incoherent,
uncollimated light; but coherent, collimated light can also be
processed by the inventions herein.
For the case where the surfaces 14 and 16 are flat, a single
angle of inclination ~ for a linear wedge is defined by the top surface
14 and the bottom surface 16. In the case of nonlinear wedges, a
continuum of angles ~ are definable; and the nonlinear wedge can be
designed to provide the desired control of light output or
concentration. Such a nonlinear wedge will be described in more
detail later.
In the embodiment of FIG. 2A a first layer 28 is coupled to the
wedge layer 12 without any intervening air gap, and the first layer 28
has an optical index of refraction n2 and is optically coupled to the
bottom surface 16. The first layer 28 can range in thickness from a
few light wavelengths to much greater thicknesses and accomplish the
desired functionality. The resulting dielectric interface between the
wedge layer 12 and the first layer 28 has a higher critical angle than at
the interface between the wedge layer 12 and ambient. As will be
apparent hereinafter, this feature can enable preferential angular
output and collimation of the light 24 from the device 10.
Coupled to the first layer 28 is a second layer 30 (best seen in
FIG. 2B) having an optical index of refraction n3 which is greater than
n2, and in some embodiments preferably greater than nl. This
configuration then allows the light 24 to leave the first layer 28 and
enter the second layer 30. In the embodiment of FIG. 2A there are
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substantially no intervening air gaps between the first layer 28 and the
second layer 30. In the preferred form of the invention illustrated in
FIG. 2A, nl is about 1.5, n2<1.5 and n3>nl. Most preferably,
nl=1.5, n2<1.5 (such as about one) and n3?nl.
In such a multilayer configuration for the device 10 shown in
FIG. 2, the wedge layer 12 causes the angle of incidence for each
cyclic time of reflection from the top surface 14 to decrease by the
angle of inclination 2~ (relative to the normal to the plane of the
bottom surface 16). When the angle of incidence with the bottom
surface 16 is less than the critical angle characteristic of the interface
between the wedge layer 12 and the first layer 28, the light 24 is
coupled into the first layer 28. Therefore, the first layer 28 and the
associated optical interface properties form an angular filter allowing
the light 24 to pass when the condition is satisfied: 8<6c = sin-1
(n2/nl). That is, the described critical angle is higher than for the
interface between air and the wedge layer 12. Therefore, if the two
critical acgles differ by more than 6~, nearly all of the light 24 will
cross into the interface between the wedge layer 12 and the first layer
28 before it can exit the wedge layer 12 through the top surface 14.
Consequently, if the two critical angles differ by less than ~, a
substantial fraction, but less than half, of the light can exit the top
surface 14. If the two angles differ by more than ~ and less than 6~,
then substantially more than half but less than all the light will cross
into the wedge layer 12 and the first layer 28 before it can exit the
wedge layer 12 through the top surface 14. The device 10 can thus be
constructed such that the condition 8<8c is satisfied first for the
bottom surface 16. The escaping light 24 (light which has entered the
layer 28) will then enter the second layer 30 as long as n3>n2, for
example. The light 24 then becomes a collimated light 25 in the
second layer 30 provided by virtue of the first layer 28 being coupled
to the wedge layer 12 and having the proper relationship between the
indices of refraction.
In order to generate an output of the light 24 from the
device 10, the second layer 30 includes means for scattering light,
such as a paint layer 33 shown in FIG. 2E or a faceted surface 34
shown in both FIGS. 2B and 2C. The paint layer 33 can be used to
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preferentially project an image or other visual information. The paint
layer 33 can comprise, for example, a controllable distribution of
particles having characteristic indices of refraction.
By appropriate choice, light can also be redirected back
through the wedge layer 12 and into ambient (see light 29 in FIGS. 2A
and 2C) or output directly into ambient from the second layer 30 (see
light 29' in FIG. 2F).
In other forms of the invention a fiuther plurality of layers with
associated "n" values can exist. In one preferred form of the invention
the index of the lowest index layer can replace n2 in equations for
numerical aperture and output angle (to be provided hereinafter).
Such further layers can, for example, be intervening between the
wedge layer 12 and the first layer 28, in~erveniag between the first
layer 28 and the second layer 30 or be overlayers of the wedge layer
12 or the second layer 30.
In certain embodiments the preferred geometries result in
output of light into ambient without being reflected back through the
wedge layer 12. For example, in FIG. 2F the device 10 can include a
translucent layer 37. In another form of this embodiment shown in
FIG. 2G, a refracting layer 38 is shown. The refracting layer 38 can
include flat facets 39 for providing a collimated output. Also shown
in phantom in FIG. 2G is a transverse lenticular diffuser 83 which will
be described in more detail hereinafter. The diffuser layer 83 can be
used with any of the invention geometries, including above the wedge
layer 12 as in FIG. 6A.
In yet another example shown in FIG. 2H, the refracting layer
38 can include curved facts 41 for providing a smoothly broadened
output over a desired angular distribution. In a further example shown
in FIG. 2I, the refracting layer 38 includes variable angle facets 42.
These facets 42 have facet angles andlor curvature which are varied
with position across the facet array to focus output light in a desired
manner. Curved facets would enable producing a softly focused
:egion within which the entire viewing screen appears to be
illuminated. Examples of the application to computer screen
illumination will be described hereinafter. In FIGS. 2J and 2K are
shown, respectively, a single refracting prism element 43 and the
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prism element 43 with an integral lens 44 to focus the output light.
FIGS. 2L and M show the faceted surface 34 with the facets angularly
disposed to control t<he output distribution of light. In FIGS. 2K and
2L the light is output to a focal point "F", while in FIG. 2M the
output is over an approximate viewing range 45. FIGS. 2N and 20
illustrate flat reflecting facets 48 and curved reflecting facet 49 for
providing a collimated light output or focused light output,
respectively.
As shown in FIGS. 2A and C the faceted surface 34 optically
reflects and redirects light 29 through the second layer 30, the first
layer 28 and then through the wedge layer 12 into ambient. Only a
fraction of each facet is illuminated, causing the output to appear
alternately light and dark when viewed on a sufriciently small scale.
Since this pattern is typically undesirable, for the preferred
embodiment shown in FIG. 2B the period of spacing between each of
the faceted surfaces 34 is preferably large enough to avoid diffraction
effects, but small enough that the individual facets are not detected by
the intended observing means. The spacing is also chosen to avoid
forming Moire interference patterns with any features of the device to
be illuminated, such as a liquid crystal display or CCD (charge
coupled device) arrays. Some irregularity in the spacing can mitigate
undesirable diffraction Moire effects. For typical backlighting
displays, a spacing period of roughly 0.001-0.003 inches can
accomplish the desired purpose.
The faceted surface 34 in FIGS. 2B and 2C, for example, can
be generally prepared to control the angular range over which the
redirected light 29 is output from the device 10. The minimum
distribution of output angle in the layer 30 has a width which is
approximately equal to:
06 = 2 ~[(n12-n22)/(n32-n22)l'/z
Thus, since ~ can be quite small, the device 10 can be quite an
effective collimator. Therefore, for the linear faceted surface 34, the
exiting redirected light 29 has a minimum angular width in air of
approximately:
~Aair = n3~A = 2~(n12-n22)/[1-(n2/n3)2J1/z,
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As described hereinbefore, and as shown in FIGS. 2H, 2I, 2K, 2L,
2M, and FIG. 3 the facet geometry can be used to control angular
output in excess of the minimum angle and also focus and control the
direction of the output light.
Fresnel reflections from the various interfaces can also broaden
the output angle beyond the values given above, but this effect can be
reduced by applying an antireflection coating 31 on one or more of the
internal interfaces, as shown in FIG. 2B.
The brightness ratio ("BR") for the illustrated embodiment can
be determined by reference to FIG. 2D as well as by etendue match,
and BR can be expressed as:
B.R. =ou~ut brightness
source brightness
or, B.R. = illuminated area/total area
B.R. = [1-(n2/n3)2]'~2 = 0.4-0.65 (for most transparent
dielectric materials)
For example, the wedge layer 12 can be acrylic (nl = 1.49), the first
layer 28 can be a fluoropolymer (n2 = 1.28-1.43) or Sol-gel
(n2 = 1.05-1.35, fluoride salts (n2 = 1.38-1.43) or silicone based
polymer or adhesive (n2 = 1.4-1.45); and the second layer 30 can be a
faceted reflector such as polycarbonate (n3 = 1.59), polystyrene
(n3 = 1.59) epoxy (n3 = 1.5-1.~5) or acrylic (n3 = 1.49) which have
been metallized at the air interface.
The flat, or linear, faceted surfaces 34 shown, for example, in
FIGS. 2B and 2C can redirect the incident light 24 to control direction
of light output and also substantially preserve the angular distribution
of light O6 which is coupled into the second layer 30 by the angle-
filtering effect (see, for example, FIG. 4D). For example, in one
preferred embodiment shown in FIG. 2L, the faceted surfaces 34
reflect light with the flat facet angles varied with position to focus the
output light. In FIG. 2M the faceted surfaces 34 include curved facet
angles which vary with position to produce a softly focused viewing
zone 45 within which the entire screen appears to be illuminated (see
also, for example FIGS. 4F and 4G). Also show in phantom in FIG.
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2M is an exemplary liquid crystal display 47 usable in conjunction
with the invention. As further shown in FIGS. 3A and B, curved
facets 36 also redirect the incident light 24, but the facet curvature
increases the resulting range of angular output for the redirected light
29 (see for comparison for flat facets FIG. 2D). For example, it is
known that a concave trough can produce a real image, and that a
convex trough can produce a virtual image (see, for example,
FIG. 3B). In each case the image is equivalent to a line source
emitting light uniformly over the desired angular output range.
Consequently, an array of such trough shaped facets 36 can redirect
the incoming form of collimated light 25 from the first layer 28 (see
FIG. 2C), and a plurality of such line source images then form the
redirected light 29. By arranging the spacing of the curved facets 36
to less than human eye resolution, the resulting array of line sources
will appear very uniform to an observer. As previously mentioned,
the choice of about three hundred to five hundred lines/inch or 0.002
to 0.003 i3ches for the period of facet spacing provides such a result.
For a typical LCD display viewing distances of approximately twenty
inches or greater are conventional.
Other useful facet shapes can include, for example, parabolic,
elliptical, hyperbolic, circular, exponential, polynomial, polygonal,
and combinations thereof. The user can thus construct virhrally
arbitrary distributions of averaged brightness of ill~.unination using
different facet designs. For example, polygon shaped facets can be
used to produce output angular distributions having multiple peaks.
Examples of brightness distribution over various ranges of
angular output using a curved-faceted reflector are illustrated in
FIGS. 4A-4C, 4F and 4G. FIG. 4C and 4E shows the brightness
distribution in the case of a reflector having linear facets, and further
including a diffuser element 40 (shown in phantom in FIG. 2C). The
predicted performance output is shown for the various angular ranges
(see FIGS. 4A-4C) and compared with the measu.Ted angular output of
light for a commercially available source, such as a "Wedge Light"
unit, a trademark of Display Engineering. The preferred angular range
can readily be modified to accommodate any particular viewing and
collimation requirements up to the minimum angle O8 (air) described
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hereinbefore by the equation in teams ~, nl, n2 and n3. This
modification can be accomplished by progressively changing the
curvature of the curved facets 36 in the manner shown in FIG. 2M and
discussed hereinbefore. In addition to the illustrated control of the
vertical viewing angular range, modification of the horizontal viewing
range can also be accomplished by appropriate changes of the shape
of the curved facets 36. The above described angular distributions
shown in FIGS. 4A-4I are representative when the device 10 is
processing the light 24 within the numerical aperature NA = (n12 -
n22)'h. When light is outside this range, additional techniques can be
applied to help control the angular output range.
FIGS. 9A and 9B further illustrate the use of redirecting means
to provide a tightly overlapping focused illumination output and a less
overlapping focused illumination output, respectively. These concepts
can be applied practically by considering that a typical portable
computer screen 87 has a vertical extent "V" of about 150mm, while a
typical viewing distance, "D", is 500mm. A viewer at distance "D",
positioned normal to the vertical center of the computer screen 87 will
~riew different areas of the screen 87 at angles ranging from -8.5°
measured at the top of the screen 87 to +8.5° measured at the bottom
of the screen 87. This variation in viewing angle can, however, cause
undesirable effects in use of a system having such screen illumination.
Such a limited illumination angle for the screen 87 implies a limited
range of positions from which a viewer can see a fully illuminated
screen 87 (see FIG. 9A). Defining the viewer position in terms of the
angle and distance from the center of the screen 87, then the effective
angular range is substantially reduced below the nominal illumination
angle. For example, if the nominal illumination range is X20°
measured at each individual facet, then the effective viewing range is
reduced to X12° in the typical flat panel illuminator shown in FIG. 9A.
The resulting illumination between 12°-20°, either side of
center for
the screen 87, will appear to be nonuniform to the viewer.
The invention herein can be used to overcome the above
described nonuniformities by controlling the orientation of the faceted
surface 34. As illustrated, for example, in FIG. 2M both surfaces of
the facets are rotated progressively such that the flat facet surface is
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varied from 35.6° to 33.3° relative to, or parallel to, the
edges of the
planes defining the various layers of the device 10. This systematic
variation from the top to the bottom of screen 89 (see FIG. 9B) results
in the redirected output illustrated. The faceted surface 34 can fiuther
be combined with the diffuser 83 and the like to produce a variable,
but controllable light illumination output distribstion. A flat faceted
surface 168 can further be combined with a diffuser 170. Therefore,
as shown in FIG. 9B the ability to rotate the angular distributions of
light at different points on the screen 89 enable compensation for the
variation in viewing angle with position. Systematic variations in the
faceted surface 34 can further include variations in one or more facet
angles, the spacing of the facets 38, or the depth and width of the
individual facets 38. In other embodiments, the same principles can
be applied to focus the output of any faceted redirecting layer.
Examples are shown in FIGS. 2I and 2L.
In another example of overcoming nonuniformities of
illumination, an array of micro-prisms for the faceted surface 34 can
be laid over a conventional diffuse backlight 101 (see FIG. 18A).
This faceted surface 34 operates by a combination of refraction and
total internal reflection to permit only a limited angular range to be
output through the layer into ambient. This angular range depends on
the facet angles. For the case of acrylic film (n=1.49), highest
brightness is typically achieved with a prism included angle of 90-100
degrees, resulting in a viewing angle of approximately ~ 35 degrees.
Backlights using such a geometry show a sharp "curtaining" effect
which is disconcerting to many viewers. This effect can be
ameliorated by rotating the facets 38 from top to bottom of the screen
to produce a focusing effect (see FIG. 18B). Simple ray-tracing shows
that, for included angles in the range of 100°-110°, a facet
rotated by
an angle O will produce an angular distribution rotated by
approximately O/2. In the embodiment shown in FIG. 18 the
progressive variation of facet face angle can vary as position X along
the faceted surface 34 wherein, for example:
~1 = 35° - (0.133°/mm) 'X
'f2 = 35° + (0.133°/mm) 'X
WO 94120871 PCT/US94/02598
14
This progressive facet angle change will produce an angular
distribution which varies by approximately ten degrees across the
screen 89, and satisfies the generic constraints outlined above.
Whatever the desired facet shapes, the faceted surface 34 (see,
FIG. 2D) is preferably formed by a conventional process such as
molding or other known milling processes. Details of manufacture
will be described hereinafter:
Nonlinear Wedges.
In another form of the invention the wedge layer 12, which is
the primary lightguide, can be other than the linear shape assumed
hereinbefore. These shapes allow achievement of a wide variety of
selected light distributions. Other shapes can be more generally
described in terms of the thickness of the wedge layer 12 as a function
of the wedge axis "z" shown in FIGS. 2B and C (the coordinate axis
which runs from the light input edge to the small or sharp edge 26).
For the linear shaped wedge.
A (z) = Ao - C.z (1)
Ao = maximum wedge thickness (see FIG. 2A)
C = constant = tan ~
A large range of desired spatial and angular distributions can be
achieved for the light output power (power coupled to the second
layer 30). This light output power is thus the light available for output
to the ambient by the appropriately faceted surfaces 34 or 36, or even
by the diffuse reflector 33 (see FIG. 2E) or other means.
For example, if L and M are direction cosines along the x and y
axes, respectively, then Lo and Mo are the values of L and M at the
thick edge (z=0). This initial distribution is Lambertian within some
well-defined angular range, with little or no light outside that range.
This distribution is especially important because ideal non-imaging
optical elements have limited Lambertian output distributions. The
key relationship is the adiabatic invariant, A(z)cos(6c) which is
approximately equal to AOLO and which implicitly gives the position
(z) of escape. To illustrate this concept, suppose we desire uniform
irradiance so that dP/dz = constant. Suppose further that the initial
phase space uniformly fills an ellipse shown below and describes by
the following expression and sketch:
WO 94/20871 PCT/US94/02598
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a
LoZ/aZ + MOz/~z = 1 (2)
Then, dP/dL = const ~ [ 1-L2/Q2J ~z but dA/dz = [Ao/LcJ dLo/dZ
where Lc = cosAc. Therefore, [ 1-(LcA)2/(Aoa)2J ~2 dp = constant
times dz. Suppose o~ = Lc in the prefers ed embodiment. This result
can be interpreted by the substitution A/AO = sin u, so that A = AO sin
a and a + 1/2 sir. (2u) _ (x/2)(1-z/D) where D is the length of the
wedge layer 12.
If the desired power per unit length is dP/dz, more generally,
then the desired shape of the wedge layer 12 is determined by the
differential equation:
dA(z)/dz = - dP/dz (Ao/[1-(n2/nl)2]'/=) (3)
dP/dLo
Note that in all these cases the output distribution has only
approximately the desired form because it is modified by Fresnel
reflections. Note also that even when the wedge device 10 is curved,
if the curvature is not too large, it may still be useful to define an
average angle ~ which qualitatively characterizes the system.
In another aspect of the invention the geometry of the above
examples has an x,y interface between two refractive media with
indices nl and n2. The components nM,nN are conserved across the
interface so that nlM1 = n2M2, nlN1 = n2M2. The angle of
incidence projected in the x,z plane is given by sin 6e ff= N/(L2 -
N2) 1/2, Then using the above relations, sin 62e~'sin 81 eff =
(n 1/n2)[ 1 - M 12] 1/2/[ 1 _ (n 1/n2)2M 12J 1/2 = (n 1/n2)~ff For example,
for nl = 1.49, n2 = 1.35, M 1 = 0.5, the effective index ratio is
1.035(nl/n2), which is only slightly larger than the actual index ratio.
Variation of Index of Refraction Over Spatial Farameters.
In the general case of tapered light guides, the wedge layer 12
is generally along the z axis with the narrow dimension along the x
PCTIUS94/02598
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16
axis (see, for example, FIG. 2A). If we introduce optical direction
cosines (nL,nM,nM) where L,M,N are geometric direction cosines
along x,y,z, then n is the refractive index which may vary with spatial
position. For guided rays in the wedge layer 12, the motion in x is
almost periodic, and the quantity ~nLdx for one period is almost
constant as the ray propagates along z. This property is called
adiabatic invariance and provides a useful framework for analyzing
the lightguide properties.
In a first example the wedge device 10 shown in FIG. 2A has a
uniform index in the wedge layer 12 and is linearly tapered in z with
width A(z) = AO - C~z. Then, along the zigzag ray path, L(z)A(z) is
approximately equal to a constant by adiabatic irmariance. If a ray
starts at z = 0 with L = Lp, then (AO - C ~ z)L(z) is approximately equal
to LOAD. The ray will leak out of the wedge layer 12 when L = cosAc
where 6c is the critical angle = [1-(n2/nl)2]'~2. Thus, the condition for
leaving the wedge layer 12 is AO-C~z = LpAp/cos6c. This will occur
at z = (Ap/C)(1 - Lp/cos6c). Consequently, the density of rays
emerging in z is proportional to the density of rays in the initial
direction cosine LO. For example, the density will be uniform if the
initial distribution in Lp is uniform.
In a second example, the index profile is no longer uniform but
falls off both in x and in z. If the fall-off in z is much slower than in
x, the light ray path is still almost periodic, and the above adiabatic
invariance still applies. Then, as the light ray 24 propagates in z, the
path in x,nL space is alinost periodic. Therefore the maximum value
of L(z) increases and at some z may reach the critical value for escape.
The z value for escape depends on the details of the index (n) profile.
When this is specified, the analysis proceeds as in example one above.
T hus, for a parabolic index profile, the index profile has the form
n2(x) = n20~1 - 20 (~P)2~ for -p<xp, = n12 = n2p~1 _ 20~ for ~x~ > p.
Then, the critical angle at x = 0 is still given by sin2 6c = 2 0 = 1 -
(nl/np)2. Then, if we have np a slowly decreasing function of z, the
slope 8 at x = 0 will slowly increase by the adiabatic invariance of
~nLdx, while Ac decreases so that light rays will escape. The details
of the light ray distributions will depend on how the index (n) varies
with z.
WO 94/20871 PCT/US94/02598
17
NonwedQe Tapered Geometries
In the most general case the light can be input into any shape
layer Le.g." parallelepiped, cylinder or non-uniform wedge), and the
principles described herein apply in the same manner, In addition, the
index of refraction can be varied as desired in (x,y,z) to achieve the
appropriate end result when coupled to means to output light to
ambient.
For example, consider a disc-shaped light guide 46 which is
tapered in the radial direction r shown in FIG. 5. The direction
cosines in cylindrical polar coordinates are kr, lcg, kz. Light 48
propagating in this guide 46 satisfies the relationship:
~nkzdz ~ constant. (adiabatic invariance) . (4)
nrkg = constant. (angular momentum conservation) (5)
The adiabatic invariance condition is identical with that for the
wedge device 10, and the previous discussions pertinent to the wedge
device 10 also thus apply to the light guide 46. The angular
momentum conservation condition requires that as the light streams
outward from source 47 with increasing radius, the kg value
decreases. Therefore, the light becomes collimated in the increasing
radial direction. This makes the properties fundamentally like the
wedge device 10, and the light 48 can be made to emerge as light 52 at
a selected angle to face 51, collimated along the z direction.
For purposes of illustration we take the guide material to have a
constant index of refraction n. For such geometries the light rays 48
along the two-dimensional cross sectional plane taken along SB-SB
behave just as in the case of the wedge device 10 counterpart
described hereinbefore. Similarly, various additional layers 54 and 56
and other means can be used to achieve the desired light handling
features. For example, for the disc light guide 46 a preferred facet
array 56 is a series of circles, concentric with the disk 46. Thus, if the
facets 56 are linear in cross section, the light rays 52 will emerge in a
direction collimated within a full angle of 2 ~ times a function of the
indices of refraction as in the device 10 described hereinbefore.
PCT/US94102598
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Tapered Luminaires with Two Low-index Layers.
In another form of the invention shown in FIG. 6A, the device
includes a first layer 61 having an optical index of refraction nl
and a top layer surface 62 and a bottom layer surface 64 converging to
establish at least one angle of inclination ~. The first layer 61 also
includes a back surface 65 spanning the top layer surface 62 and the
bottom layer surface 64.
Adjacent the first layer 61 is layer means, such as a bottom
transparent layer means, first intermediate layer 66 of index n2
disposed adjacent to, or underlying, the bottom layer surface 64. 1n
addition, the layer means can embody a top transparent layer means,
second intermediate layer 81 of index n2 disposed adjacent to the top
layer surface 62. At least one of the layers 66 and 81 can be an air
gap, or other gas or transparent dielectric gap.
An air gap can be established by conventional means, such as
external supports, such as suspending the layers under tension (not
shcwn) or by positioning spacers 68 between the first layer 61 and the
adjacent light redirecting layer 70. Likewise, the spacers 68 can be
positioned between the first layer 61 and the second light redirecting
layer 82. Alternatively, solid materials can be used for the transparent
dielectric to constitute layers 66 and 81 and can improve structural
integrity, robustness and ease of assembly. Such solid materials can
include, for example, sol-gels (n2=1.05-1.35), fluoropolymers
(n2=1.28-1.43), fluoride salts (n2=1.38-1.43), or silicone-based
polymers and adhesives (n2=1.40-1.45). Such solid materials for the
transparent dielectric need no separate means to support or maintain it,
but can result in lower N.A. acceptance since the index is higher than
for an air gap.
The layers 66 and 81 allow transmission of light received from
the first layer 61. In this embodiment, part of the light will achieve 8c
first relative to the top layer surface 62, and light will enter the layer
81 for further processing by the light redirecting layer 82. The
remaining light will thereby achieve 6c first relative to the bottom
layer surface 64, thus entering the layer 66 for fiuther processing by
the light redirecting layer 70.
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19
In one preferred form of the invention both the layers 66 and 81
are present and can have similar, but significantly different indices
n2a and n2b, respectively. The indices are considered similar when
they establish critical angles at the interfaces 62 and 64 which are
similar in magnitude to the~wedge angle ~, for example:
arc sin(n2a/n 1 )-arc sin(n2b/n 1 ) I < 6~ (6)
In this case significant, but unequal, fractions of light will enter
each of the layers 66 and 81 for further processing by redirecting
layers 70 and 82, respectively. The larger fraction will enter the layer
having the higher of the two indices n2a and n2b. The redirecting
layer 70 processes only the fraction which enters the layer 66.
Therefore, the influence of the redirecting layer 70 on the output
angular distribution of light can be changed by varying the
relationship between the indices n2a and n2b.
In another preferred form of the invention the layers 66 and 81
can be the same transparent material of index n2 < nl. In general,
lower values of n2 will enhance the efficiency of the device 10 by
increasing the numerical aperture at the light input surface 65.
Therefore, collection e~ciency can be maximized when the layers 66
and 81 are gaps filled with air or other gases (with n2 = 1-1.01).
The thickness of the layers 66 and 81 can be selectively varied
to control the output power spatial distribution of the device 10 or to
enhance its visual uniformity. For example, increasing the thickness
of the layer 81 by 0.002"-0.030" sharply reduces non-uniformities
which tend to appear at the thicker end of the device 10. The
thickness of layers 66 and 81 can also be smoothly varied with
position to influence a desired spatial distribution of the light being
output (see FIG. 12L).
In one preferred form of the invention shown in FIG. 6A, the
light redirecting layer 70 includes a reflective layer 71 which reflects
the light back through the layer 66 and the first layer 61. The light is
then output into the first layer 61 through the top layer surface 62, and
ultimately through the light redirecting layer 82 for fiu-ther processing.
The reflective layer 71 can, for example, be any combination of a
planar specular reflector, a Partially or completely diffuse reflector, or
a faceted reflector.
PCT/US94I02598
wo 94nosn
Use of a planar specular reflector leads to the narrowest
angular distribution within the layer 81. Therefore, the reflector can
simplify design of the light redirecting layer 82 when the desired
output angular distribution is unimodal. Diffuse or faceted reflectors
can also be used for the layer 71 in order to achieve a large range of
angular distributions (see FIGS. 4H and I) or to increase uniformity
(see FIG. 4N). Diffuse reflectors are preferred if the desired angular
distribution has large "tails" (see, in particular, FIG. 4I). Faceted
reflectors can produce a bimodal angular distribution within the layer
81(see FIG. 4H). Therefore, such faceted reflectors are preferred if
the desired output angular distribution is bimodal. For example, a
bimodal "batwing" distribution is preferred from iuminaires for room
illumination because it reduces glare.
In general each facet of the layer 71 can be shaped to control
the angular distribution of the light reflected back through the layer 66
and the first layer 61 for further processing by the redirecting layer 82.
'The angular distribution within the device 10 will in turn influence the
angular distribution of the light output into ambient from the
redirecting layer 82. For example, curved facets can be used to
smoothly broaden the angular distribution, as well as providing a
diffusing effect to improve uniformity. The reflective layer 71 can
al so influence the output power spatial distribution as well as the
angular distribution. The reflectivity, specularity, or geometr~~ of the
reflective layer 71 can be varied with position to achieve a desired
output distribution. For example, small variations in the slope (see
FIG. 12L) of each element of the reflective layer 71 as a function of
position significantly change the light output distribution.
The light redirecting layer 82 has an index n3 > n2, and is
substantially transparent or translucent. The light in the low-index
layer 81 enters the layer 82 and is redirected into ambient. The
transmissive redirecting layer 82 also redirects t'he light which has
been processed by reflection from the redirecting layer 71 then
transmitted back through the low-index layer 66 and the first layer 61.
The transparency or geometry of the layer 82 can be varied with
position to further influence the output spatial distribution of the
device 10. In one preferred form of the invention the redirecting layer
WO 94120871 PCT/US94/OZ598
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21
82 includes a faceted surface at the interface with the low-index layer
81, as shown in FIG. 6A. Light entering the layer 82 is refracted by
one side 84 of each facet 85 as it enters, and then is totally internally
reflected by second side 86 of each of the facets 85. In one form of
the invention the redirecting layer 82 can be a "Transparent Right-
Angle rilm" (hereinafter, TRAF), which is a trademark of 3M Corp.,
and this product is commercially available from 3M Corp. This
TRAF operates by refraction and total internal reflection to turn
incident light through approximately a ninety degree angle, as would
be desired in a typical LCD backlighting application. The acceptance
angle of the prior art TRAF is about twenty-one degrees, which is
large enough to redirect a large fraction of light ?5 which enters the
low-index layer 81. In a more preferred form of the invention, the
facet angles are chosen to redirect more of the light 75 which enters
the low-index layer 81 by the described mechanism of refraction plus
total internal reflection. Either one or both of the facet surfaces 84
and 86 can be shaped to control the output angular distribution. For
example, the use of curved facets smoothly broadens the distribution,
as well as providing a light diffusing effect which can improve
uniformity.
In another preferred embodiment, the facet angle surfaces of
the redirecting layer 82 can be varied progressively to compensate for
the variation in viewing angle with position, when viewed from
typical viewing distances. The details of such a compensation effect
were described earlier in reference to the design of the reflecting facet
layer in the embodiment shown in FIG. 2M. Similar principles can be
applied to the design of any faceted redirecting layer, including
refracting layers and refracting/internally-reflecting layers. Examples
of embodiments which make use of such progressively varied faceted
layers are shown in FIGS. 12E (layer 140), 12G (layer 152), 12H
Gayer 166), 12K (layer 186), 12N (layer 210), 120 (layer 228), and
12P (layer 246).
In another form of invention the layers 66 and 81 can have
similar but slightly different indices n2 and n2', respectively. The
operating principles of the device 10 will be substantially similar as
long as the critical angles associated with interfaces between the first
WO 94/20871 PCT/US94102598
22
layer 61 and the two layers 66 and 81 do not differ by more than the
first layer convergence angle:
~arcsin(n2~/nl)-arcsin(n2/nl) I < ~ (7)
Therefore, in this case approximately equal fractions of the
light will enter layers 66 and 81, for further processing by the
redirecting layers 70 and 82, respectively.
All forms of the invention can further include an output
diffuser layer 40, shown in phantom in FIG. 2C or transmissive or
translucent diffuser layer 83 shown in FIG. 6A. In general this
diffuser layer 40 can be a surface diffuser, a volume diffuser, or at
least one array of microlenses having at least a section of a cylinder
(known as a "lenticular array"). These layers 40 and 83 can increase
light uniformity or broaden the angular distribution into ambient.
Lenticular arrays are advantageous because they have low back-
scattering in comparison to surface or volume diffusers, and because
they have sharper output angle cut-offs when illuminated by
collimated light. Lenticular arrays also preferentially diffuse only
those features which would otherwise run in the general direction of
the axis of each cylindrical microlens.
In one preferred embodiment shown in FIG. 10, the light
redirecting layer 110 makes use of flat facets 111 such that the output
light is highly collimated. The desired output angular distribution is
further controlled by including a lenticular diffuser 112 having an
appropriate focal ratio, with its cylindrical microlenses running
approximately parallel to the y-axis. The lenticular diffuser 112 also
diffuses non-uniformities which would otherwise appear to be running
in the general direction of the y-axis. In this embodiment a second
lenticular diffuser 113 can be included to diffuse non-uniformities
which would otherwise appear running in the general direction of the
z-axis. This second lenticular diffuser's microlenses run
approximately parallel to the z-axis (see FIG. 12H and 12N). Note
that the order of positioning of the diffusers 112 and 113 can be
interchanged without loss of optical advantage. Similarly, the
lenticular diffuser 112 and 113 can be inverted and can have concave
contours rather than convex contours shown in FIG. 10. While such
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23
changes can affect the details of the performance, the diffuser layers
112 and 113 can still provide the general advantages described.
In another preferred embodiment shown in FIG. 11, the
functions of the flat-faceted light redirecting layer 110 and the parallel
lenticular diffuser 112 in FIG. 10 can both be performed by a light
redirecting layer 114 having curved facets (see also, for example,
FIGS. 2H, 2M and 3A illustrating curved facets). These curved-facet
layers redirect the light, control the angular output by having an
appropriate facet curvature, and act as a diffuser for non-uniformities
running in the general direction of the y-axis. By combining these
functions in a single-layer the number of components is reduced,
which improves thickness, cost, and manufacturability. In this
embodiment, a single lenticular diffuser 115 can be included to diffuse
the remaining non-uniformities which would otherwise appear running
in the general direction of the z-axis. This type of lenticular diffuser
microlens runs approximately parallel to the z-axis. Note that the
lenticular diffuser 115 can be inverted and can have concave contours
rather than the convex contours shown in FIG.10. Again, such
changes can affect performance details, but the layers 114 and 115
perform as intended.
In all embodiments using multiple micro-structured layers, the
facet or lenslet spacings of these layers described hereinbefore can be
chosen to have non-rational ratios, in order to avcid undesirable Moire
patterns. Similarly, each layer's feature spacing can be designed to
have non-rational ratios with the apparatus to be illuminated, such as a
liquid crystal display or charge-coupled detector (CCD) array. Each
of the lenticular diffuser layers 113, 112 and 115 can be tilted up to
about 20° from the configuration shown in the figures in order to
reduce Moire interaction between layers of a liquid crystal display.
Similar lenticular diffusers can be used with non-wedge
geometries having wedge shaped cross-sections, with similar
advantages if the diffuser cross sections are approximately as shown
in FIGS. 10 and 11. Gne example is the tapered disk illustrated in
FIG. 5. In this case the lenticular diffuser analogous to layer 112 in
FIG.10 would have microlenses whose axes run in concentric circles
about the disk's axis of rotations. A diffuser analogous to the layer
WO 94!20871 ~ '~ PCT/US94102598
24
113 in FIG. 10 and 115 in FIG. 11 would have microlenses whose
axes emanate radially from the disk's central axis.
Li~,~ht Sources and Couplers
In a more preferred form of the invention shown in FIGS. 2A
and B, a faceted layer 30 has been included for optically redirecting
the light. The facets 34 can be integral to the layer 30 or a separate
facet layer. Details of operation of such a faceted layer have been
discussed hereinbefore. As shown further in FIG. 6A an input faceted
layer 74 can also be disposed between a light source 76 and the first
layer 61. The faceted layer 74 can be a prismatic facet array which
provides a collimating effect for input light 78 which provides brighter
or more uniform output light 80 into ambient.
Linear prisms parallel to the y-axis can improve uniformity by
adjusting the input angular distribution to match more closely the
input numerical aperture. Linear prisms parallel to the x-axis can limit
the output transverse angular distribution, and also improve output
brightness when used with a fluorescent lamp light source. In other
forms of the invention, diffusion of input light is desirable wherein a
diffuser 79 is used to diffuse the light distribution to spread out the
light to improve light uniformity. The diffuser 79 is preferably a
lenticular array, with cylindrical lenslets parallel to the y-axis. The
diffuser 79 can also be a standard surface or volume diffuser, and can
be a discrete film or coupled integrally to the wedge layer 61.
Multiple prismatic or diffuser films can be used in combination. Such
a film form of the diffuser 79 and the faceted film 74 can be
interchanged in position to vary their effects.
In another preferred form of the invention, a portion of a
dielectric total internally reflecting CPC portion 100 (compound
parabolic concentrator) can be interposed between the light source 76
and the first layer 61 (see FIGS. 2L, 120 and 12P). The CPC portion
100 adjusts the input light to match more closely the input numerical
aperature. The CPC portion 100 is preferably formed integrally with
the first layer 61.
Reflector elements 92 and 94 shown in FIGS. 7 and 8,
respectively, can be shaped and positioned to maximize the
throughput of light from the light source 76 to the light-pipe aperature.
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This is equivalent to minimizing the reflection of light back to the
light source 76, which partially absorbs any returned light. The light
source 76 is typically cylindrical and is surrounded by a transparent
glass envelope 93, each having circular cross-sections as shown in
FIGS. 7 and 8. Typical examples of such light sources include
fluorescent tubes and long-filament incandescent lamps. The outer
diameter of the light source 76 can be less than or equal to the inner
diameter of the glass envelope 93. FIG. 7 shows a prior art U-shaped
reflector 92 formed by wrapping a specular reflectorized polymer filin
around the light source 76 and making contact with the wedge layer
12 at each end of the film. The reflector element 92 typically is
formed into a shape which is approximately an arc of a circle on the
side of the light source 76 opposite the wedge layer 12, with
approximately straight sections connecting each end-point of the arc
with the wedge layer 12. This manner of coupling the reflector
element 92 to the wedge layer 12 is most easily accomplished when
the reflector element cross-section lacks sharp corners. In general the
light source 76 is not permitted to touch either the wedge layer 12 or
the reflectorized film, in order to minimize thermal and electrical
coupling which can reduce lamp efficiency.
In one form of the present invention shown in FIG. 8, the
reflector element 94 is advantageously designed and the light source
76 is advantageously placed to minimize the fraction of light returned
to the light source 76, and thereby increases efficiency. In one
preferred embodiment, at least a section of the reflector element 94 is
shaped such that a line drawn normal to the surface of the reflector
element 94 at each point is tangent to the circular cross-section of the
light source 76. The resulting reflector shape is known as an involute
of the light source 76.
While an involute provides maximum efficiency, other shapes
can generally be more easily manufactured. Polymer films can be
readily bent into smooth curves which include almost semicircular
arcs, as described above. It can be shown that when the cross-section
of the light source 76 and semicircular section of the reflector element
92 are concentric as shown in FIG. 7, then the semicircular section of
the reflector element 92 will return all incident rays to the light source
WO 94/20871 PCT/US94102598
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76, leading to poor efficiency. Such inefficiency is a general property
of self absorbing circular sources and concentric semicircular
reflectors. This general property can be derived from simple ray-
tracing or the principal of skew invariance. Even if the reflector
element 92 is not perfectly circular, each portion of the reflector
element 92 will tend to return light to the light source 76 if the cross-
section of the light source 76 is centered near the center of curvature
of that reflector section.
In another preferred embodiment, the cross-section of the
reflector element 94 in FIG. 8 includes one or more alinost
semicircular arcs, and efficiency is increased by displacing the center
of the light source 76 away from the center of curvature of the
reflector element 94. Ray-tracing and experiments have shown that
such preferred embodiments can be determined using the following
design rules:
1. 'The cross-section of the reflector element 94 has a
maximum extent in the x-dimension equal to the
maximum thickness of the wedge layer 12 (or
light pipe);
2. The cross-section of the reflector element 94 has no
sharp corners;
3. The radius of curvature of the reflector element 94 is as
large as possible; and
4. The light source 75 is as far as possible from the wedge
layer 12, but is far enough from the reflector element 94
to avoid contact with worst-case manufacturing
variations.
FIG. 8 shows an example of a coupler which satisfies these
above described design rules for the light source 76 with inner
diameter = 2 mm, outer diameter = 3 mm, thickness of the wedge
layer 12 (or light pipe) = 5 mm, and manufacturing tolerances which
permit a 0.25 mm spacir_g between the reflector element 94 and the
outer diameter of the glass envelope 93. In this example of a preferred .
embodiment the radius of curvature of the reflector element 94 is
2.5 mm, and the center of the light source 76 is displaced by 0.75 mm
away from the aperture of the wedge layer 12. A coupler constructed
CA 02157898 2002-O1-04
27
according to this design was found to be 10-15% brighter than the comparable
concentric coupler shown in FIG. 7.
The involute and the U-shaped reflector elements 92 and 94 previously
described are designed to output light to the aperture of the wedge layer 12
with
angles approaching ~ 90 degrees relative to the aperature surface normal. In
another preferred embodiment, the reflector element 94 is shaped to output
light
with an angular distribution which is closer to the N.A. of the device 10. As
shown in FIGS. 6B and 6C, such shapes as the reflector element 94 can include
other geometries, such as, a compound parabolic source reflector 86 and a
nonimaging illumination source reflector 88. An example of the source
reflector
88 is described in Canadian Patent File No. 2,161,667 filed April 28, 1994.
In another embodiment of the invention shown in FIGS. 6D, 12C, 12N,
and 120, the wedge layer 90 has a non-monotonic varying wedge cross sectional
thichness over various selected portions of the wedge shaped cross section. It
has
been determined that one can exert control over the light distribution being
output
by control of this cross section. Further, it has been determined that optical
boundary effects, as well as intrinsic light source effects, can combine to
give an
output light distribution with unwanted anomalies. One can therefore also
compensate for these anomalies, by providing a wedge cross section with
nonlinear changes in the actual dimensions of the wedge layer 90, for example,
near the thicker end which typically receives the input light. By control of
these
dimensions one can thus have another degree of freedom to exert control over
the
light distribution, as well as provide virtually a design to compensate for
any
boundary effect or light source artifact. Furthermore, one can vary the index
of
refraction with the wedge layer 90 in the manner described hereinbefore to
modify the distribution of light and also compensate for light input anomalies
to
provide a desired light distribution output.
Manufacture of Luminaire Devices
In one form of the invention, manufacture of the device 10 can be
accomplished by careful use of selected adhesives and lamination
WO 94/20871 PCT/US94/02598
28
procedures. For example, the wedge layer 12 having index nl can be
adhesively bonded to the first layer 28 having index n2. An adhesive
layer 60 (see FIG. 3S) can be applied in liquid form to the top surface
of the first layer 28, and the layer 28 is adhesively coupled to the
bottom surface 16 of the wedge layer 12. In general, the order of
coupling the various layers can be in any given order.
In applying the layer 12 to the layer 28 and other such layers,
the process of manufacture preferably accommodates the formation of
internal layer interfaces which are substantially smooth interfacial
surfaces. If not properly prepared such internal layers can
detrimentally affect performance because each interface between
layers of different indices can act as a reflecting surface with its own
characteristic critical angle. If the interfacial su.~faces are substantially
smooth, then the detrimental effect of uneven surfaces is negligible.
Therefore in effectuating the lamination of the various layers of the
device 10, the methodology should utilize adhesives and/or joining
techniques which provide the above described smooth interfacial
layers. Examples of lamination processes include without limitation
joining without additional adhesive layers, coatings applied to one
layer and then joined to a second layer with an adhesive and applying
a filin layer with two adhesive layers (one on each layer surface to be
joined to the other).
In a preferred embodiment lamination of layers is done without
any additional internal layer whose potential interfacial roughness will
distort the light distribution. An example of such a geometry for the
device 10 can be a liquid layer between the wedge layer 12 and the
second layer 30. This method works best if the first layer 29 (such as
the liquid layer) acts as an adhesive. Gne can choose to cure the
adhesive either before, partially or completely, or after joining
together the various layers of the device 10. The optical interface is
thus defined by the bottom surface of the wedge layer 12 and the top
surface of the second layer 30.
In another embodiment wherein a coating is used with an
adhesive layer, the first layer 28 can be the coating applied to the
second layer 30. Then, the coated filin can be laminated to the wedge
layer 12 in a second step by applying an adhesive between the coated
WO 94!20871 PCT/US94/02598
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29
film and the wedge layer 12. It is preferable to apply the low index
coating to the second layer 30 rather than directly to the wedge layer
12 since the second layer 30 is typically supplied in the form of
continuous film rolls. In practice it is more cost effective to coat such
continuous rolls than to coat discrete pieces. With this methodology it
is more convenient to control thickness of the applied low index layer.
In another embodiment, the second layer 30 is manufactured in
such a way that it adheres to the first layer 28 directly without use of
additional adhesives. For example, the second layer 30 can be
manufactured by applying a layer of polymer material to the first layer
28, and then casting this material to have the desired second layer
geometry. In another example, the nrst layer 28 can serve as a carrier
film during the embossing of the second layer 30. By use of
appropriate temperatures during the embossing process, the second
layer 30 can be heat-fused to the first layer 28. Such heat-fusing can
be accomplished using a conventional FEP first-layer film by
embossing at almost five hundred degrees F or higher.
In a further embodiment using a film and two adhesives, the
fi.~st layer 28 can be an extruded or cast film which is then laminated
to the wedge layer 12, or between the wedge layer 12 and the second
layer 30 using adhesive between the two types of interfaces. In order
to minimize the detrimental light scattering described hereinbefore,
the adhesive layer should be flat and smooth. The film can be
obtained as a low index material in commercially available,
inexpensive forms. Such additional adhesive layers can increase the
strength by virtue of the multi-layer construction having adhesive
between each of the layers.
In the use of adhesive generally, the performance of the device
is optimized when the index of the adhesive between the wedge
layer and the first layer is as close as possible to the index of the first
layer 28. When the critical angle at the wedge/adhesive interface is as
low as possible, then the light undergoes a minimal number of
reflections off the lower quality filin interface before exiting the
device 10. In addition, the index change at the surface of the first
layer film is minimized which decreases the effects of film surface
roughness.
WO 94/20871 PCTIUS94/02598
Manufacture of faceted surfaces can be accomplished by
micro-machining a mold using a master tool. Machining can be
carried out by ruling with an appropriately shaped diamond tool. The
master tool can be replicated by known techniques, such as
electroforming or casting. Each replication step inverts the shape of
the desired surface. The resulting mold or replicates thereof can then
be used to emboss the desired shape in the second layer 30. A directly
ruled surface can also be used, but the above described embossing
method is preferred. Known "milling" processes can include chemical
etching techniques, ion beam etching and laser beam milling.
In yet another method of mechanical manufacture, the faceted
surface 34 (see FIGS. 2B and 2M, for example) is manufactured by a
welding process, such as embossing or casting, using a hard tool
which has on one surface the inverse of the profile of the desired
faceted surface 34. Therefore, the manufacturing problem reduces to
the matter of machining an appropriate tool. Usually the machined
tool is used as a template to form the tools actually used in the casting
or embossing process. Tools are typically replicated by
electroforming. Since electroforming inverts the surface profile, and
electroforms may be made from other electroforms, any number of
such inversions can be accomplished and the directly machined
"master" can have the shape of the faceted surfaces 3A or its inverse.
The tooling for the faceted surface 34 can be manufactured by
single-point diamond machining, wherein the distance between
cutting tool and the work is varied to trace out the desired profile. The
diamond cutting tool must be very sharp, but in principle nearly
arbitrary profiles can be created. A given design can also require
specific adaptations to accommodate the non-zero radius of the cutting
tool. If curved facet surfaces are required, then circular arcs are
preferred to facilitate fabrication. The cutting tool is moved through
the cutting substrate and cuts a groove having the approximate shape
of the tool. It is desirable to machine the entire piece using a single
diamond tool. When this method is used for making a "focusing" type
of the faceted surface 34, the variable groove profile therefore should
be designed such that the various groove profiles can be machined by
the same tool. The required shape variations can still be accomplished
WO 94120871 PCT/US94/02598
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31
by varying the angle of the tool, as well as the groove spacing and
depth.
Design of the faceted surface 34 preferably satisfies a few
general constraints:
1. Approximately linear variation in the center of the
illumination angular distribution as a function of
position. A variation of 11 degrees (t 5.5°) from top to
bottom of typical computer screens is effective;
2. The width of the variable angular distribution of light
output should be approximately proportional to the local
illuminance in order to achieve approximately uniform
brightness to an observer. Examples given below show
the spatial distribution is approximately uniform, so the
angular cones have approximately uniform width; and
3. Spacing between grooves of the facets 38 should be
large enough or irregular enough to avoid diffraction
effects, but also be chosen to avoid Moire patterns when
used with an LCD panel. In practice these requirements
limit the allowed spatial variations.
In the manufacture of the device 10, for example, the viewing
angle depends on the tilt and curvature of each of the facets 38.
Focusing is accomplished by rotating the facet structure as a function
of position. Using the example of a 150 mm screen viewed from
500 mm away, the illumination cone can be varied by 17 degrees i.e.
~ 8.5 degrees) from top to bottom. For typical materials, acrylic and
FEP, this requires the facet structure to rotate by approximately
5.7 degrees from top to bottom of the screen 89 (see FIG. 9B).
Design constraints can result when limitations (1)-(3) are
combined with the need to machine variable curved grooves with a
single tool. For example, maintaining a constant angular width
(Constraint #1) at a constant cutting depth requires a compensating
variation in groove spacing or groove depth. Specifically, a linear
change in groove spacing can reduce the brightness variation to a
negligible level when the form tool which cuts the groove is shaped so
that portions of each curved reflector facets (see FIG. 2M) are
,. P~1IUS 94 / 02 ~ ~ 8
~~57898
32 ~P~/~$ Q 4 OCT 199
shadowed by the top edge of the adjacent facets. This spacing
variation can be small enough to satisfy Constraint #3.
Further methods of manufacture can include vapor deposition,
sputtering or ion beam deposition of the first layer 28 since this layer
can be quite thin as described hereinbefore. Likewise, the second
layer 30 can be controllably applied to form the faceted layer 30
shown in FIG. 2B (such as by masking and layer deposition).
Wedge Light Pipe as a Simple Collimator Device
In the most general embodiment the wedge layer 12 can
function in the context of the combination as a simple collimating
optical element. The substantially transparent wedge layer 12 has an
optical index of refraction nl and the top surface 14 and the bottom
surface 16 converge to establish at least one angle of inclination ~ (see
FIG. 15). The wedge layer 12 also includes the back surface 20
spanning the top surface 14 and the bottom surface 16. Adjacent to
the wedge layer 12 is the transparent first layer 28 having index of
refraction n2 including an air gap. Adjacent to the first layer 28 is a
specular reflective layer, such as the faceted surface 34 of the second
layer 30.
Substantially uncollimated light is introduced through the back
surface 20 by the source 22. The light propagates within the wedge
layer 12, with each ray decreasing its incident angle with respect to
the top and bottom surfaces 14 and 16 until the incident angle is less
than the critical angle 9c. Once the angle is less than Ac, the ray
emerges into ambient. Rays which emerge through the bottom surface
16 are reflected back into the wedge layer 12 and then output into
' ambient. By virtue of the angle-filtering effect previously described,
' the output light is collimated within a cone of angular width:
06 = 2 ~'~_ (n2-1)'~~ (Fresnel reflections will somewhat increase 08.) (8)
An area 99 to be illuminated lies beyond the end of the wedge layer
12 and substantially within the above-defined cone of width 0A.
In another preferred embodiment a light-redirecting means can
be positioned beyond the end of the wedge layer 12 and substantially
within the above-defined cone of width 08. The light-redirecting
means can be a lens, planar specular reflector, or curved reflector.
AMENDED Sii~E~t
WO 94/20871 PCTIUS94102598
~~57~9~
33
The light-redirecting means reflects or refracts the light to the area to
be illuminated. Further details and uses of such redirecting means,
such as lenticular diffusers, will be described hereinafter.
In the embodiments of FIG. 6 having two air gaps or
transparent dielectric layers, the light redirecting layers are
independent, and thus one can construct devices having layers of
different types. For example, the use of two transmissive redirecting
layers is preferred when light is to be emitted from both sides of the
device 10 or whenever maximum collimation is desired. Examples of
the redirecting layer 82 in general for all inventions for two redirecting
layers can include the examples in FIG. 12: (a) diffraction gratings
120 or a hologram 122 in FIG. 12A, (b) two refracting facet layers
124 with diffusers 126 in FIG. 12B, (c) two faceted layers 128 with
facets 130 designed to refract and internally reflect light output from
the wedge layer 12; such facets 130 are capable of turning the light
output through a larger angle than is possible by refraction alone;
(d) two refracting single facet layers 132 (prisms); (e) a top surface
redirecting layer for the wedge layer 12 having a refracting single
facet layer 134 with a curved output surface 136 for focusing. A
bottom surface 138 includes a redirecting layer for refracting and
internally reflecting light using a faceted layer 140; facet angles are
varied with position to focus output light 142 at F; (f) a top surface
redirecting layer 144 comprised of a refracting faceted layer 146 and a
bottom redirecting layer comprised of a refracting/internally reflecting
layer 148 with narrow angle output for the light, and a diffuser layer
150 can be added to smoothly broaden the light output angular
distribution; (g) a top surface redirecting layer of refracting/internally
reflecting faceted layer 152 with refracting surfaces 154 convexly
curved to broaden the output angular distribution; the facet angles can
be varied with position and thereby selectively direct the light output
a.-~gular cones to create a preferred viewing region at a finite distance;
this arrangement can further include a transverse lenticular diffuser
156 to diffuse nonuniformities not removed by the curved facet layer
152; the bottom redirecting layer comprises a refractinglinternally
reflecting faceted layer 158 with a reflecting surface 160 being
concavely curved to broaden the light output angular distribution in a
WO 94/20871 PCTIUS94/02598
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34
controlled manner; (h) a top redirecting layer, including a refracting
faceted layer 162 with curved facets 164 to broaden the output angular
distribution in a controlled manner and to improve uniformity; a
bottom redirecting layer, including a refracting/internally-reflecting
faceted layer 166 with flat facets 168 for narrow-angle output, with
facet geometry varied with position to focus output light at a finite
distance; a parallel lenticular diffuser 170 can be used to smoothly
broaden the output angular distribution in a controlled manner and to
improve uniformity; the transparent image shown in phantom can be
printed on or adhesively based to a lenticular diffuser; a transverse
lenticular diffuser 172 is used to diffuse non-uniformities not removed
by the parallel lenticular diffuser 170. The combination of a focused
flat-faceted layer 166 and the diffuser 170 cooperate to create a
preferred viewing zone at a finite distance, similar to using focused
curved facets. Also shown is an LCD component 173 (in phantom)
usable with this and any other form of the device 10 for illumination
purposes.
In other architectures, one transmissive and one reflective
redirecting layer can be combined. These are combinations of
reflective redirecting layers with the various types of transmissive
redirecting layers discussed above. Reflective redirecting layers can
be specular, partially diffuse, diffuse, faceted or any combination
thereof. These architectures are preferred when light emission is
desired from one side only, or in some cases when minimum cost is
paramount. Examples of such architectures are: (i) a bottom surface
specular reflector 174 combined with a top layer transmission
diffraction grating or transmission hologram 176; (j) a bottom surface
specular reflector 178 combined with a top surface refracting faceted
layer 180, with a diffuser 182 (shown in phantom in FIG. 12J and an
intervening image-forming layer 171; (k) a bottom layer specular
reflector 184 with a top layer refracting/internally-reflecting faceted
layer 186, with facet geometry being varied with position to focus
output light at a finite distance; a diffuser 188 is shown in phantom;
(1) a bottom layer specular reflector 190 with a top layer
refracting/internally-reflecting faceted layer 192, and curved facets
194 are used to smoothly broaden the angular output of light in a
WO 94120871 PCT/US94/02598
~5T898
controlled manner and to improve uniformity. The thickness of the
wedge layer 12 and of both top and bottom surface low-index layers
196 e(~. ., air gaps) are varied to influence the light output spatial
distribution; (m) a bottom reflector 198 is partially specular, partially
diffuse to improve uniformity; FIG. 12M shows the initial reflector
section made controllably diffuse by addition of an integral lenticular
diffuser 200; the diffuser 200 is designed to selectively reduce
nonuniformities which would otherwise appear in the output near the
thicker end, and running in the general direction of the y-axis; also
included is a top redirecting layer 202 which is refracting/internally-
reflecting and has a reflecting surface which is curved; and (n) a
bottom reflector layer 204 which is partially specular, partially diffuse
to improve uniformity; FIG. 12N shows the initial reflector section
206 which is slightly roughened to reduce specularity, and thereby
selectively reduces nonuniformities which would otherwise appear in
the output near thicker end 208; a top redirecting layer 210 is used
which is refracting/internally-reflecting with a flat-faceted layer 212,
and the facet geometry is varied to redirect light from each facet to a
common focus at finite distance; a transverse lenticular diffuser 213 is
shown in phantom; a parallel lenticular diffuser 214 is used to
smoothly broaden the output angular distribution in a controlled
manner, converting the focal zone of the flat-faceted layer 212 to a
wider preferred viewing zone; the lenticular diffuser 213 also
improves uniformity; an LCD display 216 or other transparent image
is show in phantom; (o) in a preferred embodiment an eccentric
coupler 218 uses a uniformity-enhancing lenticular diffuser 220
shown in phantom in FIG. 120. A converging tapered section 222 or
CPC (integral to the wedge layer) transforms the output angular
distribution to match more closely the input N.A. of the wedge
layer 12. The wedge layer 12 thickness is smoothly varied to
influence output spatial distribution and improve uniformity; a bottom
redirecting layer 224 is a specular or partially diffuse reflector; a top
redirecting layer 226 is a refracting/internally-reflecting faceted layer
228 with reflecting surfaces 230 convexly curved to smoothly broaden
output angle in a controllable manner; facet geometry is varied with
position to selectively direct the angular cone of light from each face
WO 94/20871 PCT/US94102598
36
to create a preferred viewing zone 232 at a finite distance; a transverse
lenticular diffuser 234 is shown in phantom; an LCD display 236 or
other transparent image is also shown in phantom; the more
converging N.A.-matching section is advantageous in combination
with the faceted redirecting layers, because the redirecting and low-
index layers do not need to overly the more converging section;
therefore, the input aperture (and thus efficiency) of the device 10 is
increased with minimum increase in total thickness of the device;
(p) another preferred embodiment for LCD backlighting uses an
eccentric coupler with a uniformity-enhancing diffuser shown in
phantom in FIG. 12P; a converging half tapered section 240 or half
CPC (integral to the wedge layer 12) transforms a coupler output
angular distribution to match more closely the input N.A. of the wedge
layer 12. A diffuser 239 (in phantom) can also be interposed between
light source 217 and the wedge layer 12. The sufficiently truncated
half CPC 240 is just a simple tapered section. A bottom reflector 242
which is partially specular, partially diffuse is used to improve
uniformity; FIG. 12P further shows an initial reflector section 244
which is slightly roughened to reduce specularity, or alternatively
shaped into a series of parallel reflective grooves, which thereby
selectively reduces nonuniformities which would otherwise appear in
the output near the thicker end; a top redirecting layer 246 is a
refracting/internally-reflecting faceted layer 248, with refracting
surfaces 250 convexly curved to smoothly broaden output angle in a
controllable manner; facet geometry is varied with position to
selectively direct angular cones of light from each facet to create a
preferred viewing zone at a finite distance; a transverse lenticular
diffuser 252 is shown in phantom. Also included is an LCD display
254 or other transparent image shown in phantom.
The more converging N.A.-matching section (such as half
tapered section 240) is advantageous in combination with the faceted
redirecting layers, because the redirecting and low-index layers do not
need to overly the more converging section; therefore, the light-
accepting aperture of the device 10 is increased without increasing the
total thickness. The advantage is also conferred by the fully-tapered
section 222 shown in FIG. 120; but in comparison the half tapered
WO 94/20871 PCT/US94/02598
21~~~s~
37
section 240 in FIG. 12P provides greater thickness reduction on one
side, at the expense of being longer in the direction of taper for
equivalent N.A.-matching effect. It can be desirable to concentrate the
thickness reduction to one side as shown, because the top surface low-
index layer can be made thicker to improve uniformity. This
configuration can be more easily manufactured because the bottom
reflector layer can be integral to the coupler reflector cavity, without
need to bend a reflective film around a corner; (q) a bottom specular
or diffusely reflecting layer 256 can be combined with single-facet
refracting top layer 258 in yet another embodiment (see FIG. 12Q);
and (r) in cases for interior lighting usage, a bimodal "bat-wing"
angular light distribution 260 is preferred; in FIG. 12R is shown a top
refracting layer 262 with facets 264 and has a curved front surface 266
to smoothly broaden angular output and improve uniformity, with
output light directed primarily into a forward quadrant; a bottom
reflecting layer 268 reflects light primarily through a back surface of a
top redirecting layer, with output directed substantially into a
backwards quadrant.
As understood in the art the various elements shown in the
figures can be utilized with combinations of elements in tapered
luminaire devices. Examples of two such combination geometries are
shown in FIGS. 13 and 14, each figure also including features specific
to the geometry shown. As illustrated in FIG. 13, two wedges 276 can
be combined and foamed integrally. This combination can provide
higher brightness than a single wedge having the same extent because
it permits two light sources to supply light to the same total area.
While brightness is increased for this device, e~ciency is similar
because two sources also require twice as much power as one source.
A redirecting film 272 with facets 274 can be a single, symmetric
design which accepts light from both directions as shown.
Alternatively, the redirecting filin 272 can have a different design for
each wing of the butterfly.
In FIG. 14 is shown a three dimensional rendition of a tapered
disk 270, such as shown in FIG. S, and is sectioned to show the
appearance of the various layers. A faceted redirecting layer 280
comprises concentric circular facets 282 overlying a tapered light-pipe
WO 94/20871 PCT/US94102598
38
portion 284. Directly over a light source 288, overlying the gap at the
axis of the light-pipe portion 284, the redirecting layer 280 takes the
form of a lens (a Fresnel lens 280 is shown, for example). Directly
below the light source 288 is a reflector 290 positioned to prevent
light from escaping and to redirect the light into the light-pipe portion
284 or through the lens. At least one opening is provided in the
reflector to permit passage of elements, such as wires or light-pipes.
Use of Ima i~n~~ or Colored Layers
All embodiments of the invention can incorporate one or more
layers which have variable transmission to form an image, or which
impart color to at least a portion of the angular output. The image-
forming layer can include a static image, such as a conventional.
transparent display, or a selectively controlled image, such as a liquid
crystal display. The image-forming or color-imparting layer can
overlay one of the redirecting layers, or alternatively it can comprise
an intermediate layer between one of the low-index layers and the
associated redirecting layer, or an internal component of a redirecting
layer. For example, overlying image-forming layers 129 are shown in
phantom in FIGS. 12C and 12G. Examples of an internal image-
forming layer 171 are shown in FIGS. 12H and 12J.
In one preferred embodiment, the image-forming layer (such as
129 and 170) is a polymer-dispersed liquid crystal (PDLC) layer. By
proper arrangement of the layers, the image or color may be projected
from the device within selected portions of the output angular
distribution. The image or selected color can be substantially absent
in the remaining portions of the output angular distribution.
Bi-modal Reflective Wedge for LCD Panel Illumination
In some applications it is desired to illuminate a single LCD
panel selectively with either ambient light or by active back-lighting.
In these applications ambient illumination is selected in well-lit
environments in order to minimize power consumption by the display.
When available environmental illumination is too low to provide
adequate display quality, then active backlighting is selected. This
selective bi-modal operating mode requires a back-illumination unit
which can efficiently backlight the LCD in active mode, and
efficiently reflect ambient light in the alternative ambient mode.
WO 94120871 PCT/US94/02598
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39
The most widespread prior art bi-modal liquid crystal display is
the "transffective display" 101, such as is shown in FIG. 16B. This
approach uses a conventional backlight 102 and a transmissive LCD
panel 103, with an intervening layer 104 which is partially reflective
and partially transmissive. In order to achieve adequate ambient mode
performance, it is typically necessary for the intervening layer 104 to
be 80-90% reflective. The resulting low transmissivity makes the
transflective display 101 inefficient in the active mode of operation.
Another embodiment of the invention is shown in FIG. 17.
This embodiment outperforms prior art transffective displays in the
active mode, and demonstrates comparable performance in the
ambient mode. In this embodiment the wedge layer 12 (index = nl)
having the bottom surface 16 is coupled to a transparent layer 28 of
index n2 < nl, which can be an air gap. The n2 layer is coupled to a
partially diffuse reflector layer 105. This reflector layer 105 is, for
example, preferably similar to the reflectors used in conventional LCD
panels used in ambient mode only, as shown in FIG. 16A. Overlaying
the wedge layer top surface 14 is a faceted redirecting layer 106, such
as a lenticular diffuser with microlenses approximately parallel to the
y-axis. A liquid crystal display panel 107 overlays the faceted
redirecting layer 106. The back surface 20 of the wedge layer 12 is
coupled to the light source 22.
The lenticular redirecting layer 106 and the wedge-layer 12 are
substantially transparent to the incident and reflective light, so that in
ambient mode the device 10 operates in a manner similar to
conventional ambient-mode-only displays. When an active mode is
selected, the light source 22 is activated, and the multiple layers act to
spread the light substantially uniformly over the device 10 by virtue of
the relationship between the indices of refraction and convergence
angles of the layers, as described before. The resulting uniform
illumination is emitted through the top surface 14 of the wedge layer
12. In a preferred embodiment, the reflector layer 105 is nearly
specular in order to maximize ambient-mode performance. In this
preferred embodiment the light emitted from the top surface is emitted
largely at grazing angles, unsuitable for transmission by the LCD
display panel 107. The redirecting layer 106 redirects a fraction of
WO 94!20871 PCTIUS94/02598
r,.
this light by a combination of refraction and total internal reflection,
as described hereinbefore. The redirecting layer 106 is preferably
designed such that at least 10-20% of the light is redirected into angles
less than 30 degrees from the LCD normal, because typically the LCD
transmission is highest in this angular range. It is sufficient to direct
only a fraction of the back-illumination into suitable angles, because
the prior art transflective display is quite inefficient in the active mode
of operation.
While preferred embodiments of the invention have been
shown and described, it will be clear to those skilled in the art that
various changes and modifications can be made without departing
from the invention in its broader aspects as set forth in the claims
provided hereinafter.
