Note: Descriptions are shown in the official language in which they were submitted.
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LIGHT DIRECTING OPTICAL STRUCTURE
Cross ReLere,nce Lo Related ApRiications
This application is a continuation-in-pan of three U.S. patent applications:
Appl. No. 08/149,219 .filed November 5, 1993 (now U. S. Patent No. 5, 396,
350) ,
Appl. No. 08/242,525, filed May 13, 1994 (Now U.S. Patent No. 5,428,468)
and Appl. No. 08/321,368, filed October 11, 1994 (now U.S. Patent
No. 6,129,439).
Back=,ilnd of the Invention
The invention relates generally to optical sttuctures which receive light
from relativety uncontrolled sources in terms of directivity and uniformity
and emit
a spatiaIIy-controiled light distribution with respect to the two orthogonal
axes
defined with respect to the direction of propagation of the tight source, and
relates
particularly to such structures utilized for residential, commercial and
industrial
2o lighting applications.
The ab'ility to control the light distn'bution from various light sources,
such
as a point source or an astended source about two orthogonal axes has not been
saccessfuity implemented in a unitary structure. For example, in typical
oflice
lighting applications using fluorescent Iighting, no single element is able to
sinu,ltaneously control the light distribution about two axes. A fluorescent
light
contains a single reflector which is only able to provide directivity along
one axis.
Ahernatively, if two axes control is attempted with the present technology, a
reflector and huninaries are required. This combination, however, provides
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efficiency losses, suffers from non-uniformity and creates a complex and bulky
arrangement.
It would be desirable to have a relatively robust, lightweight and efficient
optical structure that would fit easily within a small form factor and that
would be
able to receive light from any light generating means. Such a structure would
permit new and useful ways to provide directed light distribution for various
lighting applications.
Summary of the Invention
The present invention provides for an integrated light directing structure
which provides a spatially-directed light output along two orthogonal
dimensions
as required by the particular application.
The invention comprises a light source in combination with an optical
structure which comprises a waveguide component which accepts the light
is generated by the light source and transports the light via total interaal
reflection
(TIlt). Optically coupled to or integrally formed with a surface of the
waveguide
are a multiplicity of prisms. Each prism, due to its index of refraction,
provides an
aperture where light may escape the waveguide rather than remain confined due
to
total internal reflection. The light escaping the waveguide is reflected from
a side
face of the prism, and redirected in the desired output direction.
The geometry of the prism can be optimized to direct the output at any
desired angle from the surface of the waveguide. Advantageously, the optical
structure, which might be characterized as a light directing structure, can
have a
narrow profile, and the waveguide structure is suitable to allow light
coupling
along an light input edge, or along several light input edges. Furthermore,
there is
no restriction on the distribution and uniformity of the light input to the
waveguide.
There are many illumination applications that can take advantage of this
invention. Such applications exist in commercial and residential markets and
in
various industries such as the automotive industry and the aerospace industry.
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Exemplary residential and commercial applications include low profile interior
and
exterior lighting such as spotlights, room or office lighting and accent
lighting.
Exemplary automotive applications include low profile car headlights and
taillights,
low profile interior car lights such as reading lights and map lights and
light sources
for dashboard displays and instrument panels.
Brief Description of the Drawing
The invention will be described with respect to a drawing of several figures,
of which:
Figure 1 is an illustration of a single prism optically connected to a
waveguide and showing the characteristics of light rays;
Figure lA is an alternate embodiment of a waveguide;
Figure 2 illustrates the light distribution output of the invention about two
orthogonal axes;
Figure 3 is a cross sectional view illustrating the geometries of the prism
and waveguide;
Figures 4A and 4B is an alternate embodiment of a prism;
Figure 4C is a further alternate embodiment of a prism;
Figure 5 illustrates a waveguide and a prism in combination with a negative
lens structure;
Figure 6 is a plan view of the invention where the prisms are illustrated as a
single sheet for simplicity purposes in combination with a point light source;
Figure 7 is a plan view of the invention in combination with a point light
source and a light source homogenizer,
Figure 7A is an elevation view at the input face of a light source
homogenizer,
Figure 8 illustrates the invention in combination with a tapered input
waveguide, an involute reflector and an array of negative lenses;
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Figure 9 is an elevation view of a single light source in combination with
two light directing structures;
Figure 10 is a plan view of light directing structure remotely located from a
light source; and
Figure 11 is an elevation view of the invention optimized for illuminatin
from the ceiling of a large flat object on the wall.
Where possible, like elements have been shown with like reference
numerals.
Detailed Description
One of the basic building blocks of the apparatus according to the invention
is a prism optically coupled to a waveguide. While the apparatus typically may
contain hundreds or thousands of prisms arranged in some pattern or array, it
is
instructive to consider one prism in detail.
With reference to Fig. 1, light rays, as exemplified by light rays 110 and
111, totally internally reflect through waveguide 40. Waveguide 40 may be a
light
pipe, light wedge or any other structure known to those skilled in the art. It
may
have planar surfaces or alternatively may have non-planar sectioned surfaces
as
shown in Fig. 1A For simplicity, and for descriptive purposes, but by no means
intended to be limiting in nature, waveguide 40 is illustrated as being
planar, and
the microprisms 42 discussed below are referenced interfacing with a planar
waveguide. As is well known to those skilled in the art, total internal
reflection
occurs if light within a medium strikes a boundary and bounces back into the
medium. For such a reflection to occur, the index of refraction of the medium
has
to be higher than the index of refraction for the material on the other side
of the
boundary, and the angle of reflection must obey Snell's Law. For simplicity of
analysis one assumes that the material outside the waveguide 40 is air, with
an
index of refraction of one. The invention does not require this, however, and
may
be practiced with materials other than air outside the waveguide.
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Light rays emanating from light source 37 are bound by a "critical angle"
within waveguide 40 as determined by Snell's Law. Light rays 110 or 111, but
for
the presence of prism 42, would be totally internally reflected within
waveguide 40.
Prism 42 has an index of refraction approximately equal to or greater than the
index of waveguide 40, and ray 110 and 111 are able to exit waveguide 40 and
enter prism 42. This situation exists whether prism 42 is integrally formed
with
waveguide 40 or whether it is separately formed and then integrated with
waveguide 40 using an adhesive or other suitable means. After ray 111 enter's
prism 42, it internally reflects to make ray 112. Similarly, ray 110 undergoes
a
reflection to make ray 113. The range of angles of rays, such as 112 and 113,
is
related to the distribution of angles of light rays within waveguide 40.
Because the
rays within the waveguide are constrained, the rays exiting the prism are
found
mostly in a fairly narrowly constrained range of angles. Accordingly, this
leads to
the result that relatively undirected light can be coupled into waveguide 40
and a
is substantial portion of that light exits prism 42 as a directed light
source.
Those skiIled in the art will appreciate that some of the faces of prism 42
are critical. First face 114 preferably is planar (this, however, would not be
true
for waveguides as shown in Fig. lA) so as to have intimate optical coupling
with
the planar face of waveguide 40. Second face 32 need not be absolutely planar,
but could be curved somewhat or could be faceted, and still bring about the
interesting results described; for that reason, it might be said that second
face 32 is
merely substantially planar. Third face 361ikewise need not be absolutely
planar.
It can also be formed into either a convex, concave lens, and even an
aspherical
lens without departing from the results described. For a broadband light
source, it
is desired that the surface of third face 36 be more or less perpendicular to
the
desired light exit direction. This minimizes refraction of the exiting light;
thereby
minimizing the breaking up of a white light into colors. One way to describe
the
desired orientation is that third face 36 has a region tangent to a plane
perpendicular to the desired light output direction, a terminology that
embraces the
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possibility that the face might be planar or might be a lens. Those skilled in
the art
will appreciate that nothing in this discussion demands any particular shape
to the
fourth face 115. In this embodiment, the control of the light output would be
in
only one viewing direction. In a preferred embodiment, however, fourth face
115
is also used to reflect light from the waveguide 40 using the same principles
as face
32. In the most preferred embodiment, fifith face 120 and sixth face 122 also
reflect light rays exiting from waveguide 40. A cross section view of the
preferred
embodiment of prism 42 is shown in Figs. 4A and 4B and disclosed in detail in
referenced U.S. Patent No. 5,428,468. In this most preferred
embodiment, iight rays traveling in different directions within waveguide 40
wil!
enter prism 42 and reflect off atl prism faces. This situation arises when
multiple
light sources are used or where reflective material recycles light back into
the
waveguide as shown in Figs. 6 and 7. This embodiment is preferred because it
provides for control of the light distribution, both in intensity and
direction, about
two viewing axes, xz and yz, as shown in Fig. 2 and provides for efficient
extraction of light from waveguide 40. Furthermore, none of the foregoing
discussion demands that any of the faces of prism 42 join in a simple edge; if
something about the fabrication technique, for example, required that the edge
connection between faces be beveled or rounded, this would not depart from the
invention. In addition, it is not necessary that first face 114 and third face
36 be
parallel. Those skilled in the art will appreciate that in some applications,
a slight
taper might be preferred to distribute the iight output from prisms 42.
There is no requirement, other than perhaps ease of fabrication or
adaptation to a particular light pattern, that all prisms 42 be identically
shaped or
evenly shaped. As set forth in the referenced patent, the spacing of
prisms 42 may vary over the expanse of the waveguide to accommodate the
distance of a prism 42 from the light source. Furthermore, prisms 42 may
attach to
waveguide 40 in selected regions so as to only allow light to escape waveguide
40
at selectively desired locations as dictated by the application. Also, the
angles of
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prisms 42 may be provided over some distribution to yield more light at one
particular exit angle and less at other exit angles, or to yield intentionally
asymmetric spatial distributions of exiting light.
Because the invention may be applied to many different applications, from
automobile headlamps to lighting for art galleries, it is desirable to be able
to vary
the orientation of prisms 42 with respect to waveguide 40 to bring about a
particular desired light distribution output. Reference is now made to Fig. 3.
We
will assume for simplicity that the index of refraction of prism 42 is equal
to the
index of refraction of waveguide 40.Snell's Law determines the angular spread
of
the light propagating in waveguide 40 with respect to the critical angle 0..
Preferably, and in the case of a broadband light source, defined as 400-700
nm,
light ray 20 represents the median ray of the light output distribution. For
illustration purposes only, median light ray 20 reflects off the second face
32 and
exits prism 42 at an angle perpendicular to the tangent of the third face 36
and at
1S an angle 0 with respect to second face 32. The directional output of median
light
ray 20 also forms an angle 9 with respect to the surface 33 of waveguide 40.
Angle 6 is a function of the particular lighting application which would
specify
some light distribution output pattern. For a waveguide having light rays from
all
angles from 0 to ,, a simple relationship exists between the desired angle
of the
light output 0 with the tilt angle,O, second face 32 forms with the surface 33
of
waveguide 40. Angle 0= 20 - 45 + 0d2 where 0. is the critical angle defined by
Snell's Law and equals sin 1(n1/nt) where ni equals the index of refraction of
waveguide 40 and and n2 equals the index of refraction of the material outside
the
waveguide 40 (for example, in the case of air, n2 = 1.00). A similar
relationship
can be derived where the index of refraction of waveguide 40 and prism 42 do
not
match.
Furthenmore, the third or top face 36 is preferably perpendicular to the exit
direction for the light, to avoid refraction of the output light into colors.
In the
case where the light source is narrow band in nature, such as a LED or laser,
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however, the above equation for angle 0 does not appliy because the median
light
ray 20 is not restricted to exit the prism 42 at an angle perpendicular to the
tangent
of the third face 36.
Fig. 4 shows alternative prism shapes 42A, 42B for the cross section of Fig.
1. It should be appreciated from these figures that the first face of the
prism
(coupled with the waveguide 40) is planar, but the second face 32A and 32B
need
merely be substantially planar. Depending on fabrication techniques and the
desired light spill' pattern the second face 32A and 32B could be curved or
could be
planar in two slightly different adjoining planes, without departing from the
invention.
Fig. 5 shows a cross section of a prisms 42 in optical cooperation with a
corresponding concave lens 39. As would be obvious, if multiple prisms 42 were
arranged in any specific pattern, a lens 39 would correspond with each prism
42.
In such an embodiment, the light output of prism 41 is broadened to match the
1s requirements of the particular application. For example, a prism with an
index of
refraction of 1.45, the light output distribution is +/- 35 degrees and a
properly
positioned de-focusing element, such as a concave lens 39, would provide a
greater
angular spread to the light distribution. This embodiment could be
advantageously
used in a commercial lighting application where the preferred angular light
distribution is +/- 60 degrees. Alternatively, scattering elements, either
internal or
external to prisms 42, may be used to broaden the output distribution, but
with a
loss of efficiency
An alternate embodiment of the invention is illustrated in Fig. 6 which
represents in plan view an array of light directing prisms 42 attached to a
waveguide 40 which is optically coupled with a reflector arc light source 44,
such
as a metal halide lamp, at one edge 21. The lamp 44 is positioned
substantially a
focal length away from the edge to allow the focused rays to couple
efficiently into
waveguide 40. Preferably, lamp 44 also comprises a light filtering device to
selectively transmit and or reflect various spectural content of the lamp. For
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example, if waveguide 40 is a plastic material, infared light may provide
unwanted
heat buildup. Additionally, light reflecting means 45, such as specular or
diffusive
reflectors, define an aperture 47 along edge 21. Preferably, reflecting means
45 is
also employed along the oppositely disposed edge of waveguide 40 to recycle
light
rays back into waveguide 40 that did not escape into prisms 42.
In order to improve the uniformity of the light emitted from the waveguide
40, a light source homogenizer 46, preferably made of the same material as
waveguide 40, altows the light rays from point light source 44 to uniformly
fdl the
width of waveguide 40 shown in Fig. 7. This embodiment prevents uneven light
brightness within waveguide 40 and results in a more uniform extraction of
light
from waveguide 40 by prisms 42. Light source homogenizer 46 may be trapezoid-
shaped or any other appropriate shape. A light reflecting material 45,
specular or
diffusive in nature, at the end away from light source 44 may also be employed
to
recycle light. The light source may be a parabolically focused short arc light
is source. Alternatively the shape of the light source reflector may be
modified to
uniformly f ll waveguide structure 40 without the need for homogenizer 46.
Fig. 7A shows head-on the input face 43 of the trapezoid of Fig. 7. The
input face 43 need not be square but need merely be selected to be larger than
the
spot of light from the parabolic reflector.
Fig. 8 shows a shallow light fixture according to the invention using two
extended light sources 37 and an array of concave lenses, together with
trapezoid
prism input structures 48 and arcuate reflectors 49. Alternatively, a
reflective
material may be substituted for one of the two trapezoidal prism input
structure 48,
light source 37 and reflector 49 combination to recycle light within waveguide
40.
The trapezoidal prisms 48 are tapered sections optically coupled with
waveguide
40 which gather light from the extended light sources through a fairly large
area,
and through TIR the light is guided into the thin main waveguide 40. Because
the
smallest commercially available fluorescent bulbs are 10-12 mm, the tapered
structure is needed to reduce the thickness of waveguide 40 so that waveguide
40
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has a thickness much less than dictated by the diameter of the light source.
To
maximize the efficiency of transfer of light into the waveguide, an involute
or
arcuate-shaped reflector 49 is preferred to avoid redirection of light back to
the
source. Reflector 49 redirects the light that was not directly coupled into
prism 48,
and through one or two reflections the light has additional opportunities to
enter
prism 48. Alternate configurations of reflector 49 are possible as is known to
those
skilled in the art. An array of negative lens structures 39 may be used to
efficiently
spread the light output of prisms 42 to a wider distribution angle. If the
dianieter
of light source 37 is dL and the height of the input face of trapezoid prism
48 is D,
a typical ratio is D: dL of about 2: 1. The waveguide thickness dW is much
less
than dL, and may be 2 mm.
It should be appreciated that this structure offers previously unavailable
architectural opportunities. For example, the light fixture is quite shallow
and so
does not require a thick ceiling. Where dropped ceilings are used the dropped
ceiling need not be positioned very far down from the higher fixed ceiling,
therefore offering cost-effective building techniques, heretofore unavailable
because of bullcy lighting fixtures. Other benefits include improved light
uniformity
over two dimensions, less glare in peripheral vision of those in the space
being
illuminated, more efficient use of the light since it can be directed to the
areas of
interest utilizing appropriately shaped prisms and lenses. Furthermore, the
prism
coverage and size may be varied across the waveguide to provide uniform
illumination in the target area.
While one embodiment may comprise two extended light sources as
discussed above, an alternative as shown in Fig. 9 may be used. Here, muftiple
waveguides structures 40 may share a common light source 37. This arrangement
increases the light efficiency since more of the light from light source 37 is
directly
coupled to one or another of the adjacent waveguides 40. Preferably,
reflectors
surround light source 37 to couple otherwise stray light to the waveguides
through
one or more reflections. As would be obvious to those skilled in the art,
additional
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waveguides 40 could be coupled to light source 37 and additional light sources
could be coupled to other available edges of waveguide 40.
Fig. 10 repesents an array of light directing prisms 42, waveguide 40, in
combination designated as a light directing structure 53 and an additional
waveguide structure 54 optically coupled to a light source 55. This permits
light
source 55 to be remote from structure 53. For example, structure 53 may be
relatively inaccessible to users and yet the light source 55 can be
accessible. Such
an occurrence may arise when structure 53 may be in a ceiling and the light
source
55 may be at floor level to facilitate maintenance of the light source 55.
Another
example would be roadway signage, where structure 53 illuminates road signs
high
above the roadway. Here, light source 55 could be at the road surface. One
advantage is easier servicing when the lamp 55 must be replaced. Another
advantage is that the heat source is removed from the illumination source. One
example of additional waveguide structure 54 is a bundle of optical fibers
arranged
iS along an edge of waveguide 40. The fiber bundle could optionally be
replaced by
other waveguide structures which transmit light with low loss through total
internal
reflection. Depending on the source, more than one light directing structure
could
be coupled to the light source. This could be useful in automotive lighting as
well
as commercial room lighting. Preferably at least 60% and more preferably 70%
of
2o the light from the source is distributed to the desired location.
Fig. 11 shows a Iight source according to the invention optimized for
illumination from the ceiling of a large flat object 22 on a wall. Most light
fixtures
fail to provide appropriate lighting for such objects. This invention allows
for
asymmetric light distribution as required by this application. This is
accomplished
25 by selecting the shapes of the prisms of the film, and by selecting how
many prisms
of each shape are provided on the film. A structure 53 only "d" inches wide,
of
shallow vertical dimension, can nonetheless provide even illumination across
an
object (e.g. a painting) that is "D" inches tall where D is much larger than
d.
