Note: Descriptions are shown in the official language in which they were submitted.
- ~;CA 02460450 2004-03-29
OPTICAL PROJECTION APPARATUS AND METHOD
FIELD OF THE INVENTIGN
The present invention was made With U.S. Government .
support, and the Govexnrnent has certain rights in the
invention.
This invention relates to the field of optical display
and, muse particularly, to an optical system and method for
displaying an image. In one preferred form of the invention,
do image is projbcted and displayed on a solid panel display
device.
BACKGROUND OF THE INVENTION
In the field of image projection of a rectilinear object
to a proportionately onlarged or reduced rectilinear image (as
represented by conventional photographic enlargers and slide
projectors), the entire image is projected typically upon a
single plane (e. g., fn the enlarger, to t:he photographic
paper; and from the slide projector, to the screen). A more
difficult task arieeg when an image must be projected into a
display device having two separate image surfaces fox the
vertical and horizontal components of the image; each. of which
requires independent magnification and focus of the vertical
and of the horizontal image components. The problem i$
further complicated when orie of the image surfaces is tilted
with respect to the projection bxis; the tilt being so
significant that conventional image focus will not be
sustained along the full image surfaces. The two disparate
image surfaces must be illuminated in sur_h a manner that the
vertical and the hoxixontal image compon<?nts maintain
independent focus along their respective tilted surfaces.
Further, since projected images generally expand (or enlarge)
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over progressively greater projected field distances, tilted
image surfaces are also subject to '°keystonxnq", whereby one
dimension (say, the horizontal "width") is enlaxged
progressively more as viewed from the "tvp" or the "bottom" of
the image.
An example of a device which reguires such image handling
is represented in U.S. patent No_ 5,381,5Q2 entitled, "Flat or
Curved Thin pptical Display Panel". Figure 1 illustrates the
type of panel construction described in the '502 Patent. The
panel comprises a.stack of thin waveguide-like transparent
lamina 111 each of typical thickness t_ When the stack is cut
at an acute angle S, each lamination exhibits a height h at
the display surface such that h = t sec S. Thus, with s
measuring typically about 70°, h is significantly larger than
t. Alsv, the full display height H is larger than the bags
thickness T by the same factor, sec S.
The device of the '502 Pe~tent is called a "polyplanar
optic display" (POD). The rightmost portion of the PoD is
represented primarily in Figure 1 a$ an isometric view. The
full width W is typically wider than its display height H.
The portion which is detailed serves to describe the t~peration
of the POb and is useful in understanding its relationship to
the present invention. Each lamination (of thickness t) of
th4 panel is a transpax'ent sheet (glass or plastic) of nominal
optical index of refraction n~, separated by thin coatings of
index of z'efraction n=, where ni > n?. Light entering the
laminations at the base is separated into sheets and is
Confined to its respective sheets by total internal reflection
at the interfaces_ Thus, light focused at the base will
retain "vertical" resolution elements of thickness t (in the
"T"-direction) throughout its propagation "upward" to the
display surface, where each thickness t is displayed as a
corresponding resolvable height h. In the width W direction,
however, there is no confinement of the input illumination,
and each sheet propagates its respective slice (fn the width
direCti4n) as ~,rould a continuous transparent medium. This
requixea that the horizontal image components be focused over
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varying distances Corresponding to the tipped viewing surfaGe_
While the vertical component of the prvje~cted image must focus
near the base, the horizontal information must focus near the
sloping plane of the display surface; those components at the
"bottora" of the display focusing close to the base, and those
higher focusing at progressively greater distances to
represent image elements approaching the top of the display.
Also, while propagating through the lamina, the horizontal
components expand progressively as an extension to the
expanding illuminating field. Unless corrected, this
generates keystvning, whereby (in this example) the top of the
displayed image becomes wider than that at the bottom.
It i$ among the objects of the present invention to solve
image handling problems of the type described above and alsa
to provide image projection that can be used in conjunction
with d POD type of display panel.
SUMMARY OF THE INVEN'T'ION
In over form of the invention, an optical system is
disclosed for displaying an image of an object. A dispiay
device is provided and has an input surface and an output
surface. Means are provided fox illuminating the object so
that light from the object is directed taward said input
surface. Ansmorphic aptical means is disposed in the light
path betvaeen the object and the input surface, the anamorphic
optical means being operative to focus one directional
component of the image at the input surface and to focus a
dffferent di:~ectianal component of the image at the autput
surface.
In an embodiment of the invention, the display device is
a panel device formed of a solid materia.I arid having disparate
imaging surfaces for sand different directional components, at
Ieast one of said imaging surfaces being non-perpendicular to
Lhe optical axis of the light. Is~ this ~ambodiment the object
i8 a Planar object tipped with respect to said optical axis by
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an angle that satisfies the Scheimpflug u~andition for said at
least one of said imaging surfaces, the :angle taking into
account the refractive effect of the solid material on said
light.
Also, in this embodiment a tel.ecent:ric optical component
can be disposed in the path of the light to Correct for
key$toririg of said image.
Further features and advantages of the fnvention will
become more xeadily apparent from the following detailed
description when taken in conjunction with the accompanying
drawin~a.
BRIEF DESCRIPTION OF° THE DRAWINGS
Figure 1 is an isometric view, in partially broken away
form, of a prior art pOD display panel.
Figure Z is a diagram of the projected optical field, and
the POD displgy, in accordance with an ezrtbodiment of the
appnratns of the invention and which can be used in practicing
an embodiment of the method of the invention,
Figuxe 3 ehos~fs a cro8s sectional view of a POD, and is
useful in understanding determination of imaging surfaces and
the determination of tilt.
Figure 4 shows an embodiment of an apparatus and
techn~.gue for practicing the invention using a scanning laser
beam to form the image.
Figure 4A illustrates a lens for providing focusing on a
sloping image plane that can be used in the Figure 4
embodiment.
DETAILED DESCRIPTIC>N
IMAGE PROJECTION: T4 develop relationships between the
object and its projected image, the entire projected field is
repre3ented in Figure 2, with the POD 205 positioned such that
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the rays (propagating from left to xight) remain "unfolded"
until they arrive at the POD base. (If a "folding" reflective
eleatent is interposed in the ray path, the rays may be
directed "upward" such that the PC?U can be pos i t ~,flned f or
typical upright viewing).
The principles hereof relate to the transfer of
information from the object surfaC$ to the image surfaces of
the PAD. As such, the manner of illuminating the object is
independent of this transfer. The object will be assumed
conventional; either transmis:give or reflective; illuminated
with incoherent or coherent Light. One exception to this
independence is the case of illumination of the Foo by a
scanned laser beam, wherein the "abject" may be virtual; that
ia, contained in the ~coftware which addresses the laser beam
inten$ity while it is scanned. 'his case will be dfscnaaed
subsequently.
The object can be one of a variety of Light valves which
may be t~tatic for projection of a still picture (such ass a
photographic slide), or dynamic, forming moving images or
Changing data (such as by any of the elecaronically controlled
light valves). Typically, the object is planar and it
exhibits spatial information which is to be projected to n
distant forage surface. At the left side of Figure 2 is
illustrated a plane object ".li.ght v$Ive" 2I1, oriented at the
origin of an x-y--z coordinate system as shown, the Abject is
tipptd such that its plane farms an anglf: 13 with the z-Axis.
This will be discussed subsequently. Assuming a transmissive
abject, transmitted rays prop$gate to the rigk~t (in the z-
direction), encountering anamorphic (cyl~Lndrical) lenses Ch and
Cue. ~ach lens provides optical power in only the horizontal or
the vertical direction. The Focal lgnqths of Ch and Cv are
Selected to satisfy the desired im$ge di~atances and the
required magniffcationg (from the object width to the display
width W, anr.1 from the object height to the base thickness '~ of
the ppD), Zn an exemplary case, the required vertical
magnification is m - 6.1 and the horizontal magnifiratian is
s 18.5. The well-known "thin lens" relationship for the
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focal length f is given by
= av _ v (1)
u..v , m+1
in which a is the object-lens distance, v is the lens-image
distance, and m = v/u;~the image/object magnification. In
application for the differing image distances and
magnifications of these systems, these and related eguations
are separated into quadratuxe directions (with subscripts h
etnd v) to represent the independent horizontal and vertical
image components.
Figure 2 also shows a lens L~ d ose to the POD, operating
ae a telecentric element to rectify keystoning. Zt is
provided with a long focal length, whereby its fecal point is
pottitioned near the source of diverging rays (in the vicinity
of C~) so that it operates on the arriving diverging ray
bundles to collimate them_as they propagate into the POD.
This additional optical power, positioned close to the focal
regions in the POD, also shortens slightly the oxiginal focal
lengths, as ca7.GUlated per Equation 1 for CA and C~ alone.
Considerations for.this arid other factors relating to the
development of the focal surf$ces in the Pdb are now
discu$$ed.
The horizontal projection components must accommodate the
tilting of the hori~ontnl image plane in the POD (due to the
differing propagation lengths within the lamina). The
projected image surface of Figure 2 (slang the Tilt Axisj is
determined by (subsequently described) successive calculation
of the optical paths within the POD, allowing for appropriate
depth of focus of the horizontal components. This reveals the
effective tilt of the i.mz~ge plane which the incoming light
must match to provide uniform horizontal focus over the entire
image surface_
T~tis is achieved by instituting the optical arrangement
known as the SGheimpflug condition, established among the
object plan~3, the image plane and the principal plane of the
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hori2ontal imaging lens C~; accomplished by orienting these
planes such that they intersect at a single line. That is,
with the Ch plane (thin lens approximation) normal to the
projection axis and the effective image plane oriented as
determined above, the object plane is tipped so that it
intersects the intersections of the other two. 'this is
represented by the Scheimpflug rule,
tank = m tans ( 2 )
where a is the image plane tilt angle, f3 is the object plane
tilt angle (both with respect to the axis) anti m is the
image/object magnification. This, too, is separated into
quadrature directions with appropriate,aubscripts tv represent
the individual magnifications and a-tilts.
LIGHT PROPAGATION WITHIN A TYPICAL POD: Figure 3 is a
section view of a generic POP (e. g. 205), showing its outline
in told solid lines and several (horizontal. component) image
suriaoes. In this Figure, the wl.dth (or horizontal) dimension
appears in-~tnd-out of the plane of the paper. The axial
dimensions are identified in the z-direction, as is the focal
tolerance ez. The POD base thickness T cmrregponds to that in
Figures 1 and 2. Illumination, propagating from left to
right, txaver8e~s the kaystone-correcting lens LL (not shown)
and encounters the sloping base of the 1't7D. This a~ tilt is
determined by application of Equation (2) for the vertical
component, after iterative determination.of the tilt of the
horizontal image surface a~ and the tilt of the object plane ii.
(The object plane tilt must satisfy Equation (Z) for both
vertical and horizontal components; saGh having differing
magnifications). The locations and effective tilts of the
horizontal image surfaces axe established following the
sequence of lines numbered (0) to (4), as. follows:
Line (O) is the d awing surface of the PC~D; as
illuatrgted in Figures 1 and 2. This surface and the total
axial distance Zt remain the same, independent of the base tl.lt
~ (Note that in Figure 1, there is shown no base tilt; i.e.,
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)
Line (1) is the corresponding focal surface in n = 1
refractive index material (aix). xn typ:f~cal n = 1.5 material
(glass, plastic), it is extended by 1.5x to the viewing
surface, line (0). This type of Consideration allows the
optical System to be calculated as though the image distances
are Completely in air.
Allowing sez focal tolerance in air at the ends of line
(1j forms line (2), the design focal surface which will image
effectively on to the ideal line (1). T)ais reduces
significantly the slope of the image plane and the
corresponding Schelmpflug tilt of the object plane. The image
tilt arh is established at line (2) .
Line (3) is the focal surface inside the » = 1.5
materiel, resulting from focusing on line (2) in air. Note
that the ez near the $ase (top left] remains in air, while at
the other end (bottom right) ft extends to nex inside the
material.
Finally, line (4) is determined analyt).cally as the
surf~etce to which cylinder lens Ch must be focused such that
with the additional keystone correcting .lens L~, the image
distance is r~hortened slightly to line (~!) in air. It is then
(par above) extended inside the higher i»dex POD material to
Sine (3).
FOCUS AND KEYSTONE CORRECTION: with the horizontal image
tilt angle cps established at line (2j of Figure 3, vne can now
determine the object tilt angle !3 by application of Equtation
(2), given mh. This prtwides horizontal component focus ove~-
the entire plane of line (x). (Line (2) is on the surface
Identified in bold dashed lines on the Projected Tilt Axi$ of
Figure 2.j mhen, by re-application of Ecluatian (2) for the
known my, one determines the base tilt angle c~v in Figure (3)
foz~ vertical foCUS over the entire POD base. Vertical focus
is then transported via the waveguides to the viewing surface,
and joined by the above-described horizorrttal components as a
fully focused image.
To determine fh (the focal length of Ch) per Equation (1),
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v- is taken to the vertical center of line (~} in Figure 3;
effectively before the addition of telecentric lens Lt
refocuses line (4) to line (2). Also, for calculation of m~
before the addition of L~, the image width is taken as greater
than W by the ratio of vh to the distance from Cn to Lt. When
L~ is added, it is to collimate the princ,ipdl rays of the
focusing beams to width W. The focal length of LL would
normally be taken as the distance from Cn to Lt if there were
no li-tilt of the object. With tilt, however, minor additional
keystoning develogs; accommodated by reducing the focal length
of L~ appropriately. numerically, fox an exemplary design, its
focal length is shortened by approximately 12$ to provide s
rectilinear focused image_
PROJECTION OF SCANNED LASER HEAM(S): An alternate method
of illuminating and addrass.lng a POD-type display device, as
expressed earlier, is by scanning a Iaser~ beam (or beams) in
typirbl raster or line segment format, while modulating the
intensity (or intensities} of the beant(s), and prvjeoting the
appropriately focusing array of beams into the display device
to torna an image. This is s relatively conventional "laser
projection system" in which a display screen is mounted
typically perpendicular to the projection antis. However, in
this di9play system, the vertical and horizontal image
Bur~pGeB 8rt3 not only disparate, but may be Lilted with
respect to the axis. Also, there is normally no "real" plane
object (as in the above described systems} which may be placed
into the Scheimpflug condition to render the focus uniform:
This "virtual" object exists only in the video signal.
Furthermore, even if the depth of focus (later defined) is
sufficiently great to accommodate the differing image
distances, keystoning would develop, unless compensated.
To resolve this situation, It is first assumed that tho
Idser beam scanning process is well implemented, ;~ollow.tng
well known x. y, z, t (t=time} raster car segment scanning and
intensity modulation procedure. The scanned image cz~n be
considered for this application as integrated over time into a
Stationary image. An analog to this process is, therefore,
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that of a photographic slide projectdr, as viewed from the
principal plane of the projection lens to the screen.
Everything before the lens is xeplaced b~,r the scanned and
modulated laser beam. ExcE~pt far the di:EfraCtion-limited
characteristics of coherent beam propagation, the flux from
the aperture of the projection lens to the screen is analogous
to the time-integrated flux during each :frame of the scanned
and mode later! laser ( s ) .
To adapt to laser operation, the above projection lens is
identified as a "scan lens" of laser scanning vernacular. If
the desired image spot size 6 is so small as to require an f-
number F of the converging beam Which is too low for its depth
of focal az to straddle tile disparate horizontal and vertical
image planes, as governed by the relations,
F ~ d/ll and Az ~ ~ f'~1 ( 3a & 3b)
then this xen~ may be anamorphic. It is implemented with lens
elements similar to Ch and ~~ of the earliez discussions,
whereby the horiaontal and vertical imagE3 components focus
over different distances. With nominal iEocal di8tarices
er~tablish~rd in a manner represented i» Figure 3, and with
horizontal and vertical laser beam scan angles into the lens
determining width W and thickness T respE=cti.vely, the
specification of en appropriate projection lens is
straightforward.
An alternative which maintains a more conventional flat .
field projection lens i8 to make the beam which illuminates
the Scanners appropriately astigmatic, such that the
subsequently-scanned horizontal and vertical forage components
are projected to their proper disparate image planes. This
is accomplished by placing an anamvrph3.c lens element into the
beam before the Scanners.
In either of the above two cases, if. the image display
planes are perpendicular to the axis, image geometry and ~Eocus
would be satisfied completely. The vertical and horizontal
scan magnitudes can also be adjusted to ~~atzsly differing
~ ' CA 02460450 2004-03-29
1~
requirements for vertical and nor,izontal image magnifications.
Keystoning can be cpntrolled in a manner discussed
earlier; by adding a telecentric lens L~ per Figure 2, to
collimate the principal ray groups and to focus them at the
center of line (2) of Figure 3. The focal length of LL is
determined by its distance to the effective nodal source of
the projected beam. Keystoning can also be nulled by
predistorting the scanned function such that it forms a
keystoned image which is complementary to that which would
at?te~rige appear on the tilted display screen. This may be
done by addressing low inertia laser scanners (e. g., acousto-
optic or gblvnnometer deflectors) which are well known in the
art to respond to variable rate electronic drive. This leaves
only the correction of defocus (if required) appearing near
rh$ top and the bottom of the display. Figure 4 shows laser'
420, inten$ity modulating components 430, beam scanning
components X40, and lens camponents C~ anc9 C~.
Unable to invoke the scheimpflug condition here (since
there ia.no object plane), a method of providing focus on a
sloping lanage plane, as illu$trated fn Figure 4, is to replace
cylindrical lens Ch (e. g. of Figure 2) with a lens of conic
cylindrical shape, C~; one shaped so that it reduces optical
poorer grt~dually from "top°' to ''bottom°' . It is essentially
a
small portion of a (solid glass or plastic) cone which is cut
therdfrom such that its radius of curvature increase-s
gY'eldu~tlly from top to bottom, as shown in Figure 4a. This
reduces optical power from tap to bottom, gradually increasing
horizontal focal length to match the tipped horizontal image
plane.
Another alternative which allows the laser scanned system
to act ucore like that described earlier and illustrated in
figure 2, is to create a synthetic object plane which may be
tilted fox Scheimpflug correction. ~t'his synthetic plane can
be formed by having the laser scanner develop a real image in
Space: located essentially as is the Object in Figure 2. The
imaging process of Figure 2 may be duplicated by converging
and propagating the flux through the lenses.
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I2
While this disclosure identifies basic optical elements
which in combination satisfy the abjectives of the invention,
it is understood that designs may be conducted by, one skilled
in the art to establish Characteristics which satisfy such
factors as variable {zoom) magnification, aberration
reduction, optical efficiency, and production effectiveness.
Zt is also understood that varidtioris to the basic disciplines
expresgad here, BuGh as the use of folding reflective and/or
Compound or c~mented optical element8 which may have
equivalent Fregnel, reflective, diffractive or hybrid optical
elements, remain within the scope of that principles of this
invention.
