CA2460450C - Optical projection apparatus and method - Google Patents

Abstract

Certain optical imaging systems exhibit disparate vertical and horizontal image focal surfaces; at least one of which is tipped with respect to the optical axis .
The projection optics which illuminates such systems must provide that the vertical image components focus upon the nominal vertical image surface, while the horizontal image components focus on the disparate horizontal image surface. Because at least one of these image surfaces may be tilted with respect to the projection axis, correction is required to maintain focus over the entire image surfaces and to eliminate keystoning. The system may also require differing vertical and horizontal image magnifications as projected upon the above disparate focal surfaces. This invention describes inter alia, the techniques for meeting these varied requirements; to project a rectilinear object field such that it forms a final focused rectilinear image in a system having tipped and disparate image planes.

Description

- ~;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) 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 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 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$toririg of said image.
Further features and advantages of the fnvention will become more xeadily apparent from the following detailed description when taken in conjunction with the accompanying drawin~a.
BRIEF DESCRIPTION OF° THE DRAWINGS
Figure 1 is an isometric view, in partially broken away form, of a prior art pOD display panel.
Figure Z is a diagram of the projected optical field, and the POD displgy, in accordance with an ezrtbodiment of the appnratns of the invention and which can be used in practicing an embodiment of the method of the invention, Figuxe 3 ehos~fs a cro8s sectional view of a POD, and is useful in understanding determination of imaging surfaces and the determination of tilt.
Figure 4 shows an embodiment of an apparatus and techn~.gue for practicing the invention using a scanning laser beam to form the image.
Figure 4A illustrates a lens for providing focusing on a sloping image plane that can be used in the Figure 4 embodiment.
DETAILED DESCRIPTIC>N
IMAGE PROJECTION: T4 develop relationships between the object and its projected image, the entire projected field is repre3ented in Figure 2, with the POD 205 positioned such that the rays (propagating from left to xight) remain "unfolded"
until they arrive at the POD base. (If a "folding" reflective eleatent is interposed in the ray path, the rays may be directed "upward" such that the PC?U can be pos i t ~,flned f or typical upright viewing).
The principles hereof relate to the transfer of information from the object surfaC$ 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 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 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., ) 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), 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, 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.

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.

Claims (12)

1. An optical system for displaying an image of an object, comprising:

a display device having an input surface and an output surface;
an illumination element for illuminating the object so that a light from the object is directed through projection optics that diverges the light, a resulting diverging light being directed about a principal axis toward the input surface of said display device, the output surface of said display device being tilted with respect to a plane perpendicular to the principal axis; and a telecentric optical component having positive optical power disposed in a path of the diverging light and positioned proximate said display device, said telecentric optical component having a long focal length and a focal point located in a vicinity of a source of the diverging light;
wherein said telecentric optical component acts on arriving divergent ray bundles to collimate them as they propagate through said display device, thereby substantially collimating the diverging light and nulling keystoning of the image at the output surface of the display device.

2. The optical system as defined by claim 1, further comprising an anamorphic element disposed in the path of the diverging light and positioned between the object and said telecentric optical component.

3. A method for displaying an image of an object, comprising the steps of:

providing a display device having an input surface and an output surface;

illuminating the object so that light from the object propagates through projection optics which diverges the light from an apparent source, a resulting diverging light being directed about a principal axis toward the input surface of the display device, the output surface of the display device being tilted with respect to a plane perpendicular to the principal axis; and providing a telecentric optical component having positive optical power in a path of the diverging light and positioning the telecentric optical component proximate to the display device, the telecentric optical component having a long focal length and a focal point located in a vicinity of a source of the diverging light;

whereby the telecentric optical component acts on arriving divergent ray bundles to collimate them as they propagate through the display device, thereby substantially collimating the diverging light and nulling keystoning of the image at the output surface of the display device.

4. The method as defined by claim 3, further comprising the step of providing an anamorphic element in a path of the diverging light and positioning the anamorphic element between the object and the telecentric optical component.

5. An optical system for displaying an image, comprising:

a display device having an input surface and an output surface;

a scanned field of light beams diverging from an effective nodal source and directed about a principal projection axis toward said input surface and forming the image at said output surface, said output surface being tilted with respect to a plane perpendicular to the principal projection axis; and a telecentric optical component having positive optical power disposed in a path of the scanned field of light beams and positioned proximate said display device, said telecentric optical component further having a long focal length and a focal point located near or at the effective nodal source;

wherein said telecentric optical component acts on arriving divergent field of light beams to collimate them as they propagate through said display device, thereby substantially collimating the diverging field of light beams and nulling keystoning of the image at the output surface of the display device.

6. The optical system as defined by claim 5, wherein the scanned field of light beams is a scanned field of laser beams.

7. The optical system as defined by claim 5, further comprising an anamorphic element disposed in a path of the scanned field of light beams and positioned between the effective nodal source and said telecentric optical component.

8. A method for displaying an image, comprising the steps of:

providing a display device having an input surface and an output surface;
scanning a field of light beams diverging from an effective nodal source about a principal projection axis toward the input surface and forming the image at the output surface, the output surface being tilted with respect to a plane perpendicular to the principal projection axis; and providing a telecentric optical component having positive optical power in a path of the scanning field of light beams and positioning the telecentric optical component proximate the display device, the telecentric optical component having a focal point located near or at the effective nodal source, thereby substantially collimating the diverging field of light beams and nulling any keystoning of the image at the output surface of the display device.

9. The method as defined by claim 8, wherein said step of scanning a field of light beams comprises scanning a field of laser beams.

10. The method as defined by claim 8, further comprising the step of providing an anamorphic element in a path of the scanning field of light beams and positioning the anamorphic element between the effective nodal source and the telecentric optical component.

11. A method for displaying an image, comprising the steps of:

providing a display device having an input surface and an output surface;
scanning a field of light beams diverging from an effective nodal source about a principal projection axis toward the input surface and forming the image at the output surface, the output surface being tilted with respect to a plane perpendicular to the principal projection axis; and addressing the scanning of the field of light beams to form a keystoned image which is complementary in shape to that displayed at the output surface due to a tilt of the output surface with respect to the plane perpendicular to the principal projection axis, to thereby correct a keystoning of the image at the output surface.

12. The method as defined by claim 11, further comprising the step of providing an anamorphic element in a path of the scanning field of light beams and positioning the anamorphic element between the effective nodal source and the display device.