WO1999015932A1

WO1999015932A1 - Method and apparatus for illuminating a display device

Info

Publication number: WO1999015932A1
Application number: PCT/US1998/019261
Authority: WO
Inventors: Douglas Mcknight
Original assignee: Colorado Microdisplay, Inc.
Priority date: 1997-09-19
Filing date: 1998-09-15
Publication date: 1999-04-01
Also published as: AU9487298A

Abstract

An optical system for illuminating a display device by providing substantially telecentric illumination at a display surface of the display device. In one embodiment, a first lens having a first focal length (f1) and a second lens having a second focal length (f2) are separated by a distance of approximately f1 + f2. The first lens receives light from a light source which provides light at a plurality of points of a surface of said light source; at each point, light is emitted in a predetermined pyramidal-like shape which has an axis and which defines an angle. The second lens, which is optically coupled to the first lens, provides light to the display device, which has a light modulating surface. A plurality of points at the light modulating surface receive a plurality of predetermined pyramidal-like shapes of light which are aligned to be parallel.

Description

METHOD AND APPARATUS FOR ILLUMINATING A DISPLAY DEVICE

BACKGROUND OF THE INVENTION

The present invention relates generally to illumination methods and apparatuses for visual display systems and more particularly to illumination systems and methods for use with a light modulating display device.

Display devices which use light modulating displays often require a carefully designed illumination system which illuminates the light modulating display device. One example of a light modulating display device is a liquid crystal display device of either the reflective type or the transmissive type. It is desirable to illuminate uniformly light modulating display devices in order to provide adequate and uniform contrast over the entire image and in order to efficiently use the light source. Thus the illumination system should control the direction of the light relative to display surface, and any subsequent optics after the display surface must be properly used relative to the illumination system to provide a good quality image.

Figure 1 represents an example of a light modulating system display device. This device includes a light modulating display surface 14 which is mounted on a surface 15 a. The light modulating surface 14 maybe the reflective display surface of a liquid crystal on silicon semiconductor substrate. Examples of such liquid crystal display devices maybe found in U.S. Patents 5,566,010 and 5,426,526. The illuminator 11 provides a source of light which is directed towards the beam splitter 13. Typically, the illuminator 11 provides light in a plurality of directions as shown by the rays 12. This light is reflected by the beam splitter 13 toward the light modulating surface 14 which then reflects back an image created by the light modulating process in the light modulating device. The light from this image passes through the beam splitter 13 into the optical elements 16 and 17 which may be conventional glass lenses. The resulting light 18 transmitted from the optical elements is then available for viewing by a user of the display device. Typically, the illuminator 11 has a diffuse light emitting surface. Light from each point of the surface of the illuminator 11 is incoherently radiated from the surface in a variety of directions.

Figure 2 shows the result of the illuminating rays at the light modulating surface 14 of the display device. Three points on the light modulating surface have been selected for illustration. Point 23a is illuminated by a cone of light having an axis 23 which bisects a cone of light which may be considered to be defined by light rays 21 and 22 which are the extreme edges of the cone of light which illuminates the point 23 a. It will be appreciated that beyond this extreme edge little or no light from the illuminator 11 strikes the light modulating display surface at point 23a. That is, a cone of light hitting point 23a is less than 180 degrees as shown in Figure 2. Points 24a and 29a at the bottom and top (rather than the center) of the light modulating display surface receive considerably different illumination than the point 23a due to the location of these points on the display surface. For example, point 24a receives a cone of illumination having an axis 24 which bisects the cone determined by the rays 25 and 26 which define the extreme edge of the cone of illumination. Thus at point 24a, the cone of illumination may be smaller in angular size than the cone at point 23a, and the axis 24 of this cone is not aligned or parallel with the axis 23 of the cone of illumination at point 23a. Many light modulating type materials, such as liquid crystals, modulate the light differently when illuminated at different angles. Pixel 24a, as a result of the different illumination due to the direction and angle of the cone, will appear different to a viewer than point 23a. This is true even if the intensity of the light at point 24A is the same as the intensity of light at point 23a. Furthermore, if the display is to appear to have uniform brightness to the viewer, then the light reflected from the edges of the display must be efficiently collected and redirected by the optical elements 16 and 17. Generally this is more difficult to design, and requires larger elements if the display is illuminated with diverging light as illustrated in Figure 2.

Generally, it is desirable to illuminate the display surface with light of uniform color and intensity and also, for many liquid crystal displays, with polarized light. If the light is not uniform in either intensity or angular directions, then a good quality image will not usually be generated. There has been numerous attempts in the prior art to improve illumination upon a light modulating surface. For example, the back light illuminators for laptop computers have recently included certain types of films which improve the illumination characteristics. One type of film is referred to as a brightness enhancement film (BEF) which is produced by 3M of Minnesota. This film in one embodiment is a plastic film with miniature triangular grooves on one side. The internal reflectance properties of this film are such that it reflects light which has incident angles outside a certain range (and transmits light within a range of incident angles). This film generates, at each point of the film, cones of light having a specified angular amount. In this manner, the film directs light in only certain directions from each point on the film. Moreover, it provides the benefit of recycling some of the reflected light which is then scattered into the useful direction of the cone rather than outside of the cone of light. A typical version of this film yields a full-width half - intensity of 44 degrees by 47 degrees if two such films are crossed at 90 degrees. The resulting light emitted at each point has a pyramidal-like shape in this case. Thus an illuminator 11 may utilize two such BEF films to generate at each point of the illuminator's surface a cone of emitted light. An example of such an illuminator is the aAlphalight from Teledyne Lighting.

While the use of such films in the illuminator 11 improves the illumination characteristics of the display device 10, there are still numerous problems with this display device. For example, to achieve uniform illumination the illuminator output window must be significantly larger than the display device (because of its distance from it, and the requirement that the same sized area on the illuminator output window contributes light to each point on the display area). This is wasteful of volume. It is also wasteful of power because much of the light misses the display. Furthermore, the cone angles of the light are typically rather large (e.g. approximately 40 to 50 degrees), and this may not be well matched to the modulator, or to the subsequent viewing optics.

SUMMaARY OF THE INVENTION

Optical systems for illuminating a display device and methods for illuminating a display device are described herein. In one embodiment of an optical system, a source of light provides light to an optical element which is optically coupled to the source of light. This optical element substantially telecentrically illuminates the display device.

In one embodiment of the optical system of the present invention, the optical element comprises a first lens having a first focal length and a second lens having a second focal length. The first lens and the second lens are positioned relative to each other such that a first distance of approximately the sum of the first and second focal length separates the first and second lenses. The first lens receives light from the source of light and second lens directs light towards the display device which is a light modulating display device. As a further example of this embodiment, the source of light comprises a plurality of points, each of the plurality of points emitting a first predetermined pyramidal like shape light. The display device receives at a light modulating surface of the display device a plurality of predetermined pyramidal like shapes of illumination which are substantially parallel to each other. As a further aspect of this embodiment, the first predetermined pyramidal like shape defines a first angle and the second predetermined pyramidal like shape defines a second angle, where the second angle is smaller than the first angle. Further, the light source is magnified at the display device, and the second angle determines an exit pupil for the optical system.

An example of the method of the present invention comprises the generation of light from a light source having a plurality of points, each of which emits a first predetermined pyramidal-like shape of light. The method further includes optically converting each of the first predetermined pyramidal-like shapes of light to a second predetermined pyramidal-like shapes of light having an axis and directing each of the second predetermined pyramidal-like shapes of light to a light modulating surface of a display device wherein the light modulating surface receives a plurality of second predetermined pyramidal-like shapes of light having a plurality of axes which are substantially parallel.

Various other alternative embodiments and aspects of the invention are also described.

BRIEF DESCRIPTION OF THE DRAWINGS

The various embodiments of the present invention will be described in detail with reference to the following drawings in which like reference numerals refer to like elements.

Figure 1 illustrates an optical system for illuminating a display device of the light modulating type.

Figure 2 shows an example of the illumination pattern at a light modulating surface of a light modulating display device. This illumination pattern is achieved by illumination systems of the prior art Figure 3 shows an example of an optical system according to the present invention.

Figure 4 shows an illumination system which does not provide the benefits of the present invention.

Figure 5a shows an example of a light source which may be used in an optical system of the present invention.

Figure 5b shows another example of a light source which may be used with an optical system of the present invention.

Figure 5c is a cross sectional view of the light source of Figure 5b, which cross sectional view is taken at the cross sectional indicator 5c-5c.

Figure 5d is another cross sectional view of the light source of Figure 5b taken at the cross sectional indicator 5d-5d.

Figure 6 is an example of one embodiment of an optical system for the present invention which illuminates the light modulating surface of the display device.

Figure 7 is an example of another embodiment of an optical system for the present invention which illuminates a light modulating surface of the display device.

DETAILED DESCRIPTION

The following description provides examples of the present invention. It would be appreciated, however, that other examples of the present inventions will become apparent to those in the art upon examination of this description. Thus, the present description and the accompanying drawings are for purposes of illustration and are not to be used to construe the invention in a restrictive manner. For example, the invention may be used for either reflective display devices or transmissive display devices. Further, various different types of optical elements may be utilized such as lenses or mirrors. Furthermore, various values and materials are described herein for purposes of illustrations and these are not to be used to construe the invention in a restrictive manner.

Figure 3 illustrates an optical system for illuminating a display device according to the present invention. This optical system 101 includes an illuminator 104 which provides a source of light to the optical elements 102 and 103 which may be conventional glass lenses. These optical elements 102 and 103 optically resolve light from the illuminator 104 upon a light modulating display surface 109. In one example the present invention, the light modulating surface 109 may be the surface of a reflective liquid crystal on silicon display device such as those described in U.S. Patent 5,566,010 or 5,426,526 or the light modulating surface may be a transmissive light modulating display such as a thin film active matrix liquid crystal display commonly found on laptop personal computers or video camcorder viewfinders.

As shown in Figure 3, the lens 102 has focal length fi, which is in this case being shown as equal to distances 111 and 112. The lens 103 has focal length f2, which is being shown as equal to lengths 114 and 115 for the purposes of this example. It will be appreciated that this discussion is using the conventional thin lens approximation, and the application of this discussion to the design of an optical system will be understood by those skilled in the art.

The illuminator 104 includes an aperture 105 which defines a light emitting surface which is a diffuse source of light; this light emitting surface 108 is flat and has a plurality of points which emit light, such as points 121 and 122. The edges of the light emitting surface 108 are defined by the edges 106 and 107 which correspond to edges of the surface 109 of the display device at edges 106a and 107a respectively. Examples of light sources or illuminators which may be used with the present invention will be described below. The light emitting surface 108 includes a plurality of points each of which emits light in a predetermined cone or pyramidal-like shape from the point. It would be appreciated herein that a cone of light will be considered to include a pyramidal-like shape of light as will other geometries of light emitting patterns which are controlled in terms of angular dispersion from a point of emission on an illumination surface.

The lens 102 is positioned in front of the illumination surface at a distance equal to focal length 111. The lens 103 is positioned in front of the lens 102 such as the distance between the lenses is equal to the sum of the focal lengths 112 and 114. It will be appreciated that this arrangement may be modified such that the distance between the lenses is approximately the sum of these two focal lengths, and yet the various advantages of the present invention would still be achieved while using this approximate arrangement. The display surface 109 is positioned in front of lens 103 such that the distance separating the lens 103 (as shown in Figure 3) from the display surface 109 is equal to the focal length 115. Using this arrangement, the illumination surface 108 is both magnified on the display surface 109 and is also in focus on the display surface 109. It will be appreciated that it may be desirable to position the display surface 109 at a distance which approximates the focal length 115 in order to have the illumination surface 108 out of focus on the display surface 109 (e.g. having the display surface so positioned will cause the illumination surface to be out of focus and thus any dust or other imperfections on the illumination surface 108 will not be imaged upon the display surface). It will be appreciated that the illumination surface 108, which is typically a rectangular surface, will be magnified upon the display surface 109, and the amount of magnification will depend upon the ratio between f2 and f 1 (specifically the magnification will be determined by the ratio of f2/fl. It will also be appreciated that it may be advantageous to enlarge the image of the illuminator to be somewhat larger than the display surface in order to ease assembly tolerances, or to provide a pleasing border area around the display area.

It will be appreciated that numerous techniques exist from the prior art for creating predetermined pyramidal-like shapes of light from each point of the illumination surface 108. One example uses two crossed BEF films as shown in Figures 5a or 5d. Thus at point 121 and point 122 pyramidal-like shapes of light are emitted from each of these points and these shapes are predetermined based upon the two BEF films which are utilized in the illuminator 104. Each cone is defined by extreme rays at the edge of the cone such as rays 131 and 132 in the case of the cone emanating from point 121 or rays 133 and 134 in the case of the cone emanating from point 122. It would be appreciated that these rays define an angle shown as θ 1. These angles will be the same for cones emanating from the various points from the illumination surface 108. Light from each cone is redirected by the lens 102 towards the lens 103 and then through that lens 103 towards the display surface 109. Thus, for example the ray 131 is redirected by lens 102 to create the ray 131a which transits from the lens 102 toward the lens 103. Similarly, the ray 132 is redirected by the lens 102 to create the ray 132a which transits between lens 102 and lens 103. Lens 103 redirects both these rays to create rays 131b and 132b as shown Figure 3. Similarly, rays 133b and 134b are generated from the rays 133 and 134 emitted from the point 122 to create the corresponding point 122a on the surface 109. The point 121a corresponds to the point 121 on the illumination surface in that light from point 121 is directed to illuminate the display point 121a. As can be seen from Figure 3, display points 121a and 122a are separated by a larger vertical distance than the distance vertically which separates points 121 and 122 on the illumination surface 108. Thus, the illumination surface has been magnified onto the display surface 109. Moreover, the angle between the extreme rays of each cone has shrunken; this incident angle is shown as 02 and is the same for the cones of light which are directed to the display surface 109. Thus, light rays 133b and 134b define an angle 02 which is bisected by an axis 136. Similarly, rays 131b and 132b define an angle 02 (angle 141) which is bisected by an axis 137. Also note that the axis 137 is parallel to the axis 136 and thus the two cones shown striking the display surface 109 are substantially parallel and have the same angle. As noted above, the image of the illumination surface 108 is magnified by the ratio of f2/f 1. The magnification of the illumination surface improves the light utilization efficiency of the system by allowing the use of the smaller illumination surface to achieve sufficient illumination at the display surface 109. The shrinking of the angles from the angle 0 1 to the angle 2 improves the display characteristics by eliminating wider angle marginal rays which are incident upon display points on the display surface 109. It will also be appreciated that by controlling the magnification (by controlling the ratio of the focal lens) one can also control the ratio of the angles 01 and 02. In particular in the system shown in Figure 3, the following equation is generally accurate:

(f2/fl) = (sin 01/sin 02H 01/ 02) Thus, the larger the magnification, then the smaller 02 will be relative to 01. For many liquid crystal display devices, shrinking the angle of incident light in each cone at a display point often improves contrast over the entire surface of the display device.

It will be appreciated that many of the benefits of the present invention may be achieved by having the two lenses 102 and 103 kept at a distance determined by the sum of the two focal lens as noted above. However, minor deviations from this distance may be tolerated as long the cones of like incident at the display surface 109 do not radically change their parallelism. Figure 4 shows an example of a case where the two lenses 102 and 103 have been moved so close to each other that the cones of light striking the display surface 109 are not substantially parallel. This is shown by the axes 136a and 137a which bisect the cones of light created by the extreme rays as shown in Figure 4. a n optical system according to the present invention may provide a plurality of benefits; it will be appreciated that in certain instances some benefits are achieved while others are not depending on the particular implementation. One advantage is that the light is provided substantially in parallel at the plurality of points on a display surface, including points at opposite ends of the display surface in those embodiments. Furthermore, the angle of incident light, such as 02 as shown in Figure 3 may be optimized by shrinking (or growing) this angle relative to the angle of emission, such as 1. For example, in the case of many liquid crystal display devices, an angle of 10 degrees will often be better than an angle of 45 degrees provided by many BEF films. Other types of light modulators may be improved by increasing the incident angle at the light modulating surface relative to the angle of emission from the light source. Furthermore, magnifying the light source uses the illuminator more efficiently allowing a smaller illuminator which consumes less power and provides light more efficiently as a consequence. Furthermore, contrast and color uniformity is improved by using this illumination system since the angle of illumination at each point has been optimized and light at each point is entering the display surface in a parallel fashion across substantially all the display surface of a light modulating display device. Furthermore, control of the incident angle, such as angle 02, allows control of the exit pupil of the optical system (e.g. exit pupil 249 shown in Figures 6 and 7) and this assists in avoiding vignetting of the image. Thus, a highly efficient light source is combined with good quality images by utilizing various aspects of the present invention.

Figure 5a illustrates an example of a light source or illuminator which may be used in an optical system according to the present invention. The illuminator of Figure 5a includes a light emitting device, such as a light emitting diode 202 or other light emitting device which emits light rays 210 within the chamber 201. Typically, the chamber 201 is closed and the light produced by the light emitting diode 202 cannot escape from this chamber except through the aperture at the opposite end, which aperture is covered by three films 204, 205 and 209. The internal walls of the chamber 201 are lined with a very highly reflective material which is diffusely reflective. It will be appreciated that several LEDs of different colors or of the same color may be used within the chamber 201. For example, a red LED, a green LED and a blue LED may be placed at one end of the chamber 201 and maybe turned on together to provide a way to generate white light through the mixing of the three lights or they may be turned on sequentially in time in order to generate a time sequential colored display system such as that described in copending U.S. Patent application Serial No. 08/801,994, which was filed February 18, 1997, which is hereby incorporated herein by reference. Light emitted from the LED 202 bounces around inside the box and exits through the three films 204, 205 and 209. Film 209 is a diffuser which helps to improve the uniformity spatially of the light emitted from the illuminator shown in Figure 5a. Alternatively, a diffuser lens may be used in place of a diffuser film. Films 204 and 205 are two BEF films arranged in a 90 degree cross fashion in the conventional manner known in the prior art. This causes each point from an emission surface of the film 205 to radiate a pyramidal-like shape of light as shown in Figure 5a. For example, point 206 emits a pyramidal-like shape of light defined by the extreme rays 206a and 206b which define an angle which is bisected by the axis 206c. Similarly, point 207 emits a cone of line defined by the extreme rays 207a and 207b which are bisected by the axis 207c. It will be appreciated that the illuminator shown in Figure 5 a may be used in the optical system of Figure 3. In particular the output surface of the BEF film 205 will be the illumination surface 108 in this case.

Figure 5b shows another example of a light source which may be used in an optical system according to the present invention. This light source 221 includes a light box 220 which has a front surface which includes three panels 222, 223 and 224. The panel 222 provides the light emitting surface and panels 223 and 224 contain the illumination source, such as LEDs which provide their light into the interior of the light box 220. Thus, LEDs 225 are within the panel 223 on the left side of the front of the light box 220, and LEDs 226 are on the left panel 224 of the front of the light box 220. The light emitting surface of the panel 222 corresponds to the light emitting surface 108 of Figure 3. Further details of the construction of this illuminator are shown in Figures 5c and 5d which represent respectively cross sectional views taken at the cross sectional indicators 5c-5c and 5d-5d as shown in Figure 5b. Figure 5c shows the cross sectional view of the panel 223 which houses six LEDs 225a, 225b, 225c, 225d, 225e, and 225f. These LEDs are positioned to direct light generated by the LED toward the inner back wall 227 of the light box 220. The internal walls of the light box 220 are lined with a highly reflective material which may be diffusely reflective. Light is reflected diffusely off of this surface and ultimately is reflected toward the interior wall of the panel 222. Light of the proper angle and polarization will then pass through the three films on the face of the light panel 222 to exit the light box 220. It will be appreciated that typically the light box 220 is otherwise sealed such that light will not escape from this box. It will be appreciated that various types illuminators may be used in the panel 223. For example, these illuminators may be all of the same color or they may alternatively arranged to have different colors to provide a time sequential display system. For example, LEDs 225a and 225d may be red LEDs, LEDs 225b and 225e may be green LEDs, LEDs 225c and 225f may be blue LEDs which are each controlled to provide a time sequential color display output through the panel 222.

Figure 5d shows the cross sectional view through the panel 222. The light box 220 at this cross sectional view includes a back reflective wall 227 on one side of the light box and three films on the light emitting panel 222 on the opposite side of the light box. It will be appreciated that all surfaces on the inside of the light box 220 may be lined with the very highly reflective material except for the exit window represented by the panel 222 and the light emitting surface of each LED, such as LED 225 a. The light exit window of the panel 222 includes three films 228, 229 and 230. Films 228 and 229 are conventional BEF films which are designed to provide light in a controlled angular dispersion from each point as described above. The film 230 is a type of polarizer film know as DBEF which is produced and sold by 3M of Minnesota. Further details concerning this film are described in the PCT application published as International Publication No. WO97/01774. Another example of this type of film is described in U.S. Patent No. 3,610,729. This film 230 polarizes light by reflection rather than by absorption. It will be appreciated that other conventional polarizers which polarize by absorption may be used rather than the film 230 as described herein. Each point at the light emitting surface of the panel 222 emits a predetermined pyramidal-like shape of light which has a controlled angular dispersion such that the angle of light coming from each point is less than 180 degrees as shown in Figure 5d. Thus for example point 231 emits a cone of light defined by the extreme rays 231a and 231b which form an angle bisected by the axis 231c. Figure 6 shows one example of an optical system for illuminating a light modulating display device according to the present invention. This system utilizes the illuminator 221 by fixing this illuminator within a bracket 239 which also holds a first lens 102a. .This lens is arranged in front of a reflecting mirror 103a which functions in the same manner as the lens 103 of Figure 3. Thus, the two lenses 102a and 103a of Figure 6 are separated such that the optical distance between these two optical elements is equal to the sum approximately of the first focal length f 1 and the second focal length f2 (where the first focal length is actually the secondary focal length of the lens 102a). The light from the reflecting mirror 103a is directed towards a reflecting beamsplitter 241 which may be a polarizing beamsplitter which reflects light of one polarity toward the display surface 109. The focal length 250 (f2) is also shown on the reflecting side of the mirror 103a. As is well known in the art, the light modulating surface of the display surface 109 forms an image on the display surface and this image is projected toward the lenses 243 and 245 through the beamsplitter 241. An image may be then viewed by looking at the lens 245 toward the display surface 109.

Figure 7 shows an example of another embodiment of the present invention which uses a folded light path by using a mirror 251 between the lens 102a and lens 103b. The lens 103b replaces the mirror 103a of Figure 6 and functions in the same way as lens 103 of Figure 3. Referring back to Figure 7, light from the illuminator 221 passes through lens 102a, is reflected by mirror 251 and then passes through lens 103b and is reflected by the reflecting beamsplitter 241 toward the display surface 109. As is well known in the art, the light modulating surface of the display surface 109 forms an image on the display surface, and this image is projected toward the lens 243a and 245a through the beamsplitter 241. An image may then be viewed by looking at the lens 245a toward the display surface 109. The two lenses 102a and 103b of Figure 7 are separated such that the optical distance between these two optical elements is equal to the sum of the focal lengths f 1 of lens 102a and f2 of lens 103b. Thus, a telecentric imaging system, formed by the illuminator 221, lens 102a, mirror 251, lens 103b and beamsplitter 241 creates a uniform pattern of illumination on the surface 109. The angle of light emitted from the illuminator 221 may be controlled by the telecentric imaging system to match the optimal angle of light incident at the display surface 109; typically, the optimal angle at the display surface will be shrunken relative to the angle of light emitted from the illuminator's surface. Further, controlling the angle at the display surface 109 allows control of the exit pupil 249 of the display system. Also, the pyramidal-like shape of incident light at most points on the display surface 109 may be made substantially parallel. Moreover, the efficiency of the display system may be made high by using a smaller illuminator 221 which is magnified at the display surface 109.

.Although only several particular embodiments of the present invention have been described in detail, it should be appreciated that the present invention may be embodied in other forms without departing from spirit and scope of the present invention. For instance, transmissive light modulating display systems may be employed rather than reflective display systems, such as liquid crystals on silicon display systems. Furthermore, other illumination systems may be employed such as laser diodes, cold cathode or field emitter cathodoluminescent sources. Also, various display systems may be implemented which use spatial display systems (having a triad of subpixels at each pixel location) rather than time sequential display systems. Moreover, these various displays systems may be used with what may be referred to as cover glass modulation (which is described in co-pending U.S. Patent Application Serial No. 08/801,994, filed February 18, 1997 which is hereby incorporated herein by reference). Furthermore, numerous alternative arrangements of optical elements may be employed with the present invention. For example, a lens which is used with the present invention may be a glass lens, a plastic lens, a Fresnel lens, a curved reflective element or even a holographic element. The optical pathway may include mirrors to provide a folded, more compact optical path. .Also, the many kinds of illumination sources may be used with the present invention, although illuminators having controlled pyramidal-like shapes of emitting light from the illuminator's surface are preferred. Thus, an illumination source having a reasonably flat, reasonably uniform source may be employed with the present invention; if the pyramidal-like shapes of emitting light are not controlled by the illuminator then the shape of the light "cones" is determined by the first lens.

Claims

CLAIMSWhat is claimed is:

1 . An optical system for illuminating a display device, said optical system comprising: a source of light; an optical element optically coupled to said source of light, said optical element substantially telecentrically illuminating said display device.

2. - n optical system as in claim 1 wherein said optical element comprises a first lens and a second lens, said first lens receiving light from said source of light and said second lens directing light toward said display device.

3. An optical system as in claim 2 wherein said display device is a light modulating display.

4. An optical system as in claim 3 wherein said first lens has a first focal length (fl) and said second lens has a second focal length (f2) and wherein said first lens and said second lens are positioned relative to each other such that a first distance of approximately f 1 + f2 separates said first lens and said second lens.

5. aAn optical system as in claim 4 wherein said light modulating display comprises a substantially flat surface.

6. An optical system as in claim 5 wherein said source of light is a diffuse source.

7. An optical system as in claim 5 wherein said source of light comprises a plurality of points, each of said plurality of points emitting a first predetermined pyramidal-like shape of light.

8. An optical system as in claim 7 wherein said first lens and said second lens provide, at a plurality of display points, a plurality of second predetermined pyramidal-like shapes of light.

9. An optical system as in claim 8 wherein said plurality of second predetermined pyramidal-like shapes are all substantially parallel to each other.

10. An optical system as in claim 8 wherein each of said plurality of second predetermined pyramidal-like shapes defines an axis, and wherein a plurality of said axes are substantially parallel.

1 1. An optical system as in claim 10 wherein said first predetermined pyramidal-like shape defines a first angle and said second predetermined pyramidal-like shape defines a second angle, and wherein said second angle is smaller than said first angle.

12. An optical system as in claim 10 wherein said source of light is not in focus at said display device.

13. An optical system as in claim 10 wherein said source of light is magnified at said display device.

14. An optical system as in claim 10 wherein each of said second predetermined pyramidal-like shapes defines a second angle and wherein said second angle determines an exit pupil for said optical system.

15. - n optical system as in claim 10 wherein said first predetermined pyramidal-like shape defines a first angle and said second predetermined pyramidal-like shape defines a second angle and wherein said second angle is smaller than said first angle, and wherein said source of light is not in focus at said display device and is magnified at said display device, and wherein said second angle determines an exit pupil for said optical system, and wherein said source of light is a flat, diffuse source.

16. A method of illuminating a display device, said method comprising: generating light from a light source having a plurality of points, each of said plurality of points emitting a first predetermined pyramidal-like shape of light; optically converting each of said first predetermined pyramidal-like shapes of light to a second predetermined pyramidal-like shape of light having an axis; directing each of said second predetermined pyramidal-like shapes of light to a light modulating surface of said display device wherein said light modulating surface receives a plurality of second predetermined pyramidal-like shapes of light having a plurality of axes which are substantially parallel.

17. A method as in claim 16 wherein said step of optically converting converts a first angle determined by said first predetermined pyramidal-like shape to second angle determined by said second predetermined pyramidal-like shape.

18. A method as in claim 17 wherein said second angle determines an exit pupil for an optical system for said display device.

19. A method as in claim 16 wherein said light source has a flat, rectangular surface and wherein said directing step produces a magnified image of said light source on said light modulating surface.

20. A method as in claim 16 wherein said directing step produces an unfocused image of said light source on said light modulating surface.

21. A method as in claim 16 wherein said directing step comprises reflecting light from said light source toward said light modulating surface.

22. An optical system as in claim 10 wherein said first predetermined pyramidal-like shape defines a first angle and said predetermined pyramidal-like shape defines a second angle and wherein a ratio of said first focal length and said second focal length is selected to determine said second angle.

23. An optical system as in claim 6 wherein said diffuse source comprises a mirrored chamber.

24 An optical system as in claim 6 wherein said diffuse source comprises a diffusely reflective chamber and a plurality of light emitting diodes which emit light into said diffusely reflective chamber.

25. -An optical system as in claim 7 wherein said first predetermined pyramidal- like shape is controlled by at least one film.

26. An optical system as in claim 25 wherein said diffuse source comprises a plurality of films which control said first predetermined pyramidal-like shape.

27. a n optical system as in claim 5 wherein said light modulating display comprises a liquid crystal.

28. An optical system as in claim 5 wherein said light modulating display is a transmissive display device.

29. An optical system as in claim 5 wherein said light modulating display is a reflective display device.

30. An optical system as in claim 5 wherein said display device is a time- sequential color liquid crystal display.

31. An optical system as in claim 5 wherein said display device is in a head mounted display.

32. An optical system as in claim 30 wherein said light modulating display is a liquid crystal on a silicon substrate reflective display in a head mounted display.