WO2007138501A2

WO2007138501A2 - Miniature projection engine

Info

Publication number: WO2007138501A2
Application number: PCT/IB2007/051728
Authority: WO
Inventors: Marcellinus Petrus Carolus Michael Krijn; Oscar Hendrikus Willemsen; Bart Andre Salters; Antonius Hendricus Maria Holtslag
Original assignee: Koninklijke Philips Electronics N.V.; U.S. Philips Corporation
Priority date: 2006-05-24
Filing date: 2007-05-08
Publication date: 2007-12-06
Also published as: WO2007138501A3

Abstract

A light engine (200) including a display panel (230) and a light-guide (210) arranged to provide illumination for modulation by the display panel (230). The light-guide (210) includes an out-coupling structure (220) configured to condition and provide light to the display panel (230) in a pre-determined polarization state. A light source (250) may be included to provide collimated light to the light engine (200). The light-guide (210) may be formed from an isotropic dielectric material. The out- coupling structure may include a birefringent material having a high index of refraction to light striking the out-coupling structure (220) in one direction, and having an index of refraction matched to the light-guide for light striking the out-coupling structure in another direction. The display panel may be an LCOS display panel. The display panel (230) may be arranged for reflective or transmissive operation.

Description

MINIATURE PROJECTION ENGINE

The present invention relates to optical video/data projection display systems, and more particularly relates to miniature projection display systems, construction and operation using a front or back lit lighting structure to illuminate miniature projection display panels.

Conventional optical projection systems are widely used in business, education and consumer applications. For example, slide and motion picture projectors are used to show images from film on screens. Projection television may use one to three liquid crystal display (LCD) panels or liquid crystal light valve (LCLV) panels, and other projectors may use computer-controlled LCD, for video/data display applications. The light for projector systems is provided by a light engine, generally consisting of a light source, reflector and one or more lenses to direct the light to an image gate or light valve, such as an LCD panel. A solid light pipe may be utilized in place of or in addition to the reflector and one or more lenses for guiding the light. As used herein, "light-guide" shall be used interchangeably with "waveguide," where a combination of a waveguide and LCD panel(s) is referred to as an LCD module or projection engine.

Conventional LCD modules or projection engines typically include one (1) to three (3) small, reflective or transmissive displays, generally referred to as display panels or micro-displays, for modulating light and thereby, forming an image. The image is enlarged by a set of optics to a final viewable size or for projection onto a display surface. The light for the display panels is provided by a projection lamp. Switching (modulating) is done by the display panels or micro-displays. LCD arrays are constructed from both amorphous and polysilicon as active matrix LCDs; transmissive micro-display panels (HTPS), reflective micro-display panels (e.g., LCOS). LCD panels, whether reflective or transmissive, modulate the polarization of the light according to the desired image data. Twisted-nematic LCDs typically include polarizers on each side of each LCD cell or element. Transmissive panels (active matrix LCD) use back-lit lighting created by a source (e.g., LED), and are colored and switched separately (e.g., by thin-film transistor (TFT) arrays). TFT arrays are rows and columns of thin film transistors made on a glass substrate to form pixel-addressing components of an active LCD. Transflective LCD arrays include an active matrix LCD that combines reflective and transmissive qualities. In a small projection display system or other optical systems, including but not limited to front and rear projection image systems, size and weight considerations are critical with an eye towards the market, and desirability of use by the end users. Size, weight and user interface, as well as production costs affect the projectors or projection systems position in the market where miniaturized projection system are sold.

LEDs may now be produced that are sufficiently bright to produce high quality video/data with an extensive gamut of colors having a contrast exceeding that of conventional LED-based applications, including miniaturized projection applications.

A conventional miniature light engine 100 is depicted in FIG. 1. Such miniature light engines, or waveguides, may be utilized for conventional portable projectors or like projection engines. The FIG. 1 light engine is understood to be one of the most compact available for commercial and/or consumer use, in various video/data displays for micro or miniature display or light engine needs. Light engine 100 includes red, green and blue (RGB) LEDs 113, 115, 117, which provide three light beams, namely RGB, and electronics 112 that are utilized for driving the RGB LEDs 113, 115, 117. The RGB light beams, or sub-beams, are initially conditioned by reflectors 119, 121, 123 within a collimator/integrator 114 portion. Dichroic filters 122, 124, 126 combine the light generated by LEDs 113, 115, 117, and direct it in a polarized state to a PBS 116. The collimated and polarized light is reflected by the PBS onto a microdisplay 118, such as an LCOS microdisplay. The LCOS microdisplay panel 118 as shown is illuminated color- sequentially. The small LCOS microdisplay panel reflects the modulated color-sequential light to a projection lens 120 to produce a desired image.

However, dimensions shown in FIG. 1 for the light engine 100 are still too large to realize a form- factor for a projector that may be hand-held or a projection system that may be utilized for hand-held devices, such as personal digital assistants (PDAs), MP3 players, cameras, etc. This is particularly true when real-world considerations are included such as a battery, cooling system (e.g., forced air), LED and video driver electronics, and additional device components such as MP3 player, camera, etc. circuitry are considered.

The skilled artisan will readily appreciate that miniature light engines and miniature projectors, such as hand-held, portable, battery operated projectors, are not realizable by prior systems and would fill a need by skilled artisans. For that matter, there has also been a long felt need for sufficiently bright and inexpensive LEDs to serve as light sources for front and rear projection displays, particularly in low-power systems, such as hand-held portable projectors. That is, a central need exists in the conventional art of projection display, particularly in miniature or micro-projectors, utilizing transmissive (HTPS) as well as reflective microdisplay panels, for projection engines that may be adapted for hand-held projection applications.

It is an object of the present system to overcome disadvantages and/or make improvements in the prior art.

The present system includes a light engine for use in a projection system. The light engine includes a display panel and a light-guide arranged to provide illumination for modulation by the display panel. The light-guide includes an out-coupling structure configured to condition and provide light to the display panel in a pre-determined polarization state. A light source may be included to provide collimated light to the light engine. The light-guide may be formed from an isotropic dielectric material. The out- coupling structure may include a birefringent material having a high index of refraction to light with a given state of polarization striking the out-coupling structure in one direction, and having an index of refraction matched to the light-guide for light with the given state of polarization striking the out-coupling structure in another direction. The display panel may be an LCOS display panel. The display panel may be arranged for reflective or transmissive operation. In one embodiment, the light engine may include a light analyzer arranged to block light propagated by pixels of the display panel that are in an off state. Further, a film may be arranged on a side of the light-guide, opposite the out-coupling structure. In this embodiment, the film changes the state of polarization of the light that is not coupled through the out-coupling structure due to an improper polarization state of the light. The light-guide may include a retarder foil arranged in a position proximal to a position for a light source to change a polarization state of the light within the light-guide.

The out-coupling structure may be arranged as a dot-pattern structure, a grooved structure, and other structures that operate in accordance with the present system. The light-guide may include a pre-collimating element such that light propagating through the light-guide is collimated in a direction that is parallel to grooves of the out-coupling structure. The pre-collimating element may be arranged as a parabolic concentrator. In the following description, for purposes of explanation rather than limitation, specific details are set forth such as the particular architecture, interfaces, techniques, etc., for illustration. However, it will be apparent to those of ordinary skill in the art that other embodiments that depart from these specific details would still be understood to be within the scope of the appended claims. For the purpose of clarity, detailed descriptions of well- known devices, circuits, and methods are omitted so as not to obscure the description of the present system. Similar reference numerals within different drawings are denoted to indicate similar functions and/or operation. Moreover, it should be expressly understood that the drawings are included for illustrative purposes only. The drawings may not be accurately scaled, in particular with respect to angular representations of optical light paths, and as such, should not be interpreted as limiting the scope of the present system as claimed.

It should be expressly understood that the drawings are included for illustrative purposes and do not represent the scope of the present system in which: FIG. 1 shows a system depicting a conventional LCOS projector with an LED- based light source;

FIG. 2 is a system level depiction of a first embodiment of a micro or miniature light engine or waveguide in accordance with an embodiment of the present system;

FIGs. 3 A, 3B depict side and top views of a light-guide or waveguide portion that is constructed to include an out-coupling structure in accordance with an embodiment of the present system;

FIG. 3C is a top view of a second light-guide comprising an out-coupling structure in accordance with an embodiment of the present system;

FIG. 4 A shows a polar intensity distribution radiated by an out-coupling structure constructed in accordance with an embodiment of the present principles herein;

FIG. 4B is a plot of light intensity along the cross section line of the out-coupling structure that is the focus of the distribution of FIG. 4 A;

FIG. 5 is a system level depiction of a miniaturized projection display system in accordance with an embodiment of the present system that includes a waveguide or light- guide built with a transmissive type micro-array; and FIG. 6 depicts an embodiment of the present system including a miniaturized projection display system, wherein a transmissive display panel is combined with an array of lenticular lenses and a diffractive element.

As mentioned above, waveguides or light-guides, out-coupling structures, and miniature optical projection systems including same, are constructed in accordance with the present system based on the physical principle that light from one or more LEDs is coupled into a light-guide, arranged so that light (e.g., collimated) may travel through the light-guide in accord with the principle of total internal reflection (TIR). Only particularly polarized light may exit the light-guide for use by the display panel, microdisplay, light valve, etc. in accordance with the present system.

A first embodiment of a miniature light engine 200 is depicted in FIG. 2 and includes a display panel 230, such as an LCOS panel, and an edge-lit backlight structure (light-guide) 210, having an out-coupling structure 220. The light-guide 210 is positioned between the LCOS panel 230, and a lens 240, in the embodiment shown. The light engine in FIG. 2 is illustratively shown as a reflective display panel although as a person of ordinary skill in the art would readily appreciate, a modification in accordance with the present system (e.g., placing the light-guide 210 behind the LCOS panel 230) may be applied to utilize this system in a transmissive application. Exemplary light engine physical dimensions (mm) are included in FIG. 2, and provide a physical size of the light engine 200 that is realizable in accordance with the present system however, are not intended as a limitation on the present system. The physical dimensions provided in FIG. 2 are intended merely for comparison purposes to the physical dimensions provided in FIG. 1 which do represent actual miniaturization limitations in the prior art light engine 100. However, in accordance with the present system, smaller or larger light engines may be readily fabricated.

Light-guide 210 functions as an edge-lit backlight for the LCOS panel 230. In operation, light from the light-guide 210 illuminates the LCOS panel 230 via the out- coupling structure 220. The illumination (light) is modulated/reflected by the LCOS panel 230 and is provided to the lens 240. That is, light from a light source 250, such as from one or more LEDs (e.g., high- brightness LED-based) is coupled from one or more sides, into the light-guide 210. The light source 250 may be arranged to provide pre-collimated light, such that the light may be propagated through the light-guide based on the principle of total internal reflection (TIR). In accordance with the present system, one side of the light-guide 210 is structured, shown as the out-coupling structure 220, such that the light has a chance to be coupled out of the light-guide 210 through the out-coupling structure 220 and thereby, be directed towards the LCOS panel 230. While the out-coupling structure 220 may be positioned on either side of the light-guide (top or bottom of the light-guide with reference to FIG. 2), it is illustratively arranged for a reflective application. Predominantly, only light having a predetermined state of polarization is passed out of the light-guide, due to the out-coupling structure 220 in accordance with the present system. A polarization contrast (e.g., a ratio of desired to undesired state of polarization) exceeding 100 may be obtained using the light-guide 210 of the present system. The light engine 200 may have a self-contained power supply 255 for powering the portions of the light engine 200 without an external power source.

Further operation in accordance with the present system will be illustrated utilizing a light-guide structure 310 including an out-coupling structure 320 for use in light engines such as illustratively shown in FIG. 2. Such a light-guide in accordance with the present system is depicted in a side (cutaway) view, and a top (plan) view in FIGs. 3A and 3B, respectively. In the embodiments shown in FIGs. 3A, 3B, the out-coupling structure 320 is illustratively shown as filled-grooves (e.g., etched, formed, etc.) in the light-guide structure 310. An angle of the grooves (e.g., with reference to a line running perpendicular to a top of the light guide) may be any angle as desired, for example, the angle may lay within a range of 30-70 degrees. For example, an angle of 50 degrees may be utilized, such as utilized for the results depicted in FIGs. 4A and 4B as discussed in more detail below. The light-guide structure 310, not including the out-coupling structure 320, may for example be formed of an isotropic medium such as glass, poly-methyl-methacrylate (PMMA), and/or other clear materials. While forming the light-guide structure 310, the grooves may be filled with deposits of a bi-refringent material, such as a bi-refringent plastic (e.g., poly- naphtyl-metacrylate) having a high index of refraction in one direction (e.g., perpendicular to the plane of FIG. 3A) and an index of refraction that is matched to that of the light-guide 310 (e.g., the isotropic medium) in another direction (e.g., in the plane of FIG. 3A). In alternate embodiments, the light guide may be formed from a bi-refringent material whereas the grooves are filled with an isotropic material. In yet another embodiment, both the light guide and groove filling may both be formed from a bi-refringent material. In an embodiment, light traveling in the light-guide having a polarization state perpendicular to the plane of the drawings may escape the light-guide structure 310 by the out-coupling structure 320, and thereby is directed to the panel as polarized light (e.g., LCOS panel 230 of FIG. 2). In contrast, light with a polarization state in the plane of the drawings (a "wrong" state) is not coupled out by the out-coupling structure 320 since the out-coupling structure 320 acts as an extension of the light-guide 310 and accordingly, light in this polarization state operates with TIR within the light guide structure 310 and may be lost, or in other words, not be available for modulation by the display panel.

The panel pixels (not shown in FIGs. 4) that are in an "on" state, condition light incident upon them in order that the polarization state is altered from the display panel such that the reflected light from the on pixels of the panel pass the light-guide without being scattered by the out-coupling structure 320 (e.g., are not captured or otherwise interfered with). An analyzer may be included (e.g., analyzer 295 in FIG. 2) to block light which may be reflected from pixels within the panel that are in an "off state or light that escapes the light-guide in a direction towards the projection lens.

In this or another embodiment in accordance with the present system, the light- guide 310 may recapture polarized light in the wrong state (e.g., polarization state in the plane of the drawings) by constructing the light-guide 310 to be partially birefringent. As should be apparent to a person of ordinary skill in the art, the function may be implemented in various ways, such as through an addition of a retarder foil, film, etc. 360, included in the light-guide structure 310. For example, a retarder foil 360 may be laminated onto the side of the light-guide structure 310 in a proximate side to a light source 350. In this or alternate embodiments, the retarder 360 may be located opposite the out-coupling structure and/or even on a same side as the out-coupling structure. In operation, the retarder will gradually change the state of polarization of the light traveling through the light-guide. In addition to using a foil 360, light in the wrong polarization state may be recycled by the light-guide using a technique of polarization recycling. A reflective polarizer 380 may be included in combination with the retarder foil 360 for recycling light having the wrong polarization state. For example, the retarder 360 may be formed as a 1/4 λ plate. Light with the proper or correct polarization state may cross the reflective polarizer 380, where all other light is reflected back towards the source. Due to TIR, part of the reflected light is redirected again towards the reflective polarizer 380. The retarder 360 alters the polarization state of the redirected light (recycles the light), such that a portion may be in a polarization state to enable passing through the reflective polarizer 380 and thereby, be coupled to the out-coupling structure 320 as described above. In addition, a reflector layer 390 may be arranged at an end distal to the light source 350 to reflect light that is not coupled to the out-coupling structure 320 and thereby, the light may pass back through the reflective polarizer 380 and retarder 360 and thereby, have its polarization state altered and thereby be recycled as described. In addition, light that is reflected from "off pixels of the display panel may be recaptured by the light-guide 310 and thereby, also be recycled.

In accordance with an alternate embodiment of the present system, a dot-like pattern of birefringent film may be utilized as an out-coupling structure 320' of a light- guide 310' of FIG. 3C. The light-guide 310' has an advantage in that light within the light guide need not be pre-collimated prior to use within the light guide 310'. Light sources 332', 334', 336', for example emitting in the red, green, and blue part of the visible spectrum, may be used to homogeneously illuminate the light guide. Suitable light sources, such as LEDs, may be utilized. Additionally, in between each of the light sources and light guide, a color filter may be located that transmits the light of this light source but reflects light of light sources emitting different colors. In this way, the light source will not be able to absorb any light of light sources emitting different colors. Color sequential illumination may be used. FIGs. 4A and 4B depict a plot of polar intensity distribution and intensity distribution by cross-section, respectively, for a light-guide structure, such as the light- guide structure 310 with a groove-type out-coupling structure. Light coupled out of the light-guide propagates in a direction perpendicular to the light-guide, with an angular spread perpendicular to the axial direction of the grooves, or groove-like structures. Illustratively, the angular spread may lie in an approximate range of twenty (20) to thirty

(30) degrees which performs satisfactorily for the present system.

In a direction or state of polarization which is parallel to the grooves or groove-like structures, the angular spread is larger than 20-30 degrees which may have an undesirable effect of not providing illumination to the display panel. To reduce light that does not fall incident to the display panel, the light may be pre-collimated in a direction parallel to the direction of the groove surfaces, or groove-like structures, before the light enters the light- guide. A parabolic concentrator, such as parabolic concentrator 355 of FIG. 3B may be included in the light-guide 310.

Those of ordinary skill in the art would readily appreciate that the embodiment of FIG. 2 illustratively depicts a reflective panel, while the present system is also readily implemented in transmissive display technology. Transmissive technologies include high- temperature polysilicon (HTPS) or silicon-on-insulator (SOI), and are implemented in the light engine 500 depicted in FIGs. 5 and 6 in accordance with an embodiment of the present system. For the embodiments shown in FIGs. 5 and 6, color-sequential illumination may be used with the display panels. FIG. 5 shows an embodiment of a light engine 500 in accordance with an embodiment of the present system including a light-guide 510, a polarizer 580, a transmissive display panel 530, color filters 532, 534, 536, such as red, green and blue filters, an analyzer 595, and a lens 540 for forming an image for projection. Operation of the light engine 500 is similar to the operation of the light engine 200. FIG. 6 shows an embodiment of a light engine 700 in accordance with the present system based on a transmissive display panel 730 including an array of lenticular lenses 780, a diffractive element 770, such as a grating or a holographic color splitting layer, and a light- source 750 directing light obliquely onto the diffractive element 770. White light 760, for example from an edge-lit light guide 710 (e.g., backlight) is directed to a layer such as the holographic layer 770 that splits the light into a red, a green, and a blue portions 762, 764, 766, having different angles of diffraction. An array of lenticular lenses 780 is used to redirect each of the color light portions 762, 764, 766 to pixels intended to be illuminated by that particular color, such as pixel 790.

Similar embodiments based on reflective panel technologies (e.g. LCOS) and color sequential illumination in accordance with the present system is readily apparent to a person of ordinary skill in the art.

The present light engines may be readily formed for use in projection systems of a size that may be handheld and/or adapted for use in other handheld devices such as cellular phones, PDAs, MP3 players, and other devices of the like. The present systems are also applicable to systems that utilize more or other than the red, green and blue color system, such as red, amber, green and blue color systems and to multi-display panel applications. In addition, efficiencies of the present system enable projection engines that may be suitably adapted for battery operation in front and rear projection systems.

Of course, it is to be appreciated that any one of the above embodiments or processes may be combined with one or more other embodiments or processes to provide even further improvements in accordance with the present system. The skilled artisan will understand that the above-discussion is intended to be merely illustrative of the present system and should not be construed as limiting the appended claims to any particular embodiment or group of embodiments. Thus, while the present system has been described in particular detail with reference to specific exemplary embodiments thereof, it should also be appreciated that numerous modifications and alternative embodiments may be devised by those having ordinary skill in the art without departing from the broader and intended spirit and scope of the present system as set forth in the claims that follow. Accordingly, the specification and drawings are to be regarded in an illustrative manner and are not intended to limit the scope of the appended claims. In interpreting the appended claims, it should be understood that: a) the word "comprising" does not exclude the presence of other elements or acts than those listed in a given claim; b) the word "a" or "an" preceding an element does not exclude the presence of a plurality of such elements; c) any reference signs in the claims do not limit their scope; d) several "means" may be represented by the same item or hardware or software implemented structure or function; e) any of the disclosed elements may be comprised of hardware portions (e.g., including discrete and integrated electronic circuitry), software portions (e.g., computer programming), and any combination thereof; f) hardware portions may be comprised of one or both of analog and digital portions; g) any of the disclosed devices or portions thereof may be combined together or separated into further portions unless specifically stated otherwise; and h) no specific sequence of acts or steps is intended to be required unless specifically indicated.

Claims

CLAIMS:

1. A light engine configured for use in a projection system, the light engine comprising a light-guide arranged to provide illumination for modulation by a display panel, the light-guide comprising an out-coupling structure configured to condition and provide light to the display panel in a pre-determined polarization state.

2. The light engine of claim 1, comprising a light source configured to provide collimated light to the light engine.

3. The light engine of claim 1, wherein the light-guide comprises an isotropic dielectric material and the out-coupling structure comprises a birefringent material comprising a high index of refraction to light striking the out-coupling structure in one direction, and an index of refraction matched to the light-guide for light striking the out- coupling structure in another direction.

4. The light engine of claim 1, comprising a display panel configured as an LCOS display panel.

5. The light engine of claim 1, comprising a light analyzer configured to block light propagated by pixels of the display panel that are in an off state.

6. The light engine of claim 5, comprising a film arranged on a side of the light-guide, opposite the out-coupling structure, the film configured to change the state of polarization of light that is not coupled through the out-coupling structure due to an improper polarization state of the light.

7. The light engine of claim 6, wherein the light-guide comprises a retarder foil configured to change a polarization state of the light within the light-guide and arranged in a position proximal to a position for a light source.

8. The light engine of claim 1, wherein the out-coupling structure is configured to have a dot-pattern structure.

9. The light engine of claim 1, wherein the out-coupling structure is configured to have a groove-like structure.

10. The light engine of claim 9, the light-guide comprising a pre-collimating element configured such that light propagating through the light-guide is at least collimated in a direction that is parallel to grooves of the groove-like structure.

11. The light engine of claim 10, wherein the pre-collimating element is configured as a parabolic concentrator.

12. The light engine of claim 1, comprising a display panel configured as a transmissive type.

13. The light engine of claim 12, wherein the display panel comprises one of a high temperature polysilicon (HTPS) and a silicon on insulator (SOI) structure.

14. The light engine of claim 1, comprising a reflective polarizer arranged in a position proximal to a position for a light source.

15. The light engine of claim 1, comprising: a diffractive layer arranged adjacent to the light-guide; and an array of lenticular lenses arranged between the diffractive layer and the display panel, wherein the light-guide is configured to direct light obliquely onto the diffractive layer and the array of lenticular lenses are configured to direct light portions from the diffractive layer to particular pixels of the display panel.

16. The light engine of claim 15, wherein the diffractive layer is a holographic color splitting layer configured to split light incident thereon into separate color-defined light sub-beams exiting at different angles from the holographic color splitting layer, wherein the array of lenticular lenses are configured to redirect each colored light sub-beam to the particular pixels of the display panel.

17. The light engine of claim 1, wherein the display panel is configured as a reflective display panel.

18. A hand-held projector, comprising a self-contained power source and a light engine of claim 1.

19. A light engine configured for use in a miniature imaging system, wherein the light engine comprises a light-guide arranged to provide illumination for modulation by a display panel, the light-guide comprising an out-coupling structure configured to condition and provide light to the display panel in a pre-determined polarization state.

20. A light engine configured for use in a hand-held imaging system, wherein the light engine comprises a light-guide arranged to provide illumination for modulation by a display panel, the light-guide comprising an out-coupling structure configured to condition and provide light to the display panel in a pre-determined polarization state.