WO2005045507A1

WO2005045507A1 - Projection system

Info

Publication number: WO2005045507A1
Application number: PCT/IB2004/052309
Authority: WO
Inventors: Stephen Mcclain; Jeffrey A. Shimizu
Original assignee: Koninklijke Philips Electronics, N.V.; U.S. Philips Corporation
Priority date: 2003-11-06
Filing date: 2004-11-04
Publication date: 2005-05-19

Abstract

An image projection system includes a liquid crystal on silicon (LCOS) panel (504), a projection lens assembly (505), and a wire-grid PBS (503) disposed to direct at least a portion of source light towards the LCOS panel (504), and to direct a portion of light reflected from the LCOS panel (504) to the projection lens assembly (504). In addition, a field léns (506) and a quarter-wave retarder (507) are optically interposed between the LCOS panel (504) and the wire-grid PBS (503). The insertion of the quarter wave retarder (507) between the LCOS panel (504) and wire-grid PBS (503) helps improve contrast by correcting polarization aberrations otherwise caused by the nontelecentricity of the field lens (506).

Description

PROJECTION SYSTEM

This invention pertains to the field of projection systems, and more particularly, to projection systems employing reflective image modulator panels, such as liquid crystal on silicon (LCOS) panels. Liquid crystal on silicon (LCOS) panels encode images by selectively altering the polarization state of reflected light. For example, each "off or "dark" pixel of the panel may reflect impinging polarized light without changing its polarization direction, while each "on" or "light" pixel may rotate the polarization direction of impinging polarized light by 90°. Thus, the polarization direction of the light reflected from each "on" pixel differs from that of each "off pixel. A polarizing beam splitting element can then be used to separate the light of each "on" pixel from the light of each "off pixel, thus transforming the polarization encoded image from the LCOS panel into an intensity image. In most LCOS systems, a polarizing beam splitter (PBS) cube has been used to separate the polarization encoded light reflected from the LCOS panel. More recently, however, there have been a number of proposals to use a wire-grid PBS in place of the more conventional glass or MacNielle type PBS cube. When compared to the PBS cube, the wire-grid PBS has the advantages of higher contrast and lower cost. See, for example, U.S. Patent Publication No. 2002/0171809 Al (Andrew Kurtz et al.), International Patent Publication No. WO 02/095487 Al (Simon Magarill et al.), International Patent Publication No. WO 02/01844 A2 (Peter J. Janssen et al.), U.S. Patent Publication No. 2003/0067586 Al (Tatsuo Chigira et al.), U.S. Patent Publication No. 2003/0072079 Al (Barry D. Silverstein et al.), and U.S. Patent Publication No. 2003/0081179 A 1 (Clark Pentico et al.), the contents of which are incorporated herein by reference. FIG. 1 is a simplified schematic view of a LCOS optical engine employing a wire- grid as a polarizing beam splitter. As shown, light from an illumination system 101 is passed through a pre-polarizer 102 to obtain, for example, P-polarized light. A wire-grid PBS (WGP) 103 is interposed at about a 45 ° tilt-angle between the pre-polarizer 102 and a LCOS panel 104. The wire-grid PBS 103 is generally made up of a plurality of electric conductors extending generally parallel to and spaced apart from one another on one or both surfaces of a support plate. The support plate is transparent to light over the visible range. The conductors collectively define a wire-grid, and the spacing of the wires of the grid is generally less than the wavelength of the shortest wavelength of the visible light used in the projection system. As is well understood in the art, a beam splitter transmits light of one polarization state, and reflects light of another polarization state. The preferred arrangement is to place the wires of the wire-grid perpendicular to the plane of incidence, whereby the wire-grid polarizing beam splitter passes light polarized perpendicular to the wires (nominally P- polarized light) and reflects light polarized parallel to the wires (nominally S-polarized). As such, in this example, the wire-grid PBS 103 is transparent to P-polarized light and reflective of S-polarized light. As such, the P-polarized light from the pre-polarizer 102 passes through the wire-grid PBS 103 and impinges on the LCOS panel 104. The "off liquid crystal pixels of the LCOS panel 104 reflect the P-polarized back towards the wire- grid PBS 103, whereas the "on" liquid crystals of the LCOS panel 104 effectively rotate the polarization direction of the impinging P-polarized light so as to direct S-polarized light back towards the wire-grid PBS 103. As mentioned above, the wire-grid PBS 103 in this example transmits P-polarized light and reflects S-polarized light. The P-polarized light reflected from the "off pixels of the LCOS panel passes back through the wire-grid PBS 103 largely without reflection. A small percentage of P-polarized light will be reflected from the wire-grid PBS 103, and accordingly, this light is removed by a clean up polarizer (not shown) placed between the wire-grid PBS 103 and the projection lens system 105. On the other hand, the S-polarized light from the "on" pixels of the LCOS panel 104 is reflected by the wire-grid PBS 103. Further, due to the 45° tilt of the wire-grid PBS 103, the S-polarized light reflects at substantially a right angle and is received by the projection lens system 105. The S- polarized light from the wire-grid PBS 103 constitutes an intensity image which is visually projected by the projection lens system 105. The arrangement of FIG. 1 is generally preferred for reasons relating to contrast and image quality. However, it should be noted that the pre-polarizer may be eliminated, and instead the polarizing characteristics of the wire-grid PBS 103 may be relied onto direct P-polarized light onto the LCOS panel 104. Also, the locations of the illumination system 101 and the projection lens system 105 can be switched, whereby the "on" polarized light from the LCOS panel 104 is transmitted through the wire-grid PBS 103 to the projection lens system 105, and the "off polarized light from the LCOS panel 104 is reflected by the wire-grid PBS 103. A significant expense factor in the fabrication of the LCOS engine resides in the projection lens system 105, which is generally made up of a series of high-precision and costly lenses. It is possible to reduce the costs of the projection lens system by inserting a field lens between the panel and the beam splitter. Unfortunately, however, this renders the light non-telecentric at the wire-grid PBS, which in turn reduces the contrast at parts of the image field. Accordingly, it would be desirable to provide an improved LCOS engine in which a field lens is employed between the LCOS panel and the wire-grid PBS to reduce the cost of the projection lens system, but which does not suffer from a reduction in contrast. In one aspect of the invention, an image projection system is provided which includes a reflective image modulator panel, a projection lens assembly, and a polarizing beam splitter disposed to direct at least a portion of source light towards the reflective image modulator panel, and to direct a portion of light reflected from the reflective image modulator panel to the projection lens assembly. In addition, the system includes a field ^■ lens and a quarter- wave retarder optically interposed between the reflective image modulator panel and the polarizing beam splitter. The polarizing beam splitter is preferably a wire-grid PBS, and the reflective image modulator panel is preferably a liquid crystal on silicon (LCOS) panel. Also, an orientation of the quarter- wave retarder is preferably parallel to or perpendicular to an orientation of a polarization state the source light incident on a center of the reflective image modulator panel. Further, the quarter-wave retarder may be located between the field lens and the reflective image modulator panel, or the quarter-wave retarder may be located between the field lens and the polarizing beam splitter. According to another aspect of the present invention, an image projection method is provided which includes transmitting polarized light through a field lens and a quarter- wave retarder and onto a reflective image modulator panel, and then transmitting light reflected from the image modulator panel back through the field lens and the quarter wave retarder, where the light reflected from the image modulator panel is a polarization encoded image. The method further includes decoding the polarization encoded image by separating light of a first polarization state from light of a second polarization state among the light reflected back through the field lens and the quarter wave retarder, where the light of the first polarization state is an intensity image, and then visually projecting the intensity image. The polarized light may be transmitted first through the field lens and then through the quarter- wave retarder prior to being incident onto the reflective image modulator panel, or the polarized light may be transmitted first through the quarter- wave retarder and then through the field lens prior to being incident onto the reflective image modulator panel. Also, the reflective image modulator panel is preferably a liquid crystal on silicon (LCOS) panel. Further, the polarization encoded image is preferably decoded by directing the light reflected back through the field lens and the quarter wave retarder onto a wire-grid PBS, where the wire-grid PBS either reflects or transmits the light of the first polarization state to a projection lens assembly. FIG. 1 shows a schematic representation of a conventional single-panel LCOS engine which includes a wire-grid polarizing beam splitter; FIGS. 2, 3 and 4 are optical system layouts for explaining concepts associated with the present invention; FIG. 5 is a schematic representation of a single-panel LCOS engine which includes a wire-grid polarizing beam splitter according to an embodiment of the present invention; and FIG. 6 is a schematic representation of a single-panel LCOS engine which includes a wire-grid polarizing beam splitter according to another embodiment of the present invention. FIG. 2 shows the optical system layout for the imaging module of a single-panel LCOS engine utilizing a wire-grid polarizing beam splitter (PBS) 203, a LCOS panel 204 and a field lens 206. In this figure, reference number 200 denotes the optical axis of the system, and reference number 205 denotes the projection lens. FIG. 4 illustrates the unfolded optical path of the system of FIG 2. Like reference numbers denote like elements in FIGS. 2 and 4. Referring to FIGS. 2 and 4, the system is designed to be telecentric at the LCOS panel 204, and accordingly, the chief ray 207 undergoes refraction by the field lens 206 and the system is non-telecentric at the wire-grid PBS 203. Methods of polarization ray tracing theory of birefringent media can be used to calculate the polarization eigenpolarizations corresponding to the chief rays 207 incident on the LCOS panel 204 at points away from the optical axis 200. The difference in relative orientation between the wire-grid PBS 203 seen in the first pass and then (after reflection by the LCOS panel 204) in the second pass results in different eigenpolarization states for the wire-grid PBS 203 in the two cases. Since these eigenpolarizations do not perfectly align, leakage occurs and the net system contrast is lowered at that point. Referring to FIG. 3, this problem can be fixed by inserting a quarter wave retarder 307 between the LCOS panel 304 and field lens 306, or between the field lens 306 and wire-grid PBS 303. In a double pass, the quarter wave retarder 307 functions as a half wave retarder, which acts so as to reflect the polarization state across the retardation axis. This maps the incoming wire-grid eigenpolarizations onto the outgoing wire-grid eigenpolarizations so that leakage does not occur. The orientation of the quarterwave retarder should be either parallel to or perpendicular to the orientation of the polarization state incident on the center of the panel. In FIG. 3, reference number 300 denotes the incident direction of the source light, and reference number 305 denotes the projection lens. FIG. 5 is a simplified schematic view of a LCOS optical engine 500 employing a wire-grid PBS according to an embodiment of the present invention. As shown, light from an illumination system 501 is passed through a pre-polarizer 502 to obtain, for example, P- polarized light. A wire-grid PBS (WGP) 503 is interposed between the pre-polarizer 502 and a LCOS panel 504. The wire-grid PBS 503 is generally made up of a plurality of electric conductors extending generally parallel to and spaced apart from one another on one or both surfaces of a support plate. The support plate is transparent to light over the visible range. The conductors collectively define a wire-grid, and the spacing of the wires of the grid is generally less than the wavelength of the shortest wavelength of the visible light used in the projection system. In this example, the wire-grid PBS 503 is transparent to P-polarized light and reflective of S-polarized light. As such, the P-polarized light from the pre-polarizer 502 passes through the wire-grid PBS 503. The P-polarized light from the wire-grid PBS 503 is passed through a field lens 506 and a quarter-wave retarder 507, and then impinges on the LCOS panel 504. The "off or "dark" liquid crystal pixels of the LCOS panel 504 reflect the P-polarized back towards the quarter- wave retarder 507, whereas the "on" or "light" liquid crystals of the LCOS panel 504 effectively rotate the polarization direction of the impinging P-polarized light so as to direct S-polarized light towards the quarter-wave retarder 507. The P-polarized light and the S-polarized light are then transmitted back through the quarter- wave retarder 507 and the field lens 506. As mentioned above, the wire-grid PBS 503 transmits P-polarized light and reflects S-polarized light. The P-polarized light reflected from the "off pixels of the LCOS panel 504 largely passes back through the wire-grid PBS 503. On the other hand, the S-polarized light from the "on" pixels of the LCOS panel 504 are reflected by the wire-grid PBS 503. Further, due to the tilt of the wire-grid PBS 503, the S-polarized light is reflected so as to be received by the projection lens system 505. The S-polarized light from the wire-grid PBS 503 constitutes an intensity image which is visually projected by the projection lens system 505. In a double pass, the quarter wave retarder 507 functions as a half wave retarder, which acts so as to reflect the polarization state across the retardation axis. This maps the incoming wire-grid eigenpolarizations at the wire-grid PBS 503 onto the outgoing wire- grid eigenpolarizations at the wire-grid PBS 503 so that leakage does not occur. Thus, insertion of the quarter wave retarder 507 between the LCOS panel 504 and wire-grid PBS 503 helps improve contrast by correcting polarization aberrations otherwise caused by the non-telecentricity of light at the wire-grid PBS 503. As will be appreciated by one of ordinary skill in the art, the configuration of FIG. 5 may be altered a number of different ways without departing from the spirit and scope of the present invention. For example, the pre-polarizer 502 may be eliminated, and instead the polarizing characteristics of the wire-grid PBS 503 may be relied on to direct P- polarized light towards the LCOS panel 504. Also, the locations of the illumination system 501 and the projection lens system 505 can be switched, whereby the "on" polarized light from the LCOS panel 504 is transmitted through the wire-grid PBS 503 to the projection lens system 505, and the "off polarized light from the LCOS panel 504 is reflected by the wire-grid PBS 503. Further, the LCOS panel 504 may be replaced in favor of other known or to-be-developed reflective image modulator panels, and the wire-grid PBS may be replaced in favor of other known or to-be-developed polarizing beam splitters. Still further, as shown in FIG. 6, the locations of the field lens 606 and the quarter- wave retarder 607 may be switched. Otherwise, the configuration of FIG. 6 is the same as that of FIG. 5. Namely, light from an illumination system 601 is passed through a pre- polarizer 602 to obtain, for example, P-polarized light. A wire-grid PBS (WGP) 603 is interposed between the pre-polarizer 603 and a LCOS panel 604. P-polarized light from the wire-grid PBS 603 is passed through the quarter-wave retarder 607 and the field lens 606, and then impinges on the LCOS panel 604. The "off or "dark" liquid crystal pixels , , of the LCOS panel 604 reflect the P-polarized back towards the field lens 606, whereas the "on" or "light" liquid crystals of the LCOS panel 604 effectively rotate the polarization direction of the impinging P-polarized light so as to direct S-polarized light towards the . field lens 606. The P-polarized light and the S-polarized light are then transmitted back through the field lens 606 and the quarter-wave retarder 607. The P-polarized light reflected from the "off pixels of the LCOS panel 604 largely passes back through the wire-grid PBS 603. On the other hand, the S-polarized light from the "on" pixels of the LCOS panel 604 is reflected by the wire-grid PBS 603 so as to be received by the projection lens system 605. The S-polarized light from the wire-grid PBS 603 constitutes an intensity image which is visually projected by the projection lens system 605. While preferred embodiments are disclosed herein, many variations are possible which remain within the concept and scope of the invention. Such variations would become clear to one of ordinary skill in the art after inspection of the specification, drawings and claims herein. The invention therefore is not to be restricted except within the spirit and scope of the appended claims

Claims

CLAIMS:

1. An image projection system, comprising: a reflective image modulator panel; a projection lens assembly; a polarizing beam splitter disposed to direct at least a portion of source light towards the reflective image modulator panel, and to direct a portion of light reflected from the reflective image modulator panel to the projection lens assembly; and a field lens and a quarter-wave retarder optically interposed between the reflective image modulator panel and the polarizing beam splitter.

2. An image projection system as claimed in claim 1, wherein the polarizing beam splitter is a wire-grid polarizer.

3. The image projection system as claimed in claim 2, wherein the reflective image modulator panel is a liquid crystal on silicon (LCOS) panel.

4. The image projection system as claimed in claim 1, wherein the orientation of the quarter- wave retarder is parallel to the orientation of a polarization state of the source light incident on a center of the reflective image modulator panel.

5. The image projection system as claimed in claim 3, wherein the orientation of the quarter- wave retarder is parallel to the orientation of a polarization state of the source light incident on a center of the LCOS panel.

6. The image projection system as claimed in claim 1, wherein the orientation of the quarter- wave retarder is perpendicular to the orientation of a polarization state of the source light incident on a center of the reflective image modulator panel.

7. The image projection system as claimed in claim 3, wherein the orientation of the quarter- wave retarder is perpendicular to the orientation of a polarization state the source light incident on a center of the LCOS panel.

8. The image projection system as claimed in claim 1, wherein the quarter-wave retarder is located between the field lens and the reflective image modulator panel.

9. The image projection system as claimed in claim 3, wherein the quarter-wave retarder is located between the field lens and the LCOS panel.

10. The image projection system as claimed in claim 1, wherein the quarter- wave retarder is located between the field lens and the polarizing beam splitter.

11. The image projection system as claimed in claim 3, wherein the quarter- wave retarder is located between the field lens and the wire-grid polarizer.

12. An image projection method, comprising: transmitting polarized light through a field lens and a quarter- wave retarder and onto a reflective image modulator panel; transmitting light refiected from the image modulator panel back through the field lens and the quarter wave retarder, where the light reflected from the image modulator panel is a polarization encoded image; decoding the polarization encoded image by separating light of a first polarization state from light of a second polarization state among the light reflected back through the field lens and the quarter wave retarder, where the light of the first polarization state is an intensity image; and visually projecting the intensity image.

13. The image projection method as claimed in claim 13, wherein the polarized light is transmitted first through the field lens and then through the quarter-wave retarder prior to being incident onto the reflective image modulator panel.

14. The image projection method as claimed in claim 13, wherein the polarized light is transmitted first through the quarter-wave retarder and then through the field lens prior to being incident onto the reflective image modulator panel.

15. The image projection method as claimed in claim 13, wherein the reflective image modulator panel is a liquid crystal on silicon (LCOS) panel.

16. The image projection method as claimed in claim 13, wherein the polarization encoded image is decoded by directing the light reflected back through the field lens and the quarter wave retarder onto a wire-grid polarizer.

17. The image projection method as claimed in claim 17, wherein the wire-grid polarizer reflects the light of the first polarization state to a projection lens assembly.

18. The image projection method as claimed in claim 17, wherein the wire-grid polarizer transmits the light of the first polarization state to a projection lens assembly.

19. The image projection method as claimed in claim 17, wherein the reflective image modulator panel is a liquid crystal on silicon (LCOS) panel.