Patent Application
20140368797
This relates generally to image projection systems; and, in particular, to image projection systems using a reflective imager illuminated by light transmitted through a field lens.
Reflective imager-based projectors may be subject to low screen image contrast when using a field lens approach for illumination as described. The cause of the problem is illumination light reflected off of the imager field lens optics that is captured by the projection lens optics and travels to the screen as stray light. This unwanted light can be seen when the reflective imager is set to the dark state and may significantly lower the contrast of the system. The stray light reflections are particularly prevalent in field lens illumination architectures as illustrated in the schematic representation of an image projection system 200 given in
As shown in
U.S. Pat. No. 7,760,437 discloses a projector having a projection lens unit including an optical lens adjacent a micromirror device and a light shielding plate for covering bias light to prevent formation of a ghost image in the projected image. U.S. Pat. No. 6,783,246 and U.S. Patent Application Pub. Nos. 2002/0057418 and 2002/0105622 disclose approaches for ghost light rejection through redirection of ghost reflections. Other reflective imager devices, such as reflective LCD projection displays (also known as liquid crystal on silicon or LCoS), are concerned with reflections returning from the projection lenses and not concerned with ghost images created by illumination light. U.S. Pat. No. 5,268,775 discloses a method to reduce projection lens ghost imaging using a quarter-wave retarder between a polarizer or polarizing beam splitter and a projection lens. Illumination for polarization rotating reflective imagers (LCoS), such as described in U.S. Pat. No. 6,478,429, place a linear polarizer in the illumination beam and have a linear polarizer in the projection path for illumination input when the device itself rotates the polarization state to modulate the brightness of pixels.
Methods and apparatus are provided for reducing the problem of ghost or stray light reflections in reflective imager-based projectors created by optics in the illumination light path resulting in loss of dark state contrast.
Described example embodiments use a polarized illumination source in conjunction with a one-quarter wavelength retarder (quarter-wave retarder) before and a linear polarizer after the imager field lens in the projection light path to block illumination light reflected off the imager field lens or other optics while passing illumination light modulated by the reflective imager.
As shown in
Because of the curved surface characteristic of imager field lens 118, a portion 210b of polarized illumination light 210 from light source 306 is reflected off imager field lens 118 into the pupil of projection lens system 122. The ghost imaging of such reflected light by projection lens system 122 is, however, prevented by the placement of the linear polarizer 314 between imager field lens 122 and at least part of the projection optics of projection lens system 122. For example, if light from light source 306 is laser light linearly polarized in a direction perpendicular to the propagation direction of the laser beam and oriented at or near 90 degrees to the pass axis of the linear polarizer 314, all or substantially all of the reflected light portion 210b will be blocked from passing through polarizer 314. In such case, little if any stray light will result from passage of the reflected portion 210b through the projection lens system 122 onto the imaging surface 124. If, on the other hand, light from light source 306 is elliptically polarized with a major axis oriented at or near 90 degrees to the pass axis of polarizer 306, some but not all of the reflected light portion 210b will be blocked from passing through polarizer 314. Polarizer 314 may be placed before the projection lens system 122 or be integrated as part of the projection lens system 122. Polarizer 314 may be either absorptive or reflective, and may be configured as a flat plate, cube polarizing beam splitter, or some other configuration that provides a similar polarized light filtering functioning.
The passage of the selected portions 210c of the modulated incident light 210a that are directed back through the imager field lens 118 into the pupil of projection lens system 122 is enabled by the one-quarter wavelength retarder 312 positioned between the field lens 118 and the reflective imager 120. The retarder 312 retards the portion 210a of light 210 from light source 306 that passes in a first direction through imager field lens 118 to reflective imager 120, and again retards the selected portions 210c of the modulated light 210a that are reflected from reflective imager 120 through the imager field lens 118 in an opposite second direction along the optical path 216 into the pupil of the projection lens system 122.
The illustrated retarder 312 is a broadband quarter-wave retarder that converts linearly polarized light into circularly polarized light, and vice versa. Linearly polarized illumination light 210a reflected for projection by modulating elements of reflective imager 120 (viz., light incident on DMD mirrors set to the ON-state) passes through the quarter-wave retarder 312 twice, once before incidence and once after reflectance, resulting in a linear polarization of the projected modulated light portions 210c oriented 90 degrees to the linear polarization of the light 210 incident on field lens 118 from the polarized light source 306. Polarizer 314 at the projection lens system 122 is oriented to pass the light 210c reflected back for projection from the reflective imager 120. The unwanted light portion 210b reflected from the field lens 118 is not rotated by quarter-wave retarder 312 and is blocked by polarizer 314.
The quarter-wave retarder 312 and polarizer 314 may be arranged respectively before and after any optics between reflective imager 120 and screen 124 that can potentially produce ghost reflections from the illumination input light 210. This includes but is not limited to prisms, lenses, cover glass, or aperture masks.
It is noted that when prism element 408 is a polarizing beam splitter or the like, the function of polarizer 314 may be integrated within prism element 408. In this case, prism element 408 will itself reject the ghost reflections from the optics between it and the DMD 120 without the need for a separate polarizer 314. A separate polarizer 314 may be added to, if desired, to act as a clean-up polarizer to reject any remaining unwanted light that has leaked through the prism polarizing beam splitter optic, thereby further enhancing the contrast.
Those skilled in the art will appreciate that modifications may be made to the described embodiments, and also that many other embodiments are possible, within the scope of the claimed invention.
This application claims the benefit of Provisional Application No. 61/834,131 filed Jun. 12, 2013, the entirety of which is incorporated by reference herein.
| Number | Date | Country | |
|---|---|---|---|
| 61834131 | Jun 2013 | US |
