WO2005120080A1

WO2005120080A1 - Selective optical filter for a projection display

Info

Publication number: WO2005120080A1
Application number: PCT/US2004/016606
Authority: WO
Inventors: James Edward Lyst; Eugene Murphy O'donnell
Original assignee: Thomson Licensing
Priority date: 2004-05-26
Filing date: 2004-05-26
Publication date: 2005-12-15

Abstract

The present invention provides a projection television system for use in an environment having ambient light, comprising a screen configured to display a video image to a viewer when pixels of light are projected onto it, projection optics for projecting pixels of light onto the screen, and a screen shield disposed between the screen and the viewer. In a first embodiment of the invention the screen forms a filter blocking at least a portion of ambient light to improve picture contrast. In an alternate embodiment of the invention, at least one of the screen and the screen shield form a selective filter blocking at least a portion of ambient light in one or more discrete spectral bands to improve picture contrast.

Description

SELECTIVE OPTICAL FILTER FOR A PROJECTION DISPLAY

Field of the Invention The invention is directed to a projection display system, and more particularly to a projection display system with a selective optical filter.

Background of the Invention In conventional projection display systems such as, for example, rear projection televisions (RPTVs), digital cinema, etc., white light output from a lamp is directed to a microdisplay, such as a liquid crystal display (LCD), liquid crystal on silicon (LCOS), or digital light processing (DLP) system, through a series of integrating and collimating optics. For the LCD or LCOS systems, white light is separated into its component red, green, and blue (RGB) bands of light, polarized by a polarizing beam splitter (PBS) in the case of LCOS, and directed onto the microdisplay. The microdisplay has a matrix of pixels. The microdisplay operates to modulate each of the pixels of the component RGB bands of incident light by a gray-scale factor to form a light matrix of discrete modulated light signals or pixels. The gray-scale factors are provided by a gray-scale control output from a controller based on a video input signal The light matrix is reflected or output from the microdisplay and directed to a system of projection lenses that project the modulated light onto a display screen, combining the pixels of light to form a video image. In the DLP system, the white light is separated into its component RGB bands of light, and reflected onto a DLP microdisplay. The microdisplay is a semiconductor device containing an array of hinge-mounted microscopic mirrors. Each of the mirrors corresponds to one pixel in a video image input to the microdisplay. When the semiconductor is driven by the video input signal, the mirrors are tilted or switched on and off to reflect all or some of the incident light using temporal control. The array of pixels reflected from the mirrors form a light matrix corresponding to the video-input signal. The light matrix is reflected or output from the microdisplay and directed to a projection lens system that projects the modulated light onto a display screen to form a video image. Normal room light degrades the contrast performance of many projection television

(PTV) systems when it is reflected off of the screen and/or internal surfaces of the television cabinet. An existing PTV is shown in FIG. 1. In this PTV, a projection system 60 is located in a cabinet 10. The projection system 60 projects modulated pixels of light onto a screen 20 to form a viewable image. In this PTV, a shield 30 is provided in front of the screen 20 (i.e., toward viewers of the image; not shown). As shown in Fig. 1, ambient light 90 may pass through the screen shield 30 and strike the screen 20. A portion of the light 91 is reflected from the screen back toward the viewer. Another portion of the light 92 passes through the screen and into the projection television (PTV) cabinet 10, reflects off internal surfaces of the cabinet and also exits the screen 20 toward the viewer. This reflected ambient light 91, 92 from the screen and cabinet undesirably reduces picture contrast for a viewer because dark features become lighter such that details fade away. Some projection television (PTV) makers use a tinted screen 21 having a dye incorporated in it for full-spectrum tinting, as shown in Fig. 2. These tinted screens 21 block all optical wavelengths equally, including both the ambient light 90 as well as the projected light 61 that forms the video image (FIG. 2). To prevent blocking too much of the projected light 61, tinted screens 21 typically have enough dye to block about 25% of the light incident upon them. By blocking both the ambient light and the projected light, these tinted screens 21 result in a small (e.g., 5 %) improvement for a viewer in contrast performance in a well-lit room. For example, as shown in Fig. 2, the viewable image 62 has an intensity of about 75% of the projected light 61, and the reflected ambient light 93 has an intensity of about 70% of the ambient light 90. Accordingly, it is an object of the invention to provide an optical projection system that selectively filters undesirable ambient light while enhancing contrast performance for the

viewer. Summary of the Invention The present invention provides a projection television system for use in an environment having ambient light, including a screen configured to display a video image to a viewer when pixels of light are projected onto it, projection optics for projecting pixels of light onto the screen, and a screen shield disposed between the screen and the viewer. In a first embodiment of the invention the screen shield forms a filter blocking at least a portion of ambient light to improve picture contrast. In an alternate embodiment of the invention, at least one of the screen and the screen shield form a selective filter blocking at least a portion of ambient light in one or more discrete spectral bands to improve picture contrast. By forming a filter from the screen shield, reflected ambient light passes through the filter twice, while projected light only passes through the filter once, thus blocking more of the undesirable reflected ambient light as compared to the desired projected light and enhancing contrast. Also, since the projected light is in discrete green, red, and blue spectral color bands, and the ambient light is distributed over a broader spectrum of wavelengths, selectively absorbing the light at wavelengths between the green, red, and blue bands reduces reflected ambient light with minimal effect on the projected light, thereby enhancing contrast.

Brief Description of the Drawings The invention will be described with reference to the drawings, in which: Fig. 1 is a sectional view of a projection television known in the prior art, showing the reflection of ambient light from the screen and cabinet of the projection television; Fig. 2 is a sectional view of a projection television with a tinted screen known in the prior art, showing ambient and projected light; Fig. 3 is a sectional view of a projection television with a selective optical filter according to an exemplary embodiment of the invention, showing ambient and projected light; and Fig. 4 is a sectional view of a projection television with a selective optical filter according to an alternate exemplary embodiment of the invention, showing ambient and projected light; and Fig. 5 shows a graph of the absorption spectrum for a selective optical filter according to an exemplary embodiment of the invention.

Detailed Description of the Invention In an exemplary embodiment of the invention, as shown in Fig. 3, a projection television is provided having a projection system 60 positioned in a cabinet 10. The projection system 60 projects modulated pixels of light 61 onto a screen 20 to form a viewable image 63. A tinted screen shield 31 is disposed between the screen 20 and the viewer (not shown). The viewable image 63 passes through the tinted shield screen 31 to the viewer. In the exemplary embodiment shown in Fig. 3, the tinted shield screen 31 is tinted for full spectrum tinting. This causes the screen shield 31 to become a neutral density filter that is advantageous because ambient light 90 passes through the tinted screen shield 31, and the reflected ambient light 94 passes through the tinted screen shield 31 again before reaching the viewer. Thus the reflected ambient light 94 is filtered twice by the tinted screen shield 31. Since the projected image 63 is only filtered once by the tinted screen 31, the twice filtered, reflected ambient light 94 results in a significant improvement in viewer perceived contrast performance. For example, as shown in Fig. 3, the intensity of the viewable image 63 is about 75% of the projected light 61, and the intensity of the reflected ambient light 94 is about 56% of the ambient light 90. The tinted screen shield 31 may be formed of a plastic material that is tinted by adding pigment to the plastic material prior to extruding the tinted screen shield 31. The amount and composition of pigment added to the plastic determines the transmission characteristics. An extruded screen shield providing 25% neutral density filtration may be obtained, for example, from Plaskolite of Columbus, OH , US. Because the image 61 passes through the tinted shield screen 31, the filter preferably has a filters no greater than 25 % of the transmitted light since the projected image is also blocked by a corresponding amount. The tinted screen shield 31 may alternatively comprise a full spectrum filter affixed to or deposited on a surface of an optically transparent shield. The filter may comprise a tinted plastic sheet or a layer of material such as for example, silicon oxide. In an alternative embodiment of the present invention, as shown in Fig. 4, a projection television comprises a projection system 60 disposed in a cabinet 10 and configured to project modulated pixels of light 61 onto a screen 120 to form an image 163, which passes through a screen shield 130 to a viewer (not shown). In this embodiment, at least one of the screen shield 130 or screen 120 is a selective filter. The selective filter is designed to absorb light at wavelengths between the primary colors (e.g., the band between green and blue and the band between green and red). This is advantageous because the pixels of projected light 61 from a DLP or LCOS imager will be filtered to comprise light in one of the three color bands (green, blue, or red), while the ambient light 90 and reflected ambient light 194 will be more uniformly distributed over the entire light spectrum. As shown in Fig. 5, in one embodiment the projected green light 61G has wavelengths of 530 nm to 565 nm and the projected red light 61R has wavelengths from 590 nm to 630 nm. The selective filter is designed to absorb wavelengths within the range of about 570 nm to about 585 nm that are adjacent to and between the bands of projected green and red light 61G, 61R. Similarly, projected blue light 61B has wavelengths of 480 nm to 510 nm. The selective filter is designed to also absorb wavelengths within the range of about 515 nm to about 525 nm that are adjacent and between the bands of projected green and blue light 61G, 61B. The selective filter may be formed of a plastic material with the selective filtering characteristic determined by adding pigment to the plastic material prior to extruding it into either the screen 120 or the screen shield 130. The amount and composition of pigment added to the plastic determines the transmission characteristics thereof and particularly the specific wavelengths of light absorbed by it. An extruded screen 120 or screen shield 130 providing selective filtration of specific spectral ranges of light may be obtained, for example, from Plaskolite of Columbus, OH , US. The screen shield 130 may alternatively comprise a selective filter affixed to or deposited on a surface of a transparent screen shield. The selective filter may comprise a tinted plastic sheet or a layer of material such as for example, silicon oxide. In yet another alternative embodiment of the invention, a selective filter may be formed directly onto or in the screen 120, with the screen shield 130 omitted. In this embodiment, reflected ambient light at wavelengths between the green, red, and blue bands is absorbed with minimal effect on the projected light, thereby enhancing contrast. The foregoing illustrates some of the possibilities for practicing the invention. Many other embodiments are possible within the scope and spirit of the invention. It is, therefore, intended that the foregoing description be regarded as illustrative rather than limiting, and that the scope of the invention is given by the appended claims together with their full range of equivalents.

Claims

What is Claimed is: 1. A projection television system, comprising: a screen configured to display a video image to a viewer when pixels of light are projected onto it; projection optics for projecting pixels of light onto the screen; and a screen shield disposed between the screen and the viewer and tinted to filter at least a portion of reflected ambient light to improve picture contrast.

2. The projection television system of claim 1, wherein the screen shield is a full spectrum filter.

3. The projection television system of claim 2, wherein the screen shield blocks about 25 percent of light passing into it.

4. The projection television system of claim 1 , wherein the screen shield selectively filters spectral bands of light.

5. The projection television system of claim 4, wherein the projection optics project pixels each comprising light filtered to include light in only one of a green spectral band, a red spectral band, and a blue spectral band, and the screen shield selectively absorbs light having a spectral range between the green spectral band and the red spectral band and light between the blue spectral band and the green spectral band.

6. The projection television system of claim 4, wherein the screen shield selectively absorbs light having a wavelength between about 515 nm and 525 nm and light having a wavelength between about 570 nm and 585 nm.

7. The projection television system of claim 1, wherein the screen shield comprises an extruded plastic material with pigment added to the plastic.

8. The projection television system of claim 1, wherein the screen shield comprises a filter affixed to a transparent shield.

9. The projection television system of claim 1, wherein the screen shield comprises a filter deposited on a transparent shield.

10. A projection television system for use in an environment having ambient light, comprising: a screen configured to display a video image to a viewer when pixels of light are projected onto it; projection optics for projecting pixels of light onto the screen; and a screen shield disposed between the screen and the viewer; wherein at least one of the screen and the screen shield form a selective filter blocking at least a portion of ambient light to improve picture contrast.

11. The projection system of claim 10 wherein the at least one of the screen and the screen shield selectively absorbs light at wavelengths between one or more of a green, red and blue spectral color band.

12. The projection television system of claim 10, wherein the projection optics project pixels each comprising light filtered to include light in only one of a green spectral band, a red spectral band, and a blue spectral band, and the at least one of the screen and the screen shield selectively absorbs light having a spectral range between the green spectral band and the red spectral band and light between the blue spectral band and the green spectral band.

13. The projection television system of claim 10, wherein at least one of the screen and the screen shield selectively absorbs light having a wavelength between about 515 nm and 525 nm and light having a wavelength between about 570 nm and 585 nm.

14. The projection television system of claim 10, wherein the selective filter comprises at least one of the screen and the screen shield formed from an extruded plastic material with pigment added to the plastic.

15. The projection television system of claim 10, wherein the selective filter is affixed on at least one of the screen and the screen shield.

16. The projection television system of claim 10, wherein the selective filter is deposited on at least one of the screen and the screen shield.

17. A projection system comprising: a screen configured to display a video image to a viewer when pixels of light are projected onto it; and projection optics for projecting pixels of light onto the screen at selected spectral frequencies; wherein the screen is tinted to absorb light outside of the projected spectral frequencies; thereby absorbing at least a portion of reflected ambient light to improve picture contrast.

18. The projection system of claim 17 wherein the screen selectively absorbs light at wavelengths between one or more of a green, red and blue spectral color band.

19. The projection system of claim 17, wherein the projection optics project pixels each comprising light filtered to include light in only one of a green spectral band, a red spectral band, and a blue spectral band, and the screen selectively absorbs light having a spectral range between the green spectral band and the red spectral band and light between the blue spectral band and the green spectral band.

20. The projection system of claim 17, wherein the screen selectively absorbs light having a wavelength between about 515 nm and 525 nm and light having a wavelength between about 570 nm and 585 nm.