Television – Video display – Projection device
Reexamination Certificate
1997-06-10
2001-06-12
Luu, Matthew (Department: 2672)
Television
Video display
Projection device
C348S744000, C348S745000, C348S781000, C359S205100, C359S215100
Reexamination Certificate
active
06246446
ABSTRACT:
FIELD OF THE INVENTION
The present invention is generally related to an image display system including a spatial light modulator modulating incident or transmitted light to form an optical image, and more particularly, to a display system incorporating an auto-focus system for insuring the displayed image is automatically in focus at the display image plane.
BACKGROUND OF THE INVENTION
Spatial Light Modulators (SLM's) are widely used in the industry for video monitors, graphic displays, projectors, and hard copy printers. Spatial light modulators typically modulate incident light to form a light image. This light image is directed to a screen in the case of a projector, video monitor or display, or is ultimately focused on a light sensitive material, such as a photoreceptor drum, in the case of a xerographic printer.
A recent innovation of Texas Instruments Incorporated of Dallas, Tex. is the digital micromirror device (DMD). The DMD is a spatial light modulator suitable for use in displays, projectors and hard copy printers. The DMD is a micromechanical monolithic single-chip integrated circuit, comprised of a high density area array of 16 micron square deflectable micromirrors on 17 micron centers. These mirrors are fabricated over address circuitry including an array of memory cells and address electrodes, these memory cells being controlled by a row address circuit and loaded/unloaded by column pixel data shift registers. Each mirror forms one pixel of the DMD array, and is bistable through electrostatic attraction forces, that is to say, stable in one of two deflected positions. A source of light is directed upon the mirror array, and is reflected in one of two directions by each mirror. In one stable “on” mirror position, incident light is reflected by the mirror to a collector lens and focused on the display screen, or directed to the photoreceptor drum, and comprises one pixel. In the other “off” mirror position, light directed on the mirror is deflected to a light absorber. Each mirror of the array is individually controlled to either direct incident light into the collector lens, or, to the light absorber. The collector lens may be used in combination with a light prism to ultimately focus and magnify the light image from the pixel mirrors when projected onto a display screen to produce a viewable image. If each pixel mirror of the DMD array is in the “on” position, the light image will be a bright array of pixels.
For a more detailed discussion of the DMD device, cross reference is made to U.S. Pat. No. 5,061,049 to Hornbeck, entitled “Spatial Light Modulator and Method”; U.S. Pat. No. 5,079,544 to DeMond, et al, entitled “Standard Independent Digitized Video System”; and U.S. Pat. No. 5,105,369 to Nelson, entitled “Printing System Exposure Module Alignment Method and Apparatus of Manufacture”, each patent being assigned to the same assignee of the present invention, and the teachings of each are incorporated herein by reference. Gray scale of the pixels forming the image is achieved by pulse width modulation techniques of the mirrors, such as that described in U.S. Pat. No. 5,278,652, entitled “DMD Architecture and Timing for Use in a Pulse-Width Modulated Display System”, assigned to the same assignee of the present invention, and the teachings of each are incorporated herein by reference.
Image display systems, such as those incorporating the DMD device, require that the image projected on the display screen be in focus, for both rear-screen and front-projection display systems. In some conventional auto-focusing systems, an infrared-type range-finder system is employed to determine the distance from the projector lens to the image plane, i.e. the screen, and adjust the focal length of the projector lens based on the ascertained distance. Such a system is relatively expensive and subject to some discrepancies.
The present invention sets forth to provide an inexpensive auto focus system for an image display system, and particularly, for an image display system including a micromechanical SLM such as the DMD having an array of micromechanical pixels with well defined geometric physical features.
SUMMARY OF THE INVENTION
The present invention achieves technical advantages as an auto-focus system for a spatial light modulator-based image display system whereby focus is established by sensing the sharpness of individual pixels generated by discrete features of the SLM device, rather than the sharpness of the image projected by the spatial light modulator device. In essence, the physical features of the SLM are sensed and focused upon, rather than the image formed by the pixels, these features being ascertained by sensing a predetermined pixel pattern on a display screen, such as pixels rendered in a checker board pattern. A sensor, such as a CCD camera, senses the sharpness of the pixels in the pattern whereby the focal length of the projector lens is adjusted until the pixels are in focus. A display system employing a DMD-type SLM is preferred due to the uniform rectangular geometry and spacing of the pixel mirrors which, in turn, generate a uniform rectangular “on” pixel on the display screen. The edges of the “on” pixel are sharp and can be easily focused upon using a CCD camera or the like.
The preferred embodiment of the present invention comprises an image display system, and a method for operating such display system. The display system comprises a light source generating light and a spatial light modulator modulating the light to form an image comprised of pixels on an image plane, such as a screen. An optical system, preferably including a projector lens, focuses the image on the screen, whereby a sensor senses a focus of at least one pixel projected on the screen and provides a sensor output indicative thereof. An adjustment device alters an optical parameter of the optical system, preferably the focal length of the projector lens as a function of this first output. Preferably, the sensor provides the first output as a function of the sensed sharpness of several pixels of the projected image on the screen. Ideally, the sensed pixels have a high contrast in intensity from pixel-to-pixel, such as a checker board pattern. The sensor may comprise a CCD camera or the like. This adjustment device preferably comprises a servo mechanism or other suitable device for adjusting the focal length of the projector lens.
The method according to the preferred embodiment of the present invention focuses a display system including a light source generating light, a spatial light modulator (SLM) modulating the light, and a projector lens focusing the modulated light on an image plane. The method comprises the steps of illuminating the SLM with the light to define an image comprised of pixels at the image plane. The sharpness of at least one pixel of the image is sensed. The focal length of the projector lens is adjusted as a function of the sensed pixel sharpness until the image is brought into focus on the image plane. Preferably, several pixels rendering a pattern are sensed. Preferably, the SLM is a micromechanical device whereby the pixel-members have a uniform geometry which, in turn, generate corresponding uniform shaped pixels at the image which can be focused upon by the sensor. The sensing of the pixels at the screen can be sensed one of two ways. First, by sensing pixels through the projector lens itself, or, secondly by sensing the pixels from lateral of the projector lens.
The present invention derives technical advantages in that the DMD device comprises pixel-members having a uniform geometry, and thus, generate a pixel having a uniform geometry which can be easily focused upon. For instance, a square micromirror generates a square pixel at the image plane which can be easily focused upon. By sensing, for instance, a 10×10 array of pixels comprising a portion of the image, the projection lens can be adjusted until these sensed pixels are in focus at the image plane, i.e. the screen.
REFERENCES:
patent: 3639047 (1972-02-01), Holle
patent: 4560864 (19
Heimbuch Scott
Vorgert James
Brady III Wade James
Brill Charles A.
Luu Matthew
Sajous Wesner
Telecky , Jr. Frederick J.
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