Method for increasing the resolution of a spatial light modulator

In a method for increasing the native resolution of a spatial light modulator, the light-modulating elements of the spatial light modulator are illuminated with spots of light that are smaller in size than the light-modulating elements. The spots of light are moved to different positions with respect to the light-modulating elements as different data are displayed. The light-modulating elements thus do double-duty (or triple-duty, or quadruple-duty, or more) as the spots of light are moved to different positions.

BACKGROUND OF THE INVENTION

The present invention is directed to a display method which employs a spatial light modulator, such as a liquid crystal display or digital micromirror device.

A digital micromirror device is a spatial light modulator which employs an array of tiny mirrors, or micromirrors, whose positions can be electrically controlled in order to display an image. This technology has been developed extensively by Larry J. Hornbeck and others at Texas Instruments, Inc. of Dallas, Tex., and is described by them in a sequence of patents going back years. These developmental efforts have culminated in a digital micromirror device which includes an array of memory cells and a corresponding array of pivotable micromirrors whose positions are electrostatically adjusted by the contents of the memory cells. As is perhaps best described in U.S. Pat. No. 5,096,279 to Hornbeck et al, the array of pivotable micromirrors that cooperates with the memory cells can be made using integrated circuit fabrication techniques.

As is described in the above-identified patent, in U.S. Pat. No. 5,280,277 to Hornbeck, and in an article entitled “Mirrors on a Chip” that was published in the November, 1993 issue ofIEEE Spectrumat pages 27–31 by Jack M. Younse, a negative biasing voltage is selectively applied to the micromirrors and to landing electrodes fabricated beneath them in order to obtain bistable operation of the micromirrors and simultaneous updating of the entire array of micromirrors. Sometimes the micromirrors get stuck. It is known that this problem can be cured by subjecting the micromirrors to resonant reset pulses which electrostatically dislodge any stuck micromirrors.

It is also known to make a color display using a single digital micromirror device by sequentially exposing it to red, green, and blue light impinging from a single direction. A white lamp and a color wheel can be employed for this purpose. In situations where it is economically feasible to devote three digital micromirror devices to a display, each of them can be illuminated by light of a different primary color and the resulting red, green, and blue images can then be superimposed on a screen.

Advances have also been made in other types of display apparatuses. For example U.S. Pat. No. 5,122,791 to David J. Gibbons et al discloses a ferroelectric LCD panel which is selectively backlit by red, green, and blue fluorescent tubes. The intensity or duration of the backlighting is controlled on the basis of the rank of the bits that are being displayed on the panel.

Applicant's U.S. Pat. No. 5,416,496 also employs a ferroelectric LCD that is back-lit with colored lights. The colored light may be generated in flashes whose intensity is controlled on the basis of the rank of the video information bits that are being displayed. Alternatively, instead of flashes of light, the LCD panel may be illuminated by light that is generated steadily, and whose intensity is determined by the rank of the bits that are being displayed. In the latter alternative, the pixels of the panel are turned on in accordance with the video information on a row-by-row basis, and are subsequently turned off in accordance with the same video information, again on a row-by-row basis. As a result, each pixel that is turned on and then turned off receives the same amount of light regardless of its row, so the LCD can be addressed row-by-row with video information while the LCD is being illuminated.

Applicant's U.S. Pat. Nos. 6,348,907 and 6,535,187 are directed to displays using LCDs and DMDs. These patents disclose a variety of techniques for varying and controlling the intensity of light falling on a spatial light modulator and feeding bit ranks of digital words that define an image to the spatial light modulator in a coordinated manner. The patents also disclose other advances, including displaying an image frame during multiple revolutions of a color wheel, a DMD with micromirrors having pivot axes in orthogonal directions (for illumination by light impinging in three directions), and alternatives to the use of resonant reset pulses to dislodge stuck micromirrors and electromechanical latching to update all micromirrors simulatneously.

SUMMARY OF THE INVENTION

A primary object of the invention is to provide a method for increasing the native resolution of a spatial light modulator having an array of light-modulating elements, such as micromirrors in a DMD or liquid crystal cells in an LCD.

This object can be attained by exposing an image-forming area of the spatial light modulator to light in a pattern such that the light illuminates only part of the light-modulating elements. The light pattern can then be shifted to illuminate one or more additional parts of the light-modulating elements. Different data can thus be displayed at different portions of each light-modulating element. In other words, each light-modulating element can be used to produce two or more dots in an image.

In accordance with one aspect of the invention, a method for displaying a color component of an image described by video words of a frame, with these video words for the color component having bits with different bit ranks, includes the step of turning the light-modulating elements of the spatial light modulator on or off in accordance with the values of some of the video words for a given bit rank, exposing the spatial light modulator to a light pattern that produces spots of light that are smaller in size then the light-modulating elements, turning the light-modulating elements on or off in accordance with values of others of the video words for the given bit rank, and exposing the spatial light modulator began to the light pattern, but shifted with respect to the earlier exposure. This changes the position of the spots of light, so that each of the light-modulating elements modulates more than one dot in the image formed by the spatial light modulator.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

With initial reference toFIG. 1, a display apparatus10includes a control unit12that receives multi-bit digital words14for the red, green, and blue components of a sequence of image frames. These digital words (sometimes called “video words” hereafter) will frequently be referred to hereafter as red digital words, green digital words, and blue digital words. The control unit12transfers the digital words for a frame to a digital micromirror device (DMD)16in some predetermined sequence. For example, the least significant bits of half of the red digital words may be transferred first, whereupon the micromirrors (not illustrated inFIG. 1) of the DMD16move to off or on positions depending on the values of these least significant bits. Then the least significant bits for the other half of the red digital words are transferred to the DMD16. This may be followed by the next-least significant bits of the red digital words, again in two stages, and so on until the most significant bits of the red digital words are displayed. After the red component of the frame has been displayed, the green and blue components can be displayed by feeding the various bit-ranks of the green and blue digital words to the DMD16in the same way.

The control unit12cooperates with a lighting unit18. It a shown only schematically, and includes an illumination unit20and a motor-driven color wheel22. The color wheel22includes red, green, and blue optical filters, and the illumination unit20includes one or more light sources which emits light that is then colored by these filters. The illumination unit20is preferably designed to emit light at different intensity levels in coordination with the bit ranks of the digital words that are supplied to the DMD16. For example, the light intensity when the most significant bits of the digital words are displayed is preferably greater by a multiple of a power of two than the light intensity when the least significant bits are displayed. There may be one or more intermediate intensity levels. Details of how this may be done can be found in Applicant's U.S. Pat. Nos. 6,348,907 and 6,535,187, which are hereby incorporated herein by reference.

The colored light emitted by lighting unit18travels along an optical path marked by dotted line24and then impinges on an image-forming area16′ (illustrated schematically using a dot-dash chain line) of the DMD16. This optical path extends through a spot-forming unit26and an optical system28. The spot-forming unit26includes a light pattern device30and an actuator32that is linked to the device30and that shifts the light pattern device30back and forth between a left position and a right position.

FIG. 2shows a top view of the light pattern device30. In this embodiment, it is an opaque plate34having parallel slots36in it The optical system28focuses the slots on the image-forming area of the DMD16. As a result, the micromirrors in the DMD16are illuminated by bands of light. The bands of light form spots of light on the individual micromirrors.

FIG. 3Ashows bands38of light on micromirrors40when the light pattern device30is in its left position. As a result of the bands38, each micromirror40receives a light spot42that covers some but not all of its surface. When the actuator32moves the light pattern device32to its right position, the bands38of light and thus also the light spots42are shifted. As will be seen inFIG. 3B, the light spots42now shine on the right portions of the micromirrors40. This shifting of the light spots42permits each micromirror40to control two spots of light rather than one in the image formed by DMD16.

Returning now toFIG. 1, the light reflected by the micromirrors40when they are in their on positions travels along an optical path marked by dotted line44to a mirror46, which reflects the light to an optical system50. The optical system50focuses the light on a screen52.

The control unit12feeds half of the column data to the DMD16when the light pattern device30is in its left position and feeds the other half of the column data to the DMD16when the device30is in its right position. Accordingly, for each bit rank of the video words for each color component, the control unit12must write data into the DMD16twice. For example, the control unit12may feed half of the data for the least significant bit of the red digital words for a frame to the DMD16while the light pattern device30is in its left position, followed by the other half of the data for the least significant bit of the red digital words for the frame while the device30is in its right position, followed by half of the data for the next-least significant bit of the red digital words for the frame while the device30is in its left position, followed by the other half of the next-least significant bit of the red digital words for the frame while the device30is in it is right position, and so on. The bit ranks for the green and blue digital words would be split in the same way.

It was noted previously that the lighting unit18preferably emits light at different intensity levels. Increasing the intensity of the impinging light when the higher-order bit ranks are displayed reduces the time needed to display them, and correspondingly increases the time available for writing data twice to the DMD16for each bit rank of each color component. An example is shown inFIG. 6, where it is assumed that the video words for the red, green, and blue color components of an image have six bits. In this example, the light intensity I is set at a low level L during display of the three lower-order bit ranks and at a higher level H during display of the three higher-order bit ranks.

A drawback to the arrangement described above is that the light pattern device30intercepts half or more of the light that would otherwise reached the image-forming area16′ of DMD16. Although not shown, this drawback could be reduced by enclosing the lighting unit18and the light pattern device30in a highly-reflective chamber and by making the back side of the device30itself highly reflective. The light intercepted by the device30would therefore be reflected back into the chamber, and a portion of this light returned to the chamber would be re-reflected to the device30.

FIG. 4illustrates a side view of a modified light pattern device52. It comprises a transparent substrate that supports parallel rod-shaped lenses, each of which collects light over a broader area than the slits36shown inFIG. 2. The rod-lenses of the device52focus the incoming light into the bands38shown inFIGS. 3A and 3B.

In the embodiment shown inFIG. 1, the light pattern device30is physically moved between left and right positions. A more elegant solution would be to use a polarizer and a large liquid crystal cell on one side of the light pattern device32to switch the polarization of the light bands42back and forth between two directions of polarization, and a double-refracting crystal on the other side of the device30to direct the light of each polarization to different positions on the DMD16.

It will be apparent of those ordinarily skilled in the art that the arrangement shown inFIG. 1could readily be modified to provide more than two light spots42on each micromirror40. For example, a second spot-forming unit26could be added, with the slots36of the light pattern device30in the second unit26being perpendicular to the slots36in the other unit30. It will also be apparent that an LCD instead of a DMD could be used as the spatial light modulator. Liquid crystal cells instead of micromirrors would then be the light-modulating elements of the spatial light modulator.