WO1991012560A1

WO1991012560A1 - Spatial light modulator with improved aperture ratio

Info

Publication number: WO1991012560A1
Application number: PCT/US1991/000919
Authority: WO
Inventors: Ronald F. Zampolin; Bryan E. Loucks
Original assignee: Greyhawk Systems, Inc.
Priority date: 1990-02-15
Filing date: 1991-02-15
Publication date: 1991-08-22
Also published as: US5083854A

Abstract

A light energy source (20) produces low intensity light. The low intensity light rays are made substantially parallel by collimating lens (30). Filter (32) is placed to receive light from collimating lens (30), and to pass wavelengths to which a photoconductor in light valve (34) has maximum sensitivity, and to absorb or reflect other wavelenghts. The filtered, collimated light propagates through aperture mask (36) and is focused into the pupil of the matrix modulator (38) by lens array (40). The focused light is amplitude modulated by the matrix modulator (38). A lens array (42) collects light from the matrix modulator (38) and relays it to the photoconductive surface (44) of light valve (34). Thus, the illuminated image of the aperture mask (36) is magnified at photoconductor (44).

Description

SPATIAL LIGHT MODULATOR WITH IMPROVED APERTURE RATIO

BACKGROUND OF THE INVENTION Field of Invention.

The present invention relates to an optically addressed spatial light modulator. More specifically, the present invention relates to a spatial light modulator in which a matrix of light modulating elements is used to address a single photoaddressed liquid crystal light valve.

Summary of the Prior Art.

Spatial light modulators consisting of a matrix of electrically addressed liquid crystal elements can be used to modulate high intensity light and with suitable optics project an image onto a screen where it may be viewed. The quality of the image is influenced by the brightness and contrast of the projected image.

The matrix of liquid crystal elements consists of an active area (region where the liquid crystal can modulate the input light) , and an inactive area (region composed of electrode data lines, busbars, electrical devices and the isolation separating adjacent picture elements) . The ratio of active to inactive area is defined as the aperture ratio.

As the number of pixels per unit area is increased to achieve higher resolution, the number of components in the inactive area is also increased. The process of lithography used to define the components making up the inactive area is constrained as to minimum feature size. Consequently, the inactive area will increase and the aperture ratio, picture brightness, and contrast will accordingly be reduced.

Prior art addresses the brightness problem by separating the light used to create projected images from the light transmitted through the matrix liquid crystal element. This is accomplished by modulating a low intensity light source by a matrix addressed liquid crystal light valve. The light that is thus modulated falls onto a photoaddressed liquid crystal light valve. The image of the addressing matrix is thus reproduced at the photoconductive surface. This device has been described in U.S. Patent Application Serial No. 436,447 (hereafter referred to as Haven et al.) , filed on November 14, 1989, and assigned to the same assignee as the present application .

Another problem with the prior art is that the optical properties of a photoaddressed light valve are spatially modulated according to the intensity of the addressing light at each picture element. Therefore, any light falling outside defined pixel boundaries will degrade system contrast. Prior art requires expensive collimating optics together with fiber-optic substrates to provide good contrast.

An additional problem with prior art is the effect of diffracted energy outside the pixel boundaries as the pixel size in the matrix modulator becomes small (higher resolution) . Since the active area in the matrix modulator defines the pixel boundaries, contrast will be degraded as resolution increases.

Additional prior art, represented by U.S. Patent No. 4,836,652 by Oishi et al. is directed toward a similar -3- application, that of electrographic imaging using light projected through matrix addressed liquid crystal shutters to address a photosensitive medium. In this case, light is focused by an array of lenses and directed through the matrix modulator element onto a photosensitive medium, with unwanted or diffuse light blocked by a photomask. The disadvantages of the approach outlined by Oishi et al. are (1) the glass substrates must be thin, and consequently fragile, so that light directed from the matrix element does not diffuse into adjacent elements and degrade contrast, (2) the photosensitive medium must be in close proximity to the photomask to minimize diffraction effects from the photomask and consequent loss of resolution and contrast, (3) the size of the light opaque region of the photomasks aperture must be large to satisfy the geometry constraints on the lens design. Therefore, no aperture ratio improvement can be realized.

Earlier prior art in this area relies on the use of lens array (Selfoc) disposed between the matrix modulator and the photosensitive medium, to image the matrix onto the medium. This limits the projected geometry to that of the matrix aperture itself. Therefore no aperture ratio improvement can be realized.

OBJECTIVES OF THE INVENTION It is an objective of the present invention to provide ar. improved spatial light modulator with favorable aperture ratio.

It is another objective of the present invention to provide optical aperture boundaries that are independent of the addressing means.

It is another objective of the present invention to provide means to optically relay addressing means from a matrix liquid crystal device to a photoaddressed liquid crystal device.

It is another objective of the present invention to provide means to collimate light input to a matrix addressed liquid crystal light modulator so as to improve uniformity and contrast.

It is another objective of the present invention to provide a spatial light modulator whereby the focused image of the aperture in the pupil of the light modulating element is small relative to the pupil of the light modulating element.

SUMMARY OF THE INVENTION

The attainment of these and related objective may be achieved through the use of the novel spatial light modulator system herein disclosed. A spatial light modulator in accordance with this invention has a first lens array disposed adjacent to a matrix of light modulating elements, and in proximity to a liquid crystal light valve containing a photoσonductive element. Low intensity light is collimated by a lens element to illuminate an optical aperture which has geometric configuration similar to that of the matrix element. The first lens array es light from the illuminated aperture mask into the pupil of a matrix addressed spatial light modulator element. The second lens array collects light emitting from the matrix of light modulating elements, and es the image of the aperture onto the photoconductive element of the liquid crystal light valve so as to preserve the geometrical and positional configuration of the optical aperture, and magnify the size of the illuminated region on the. photoconductor. The design of the optics is such that when adjacent apertures are imaged onto the photoconductor, the proximate boundaries of the illuminated regions are contiguous. The attainment of the foregoing and related objects, advantages and features of the invention should be more readily apparent to those skilled in the art, after review of the following more detailed description of the invention, taken together with the drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a cross section view of the preferred embodiment spatial light modulator.

Figure 2 is an enlarged . schematic view of the addressing optics.

Figure 3A is an illustrative diagram of the energy distribution at a light modulating element.

Figure 3B is a illustrative diagram of the energy distribution at a pixel in the photoconductor plate.

Figure 4 is an illustrative diagram of the improved aperture ratio.

DETAILED DESCRIPTION OF THE INVENTION Referring to Figure 1, a cross sectional view of the spatial light modulator of the preferred embodiment is shown. A light energy source 20 such as an incandescent lamp produces low intensity light- Light rays from light energy source 20 are made substantially parallel by collimating lens 30. Collimating lens 30 is typically a molded aspheric single element lens, but lens 30 could be any combination of lenses producing well collimated light. Filter 32 is placed to receive light from collimating lens 30, and to pass wavelengths to which a photoconductor in light valve 34 has maximum sensitivity, and to absorb or reflect other wavelengths. For a photoconductor such as amorphous silicon, the range of wavelengths of maximum sensitivity is between 650 nm and 750 n . Monochromatic light produced by filter 32 also eliminates some chromatic aberrations in the lenses, thereby increasing resolution.

The filtered, collimated light propagates through aperture mask 36 and is focused into the pupil of the matrix modulator 38 by lens array 40. In the preferred embodiment, lens array 40 is a SMILE array made by Corning Glass Works. Lens array 40 can be a gradient index (GRIN) array such as a Selfoe array made by Nippon Sheet Glass Co., an array of graded index polymers, an array of Fresnel zone plates, or an array of holographic optica1 elements.

The focused light is amplitude modulated by the matrix modulator 38. In the preferred embodiment, matrix modulator 38 is a thin film transistor (TFT) addressed liquid crystal light valve. The matrix modulator could be any transmissive matrix addressed light valve. A lens array 42, similar in construction to lens array 40, collects light from the matrix modulator 38 and relays it to the photoconductive surface 44 of light valve 34. Lens array 42 is typically an imaging lens with 2:1 conjugates, and thus a magnification of 2:1. Thus the illuminated image of the aperture mask 36 is magnified at the photoconductor 44. The magnification and focal length can be chosen such that when the aperture is rectangular, the proximate boundaries of adjacent elements are contiguous, or slightly overlapping, to produce a tiled effect, as shown in Fig. 4. For circular or hexagonal apertures, the effect is a close packed hexagonal array. A slight overlap may be desirable because it corrects for possible lens defects which would shift placement of the imaged aperture.

Referring to Fig..2, a representative design of a single element of the spatial light modulator is shown for analysis. Figure 2 is one example of a design which produces the desired result, and other designs, particularly of light paths, are possible. Collimated light passes through the aperture 50 in aperture mask 36 which defines the object to be imaged onto the photoconductor 44. The size of the aperture should be approximately equal to that of the matrix modulator to maximize transmitted intensity. The light is collected by a lenslet 52 which brings the light to a focus at the pupil 53 of the light modulating element 54. An example of an appropriate lenslet 52 would have an effective focal length of approximately 2.0 mm and be operated at an f/# of typically 25 and would focus the light approximately 1.6 mm to the right of the lenslet 52. A goal with lenlet 52 would be to minimize the f/#, hence producing a smaller ed spot, by reducing the focal length. The ray bundle emerging from light modulating element 54 should fill the entire lenslet 56, thereby increasing the effective beam diameter and reducing diffraction effects. The lenslet 56 operates at about 2:1 conjugate and hence magnifies the image to produce some overlap of images at the photoconductor 44. An appropriate imaging lenlet 56 would have an effective focal length of 4.2 mm and be operated at an f/# of 50.

The focused image of the aperture 50 is substantially smaller than the pupil 53 of the light modulating element 54. In the constructed device, approximately 95% of the energy in the diffracted light is within a 32.5 urn spot in the focal plane. This allows a greater margin for misalignment for low resolution modulators with large apertures, and less demands on maintaining intrinsically large aperture ratio during the process of manufacturing high resolution matrix light modulators.

Using the preliminary design presented herein, diffraction analysis can be done based on the f/# numbers of lenslets 52 and 56 to yield intensity distributions as a function of diffracted spot size at the light modulating element 54 and the photoconductor 44. Diffraction occurs at both the sguare aperture 50 and the second lenslet 56. Since these apertures are both symmetric, the intensity distribution can be viewed in one dimension and translated into the other. Fraunhoffer diffraction produced by a square aperture yields an intensity distribution I(x) in the focal plane of a lens described by:

I(x)= 1(0) [2f*sin(dxk/2f)/dxk]² where x is the linear distance in the focal plane, d is the lens diameter, f is the focal length of the lens, k is the wavenumber of the light, and 1(0) is the input intensity of light. Figure 3A depicts the intensity distribution at the matrix light modulator 54 versus the spot radius. Approximately 95% of the energy entering the square aperture is focused into the light modulating element. This spot size, approximately 32.5 urn, provides a good margin for alignment error. Fraunhoffer diffraction also occurs at the second lenslet 56. The intensity distribution I(x) at the photoconductor 44 is given by the equation: I(X)=I(0) [4F*J₁(2kx/F)/2kx]² where J,(S) is the first order Bessel function, F is the f/# number of the lens, x is the linear distance in the focal plane, and k is the wavenumber of the light. Figure 3B depicts +-he intensity distribution at the photoconductor versus spot radius. Approximately 98% of the energy transmitted through the lenslet 52 reaches the photoconductor 44. Consequently, of the energy transmitted through the aperture 50, approximately 93% reaches the photoconductor 44.

The distribution of light energy at the photoconductor determines the degree of modulation within the optically activated light valve. This effect is well described in the literature, and specifically in Haven et al. Therefore, in order to achieve high contrast of the activated pixels defined by the projected image of the optical aperture, no diffracted energy should occur outside the pixel boundary. Figure 3B indicates that 98% of the diffracted energy occurs within a 79.3 urn spot. Therefore, the maximum possible resolution at 100% aperture ratio for this spatial light modulator design is 6.3 line pairs per millimeter.

Referring to Fig. 1, high intensity light from lamp 60 is filtered to desired color by filter 62, polarized by polarizing beam splitter 64 and directed to the reflective surface of light valve 34. Where an illuminated image is defined on the photoconductor 44, the polarization of the high intensity light is rotated and transmitted through the polarizing beam splitter 64, appropriate optics 66, and onto viewing screen 68. This process is well described in Haven et al.

The image thus produced will have maximum brightness due to the 100% aperture ratio and will have high contrast due to containment within pixel boundaries of the light energy diffracted from apertures.

The foregoing descriptions of specific embodiments of the present invention have been presented for purposes of illustration and description. The are not intended to be exhaustive or to limit the invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teaching. The embodiments were chosen and described in order to best explain the principles of the invention and its practical application, to thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the Claims appended hereto and their equivalents.

Claims

What is claimed is:

1. A spatial light modulator system, comprising: light source means; filter means for selecting light of specified wavelengths from said light source means; collimating means placed between said filter means and said light source means for making light from said light source means substantially parallel; aperture mask means placed to intercept collimated and filtered light; a first plurality of lens elements means placed for focusing light transmitted through said aperture mask means; a plurality of light valve elements means placed to amplitude modulate light from said first plurality of lens elements means; photoaddressed light valve means to receive a light from said plurality of light valve elements means; and a second plurality of lens elements means placed between said plurality of light valve elements means and said photoaddressed light valve means to relay light from said plurality of light valve elements means to said photoaddressed light valve.

2. The system of claim 1 wherein said photoaddressed light valve means is a reflection mode photoaddressed liquid crystal light valve.

3. The system of claim 1 wherein said aperture mask means is a mask with a plurality of apertures.

4. The system of claim 3 wherein said plurality of apertures are either rectangular, and form a rectagular pattern on said mask, or hexagonal or circular, and form a hexagonal pattern on said mask.

5. The system of claim 1 wherein said plurality of light valve elements are formed by a matrix addressed light valve.

6. The system of claim 1 wherein said first 5 plurality of lens elements provide to focus light transmitted through said aperture mask to focal points in pupils of said light valve elements, said focal points being substantially smaller than the pupils of said light valve elements.

0 7. The system of claim 1 wherein said second plurality of lens elements means relay and magnify aperture images in order to project images with contiguous or slightly overlapping boundaries onto said photoaddressed light valve.

5 8. The system of claim 7 wherein said second plurality of lens elements have magnifying conjugates.

9. The system of claim 1 wherein said collimating means is one of the following: a SMILE array, a GRIN array, or a Selfoc array, or another array of o microlenses.

10. The system of claim 1 wherein said first plurality of lens elements means and said second plurality of lens elements means are each one of the following: a SMILE array, a GRIN array, a Selfoc a r?y, 5 or another array of microlenses, fresnel zoneplates, or holographic optical elements.

11. In a projection system having means for providing a source of light energy, means for collimating and filtering said light energy, splitting means for 0 passing light energy of a first polarization and reflecting light energy of a second polarization, spatial light modulator system for modulating the polarization of reflected light energy and reflecting light energy back to splitting means, and means for imaging light energy onto a screen, the improvement comprising a spatial light modulator system which consists of a plurality of trans issive spatial light modulators imaged onto a photoaddressed liquid crystal light valve by an aperture mask and a plurality of lenses.

12. A method for providing a spatial light modulator with high aperture ratio, comprising the steps of: collimating and filtering light from a light source to produce collimated and filtered light; projecting said collimated and filtered light through a plurality of apertures in an aperture mask to produce a plurality of images of apertures; focusing said plurality of images of apertures into pupils of a plurality of transmissive light modulating elements to produce a plurality of modulated images; and projecting and magnifying said plurality of modulated images onto a photoaddressed spatial light modulator so that the boundaries of adjacent singular modulated images are contiguous or slightly overlapping.