ILLUMINATION SYSTEM FOR DISPLAYS PANELS

Abstract: Binary optics are used in an illumination system for a color projection display. In one embodiment a broad spectrum light source illuminates a multilevel optical phase element which disperses the broad spectrum light from the light source by diffraction. A display having a number of pixel elements, each capable of transmitting a predetermined spectral region, is positioned within the near field region of the multilevel optical phase element so as to receive the light dispersed by the multilevel phase element.

Description: ILLU INATION SYSTEM FOR DISPLAYS PANELS Background of the Invention Currently used techniques for color projection displays tend to be relatively inefficient in their light utilization. Such low efficiency limits the brightness of the display, which in effect limits the acceptable amount of ambient lighting in a viewing environment. In certain presently used designs, light from a spectrally broad source is collected by a condensing lens and illuminates a spatial light modulator system. The spatial light modulator system comprises a two-dimensional array of pixels and the amount of light transmitted through each pixel is controlled electronically. A projection lens then images the array of pixels on a viewing screen, the magnification of the displayed image being determined by the particular characteristics of the projection lens. The light impinging on each pixel of the spatial light modulator is spectrally broad (i.e., white light) . Therefore, unless the system is modified to distinguish colors, the display is only capable of displaying black and white images. In many current systems used to modify such a system so that it is capable of displaying color images, each pixel of the spatial light modulator is divided into three sub-pixels having equal areas. Each of the three sub- pixels is covered with a micro-color filter having a different spectral transmittance. For example, the filters are chosen such that one filter transmits only red light, another filter only green light, and the third filter only blue light. The transmittances of the three sub-pixels of each pixel of the spatial light modulator can be controlled independently, resulting in the ability to display a color image. The inefficiency of the approach can be seen by considering the following factors. The light illuminating a full pixel essentially is white light and, consequently, the light impinging each sub-pixel is also white light. The red filtered sub-pixel transmits only red light, absorbing all of the incident green and blue light. Likewise, the other two sub-pixels transmit only its corresponding color, absorbing the other two colors. It is apparent that this approach utilizes, at most, only one third of the available light impinging on the modulator, and absorbs the rest . Furthermore, state-of-the-art microcolor filters required to produce acceptable color images are approximately only 33% efficient in transmitting the color that they are designed to transmit. Therefore the overall light utilization of current color projection displays is about 10%. One approach for improving the efficiency of color projection displays is found in U.S. Patent No. 5,161,042 issued on November 3, 1994 to H. Hamoda. In accordance therewith, the spectrally broad input light is supplied to three dichroic mirrors which reflect three different color components, e.g., red, green, and blue, in different directions, i.e., at different angles with respect to each other. The reflected components are then supplied to an array of lenses for focusing the different color components so as to converge light beams of similar wavelength ranges for transmission through a liquid crystal display element so as to form combined color images on a display screen. A further U.S. Patent No. 5,264,880, issued on November 23, 1993, to R.A. Sprague et al . , discloses a similar approach to that of Hamoda wherein the dichroic mirrors are replaced by a phase grating for dispersing the color components of light received thereat into a spectrum of different colors at different angles relative to each other. While such approaches can be used, the losses of energy of each color component are sufficient to reduce the efficiencies of such systems and to show the need for further improvement in such display systems. Such improved display systems should minimize such losses so as to provide for substantially the total use of the received energy across the color spectrum in the imaging display process resulting in an improvement of the efficiency of the system. Summary of the Invention The invention relates to a color projection display in which received light, having a relatively broad spectrum, illuminates a multi-level optical phase grating so as to disperse each of the color components contained therein into a plurality of different diffraction orders. In one embodiment, the diffraction orders of each color component are then focussed onto a zero-order phase shift element which phase shifts only the undiffracted light (i.e., the zero diffraction order) with respect to the diffracted light (i.e., the higher level diffraction orders) . The output of the zero-order phase shifter is then imaged onto a display having a plurality of pixels, each pixel having sub-pixel regions assigned to transmit different color components of light. The depths of the phase grating element and the zero-order phase shifter are suitably selected so they are practical for manufacture and so the area of chromaticity space for the color components at the image plane is maximized. The use of such a combination of multi-level phase grating and a zero-order phase shifter, having suitably determined depths, provides desired color components at each pixel in which essentially little or no energy is lost. These color components are then suitably combined to provide a color image at each of the pixels of the display which is considerably brighter than that available using prior known systems. In another embodiment, a broad spectrum light source illuminates a multilevel optical phase element which disperses the broad spectrum light from the light source by diffraction. A display having a number of pixel elements, each capable of transmitting a predetermined spectral region, is positioned within the near field region of the multilevel optical phase element so as to receive the light dispersed by the multilevel phase element. In one embodiment, the multilevel phase element is periodic in two dimensions, thereby concentrating the light in two dimensions. In yet another embodiment, a method for displaying a color image is disclosed. The method for displaying a color image includes illuminating a multilevel optical phase element with a broad spectrum light source. The multilevel phase element disperses light from the light source by diffraction. A display having a plurality of pixel elements, each transmitting a predetermined spectral region, is positioned within the near field region of the multilevel optical phase element to receive the dispersed light from the multilevel optical phase element. Brief Description of the Drawings The above and other features of the invention, including various novel details of construction and combination of parts, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular illumination system for color display panels embodying the invention is shown by way of illustration only and not as a limitation of the invention. The principles and features of this invention may be employed in varied and numerous embodiments without departing from the scope of the invention. FIGs. 1 and 1A are block diagrams of an embodiment of a display system using the technique of the invention; FIG. 2 is a perspective diagram of an embodiment of the multilevel optical phase element; FIG. 2A is a perspective diagram of another embodiment of a multilevel optical phase element of the invention; FIG. 3 shows a graph of optimized phase grating depths of three phase levels for a normalized pixel dimension for red, green and blue color channels; FIG. 4 shows the effective phase grating depths of three phase levels for a normalized pixel dimension for the wavelengths of the red, green and blue color components; FIG. 5 shows the percent efficiencies of the spectral content for the red, green and blue color components; FIG. 6 shows the area of the chromaticity space covered when using a particular embodiment of the invention on a standard 1976 CIE chromaticity graph space; FIG. 7 is a block diagram of another embodiment of a display system of the invention; FIG. 8 is a block diagram of yet another embodiment of a display system of the invention; and FIGs. 9 and 9A are block diagrams of a multilevel phase element and its complex conjugate, respectively. FIG. 10 is a flow chart illustrating a lithographic method of fabricating the phase plates. FIG. 11 is a flow chart illustrating a direct writing method of fabricating the phase plate. FIG. 12 is a flow chart illustrating a deposition method of fabricating the phase plates. FIG. 13 is a graphical representation of a constant amplitude wavefront interfacing with an aperture in a mask element. FIG. 14 is a schematic diagram of a transmissive display system employing a mask element. FIG. 15 is a foreshortened cross sectional schematic diagram of the masked plate 110 of FIG. 14. FIGs. 16A-16B are schematic front views of the masked plate 110 of FIG. 14. FIG. 17 is a perspective cross sectional schematic diagram of another preferred embodiment of the invention employing a reflective box light source as a backlight. FIGs. 18A-18B are foreshortened cross sectional views of the masked plate 210 of FIG. 17. FIG. 19 is a schematic diagram illustrated a preferred embodiment of a display system having a light pipe. FIG. 20 is a schematic diagram of a preferred embodiment of a display system having multiple light sources . FIG. 21 is a schematic diagram of a preferred embodiment of a display system having a changed aspect ratio. FIG. 22 is a schematic diagram of a preferred embodiment of display system having a multiple aperture light pipe in accordance with the invention. Detailed Description of Preferred Embodiments of the Invention To increase the light utilization of color displays, the novel technique of the invention can be considered, in' a conceptual sense, as effectively concentrating all of the light of each color component in a received spectrally broad light onto appropriate sub-pixel regions at a color image plane. For example, all of the incident red light is concentrated in a manner such that it only illuminates the sub-pixel regions corresponding to the red component thereof, all of the incident green light is concentrated in a manner such that it only illuminates the sub-pixel regions corresponding to the green component thereof, and all of the incident blue light is concentrated in a manner such that it only illuminates the sub-pixel regions corresponding to the blue component thereof. By so doing, the use of micro-filters is not necessary, and the theoretical light utilization efficiency of such technique approaches 100% (ignoring light loss through the system optics by reflection and absorption) . The invention achieves such light utilization based on a technique referred to as aperture filling. Aperture filling is described, for example, in the article, "Aperture filling of phase-locked laser arrays" by G.J. Swanson et al . , Optics Letters, Vol. 12, April 1987. This article describes a method for increasing the energy in the central lobe of a far-field pattern of a phase-locked laser array. In accordance with the invention, the underlying physics of this technique is modified and extended in a unique manner to solve the color display problem of light utilization. In accordance with the basic physics behind aperture filling, a binary amplitude grating (a grating having a transmittance of 1 or 0) with a fill factor (the ratio of the transmitting area to the total area) of greater than or equal to 0.25, has, aside from a phase shift of the zero order, a Fourier transform identical to that of a binary phase grating having the same fill-factor as the binary amplitude grating. By placing a zero-order phase shift element in the transform plane of a focal imaging system, the light from an aperture with a fill-factor of > . .25 can be uniformly spread out to fill the entire aperture. Further, by invoking reciprocity, light from a uniform aperture can be concentrated to produce an underfilled aperture with a fill-factor of .25. In making use of such concepts for improving the color projection display efficiency, the above phenomenon can be modified to substantially improve the light throughput thereof. A system embodying the technique in accordance with the invention is shown in FIGs. 1 and 2, wherein a multi-level, e.g., a three-level, phase grating is illuminated with a spectrally broad light from a source 10, such as a tungsten halogen bulb or a xenon arc lamp. Alternatively, the light source may comprise three separate color component sources. For example, three light emitting diodes (LEDs) or three laser sources, each emitting a separate color such as red, green, and blue color components. For the purposes of the particular description of a preferred embodiment of the invention, it is assumed that the illuminating source 10, whether a single broad spectrum source or separate color sources, primarily includes color components of the three wavelength regions, e.g., red, green, and blue. The lateral dimension of each phase level, in one embodiment, is assumed to be equal to the lateral dimension of a sub-pixel region of the spatial light modulator. For illustrative purposes only, FIG. 1A shows only two greatly magnified grating periods, each having corresponding three phase depth levels, occupying the entire aperture. It should be understood that a large plurality of grating periods, each corresponding to a pixel of the overall color image, would normally occupy an aperture. If it is assumed that a first phase depth level measured with respect to a second phase depth level at each grating period of the phase grating 11 is equal to an integral number of wavelengths of red light plus one-third of such wavelength, i.e. (m+0.33), where m is an integer, and the third phase depth level, again measured with respect to the second phase depth level, is an integer multiple of the wavelength of red light, the red light that is illuminating a three-level phase grating will in effect encounter a binary phase grating with a fill-factor of 33%, and a phase depth of 0.33 wavelengths. The red light will be dispersed from the phase grating 11 into a zero diffraction order and a plurality higher level positive and negative diffraction orders which are focussed on a zero- order phase shifter 13 via lens 12. If the zero diffraction order (undiffracted) is then phase shifted by about 0.33 wavelengths of red light by phase shifter 13, the red light exiting the system will be concentrated via a lens 14 so as to fill only 33% of the output imaging plane 15 (FIG. 1A) . The same methodology as applied above to the red light range can also be applied to the green and blue light ranges. The second phase depth level at each grating period equals zero wavelengths of green light by definition, and the first and third phase depth levels equal (n - 0.33) and (n' - 0.33) wavelengths of green light, respectively, wherein n and n' are integers. The green light illuminating the phase grating 11 alsc effectively encounters a binary phase grating with a fill- factor of 33%, and a phase depth of 0.33 wavelengths. If the zero diffraction order is also effectively shifted by about 0.33 wavelengths, the green light exiting the system is concentrated so as to fill the 33% of the output imaging plane that is adjacent to the 33% of the output plane occupied by the red light (FIG. 1A) . For the blue light, the phase depth of level of each grating period, again measured with respect to the second ' phase depth level, equals (p'+.33) wavelengths of blue (where p' is an integer) , and the first phase depth level is an integer multiple of wavelengths of blue light. The blue light illuminating the grating also in effect encounters a binary phase grating with a fill-factor of 33%, and a phase depth of 0.33 wavelengths of blue light. If the zero diffraction order is also effectively shifted by about 0.33 wavelengths, the blue light exiting the system is concentrated so as to fill the remaining 33% of the imaging plane not occupied by the red light and the green light (FIG. 2) . The above conditions for three discrete wavelengths can in theory be met to any level of accuracy. However, in practice, the accuracy is limited by the physical depths of the grating levels that can be practically manufactured. Furthermore, the system can be designed to operate over the entire visible spectrum, rather than at only three discrete wavelength regions. The area of chromaticity space spanned by a particular embodiment of the invention depends on the relative depths of the three phase level regions of each grating period corresponding to each pixel, and the depth of the zero- order phase shifter. Since the phase depths are relative, and measured with respect to the second phase depth level, the second phase depth level is zero by definition, thereby leaving three variables: the depths of phase levels 1 and 3 with respect to phase level 2, and the depth of the zero order phase shifter. These three parameters in effect define the performance of the overall system, with the measure of performance being defined as the area of chromaticity space that is so covered. The three depth parameters are most easily optimized by performing a "global search" process that spans the range of practicable manufacturable depths. The goal thereof is to select relative depths which will maximize the area and the location of the spanned chromaticity space. An approach to such process is discussed below. In considering the first phase level of the grating period, the phase shifts (in waves) Φ # , Φ^ and Φ^ of the red, green, and blue light can be expressed as: i = (T7-1) Φέ - -?(i7-l) where 17 is the index of refraction of the phase grating, and d x is the depth of the first phase level with respect to the second phase level. As mentioned above, it is desired that the phase shift « = m + 0.33, while the phase shift Φ G = n - 0.33, and the = p, where m, n, and p are all integers. In a similar manner at the third phase level, having a depth of d 3 with respect to the second phase level, the phase shifts are: έ = η - l ) Here, it is desired that the phase shift Φ* = m ' , the phase shift Φ G = n ' - 0.33, and the phase shift Φ B 2 = p ' + 0.33, where m' , n' , and p' are all integers. Because the first and third phase levels of the grating are referenced in depth to the second phase level of the grating, by definition, d 2 = 0, and at the second phase level the phase shifts at all three wavelengths is zero: Φ^ = 0 φ σ = 0 In addition, at the zero-order phase shifter having a depth of d 4 , a phase shift of about one-third wavelength of each color is required so that at the phase shifter: Φ'r. = • ι: where Φ 4 R = r+0 . 33 , Φ G = s +0 . 33 and Φ= = t+0 . 33 (where r , s , and t are integers) . Since the depths of d l t d 2 , d,, and d 4 must be within practical manufacturable limits, the following practical limitations can be imposed thereon: -5 μm <, d 1 <_ +5 μm -5 μm <_ d 3 < . +5 μm - 5 μm <. d 4 <. +5 μm and the value of η can be assumed at a conventional value, for example, of 1.5. Using the above equations, those in the art can then utilize a well known global search algorithm technique, in which the values of the depths d x , d 3 , and d 4 are changed in steps, Δd, of approximately 0.01 μm, and used to determine in each case the area of the chromaticity space that can be spanned for each set of parameters. The depths d l f d 3 , and d 4 for the solution providing a maximized area can then be used as the practical physical depths for the three phase level regions at each phase grating period and the practical physical depth of the zero-order phase shifter 13. In accordance with a specific embodiment of the invention, such a process was used to determine the three optimum depth parameters for a system operating with a uniform spectral source covering a 0.40-0.68 μ wavelength region, using both multilevel phase grating and zero-order phase shift substrates assumed to have an index of refraction of 1.5. Exemplary results for optimized sub- pixel phase grating depths of an exemplary pixel having a normalized pixel dimension are shown in FIG. 3, with the red channel having a phase grating depth 16 of 1.84 μm relative to the green channel, and the blue channel having a phase grating depth 17 of 4.0 μm relative to the green channel . To illustrate how such an optimized phase grating design conforms to the theory described above, the following three discrete wavelengths can be considered: red=0.66 μm, green=0.54 μm, and glue=0.46 urn . The effective phase grating depths (modulo one-wave) of the three sub-pixels at these three phase level regions are shown in FIG. 4, where the solid line IB represents red, the dashed line 19 represents green, and the dot-dash line 20 represents blue. It should be noted that in the first sub-pixel region, the phase grating depth for red is approximates one-third wavelength of red light, and the phase grating depths for green and blue are essentially zero. Similarly, in the second sub-pixel region, the effective phase grating depth for the green approximates one-third wavelength of green light, and the phase grating depths for red and blue are approximately zero. In the third sub-pixel region, the effective phase grating depth for blue approximates one-third wavelength of blue light, while the phase grating depths for red and green are approximately zero. The optimized depth for the zero-order phase shifter 13 is 0.36 μm, which depth corresponds to 0.27 wavelengths of red, 0.33 wavelengths of green, and 0.39 wavelengths of blue. For this example, it is noted that the optimum phase depth is less than one wave for all three wavelengths. FIG. 2 is a perspective view of a multilevel phase element which repeats periodically in the x-direction. FIG. 2A is a perspective view of a multilevel phase element which repeats periodically in both the x and y directions. Such a configuration permits the incident light to be compressed both in the x-direction, as in the prior embodiment, and also in the y-direction. Methods for forming such multilevel phase elements are well known to those skilled in the art. In particular a method for forming such multilevel diffractive optical elements is disclosed in U.S. Patent 4,895,790 to Swanson and Veldkamp, the teachings of which are incorporated herein by reference. The system's ability to concentrate the visible spectrum into three color channels is illustrated in FIG. 5 for the above-mentioned 0.4-0.68 μm wavelength region. The solid curve 21 represents the percent efficiency of the spectral content of the red channel, the dashed curve 22 represents the percent efficiency of the spectral content of the green channel, and the dash-dot curve 23 represents the percent efficiency of the spectral content of the blue channel . It should be noted that the red channel efficiency peaks at the wavelength of 0.66 μm, the green channel efficiency peaks at 0.54 μm, and the blue channel efficiency peaks at 0.46 μm. The red channel has a secondary peak in the far blue region of the spectrum. This blue light, in effect "leaking" into the red channel, tends to limit the area covered in chromaticity space. In some cases, it may be desired or required to remove this unwanted blue light from the red channel by conventionally filtering the red channel and such removal can be achieved with a blue-blocking micro-filter, albeit at the cost of losing a minimal amount of the blue light energy. As is well known to the art, the spectral content of these three color channels can then be used to determine the area of chromaticity space spanned by the system. FIG. 6 shows a standard 1976 CIE chromaticity space graph 25 which is well known to the art. The area of the chromaticity space spanned by the embodiment discussed above is depicted by three vertices of a triangle, defined by the plus signs, in the graph. This area will be covered using essentially 100% of the received source illumination. Another embodiment of the invention does not require either the zero-order phase plate 13 or the auxiliary optics 12, 14 of the previous embodiment shown in FIG. 1. In this embodiment a broad spectrum light source illuminates a multilevel optical phase element which disperses the broad spectrum light from the light source into diffraction orders. A modulation display, having a number of pixel elements, each capable of transmitting a predetermined spectral region, is positioned within the near field region of a multilevel optical phase element so as to receive the light dispersed by the multilevel phase element . In this embodiment, the free-space propagation of light from the multilevel phase element produces a 1/3 wavelength phase shift of the undiffracted light with respect to the diffracted light. Because of this, the phase plate 13 and auxiliary optical elements 12, 14, which were required to produce the same zero-order phase shift in the previous embodiment, are not required in this embodiment . To understand how free-space propagation provides the required 1/3 wavelength zero-order phase shift, assume that the amplitude transmittance of the phase grating is expressed as : t(x) = a σ + ∑a n exp[i2τri x] where T is the grating period, a 0 is the amplitude of the undiffracted light, a n is the amplitude coefficients of the various diffracted orders, and n is an indexing parameter. If a unit amplitude plane wave illuminates this phase grating, the light amplitude distribution, U z , at a distance Z from the grating plane is described by: n < UΛx) J a π exp [i2π — x] exp [ -iπλ ±L- Z] where λ is the wavelength of the unit amplitude plane wave. The irrelevant constants have been omitted from this equation. Thus the free-space propagation over the distance Z has the effect of introducing phase shifts to the diffracted components with respect to the undiffracted componen . A distance, Z 1/3 , is defined by the equation: 2 2 J l/3 3 λ where λ is the central wavelength of the spectral distribution. Substituting this equation for Z 1/3 into X in the previous equation, results in the following light distribution: ϋ. (x) ∑ a n exp [ i2τΞx] exp [ -i — πn 2 ] The resulting phase shift (for all values of n that do not result in an integer when divided by 3) is equal to an integer number of wavelengths plus 1/3 wavelength. Because the integer number of waves of phase shift are irrelevant, all of the values of n (that do not result in an integer when divided by 3) effectively see a 1/3 wave phase shift with respect to the undiffracted light. For values of n that do result in an integer when divided by 3, the result is an integer number of wavelengths of phase shif . However, for the grating described above, all the values of a n (for n divisible by 3) are zero. Thus, the net result of free-space propagation over the distance Z 1/3 is to produce a light distribution where the undiffracted light is phase-shifted by 1/3 wavelength with respect to the diffracted light. It is at this location that the modulation display, such as a liquid crystal light modulator, is placed. With such a positioning, no phase shift element 13 or additional optics 12, 14 are needed. It should be noted that the propagation distance Z 1/3 is a function of wavelength. Therefore, the free-space propagation just discussed is strictly accurate at only one wavelength. However, acceptable performance over the whole visible spectrum may be achieved by choosing the Z 1/3 distance to correspond to the wavelength at the center of the spectrum. That is, Z 1/3 should be chosen such that 2 T 2 2 -1 \ - < -Z T 7 - 1 ι 1/ r /33 *<- -J \ J Λ long J Λ short where λ long is the longest wavelength of interest and λ short is the shortest wavelength of interest. The above analysis assumes that the illumination source is a point source at infinity, resulting in plane wave illumination. If the illumination source is such that the approximation of a point source at infinity which was just discussed is not valid, an embodiment which describes a physically extended illumination source must be considered. In this embodiment, shown in FIG. 7, an illumination source 10' has a dimension (for purposes of discussion referred to as the x-dimension) of S c . A condensing lens 30 having a focal length of F c , is positioned adjacent the illumination source 10', at a distance of F c . This configuration results in an angular source extent in the x- dimension of Θ « S c /F c . A figure of merit for performance is the ratio b/T, where b, the blur, is the physical extent of the light pattern at the display location in the x-dimension and T is the grating period. For good performance, this ratio is less (typically much less) than 1/3. If b is given by the expression b = z ιn θ c , the figure of merit becomes: ≥ T - 23Lλ„Θ c where λ 0 is the center wavelength. For example, if an acceptable figure of merit is 1/6, the center wavelength is .55 micrometers, and the grating period is 48 micrometers, the resulting value for the angular source size is θ 0 = 2.9 milliradians. Thus, for a 50 mm focal length condensing lens, a source with a size no larger than 145 μm in the x- dimension is required for this figure of merit to hold. Hence, a source 10' with a physical dimension smaller than most commercially available broad-spectrum incoherent sources should be used to obtain good performance. Although broad spectrum incoherent sources of the dimension just described are not generally available, three spatially coherent monochromatic sources, such as three different wavelength LED's or laser diodes, could readily be used as the illumination source 10' . As discussed with respect to the previous example, the modulation display is preferably placed at a distance Z 1/3 from the multilevel phase element 11. Referring to FIG. 8, an embodiment which increases the extended source performance includes a lenslet array 40 (in one embodiment cylindrical lenslets) , placed between the condensing lens 30 and the multilevel phase element 11. The focal length of each lenslet 42 is F m , and the distance between the lenslet array 40 and the multilevel phase element 11 is Z s + F m . Thus, Z s is the distance between the imaged source 22 and the multilevel phase element 11. Each lenslet 42 focusses an image 44 of the extended source, S c ,.. at a distance F m from the lenslet array 40. Each of these imaged sources 44 is of physical dimension, S m , in the x- dimension, where S m = (F m S c ) /F c centered about the optical axis of the respective lenslet 42. For an image 44 of dimension S ra that lies on the optical axis of the condensing lens, a Fresnel diffraction calculation indicates that the light amplitude distribution at a distance Z from the multilevel phase element 11 is given by the expression: in which the irrelevant constant factors have been neglected. The first exponential term in the series is the wavefront curvature introduced by the lenslet 42. The first exponential term after the summation sign represents the phase shifts incurred by the various diffraction orders. Again, what is desired is for all of the phase shifts for value of n which do not result in integers, when divided by three, be equal to 1/3 wave. For this to be the case, Z must be given by the expression: 2r 2 Z„ 3XZ -2T 2 Hence, in a manner similar to the case without lenslets, the value of z should be chosen such that 2 T 2 Z„ 2 T 2 Z, . <Z< . 3 λ ion ,Z s -2 T 2 3λ 3hort Z ε -2 T 2 wherein T is the periodicity of said multilevel optical phase element, Z s is equal to the distance between said multilevel optical phase element and said lenslets minus the focal length of said lenslets, λ long is the longest wavelength of interest and λ short is the shortest wavelength of interest. Comparing this relationship with the relationship previously shown for Z 1/3 (the optimum distance for the plane wave illumination case) : As Z s approaches infinity, the distance, Z, approaches Z 1/3 , as is expected. For finite source distance Z s , the optimum Z distance is greater than the plane wave distance Z 1/3 . The last exponential term indicates that the period of the light distribution at the optimum Z distance is no longer equal to the period of the original phase grating. In effect, free-space propagation from a source 10' located a finite distance from the grating 11 results in a magnification. This magnification, M, is given by the equation: M Z. Note that for a finite source definition, Z s , the magnification factor is greater than one. The angular source size of the extended source 10 as seen at the grating 11, S m /Z s . This angular source size results in a new blur dimension, b n , given by the expression: S r 2 b = - LZ 3λZ s -2T 2 Because of the magnification described above, the new period of the image pattern is, T n : 3λZ T T = MT = s 3XZ -2 T 2 The resulting fractional blur of the image pattern can now be described by the relationships: = 2 T q _ 2 T F m T 3λZ„ m 3λZ„ P c The fractional blur, with the lenslet array 40 in position, can be directly related to the fractional blur, b/T, without the lenslet array 40, according to the expression: This relationship clearly shows that the blurring can be dramatically reduced by the proper introduction of the lenslet array 40. The reduction factor of the blurring is the ratio F m /Z s . So far, consideration has only been given to the one lenslet 42 centered on the optical axis (OA) of the condensing lens 30. Additional lenslets placed adjacent to the original lenslet 42 behave in a manner identical to that described above for the original lenslet 42. However-, because of a coherent interaction between the light traversing different lenslets, an additional constraint is placed on the allowable center-to-center spacings of the lenslets. The minimum center-to-center spacing distance of the lenslets, L, is given by the expression: Z+Z L = pT ( — ) where p is a positive integer. In another embodiment, the multilevel phase element 11' shown in FIG. 9, and discussed above with respect to FIG. 3, includes a double step having an aggregate height of 4.0 μm. The first step is 1.84 μm measured from the base of the phase element and the second step is 2.16 μm measured from the top of the first step to the top of the phase element. A phase element constructed with these dimensions functions as described above. If instead of the phase element 11' shown, a complex conjugate phase element 11' ' as shown in FIG. 9A is constructed, the complex conjugate phase element 11' ' performs equivalently to the phase element 11' . The reason for referring to phase element 11' ' as a complex conjugate phase element becomes readily apparent if the complex conjugate phase element 11' ' is placed adjacent the phase element 11' such that the steps are aligned. Light passing through both phase elements is unaffected, and thus, just as the integration of a wavefunction by its complex conjugate equals one, phase element 11' ' acts as the complex conjugate to phase element 11, thereby permitting the incident light to pass both elements through unaffected. Unlike the phase element 11'., the complex conjugate phase element 11' ' Z 1/3 is defined by. z = τ2 wherein λ is the central wavelength of the spectral distribution. Acceptable performance over the whole visible spectrum may be achieved by choosing the Z 13 distance to correspond to the wavelength at the center of the spectrum. r 2 ,„ . r 2 3λl„ong <z - 3< 3 J shore where λ iong is the longest wavelength of interest and λ short is the shortest wavelength of interest. Similarly, when lenslets are employed in conjunction with the complex conjugate phase element 11'', Z must be chosen such that: T 2 Z, T 2 Z < Z< . 3λ Jon? Z s -T 2 3λ short Z s -T 2 wherein T is the periodicity of said multilevel optical phase element, Z s is equal to the distance between said multilevel optical phase element and said lenslets minus the focal length of said lenslets, λ long is the longest wavelength of interest and λ shorc is the shortest wavelength of interest. Thus, using a complex conjugate phase element permits a closer spacing than is permitted by the non- complex conjugate phase element. The above-described phase plates can be fabricated using a number of techniques including, but not limited to, lithography, direct writing, deposition, diamond turning, grating ruling engine, or laser ablation. FIG. 10 is a flow chart illustrating a lithographic method of fabricating the phase plates. The phase plate is preferably fabricated using a lithographic mask along with standard photoresist and etching techniques used in semiconductor fabrication processes. Preferably, an optical substrate (which will become the phase plate) is coated with a layer of photoresist. A lithographic mask is then used to selectively expose the photoresist over desired areas of the substrate. The photoresist is then developed, where it is removed from the exposed areas. The substrate is then etched by one of many etching processes (e.g., reactive ion etching, ion milling, chemical wet etch) to the desired depth of one of the phase levels . The residual photoresist is then washed off, leaving the substrate surface with two phase levels. Repeating the above procedure for a second lithographic mask results in a substrate having three phase levels. The process can be continued until the desired number of phase levels are achieved. FIG. 11 is a flow chart illustrating a direct writing method of fabricating the phase plate. Direct writing is a variation of the above fabrication procedure to expose the photoresist . Direct writing refers to the process whereby an exposing beam, such as electron or laser beam, is scanned across the surface of the substrate. The beam is turned on or off depending on whether or not the photoresist is to be exposed at the particular substrate location. There are two methods of direct writing which can be used. The first direct writing method uses direct writing to essentially duplicate the lithographic mask technique. Each direct write iteration results in an addition of a phase level to the substrate surface. A second method of • direct writing varies the expose energy of the exposing beam as a function of position. In other words, instead of simply turning the beam on or off, the beam energy can be modulated to the correct gray scales in the photoresist. This process will result in a photoresist profile that has as many levels as the desired phase plate. If done properly, a single etching step results in the photoresist profile being transferred onto the substrate. FIG. 12 is a flow chart illustrating a deposition method of fabricating the phase plates. In this fabrication process the phase levels are deposited onto the substrate surface, rather than etched into the substrate. In this process, the optical substrate is coated with a layer of photoresist. A lithographic mask is then used to selectively expose the photoresist over the desired areas of the substrate. The photoresist is then developed, where it is removed from the exposed areas. Material is then deposited on the surface of the substrate by one of many deposition processes (e.g., evaporation, sputtering) to the desired depth of one of the phase levels. The residual photoresist is then washed off leaving a substrate surface with two phase levels . The above procedure can be repeated with a second lithographic mask which leaves a substrate with three phase levels. The process can be continued until the desired number of phase levels are achieved. Any of the above processes can be used to fabricate a master element. The master element can then be used to produce a mold from which numerous replicas can be made. Potential replication processes include injection molding, compression molding, embossing, and precision glass molding. Although the lenslet array described above is optically efficient, they require a complex fabrication process. Consequentially lenslets are expensive to manufacture. An optical system is therefore needed that uses less complex, and therefore inexpensive, optical elements. Other preferred embodiments of the invention employ a mask element in place of a lenslet array. FIG. 13 is a graphical representation of a constant amplitude wavefront interfacing with an aperture in a mask element. The mask element 50 includes an aperture or opening 56. The opening 56 may be either a slit or square, as will be described in further detail below. Upon striking the mask element 50 only light incident on the opening 56 passes through the mask element 50. The opening 56 appears to be an individual light source with wavefronts 70 propagating outwardly. In the image plan defined by the mask element 50, the exiting wavefront 71 is in phase, but the amplitude varies in a step fashion. As the wavefront propagates from the opening 56, the wavefront shifts in phase because the varying distance from the opening 56 to another image plane is manifested as a phase shift . Where there are multiple openings 56 on the mask element 50, multiple wavefronts propagate from the various opening 56 and interfere with each other. In that case, the wavefronts of light at the image plane of the phase plate 80, now have a constant amplitude, but are out-of- phase. That is because light incident on any particular point on the phase plate 80 has arrived from multiple openings 56 of the mask 50, each of which is located at a different distance from the point of interest . The phase plate 80 disperses the incoming wavefront into wavelength components. Illustrated are red components 82, green components 84 and blue components 86 as dispersed by the phase plate 80. The dispersed color components 82, 84, 86 impinge on respective sub-pixels 92, 94, 96 of an LCD panel 90. FIG. 14 is a schematic diagram of a transmissive display system employing a mask element . The display system 100 includes a light source 105, a masked plate 110-, a phase plate 120, an LCD panel 130. In addition, other optical elements can be disposed between the light source 105 and the phase plate 120. The light source 105 is preferably a lamp with a parabolic reflector. The parabolic reflector insures that the light source 105 generates parallel light rays. The parallel light rays impinge on the masked plate 110 and the wavefronts of light from the masked plate 110 impinge on the phase plate 120. As described above, the phase plate 120 separates the impinging light into red, green and blue wavelength components and directs those wavelength components onto respective subpixels of the liquid crystal display panel 130. By driving the liquid crystal display panel 130 with control circuitry (not shown) , the subpixels can be valved so as to generate a color image across the face of the display panel 130. The displayed image can either be projected onto a viewing surface or directly viewed by a viewer. FIG. 15 is a foreshortened cross sectional schematic diagram of the masked plate 110 of FIG. 14. The masked plate 110 includes a glass substrate 112 having an alternating pattern of mirrors 114 and openings 116 on the distal side (relative to the light source 105) . The mirrors 114 can alternatively be placed on the proximal side of the glass substrate 112 (relative to the light source 105) . The glass substrate 112 can be heat absorbing, liquid cooled, or coated on either side with an infrared and ultraviolet rejection filter. The glass substrate 112 can also be coated on either side with dichroic notch filters. Preferably, there is also a color rejection filter to reflect or absorb wavelength of light that are not desired for display, such as an orange and yellow rejection filter. Although the color rejection filter can be placed anywhere before the LCD panel 130, it is preferably integrated with an infrared and ultraviolet ■ filter as a single filter. As illustrated, the linear ratio of the area of the openings 116 relative to the mirrors 114 is 50%. Consequently, in the illustrated section only one third of the light exiting the light source 105 can exit the masked plate 110 on the first attempt. The mirrors 114 reflect the light back into the light source 105 to be reclaimed and reflected off from the parabolic reflector, thereby increasing the chance that an individual light ray eventually exits an opening 116 instead of being dissipated within the light source 105 and mask element 110. FIGs. 16A-16B are schematic front views of the masked plate 110 of FIG. 14. FIG. 16A illustrates a masked plate 110' having slit openings 116' between mirror bars 114' . FIG. 16B illustrates a masked plate 110'' having, a mirror 114'' perforated with square openings 116' ' . Regardless of which embodiment is used, there is always a 2:1 ratio of mirrors 114 to openings 116 in either or both directions. In the one-dimensional grating of FIG. 16A, one third of the light from the light source 105 (plus any reclaimed light) is passed through the masked plate 110' . For the two-dimensional grating of FIG. 16B only one ninth of the light from the light source 105 (plus any reclaimed light) exits from the masked plate 110'' . The efficiency of the attempt to reclaim light is dependent on the quality of the mirrors 114 on the masked plate 110 and the reflectivity and shaping tolerances of the reflector of the light source 105. Although less light escapes from the two-dimensional grating of FIG. 16B, that light can be concentrated onto individual pixel areas of the liquid crystal display panel 130. The one-dimensional grating of FIG. 16A, however, wastes light by placing light energy in the black areas between the pixel electrodes. The choice between a one- dimensional grating and a two-dimensional grating is thus a system tradeoff between light collection efficiency and transmission efficiency through the LCD panel 130. Preferably, the openings 116 in the masked plate 110 are much larger than the wavelengths of the light. Consequently, the diffraction of light from the openings 116 is limited. However, the 2:1 ratio of mirror to openings efficiently handles any diffraction by assuring that the wavefronts mesh together on the phase plate 120. That is because the diffractive orders from the mask element 110 match the phase gratings on the phase plate 120. Although a 1:2 ratio is illustrated for a three color display, a 1:1 ratio can be used for a monochrome display. It should be noted that there is a fundamental limit of a 1:3 ratio between openings and mirrors, which allows the use of four different areas of the light spectrum such as red, green, blue and black colors in a display. FIG. 17 is a perspective cross sectional schematic diagram of another preferred embodiment of the invention employing a reflective box light source as a backlight. Fluorescent lamp elements 215 (or another suitable light source) are encased within a reflective box 217, which, for example, can be either a specular reflector having a mirrored inside surface or a non-specular reflector having a highly reflective white inside surface. Light from the reflective box 217 passes through a masked plate 210, which is fabricated as described above. A phase plate 220 is separated from the masked plate 210 and an LCD panel 230 is separated from the phase plate 220. The reflective box 217 for a LCD panel having a pixel pitch of 16-24 μm is less than 0.42 cm in thickness, the phase plate 220 is about 0.7 mm in thickness and the LCD panel 230 is about 1.5 mm thick. The spacings between the elements are on the order of 1 mm with a tolerance of about 0.025 mm. The LCD panel 230 can include one or two polarizers. As with the embodiment of FIG. 14, the reflective box display system 200 can include a viewing lens for head-mounted or other direct viewing of the image formed on the LCD panel 230. FIGs. 18A-18B are foreshortened cross sectional views of the masked plate 210 of FIG. 17. As illustrated the masked plate 210' , 210'' is coated with a diffusing surface to scatter incident light from the reflective box 217. The diffusing coating helps to more evenly distribute the light from the reflective box 217. As illustrated, the diffusing coating 218 can be on the proximal side of the glass substrate 212 (FIG. 18A) or on the distal side of the glass substrate 212 (FIG. 18B) . The defusing coating can also be on either side of the mirrors. In either embodiment, the mask includes alternating mirrors 214 and openings 216 as described above; the openings 216 can either be one- dimensional slits or two-dimensional squares. In either case, there is a 1:2 ratio of openings 216 to mirrors 214, in either or both directions. As above, the glass can be heat absorbing, coated on either side with infrared and ultraviolet rejection filters, or coated on either side with dichroic notch filters. A continuing problem is the need to evenly distribute light across the face of the LCD panel. One approach to solving this problem is to use a light pipe as will be described in more detail below. FIG. 19 is a schematic diagram illustrated a preferred embodiment of a display system having a light pipe. The display system 300 includes a parabolic light source 305 which is a lamp with a parabolic reflector. Alternatively, the light source can include a elliptical reflector. Where the light source 305 includes a parabolic reflector a condenser lens 310 is used to focus the light from the light source 305 through an aperture 324 of a light pipe assembly 330. The aperture 324 is formed in a mask 320 having blocking elements 322. The light pipe terminates at an abutting phase plate 340 as described above. The image- plane of a LCD panel 350 is separated from the image plane of the phase plate 340 by a distance z. A projection lens 360 projects the image form on display panel onto a viewing surface and disposed at the center of the opening to the light pipe 330. The light pipe assembly 330 includes four reflective surfaces 332 which cooperate to create a light guide or pipe. The light pipe is dimensioned so as to evenly mix the light from the aperture 324 before the light reaches the phase plate 340. The light pipe is a distance 1 long and has a width w. The ratio of the width w to the length 1 is where ø is the angle from the end edge of the light pipe 330 and the center of the aperture 324. However, w _ T 1 21 z ' when T' is the distance between pixel centers. The length of the light pipe 330 is thus 7= wz 2 The aperture is preferably square with a dimension d which is defined as The light pipe 320 is preferably a rod having a rectangular cross section and fabricated from optical glass with reflective surfaces on the peripheral surface of the rod. The output ratio of the light pipe 320 must match the square aperture 324 at the proximal end and the rectangular phase plate 340 at the distal end. FIG. 20 is a schematic diagram of a preferred embodiment of a display system having multiple light sources. As described above, the display system 400 includes a light pipe assembly 330 with a phase plate 340, an LCD panel 350 and a projector lens 360. Unlike the embodiment of FIG. 19, the display system 400 of FIG. 20 includes multiple light sources 405 which can be used in a linear or rectangular array. The use of multiple lamps permits the use of small, efficient, and less expensive light sources. Furthermore, the light output of the system can be tuned by varying the number of lamps in the array. Illustrated are three lamps 405a, 405b, 405c. Depending on the type of reflector used in the light sources 405, a matching lens array 408 (shown in phantom) may be needed to collimate the light into parallel light rays. As illustrated, there is one lens 408a, 408b, 408c for a respective light source 405a, 40b, 405c. A condenser lens 310 focuses the parallel light rays from the light ■ sources 405 onto the aperture 324 and subsequently through the light pipe 330. FIG. 21 is a schematic diagram of a preferred embodiment of a display system having a changed aspect ratio. The display system 500 incorporates a light pipe assembly 530 which changes the aspect ratio from a square aperture 524 to a rectangular face plate 340. In all other respects a display system 500 is identical to the display system 300 of FIG. 19. The above described light pipe application requires a relatively long light pipe assembly, which restricts their application with head-mounted display systems. Instead, they are primarily useful on projection display systems. The length of the light pipe, however, can be reduced by increasing the number of apertures into the light pipe. FIG. 22 is a schematic diagram of a preferred embodiment of display system having a multiple aperture light pipe in accordance with the invention. The display system 600 includes a light source 305, a condenser lens array 610 and a light pipe assembly 630. The light pipe assembly 630 includes a mask 620 having four apertures 624a, 624b, 624c, 624d. There is one condenser lens 610a, 610b, 610c 610d for a respective aperture 624a, 624b, 624c, 624d. The light pipe assembly 630 has a length 1' which is equal to 1/4. This is because the four apertures 624a, 624b, 624c, 624d of the mask 620 result in a corresponding decrease in the length of the light pipe 630. Equivalents Those skilled in the art will know, or be able to ascertain using no more than routine experimentation, many equivalents to the specific embodiments of the invention described herein. These and all other equivalents are intended to be encompassed by the following claims.

Claims: Claims What is claimed is: 1. A method for displaying an image comprising: illuminating a diffractive optical element with light having a plurality of wavelength regions, said diffractive optical element spatially dispersing the light into the plurality of wavelength regions; optically coupling the dispersed light to a display having a plurality of pixel regions, each said pixel region receiving a selected wavelength region; and actuating the pixel regions to form an image. 2. The method of Claim 1 further comprising providing a liquid crystal display, the display having the plurality of pixel regions and wherein the plurality of wavelength regions comprises red, green and blue. 3. The method of Claim 1 further comprising providing a light source that generates the plurality of wavelength regions wherein the light source is a single broadband source or comprises a plurality of subsources, each subsource generating one of said plurality of wavelength regions. 4. The method of Claim 1 further comprising providing a diffractive optical phase element having a multilevel structure extending in a first direction or wherein the structure is multilevel in two orthogonal directions . 5. The method of Claim 1 wherein said display is positioned at a distance Z from said optical phase element, wherein Z is determined by the relationship: <Z>2. * Long λ SΛort wherein T is the periodicity of said optical phase element, X^ g is the longest wavelength of said plurality of wavelength regions and λ short is the shortest wavelength of said plurality of wavelength regions 6. The method of Claim 1 further comprising providing a light source generating the plurality of wavelength regions wherein the light source is a laser system, a light emitting diode system, a halogen lamp or an arc lamp. 7. The method of Claim 1 wherein the optically coupling step comprises coupling a plurality of colors to each pixel region, each pixel region comprising subpixel region of a pixel element, each pixel element having at least two subpixel regions. 8. The method of Claim 1 further comprising providing a phase shift element positioned between the diffractive optical element and the display. 9. The method of Claim 1 further comprising providing diffractive optical element having a plurality of optical phase sub-elements, said sub-elements have different depths within the diffractive optical element to provide a multilevel structure, the structure defining a chromaticity space of light incident on the display. 10. the method of Claim 1 further comprising providing a projection lens for projecting light from the display onto an imaging surface, and providing a lenslet array positioning between a light source and the diffractive optical element. 11. The method of Claim 1 further comprising providing a light source and a masking element positioned along an optical path between the light source and the diffractive optical element, the masking element having at least one aperture, or an array of apertures. 12. The method of Claim 1 further comprising providing a light pipe positioned to optically couple light from a source to the diffractive optical element . 13. The method of Claim 11 further comprising one or more lenses to focus light from the source onto corresponding apertures of the masking element . 14. The method of Claim 1 further comprising providing a light source, a liquid crystal transmission display and a projection lens to provide a color projection display system. 15. A display device comprising: an optical phase element that spatially disperses light into a plurality of wavelength regions; and a display having a plurality of pixel regions and optically coupled to the optical phase element, each said pixel region receiving a selected wavelength region. 16. The display device of Claim 15 wherein the display further comprises a liquid crystal display, the display having the plurality of pixel regions comprise red, green and blue. 17. The display device of Claim 15 further comprising a light source that generates the plurality of wavelength regions wherein the light source is a single broadband source or, alternatively, comprises a plurality of subsources, each subsource generating one of said plurality of wavelength regions. 18. The display device of Claim 15 wherein the optical phase element further comprises a diffractive multilevel structure extending in a first direction or wherein the structure is multilevel in two orthogonal directions . 19. The display device of Claim 15 wherein said display is positioned at a distance Z from said optical phase element, wherein Z is determined by the relationship: <Z)2. 3 λ i/θng λ SΛorc wherein T is the periodicity of said optical phase element, X^,^ is the longest wavelength of said plurality of wavelength regions and λ shorC is the shortest wavelength of said plurality of wavelength regions 20. The display device of Claim 15 further comprising a light source generating the plurality of wavelength regions wherein the light source is a laser system, a light emitting diode system, a halogen lamp or an arc lamp. 21. The display device of Claim 15 further comprising a phase shift element positioned between the diffractive optical element and the display. 22. The display device of Claim 15 wherein the optical phase element further comprises a diffractive optical element having a plurality of optical phase sub- elements, said sub-elements have different depths within the diffractive optical element to provide a multilevel structure, the structure defining a chromaticity space of light incident on the display. 23. The display device of Claim 15 further comprising a projection lens for projecting light from the display onto an imaging surface, and providing a lenslet array positioned between a light source and the diffractive optical element. 24. The display device of Claim 15 further comprising a masking element having an aperture and a light pipe to direct light from a source to the optical phase element . 25. A method of fabricating a diffractive optical element for a display panel comprising: providing a matrix display panel having a plurality of pixel regions, each pixel region having an aperture area; providing an optically transmissive substrate; forming an optical phase element with the optically transmissive substrate, the optical phase element having a periodic structure that diffracts light incident upon the element into a plurality of wavelength regions, the plurality of wavelength regions corresponding to a selected plurality of pixel regions. 26. The method of Claim 25 further comprising forming a lithographic mask and etching the optically transmissive substrate to form the periodic structure. 27. The method of Claim 25 further comprising depositing material on the optically transmissive substrate to form the periodic structure. 28. The method of Claim 25 wherein the forming step further comprises stamping the substrate with a mold. 29. The method of Claim 25 wherein the step of providing an optically transmissive substrate comprises providing a plastic material. 30. The method of Claim 25 further comprising direct writing the periodic structure on the optically transmissive substrate.

Inventors: SWANSON GARY J, SWANSON, GARY, J.
Applicants: MASSACHUSETTS INST TECHNOLOGY, MASSACHUSETTS INSTITUTE OF TECHNOLOGY
Application year: 1995
Main classification: H04N9
IPC codes: G02B27, G02F1, G03B33, H01L21, H04N5, H04N9