Patent Application
20050259302
1. Field of Invention
The present invention related to holographic light panels (HLPs) embodying edge-lit and steep reference angle holograms, for use in illuminating electronically-switched pixelated display screens (e.g., liquid crystal displays), flat panel displays, as well as transparencies and holograms, and also to methods of making such holographic light panels and the holograms embodied therein.
2. Brief Description of the Prior Art
Many objects, such as transparencies or flat panel displays, require a broad area illumination source. Prior art optical schemes for achieving such illumination typically requires considerable packaging volume, can involve multiple optical elements, are costly and/or inefficient. Manufacturers of flat panel displays, and in particular active matrix liquid crystal displays (AMLCD's), strive for system designs which produce bright, uniform illumination, are thin, lightweight, inexpensive, and energy efficient. Energy efficiency is particularly important for portable displays, such as in notebook computers, to conserve battery life.
For backlighting flat panel displays, various direct lighting solutions at the rear of the display have been used, such as tubular or serpentine fluorescent lamps disclosed in U.S. Pat. Nos. 5,285,361 and 5,280,371, leaking woven fiber optic materials and electroluminescent panels. Backlighting with flat fluorescent lamps is not attractive because of problems with uniformity of light from the tubes and because the tubes are relatively bulky and require too much electrical power for the typical LCD environment (see e.g., Hathaway, Proc. SID 1991, which also describes using a wedge light pipe). Other solutions include variations on the use of edge-lit light pipe or waveguiding structures, textured structures and diffusers are disclosed in U.S. Pat. Nos. 5,359,691; 5,349,503; 5,339,179; 5,335,100; 5,303,322; 5,288,591; and 5,280,372).
An additional problem with displays such as AMLCD's is that in order to spatially intensity modulate light from the backlighting system, a pixelated array of the discrete liquid crystal elements surrounded by opaque interstitial regions which reflect and/or absorb light incident thereon. Most lighting solutions flood the entire display, both transmissive windows and opaque interstices, with light, thus wasting typically around 50% of the available light, which is lost to the opaque interstices.
Furthermore, many color flat panel displays employ a subpixel array of “absorptive-type” red, green, or blue filters made from absorptive-type pigments and dyes, which spectrally filter spatial intensity modulated “white” light produced from the backlighting system, thus allowing only a small portion of the input light to actually be transmitted through the filters to the LCD layer. Absorptive color filters are used for each subpixel to select the appropriate color bandwidths (red, blue or green) for that pixel from the white light illuminating the pixels. This process is very inefficient and typically absorbs most of the incoming light, requiring stronger illumination light sources, and, in battery operated systems, wasting precious battery life.
Some of these problems have been addressed by proposing solutions involving holographic optical elements (HOEs). For example, in UK Patent Application number GB 2 260 203A, Webster suggests the use of an edge-lit holographic light panel comprising a pixelated transmission-type modulated hologram mounted onto a transparent substrate having the same refractive index as the hologram. The hologram has recorded within it repeated sequences of discrete light diffractive gratings arranged in an array, where each discrete grating is arranged to couple a fraction of the incident light within a particular wavelength to a subpixel of an electrically addressable spatial intensity light modulation panel representative of the color of subpixel of the multicolor display. While in theory this prior art holographic light panel design provides advantages over prior art displays employing absorptive-type color filters, it suffers from a number of shortcomings and drawbacks.
First, the light diffractive transmission gratings employed in this prior art light panel exhibit significant objectionable dispersion of the incoming light, whereas in such an application strong wavelength selectivity would be more desirable. Additionally, the illumination light must necessarily make multiple bounces within the substrate, resulting in significant efficiency loss. The accuracy required of the incoming light for it to bounce correctly along the substrate and couple into the hologram is very difficult to achieve in commercial practice, making the holographic light panel impractical.
Thus, there is a great need in the art for an improved holographic light panel that can be used in various backlighting and frontlighting applications, while avoiding the shortcomings and drawbacks of prior art holographic light panel systems.
Accordingly, it is a primary object of the present invention to provide an edge-lit holographic illumination or light panel )HLP) which can be used in a diverse range of backlighting and frontlighting applications while avoiding the shortcoming and drawbacks of prior art holographic light panel systems.
A further objection of the present invention is to provide a holographic light panel for producing a pixelated pattern of illumination for use in monochromatic or color display applications.
A further objection of the present invention is to provide a method of making such a holographic light panel in which an array of spectrally-tuned, narrow-band volume holograms are embodied for carrying out spectral filtering functions.
A further objection of the present invention is to provide a flat panel display system, in which an edge-lit holographic light panel is used to illuminate its electrically-addressable pixelated spatial intensity modulation (SLM) panel.
A further objection of the present invention is to provide such a flat panel display system, in which the holographic light panel is realized as a grazing incidence, single-pass reflection-type volume hologram of either the transmission or reflection type.
A further objection of the present invention is to provide a method of making such a holographic flat panel display system.
A further objection of the present invention is to provide a holographic light panel which has no inherent structure to produce undesirable moire effects when used in image display applications.
A further objection of the present invention is to provide a holographic light panel, in which a light beam transmitted through its substrate at a grazing incidence angle is diffracted with a high degree of diffraction efficiency along its first diffractive order.
A further objection of the present invention is to provide a holographic light panel which allows a significant reduction in the physical volume necessary for the illumination of flat panel displays, transparencies, holograms, and various other objects.
A further objection of the present invention is to provide a holographic light panel, wherein the light entering the panel at a very steep angle is redirected by a slanted-fringe volume hologram to be emitted over a wide area.
A further objection of the present invention is to provide a holographic light panel, wherein a large area illumination source is created and contained within a thin package.
A further objection of the present invention is to provide a flat panel image display system, in which a holographic light panel of the present invention in provided for backlighting the electrically-addressable spatial intensity modulation panel thereof.
A further objection of the present invention is to provide a flat panel image display system, in which a holographic light panel of the present invention is provided for frontlighting the electrically-addressable spatial intensity modulation panel thereof.
A further objection of the present invention is to provide a novel system and method for recording holographic light panels of the present invention.
These and other objects of the present invention will be described in greater detail hereinafter.
In order to more fully understand the objects of the Present Invention, the following Detailed Description of the Illustrative Embodiments should be read in conjunction with the accompanying Drawings, wherein:
Referring now to the accompanying Drawings, the Illustrative Embodiments of the Present Invention will now be described in detail, wherein like structures in the figures shall be indicated by like reference numerals.
Brief Overview of Holographic Light Panel Hereof
The present invention is directed to a novice device capable of producing a plane of unpatterned or patterned (e.g., pixelated) light of a specified spectral distribution (e.g., broad-band, narrow-band, etc.), for use in various types of illumination applications. In general, the device comprises at least one volume diffractive optical element, and an optically transparent substrate for supporting the same. The function of the optically transparent substrate is to receive a light beam produced from a light source, and to directly transmit the received light onto the volume diffractive element in a single-pass manner, at a very steep, grazing incidence angle (i.e., greater than the critical angle for the material, and typically approaching 90 degrees to the normal to the face of the device).
In general, the volume holograms incorporated in the holographic light panels (HLPs) hereof contain fringes which are neither parallel to the large area boundary surfaces of the holographic material as in standard reflection holograms, nor are perpendicular thereto as in standard transmission holograms. Rather, the fringes are ‘slanted’ with respect to the aforementioned boundary surfaces. With respect to some embodiments of the present invention, terms “substrate referenced”, “edge-lit”, or “edge-illuminated” hologram shall be used herein to describe holograms with slanted fringe structures whose recording reference beams as well as playback reconstruction beams pass at an angle nearly parallel to the plane of the hologram, with respect to the holographic medium, using passing first through a substrate associated with the hologram, prior to entry into the hologram. This angle is greater than the critical angle for the substrate carrying the hologram.
With respect to other embodiments of the present invention, the term “steep reference angle hologram” shall be used to describe holograms where the playback (i.e., reconstruction) beam for the hologram enters the hologram from its air/face surface or where the reconstruction beam passes into a substrate attached to the hologram at a large angle (nearly parallel to the plane of the substrate, but entering via the face, not the edge), at an angle less than the critical angle for the substrate, and then passes from the substrate to the hologram. A steep reference angle hologram usually comprises a thicker package than is achieved with a true substrate referenced, or edge-lit hologram. Steep reference angle holograms can be used in many (though not all) of the applications of edge illuminated holograms, without many of the engineering restrictions imposed by the edge-lit regime necessary to achieve commercially acceptable quality.
While many of the figures shown in the accompanying Drawings depict the light from the light source as entering the optically transparent substrate through its edge (which may or may not be bevelled), it is understood that such light can be made to travel through the substrate at a steep angle via other means, such as by sending it through a prism or diffractive grating affixed to the face of the substrate. Notable, the most of the useful light travelling through the substrate passes out of the substrate and into the hologram directly, without bouncing or waveguiding within the substrate. The function of the volume diffractive optical element is to diffract the transmitted light beam in a manner to produce from the front surface of the holographic light panel, either plane of patterned (e.g., pixelated) or unpatterned light of a specified spectral distribution. Hereinafter, the term “holographic light panel”, “HLP”, or “light panel” shall be used to describe the volume diffractive optical element used in the holographic light panel, even though it may have been created by non-holographic means.
In a typical configuration, the holographic light panel will approximate a rectangular parallelopiped, comprised of four edges and two faces having larger surface areas. The light entering the holographic light panel interacts with the hologram embodied therein, and is then reemitted in a controlled pattern from the face of the device, creating the appearance that the face of the holographic light panel is a new light source. Within the hologram there is a fringe pattern consisting of variations in refractive index of the enabling medium (e.g., polymer material, gelatin, etc.). The structure of the slanted fringes constituting the hologram control the emitted light pattern. In some embodiments, two or more consecutive holograms may be used to achieve the desired emitted light pattern.
In general, the holographic light panels of the present invention are thin, flat, and inexpensive to manufacture, and can produce a plane of unpatterned or patterned (i.e., pixelated) light from a broad surface area. The plane of unpatterned or patterned light can be “white” light, multi-colored, or monochromatic light, depending on spectral and temporal composition of the light entering the edge of the holographic light panel. The unpatterned light emitted from the holographic light panel will have an intensity distribution which is contiguous over the spatial extent (x,y) of its light emitting surface, whereas patterned light will have an intensity distribution which varies thereover in order to satisfy the requirements of any specific application to which the present invention is applied.
In other embodiments of the present invention, the holographic light panel can be designed to produce a light beam or multiple light beams which can be narrow, highly directed or wide angle or even diffused within a controlled emission angle. As will be described in greater detail hereinafter, such holographic light panels can be used anywhere broad areal illumination is desired or required. Examples of such applications include, but are certain not limited to: the conversion of standard holograms into edge-lit holograms; flat-panel type image displaying systems; fingerprint and footprint image detection systems; biological-tissue image detection systems; access-control systems; and the like.
Construction of a Basic Configuration of the Holographic Light Panel
Hologram 3, containing a previously recorded slanted fringe pattern, diffracts light reaching it from light source 1, redirecting the light in the general direction of object 4, thus illuminating object 4, with a predetermined light pattern dependent on the fringe structure recorded in hologram 3. Object 4 may be, for example, a transmissive flat panel display, a transparency, another hologram, etc. Object 4 may be in direct contact with substrate 2, or optically coupled by an intermediate layer such as an adhesive, or an index matching fluid, or object 4 may merely be sufficiently proximate to substrate 2 to achieve a proper amount of illumination of object 4 to allow its intended performance. Note that the light emitted from the hologram may be collimated, converging, or diverging; may have spatial structure, such as pixelation; and may be directed generally perpendicular to the plane of the hologram, or at an angle with respect to the normal to the plane of the hologram, depending on the construction configuration which formed the fringe structure within the hologram. Depending on the application, the space 5 between the object to be lit 4 and the substrate 2 may be filled with air; filled with a material to index-match object 4 to substrate 2 to minimize reflection losses and/or to reduce or eliminate undesirable moire fringes; or non-existent, in the case where object 4 is laminated to or closely pressed against substrate 2. As shown, a viewer or detection system 6 is located on the opposite side of the object from the HLP.
Other configurations of the holographic light panel system are shown in
The HLP depicted in
Advantages and Uses of the Holographic Light Panel
One advantage of the HLP is that the light exiting therefrom can be shaped to be sent out in small solid angles or large solid angles, and can be contiguous or emitted in discrete areal sections, corresponding to a pattern of such as stripes or dots (pixels).
These discrete light patterns (arranged as stripes or dots) may be monochromatic, or in pattern of alternating colors, such as red, green or blue triads, or white. This feature can offer several advantages. For example, in an active matrix liquid crystal display (AMLCD) panel, each pixel region is surrounded by opaque interstices which contain electronic components, such as thin-film transistors (TFTs), which control the liquid crystal polarization state for the adjacent light intensity modulation “window”, by either blocking light or allowing light to pass through the window by way of polarization filtering. Prior art backlighting and frontlighting system designs flood the entire surface, windows and interstices with light, wasting considerable light which is blocked by the interstices. In contrast, an HLP as taught herein can direct light in a pixelated pattern so that the light emitted from the hologram is directed only to the windows, and not the opaque interstices, providing a significant improvement in the light transmission efficiency of the overall holographic light panel.
In addition, the pixelated pattern of light emitted by the hologram need not be monochromatic, but rather can be made, as described herein, polychromatic such as an alternating red, green, blue light pattern. This is achieved by forming individual spectrally-tuned holograms at the subpixel regions of the holographic light panel, which spatially correspond to the actual subpixel structure of an electrically addressable spatial light modulation panel (e.g., AMLCD). Such a colored (red, blue, green) illuminator can be used to improve the efficiency and reduce the cost of manufacture of flat panel displays such as active matrix liquid crystal displays. In addition, the holograms can polarize incoming light, thus diminishing or eliminating the need for a separate polarizer in the spatial-intensity modulation component of an image display system.
In one embodiment of the present invention, a monochromatic electrically addressable spatial light intensity modulating (SLM) panel is used to carry out the spatial intensity modulation function of the image display system by controlled light transmission (or reflection), whereas a RGB pixelated HLP illuminator would carry out the spectral filtering function within the display system by diffractive means. A brightness advantage over current color SLMs by a factor of 10× or more is expected by shaping the light to match the specific pixel size requirements of each display. Additional brightness is expected because the invention will generate color images without the use of absorptive-type spectral filters. Also, as spectral filtering occurs within the holographic light panel, rather than within the spatial intensity modulation panel, there are no red, blue, green (RGB) point failures typically found within in conventional prior rat SLM panels.
As shown in
In
During operation of the flat panel display of
Thereafter, these diffracted light rays travel again through the substrate 422, and thence through the monochromatic LCD panel where they are spatial intensity modulated on a subpixel by subpixel basis in order to impart graphic information thereonto in a conventional manner for subsequent display in either the direct or projection mode. The diffracted light rays within the red spectral band are transmitted through the corresponding “red pixel” windows of the monochromatic LCD panel; the diffracted light rays within the “green” spectral band are transmitted through the corresponding “green pixel” windows of the monochromatic LCD panel; and the diffracted light rays within the “blue” spectral band are transmitted through the corresponding “blue pixel” windows of the monochromatic LCD panel. As the light from the pixelated hologram hereof produces linearly polarized light that has been spectrally filtered in accordance with a pixelated spatial filter pattern, it is possible to use a monochromatic SLM panel having one linear polarizer (i.e., the analyzer), in contrast with two linear polarizers requied by conventional panels. This aspect of the present invention will result in a marked decrease in manufacturing costs of the system.
The function of the optional light diffusing panel 424 is to control the angle of spread (field of view) of the emitted light, and/or to depixelate the light produced from the discrete pixels of the monochromatic SLM 423. It also increases the transmission efficiency of the panel and increases image contrast as observed off-axis. As a result, the sensation of seeing discrete dots displayed from the display panel is lessened or eliminated, and display brightness and image fidelity increased.
In
In general, there are several different ways in which to fabricate the pixelated (reflection or transmission) holograms incorporated into the HLP-based color display systems of the present invention.
According to a first illustrative recording method, a single master hologram is made in which the pattern of red, green and blue spectral filtering diffraction regions are realized therein.
According to a second illustrative recording method, a two separate master holograms are made, where in the first hologram, the pattern of red and green and blue spectral filtering diffraction regions are realized therein during the first stage of the mastering process; and where in the second hologram, the pattern of blue spectral filtering diffraction regions are realized therein during the second stage of the mastering process. Once made, copies of these pixelated holograms are spatially registered and then optically and mechanically coupled together by way of lamination or other suitable techniques.
According to a third illustrative recording method, three separate master holograms are made, where in the first master hologram, the pattern of red spectral filtering diffraction regions are realized therein during the first stage of the mastering process; where in the second hologram, the pattern of green spectral filtering diffraction regions are realized therein during the second stage of the mastering process; and where in the third hologram, the pattern of blue spectral filtering diffraction regions are realized therein during the third stage of the mastering process. Once made, copies of these pixelated master holograms are properly registered and optically and mechanically coupled together by way of lamination or other suitable techniques.
Details of such holographic recording processes will be described hereinafter.
Procedures for Making “Non-pixelated” HLPs
Procedures for making non-pixelated HLP devices will now be described in detail. While construction of HLP holograms as described herein follows basic well-known holographic principles, the primary difference between the construction of the HLPs hereof and standard holograms resides in use of strict index matching volume techniques taught in Applicants copending U.S. application Ser. Nos. 08/594,715, 08/546,709 and 08/011,508. As disclosed in said copending Applications, Applicants have developed a technique for index matching the substrate to the recording medium when the index of refraction of the substrate is less than the recording medium (referred to as Case 1), and another technique for index matching when the index of refraction of the substrate is greater than (or equal to) the recording medium (referred to as Case 2).
Index Matching: Case 1
In U.S. application Ser. Nos. 08/594,715, 08/546,709 and 08/011,508, Applicants teach that for Case 1 recording situations, the highest quality edge-lit holograms can be achieved by carefully matching the index of refraction of the recording medium with the index of refraction of its associated substrate. The degree of matching required is a function of the steepness of the reference beam angle and the light transmission into the recording medium, which is derived by combining the well known Fresnel reflection equations with Snell's Law at the substrate-recording medium interface. In practice, the best index matching in this case is achieved by choosing a substrate whose index of refraction is equal to or slightly less than the index of refraction of the recording medium. For example, in accordance in with this index matching technique, Applicants have discovered that BK10 glass works well with DuPont holographic recording material designated HRF 352. The concept works well with any well-matched substrate and recording medium. Typically, Applicants have found that is desirable to maintain the mismatch in indices of refraction between the substrate and the recording medium to less than 0.02 for angles of incidence of the recording reference beam greater than 80 degrees where a relatively high light transmission efficiency is required. If an intermediate layer, such as a glue or an index matching fluid, is used between the recording medium and the substrate, then care must be taken to select the index of refraction of the intermediate layer to be either: equal to the substrate or equal to the recording medium, or between the index of refraction of the recording medium and the substrate.
Due to the steep angles used in the recording process of the HLP, the optical path length in the material is comparatively quite long compared with standard holographic geometries. This means that the quality of the final hologram is more significantly affected by the size of the scattering centers within the recording medium, and thus Applicants have found that better results are achieved when using low scatter recording materials such as the family of DuPont holographic recording photopolymers.
Index Matching: Case 2
In U.S. application Ser. Nos. 08/594,715, 08/546,709 and 08/011,508, Applicants also teach that for Case 2 recording situations, it is best to use a “gradient-type” index matching region at the interface between the substrate and the recording medium. This type of indexing matching region can be achieved during the recording of edge illuminated holograms when using photopolymer recording materials which contain migratory monomers. During such recording process, applicants have discovered that under particular conditions the action of the signal wave (object beam) can increase the refractive index of the recording layer near the boundary between the recording material and the substrate by attracting migratory monomer toward this boundary. This increases the ability of the reference wave to couple into the recording medium when it is incident at an angle close to grazing incidence. At locations of high reference signal strength in the recording medium, the refractive index increases in that locality, thus enabling the penetration of the reference wave.
Systems for Making Edge-lit HLPs
The recording system shown in
In each of the holographic recording systems shown in
Depending on the application, and the desired reconstruction geometry, reference light beam 10 may be collimated, converging, diverging and/or anamorphically shaped so that it may have different properties along each of two perpendicular axes. For example, to make more efficient use of light going into a substrate edge which is long in one dimension and thin in the other, the reference light beam may be collimated in the thin direction and diverging in the long dimension. Reference light beam 10 then passes through substrate 12 and substrate/recording medium interface 14 and into recording medium 13.
During Case 1 recording processes, the relative amount of light from the reference beam that is transmitted into the recording medium depends on the relative refractive indices of the substrate and recording medium, the angle of incidence of the beam, and the polarization state of the beam. Inside the recording medium, reference beam 10 interferes coherently with object beam 11 to form, within recording medium 13, a holographic fringe pattern, with slanted fringes. Notably, each “slanted fringe” formed in the recording medium is the effect of a localized change or modulation in the bulk index of refraction of the recording medium caused by a change in the optical density of the recording medium during the recording process, such changes in optical density of the recording medium are in response to the light intensity pattern created by the interference of the object and reference light beams within the recording medium. The angle of slant of the fringes is typically in the neighborhood of between 35 and 55 degrees to the optical axis of the object beam. Object beam 11 may typically be collimated, converging or diverging light, or may have some other wavefront form. In fact, the object beam may have scattered off of a real object before reaching the recording medium; it may comprise the real image from another previously made hologram; or it may have passed through a mask, diffuser or other optical element, as will be described further below.
In case 2 recording processes, increasing the refractive index at the interface can be achieved by either reference or signal wave activity. Such an increase can be achieved by, for example, exposing the recording layer to a diffuse page of signal wave (e.g., passing the object beam through a diffusing material) on its own prior to exposure to the holographic patterns. Since monomer will migrate toward the incoming light, the bulk index of the recording layer is thus increased. The bulk index increases because polymer occupies less volume than monomer.
It is noted that signal-wave gated holograms can have zero noise background, since interference patterns are only present where the reference wave is permitted to leak in. This process of index matching by light induced effects throughout the bulk of the recording layers is distinct from localized index matching induced by the evanescent field of the reference wave near the interface between recording medium and substrate. In either method, the effects are to be employed just prior to the recording of the holographic pattern.
After recording of the holographic fringe pattern using either the Case 1 or Case 2 scheme, the recording material is processed to stop the exposure sensitivity, and fix the fringe pattern formed in the recording material. Depending on the processing required for the recording material, it may be necessary to delaminate the recording material from the substrate for processing. For example, materials such as dichromated gelatin and silver halide require wet processing, which may be better achieved by delamination from the substrate, particularly if glass plates coated with gelatin were used, with the gelatin-air surface laminated to substrate 12. Other materials, such as the DuPont photopolymer family, are processed by exposure to ultraviolet light and, optionally, subsequent baking. This process does not require that the recording material be delaminated from substrate 12, however, for cost factors or other reasons, it may be advantageous to use a different substrate for playback than when recording. Other recording materials may require no post-processing at all.
Once a “perfect” hologram (HLP master) has been produced for the monochromatic or color display application, large numbers of low-cost copies can be produced that will have the same properties as the HLP master, thus significantly reducing the manufacturing costs of flat panel displays.
Systems for Replaying Recorded Edge-lit HLPs
In
In
Referring to
In
In
The methods described above are useful for making holographic illuminators which emit an areal field of structured light from their surface. In many applications, such as Grey scale and color flat panel display systems, it is desired that the light emissions from the holographic light panels are segmented, striped, pixelated, or otherwise structured.
Making Pixelated HLPs for Grey-scale Flat Panel Display Systems
In
In
Surprisingly, Applicants have discovered that the reflection edge-lit holograms hereof can be made sufficiently thick to maintain excellent filtering properties even though the fringes within the hologram are slanted with respect to the plane of the hologram. Thus, in monochromatic LCD systems of the type shown in
In
Method for Recording Holograms H1 and H2
In some cases, it may be mechanically or otherwise inconvenient to locate the spatial mask 5 proximate to the holographic recording medium 52 during the recording of the reflection or transmission volume holograms for the HLPs hereof. Thus, in such cases, it may be desirable to use a holographic-type spatial mask “(H1)” in the HLP recording system hereof, such a holographic spatial mask can be made by producing an H1 hologram of a spatial filter (e.g., apertured plate, etc.) and thereafter using the image of the H1 hologram as the object for an H2 hologram. One advantage gained by using an H1 hologram (as a spatial mask), is that one can achieve an HLP having a wider field of view than the HLP produced by the one-step recording system shown in
As shown in
In
In
In
In
Making Pixelated HLPs for Flat Color Display Panels
When making a color flat panel image display system employing active matrix liquid crystal display panel, each pixel region in the color display panel is divided into three subpixels, each subpixel corresponding to the color red (R), blue (B), or green (G), in additive-primary type color systems. In subtractive-primary color systems, the subpixels associated with each pixel in the color display will correspond to yellow (Y), cyan (C) and magenta (M). In the illustrative embodiments, the additive primary color system is employed.
Each subpixel in the HLP of the illustrative embodiment embodies a slanted-fringe volume hologram. The function of each “red” subpixel region in the HLP is to produce spectrally-filtered light within the red spectral band. The function of each “green” subpixel region in the HLP is to produce spectrally-filtered light within the green spectral band. The function of each “blue” subpixel region in the HLP is to produce spectrally-filtered light within the blue spectral band. Collectively, these arrays of microscopic volume reflective holograms provide a system of color generation, operating on principles of diffraction. As this system of color generation does not employ absorptive-type spectral filters, its light transmission efficiency is substantially greater than the light transmission efficiency of prior art absorptive color generation systems, and its manufacturing cost is significantly less.
In order to make the pixelated HLP for this color display system, a spatial mask is used having (subpixel) light transmitting apertures that correspond to the actual subpixel locations of the spatial light modulator (e.g., AMLCD) used in the final color display system under design. In general, since the red green and blue subpixel regions in the monochromatic active matrix LCD are spatially periodic, one mask can be used to record each of the three subpixel patterns within the hologram of the HLP. It is understood however that it will be necessary to register the spatial mask at each stage of the holographic recording process in order to register the subpixel regions of the mask with corresponding subpixel regions in the recording medium that correspond to the subpixel regions along the monochromatic LCD panel, forming the SLM component of the HLP. Alternatively, one can use a different mask to realize a different pattern of mini-holograms corresponding to a particular subpixel color (R, G, B). In either embodiment of the present invention, each of the three subpixel arrays of mini-holograms is spectrally tuned to a different wavelength band (e.g., R, G, or B) corresponding to the color band of light which is to emanate from the spatially-registered subpixel pattern on the monochromatic LCD panel.
System for Recording Pixelated HLPs for Color Display Panels
A three color HLP may be constructed using the holographic recording system schematically illustrated in
During each primary color recording stage, the pixelated spatial mask 412 is translated with respect to the substrate 413 under computer control, for example. During recording of the RED holographic pixel array, the apertures in the spatial mask 412 are aligned with the red subpixels on the monochromatic SLM panel so that only an array of discrete volume holograms tuned to the red spectral band are formed in the holographic recording medium at locations that physically correspond to the red subpixels on the monochromatic SLM panel. During recording of the Green holographic pixel array, the apertures in the spatial mask 412 are aligned with the green subpixels on the monochromatic SLM panel so that only an array of discrete volume holograms tuned to the green spectral band are formed in the holographic recording medium at locations that physically correspond to the green subpixels on the monochromatic SLM panel. During recording of the blue holographic pixel array, the apertures in the spatial mask 412 are aligned with the blue subpixels on the monochromatic SLM panel so that only an array of discrete volume holograms tuned to the blue spectral band are formed in the holographic recording medium at locations that physically correspond to the blue subpixels on the monochromatic SLM panel. During each such recording stage, the reference beam originates from the same location. Depending on the application, and the film and processing technique used, the reference beam angle for each color may have to be adjusted to compensate for chromatic aberrations. After completing the three primary color recording stages, the selectively exposed holographic recording medium (e.g., panachromatic film) is then processed using conventional techniques. When replayed using a white light reconstruction beam, or a light source or sources having discrete red, green and blue spectral emissions, the hologram will emit discrete beams of red, green and blue light spatially corresponding to the red, green and blue subpixel regions of the monochromatic SLM panel. Depending on the pixel or stripe configuration provided by the monochromatic SLM panel to be employed in the flat panel display system under design, three different masks may need to be used, if the pixel spacings differ from color to color for a particular display configuration.
In order to eliminate the problem of multiple exposures of the same region with the reference beam, an additional mask 410, registered to the apertures of mask 412, is placed between the substrate 413 and recording material. During each of the three primary stages of the holographic recording process, the mask 410 is moved to a different registration location for the recording of each array of spectrally-tuned volume holograms.
Preferably, spacial masks 410 and 412 are identical and consist of optically transparent or “open” windows in an opaque material. Such spatial masks can be made by using any one of a number of well known techniques, such as punching holes in a sheet of metal, or, for example, depositing chrome on glass. For an AMLCD illuminator, the hole locations would correspond to all of the subpixel locations for a single color. Mask 410 and mask 412 should be closely index-matched to recording medium 411 according to the index matching principles noted elsewhere herein. Mask 412 should also be index-matched to substrate 413. Typically an index matching fluid would be used for this purpose. If the masks are made on glass, the glass should be of the same material as substrate 413. Each of
Masks 410 and 412 should be mechanically established so that their position with respect to each other remains constant, but can change relative to recording medium 411. Depending on the application, setup, mask type, and recording medium, it may be more desirable to move either the masks or the recording medium, or remove, replace and reposition the masks with respect to the recording medium in between exposures.
A method for recording the RGB-type HLP of the present invention will now be described in detail with reference to the recording system configurations shown in
In the illustrative embodiment, it is assumed that an active matrix liquid crystal display will be use to spatial intensity modulate the discrete set of finely-focused pixelated light beams produced by the HLP. Also a method of recording a three color (RGB) holographic array will be described using a single spatial mask pattern with symmetrically arranged apertures, that is moved under computer control with respect to the holographic recording medium in order that the light transmitting apertures are registered with regions on the recording medium that will spatially correspond with the subpixel regions of the monochromatic SLM panel when the constructed HLP and monochromatic SLM are assembled together to produce the final product. It is understood however that some applications may require different masks for each of the different additive primary colors employed in the color system.
In the illustrative example to be described below, masks 410 and 412 are movable in the x direction relative to holographic recording medium 411. However, it is understood that some applications may require motion of the mask in the y and/or x and y directions. Also some applications may require that there is a spacer disposed between mask 410 and recording medium 411 so that upon replay, the image of the “windows” (i.e., light transmitting apertures) in spatial mask 410 fall or otherwise focus precisely within the corresponding subpixel regions of the SLM display panel (e.g., AMLCD). It may also be helpful to laminate or otherwise affix the holographic recording medium 411 to a substrate of the same material as substrate 413 to give it mechanical integrity. During each stage of the multi-stage holographic recording process, the object and reference beams should have the same relative wavefront (or F/#). Also to ensure proper index matching between the substrate and recording medium, it may be desirable to submerge the entire exposure rig in a tank filled with index matching fluid during the recording process. (This technique may be used to realize any embodiment of the present invention).
As shown in
As shown in
During the third stage of the holographic recording process, shown in
After the carrying out the above three stages of exposure, the recording medium 411 is then processed and fixed as a hologram using conventional techniques well known in the art. The hologram is mounted on a substrate for replay using a grazing incidence laser beam produced from either a white light source or a RGB light source at the same location as the recording reference beam.
Having constructed the RGB-type HLP described above, the HLP is then laminated, affixed, adhered or otherwise appropriately arranged with respect to the rear surface of the monochromatic SLM panel, for which the HLP has been designed. Index matching should be taken into consideration when laminating such panels together in order to reduce reflection losses at the hologram-substrate interface. The overall structure, together with the multi-spectral light source and beam shaping optics, can be assembled as an integral unit capable of being mounted within virtually any type of image display housing using techniques well known in the art.
During replay of the RGB-type HLP, a three-color pixelated light pattern will be emitted from the hologram at locations on the surface of the hologram that spatially correspond to the location of corresponding subpixels on the monochromatic SLM panel. In this way, the red subpixelated light pattern is projected through and intensity modulate by the red subpixels of the monochromatic SLM panel; the green subpixelated light pattern is projected through and intensity modulated by the green subpixels of the monochromatic SLM panel; and the blue subpixelated light pattern is projected through and intensity modulated by the blue subpixels of the monochromatic SLM panel. When transmitted through the light intensity modulating subpixel regions on the monochromatic SLM panel, mounted to the HLP, the light projected from these subpixel patterns is spatial intensity modulated in accordance with incoming image display information and the resulting light distribution projected therefrom is fused together on a subpixel-by-subpixel basis, to form the color image to be displayed. Notably, particular color to be imparted by any one pixel in the resulting displayed image is comprised of the light intensity produced from the associated red, green and blue subpixel regions. As light energy absorptive mechanisms are avoided in the color generation method employed in this display system, the light transmission efficiency of the system can be significantly improved over that of prior art systems.
In the above-described embodiment of the RGB HLP hereof, the holograms in each of discrete R, G and B set of holograms have been simultaneously recorded within the recording medium during a single recording stage. It is contemplated, however, that the reference and/or object beam used to form such holograms can be focused down to the size of each subpixel, and scanned (e.g., according to a raster pattern) in order to expose each subpixel location within the recording medium, one at a time. The light beam(s) could be modulated during scanning using techniques (e.g., acousto-optic modulators) well known in the laser scanning industry, so that, for example, a red subpixel region along the holographic recording medium is not exposed by a laser beam used to form a blue subpixel region therein.
In the illustrative embodiment of the RGB-type HLP described above, the holograms in each discrete set thereof are recorded in a single layer of panchromatic film. One alternative method would involve recording discrete sets of hologram associated with two subpixel color patterns of the RGB HLP in a first layer of recording medium (e.g., in solid or liquid phase), while the third discrete set of hologram associated with the third subpixel color pattern is recorded in a separate layer of recording medium. Once recorded, these layers can then aligned or registered with respect to each other, and then held in place using lamination or other techniques known in the art.
An alternative method for making the RGB HLP hereof involves separately recording three discrete sets of holograms spectrally-tuned to the additive primary colors red, green, and blue on three separate layers of holographic recording medium during three recording stages. Thereafter, these three layers are aligned and fixed into place with respect to one another so that the red, green and blue subpixel regions thereof are in proper spatial relationship to each other and in registration with the corresponding subpixel regions along the monochromatic SLM panel for which the HLP is being designed. These aligned layers can be laminated or otherwise mechanically and optically coupled together, or to spacers disposed between each layer, or by mechanically framing or fixturing each layer in such a way that the subpixel patterns of each layer are properly aligned. The stack of pixelated holograms layers are then mounted to a substrate as described hereinabove to produce an RGB-type HLP of composite construction.
Method of Converting to an Edge-lit HLP to a Face-lit HLP
Various techniques have been described above for constructing edge-lit HLPs, for example, for use with both monochromatic and color flat panel image display systems. However, there will be some applications where the amount of light required to illuminate an object (e.g., SLM panel, film structure or transparency, etc.) is more than can be easily transmitted through the substrate edge of an edge-lit HLP without resorting to higher power lamps or inconvenient light preconditioning optical schemes that can add unwanted volume to the system packaging. Thus in some cases it is will be desirable to replay the HLP hologram using a light beam that is forced to enter the face of the substrate or the recording medium, at a steep angle, but not with the grazing incidence associated with an edge-lit or substrate guided system. While this illumination technique increases thickness of the overall system packaging, this drawback may be an acceptable trade-off in some instances in order to provide more light for illuminating the HLP hologram during its replay mode.
In accordance with an alternative method of HLP hologram recording, an original H1 hologram is first made using the recording system shown in
In
In
Once constructed, the H3 hologram is affixed to the H2 hologram or an appropriate substrate therebetween as shown in
In
While the above-described conversion method has been illustrated in connection with an edge-lit reflection type HLP, the method can be readily used to convert an edge-lit transmission-type HLP into a face-lit transmission type HLP.
Method and System for Making a White-light Emitting HLP
In some applications (e.g., image illumination or display systems), it would be advantageous for an HLP emit a pixelated pattern perceived as “white” pixels, rather than a subpixel pattern of red, green and blue light required in color display systems. Below will be described a method of creating an HLP capable of emitting white light pixel patterns.
According to this method, an H1 hologram is first made using the recording system shown in
As shown in
Notably, in the HLP embodiment shown in
While the particular illustrative embodiments shown and described above will be useful in many applications in back and front lighting art not limited to the use of SLMs, further modifications to the present invention herein disclosed will occur to persons with ordinary skill in the art. All such modifications are deemed to be within the scope and spirit of the present invention defined by the appended Claims to Invention.
| Number | Date | Country | |
|---|---|---|---|
| 0020975 A1 | Jan 2003 | US |
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| Parent | 08885646 | Jun 1997 | US |
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| Parent | 08812381 | Mar 1997 | US |
| Child | 08885646 | Jun 1997 | US |
| Parent | 08375069 | Jan 1995 | US |
| Child | 08636688 | Apr 1996 | US |
| Parent | 08095748 | Jul 1993 | US |
| Child | 08375069 | Jan 1995 | US |
| Parent | 08594715 | Jan 1996 | US |
| Child | 08812381 | US | |
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| Child | 08546709 | Oct 1995 | US |
| Parent | 08011508 | Jan 1993 | US |
| Child | 08373878 | Jan 1995 | US |
| Parent | 08597491 | Feb 1996 | US |
| Child | 08812381 | US |
| Number | Date | Country | |
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| Parent | 08636688 | Apr 1996 | US |
| Child | 08812381 | Mar 1997 | US |
| Parent | 08011334 | Jan 1993 | US |
| Child | 08095748 | Jul 1993 | US |
| Parent | 08011508 | Jan 1993 | US |
| Child | 08095748 | Jul 1993 | US |
| Parent | 07902881 | Jun 1992 | US |
| Child | 08095748 | Jul 1993 | US |
| Parent | 07841576 | Feb 1992 | US |
| Child | 08095748 | Jul 1993 | US |
| Parent | 08546709 | Oct 1995 | US |
| Child | 08594715 | Jan 1996 | US |
| Parent | 08394470 | Feb 1995 | US |
| Child | 08597491 | Feb 1996 | US |
| Parent | 08198998 | Feb 1994 | US |
| Child | 08394470 | Feb 1995 | US |
