Abstract:
A method and apparatus for minimizing diffraction effects on flat panel displays. The flat panel display includes a plurality of display elements, each display element including a first structural feature having an edge and a second structural feature disposed on the first structural feature. The edge of the first structural feature causes incident rays from a point light source to generate diffraction effects due to the Fresnel reflectance difference in the boundary between the first and second structural features. The display further includes an edge texture on the edge of the first structural feature to minimize the diffraction effects into a more diffused diffraction pattern.

Description:
FIELD OF THE INVENTION 
     The invention relates generally to flat panel displays. More particularly, the present invention relates to a method and apparatus for minimizing diffraction effects in flat panel displays. 
     BACKGROUND OF THE INVENTION 
     Flat panel displays enjoy wide appeal as computer screens, television screens, electronic game displays, avionics or vehicular displays, and as displays in a variety of other applications because of their light weight, small footprint, relatively sharp resolution, and low power consumption. Unfortunately, flat panel displays can exhibit undesirable visual effects previously unseen with other types of display devices. One such visual effect is the presence of bright bands, or multiple images, surrounding the mirror image of a small, non-diffused, intense light source, such as the sun. 
     The visual anomalies, as shown on a flat panel display  10  in FIG. 1, generally lack well-defined features and instead, appear as broad, orthogonal bands  24  extending vertically and horizontally away from the mirror or specular image  20  of the small, intense light source  18  and may also include multiple secondary images  22  of the light source  18 . Bands  24  and the secondary images  22 , collectively referred to as a diffraction pattern, always surround the specular image  20  of the light source  18 , with the diffraction pattern being brightest near the specular image  20 . 
     For example, in the case of flat panel display  10  being a liquid crystal display (LCD)  240  (see FIG.  2 ), it is the sharp boundary between a black matrix  244  and a color filter  246  that is the primary source of the diffraction pattern. An incident ray  80  impinging on a region of LCD  240  where no black matrix  244  is present experiences Fresnel reflectance and transmittance to generate a reflection ray  82  and a transmission ray  84 , respectively. At regions of black matrix  244  away from its edges, an incident ray  86  will experience Fresnel reflectance to generate a reflection ray  88 , but no transmission ray because black matrix  244  is optically opaque. Thus all points on LCD  240  not on an edge of black matrix  244  lead to Fresnel reflectance that is specular because only a small refractive index difference exists between a front substrate  242  and the color filter  246 . 
     In contrast, incident rays  90 ,  100  impinging on an edge of black matrix  244  leads to Fresnel reflectance and transmittance to generate respectively reflection rays  92 ,  102  and transmission rays  94 ,  104  as described above, but also reflection-mode diffraction rays  96 ,  106  and transmission-mode diffraction rays  98 ,  108 . It is at these edges that the difference in the Fresnel reflectance at the boundary of the black matrix  244  and color filter  246  leads to the diffraction pattern. Specifically, it is the diffraction rays exiting the front of LCD  240 , e.g., reflection-mode diffraction rays  96 ,  106 , that are problematic since LCD  240  is viewed from a viewing direction  258 . 
     Typically, the appearance of the diffraction pattern, i.e., intensity, shape, color, etc., is directly correlated to the physical construction of the flat panel display, and not its operation. For example, displays of the same design exhibit similar diffraction patterns, while displays of a different design may exhibit a significantly different diffraction pattern. Similarly, one display design may generate predominantly chromatic diffraction patterns, while other display designs may lead to strong spectral dispersion with repeating bands. Furthermore, the brightness of the diffraction pattern may vary according to the display design. 
     In addition to the sun, other small, intense light sources (also referred to as point sources) such as incandescent or arc lamps can also generate diffraction patterns on flat panel displays. Basically, light sources with limited angular size are potential diffraction pattern generators, while spatially extended sources like fluorescent or diffused lamps or sunlight reflecting from diffuse surfaces such as clothing or clouds do not generate diffraction patterns as shown in FIG.  1 . This is not to say that spatially extended sources, also referred to as area sources, do not produce diffraction patterns. Instead, each point on the area source generates its own diffraction pattern with a slight lateral displacement thereof. Then, when the diffraction patterns from all the points visually superimpose, the peaks in each of the diffraction patterns merge to form secondary images of the area source. Hence, area sources do not produce significant diffraction effects as observed with point sources such as the sun. 
     Moreover, it has been observed that changing the orientation of the flat panel display and/or viewing direction does not eliminate the diffraction pattern. For example, when the flat panel display is rotated relative to the viewer and the light source, the diffraction pattern also rotates with the display. Similarly, when the viewer moves relative to the display and light source, the diffraction pattern will also shift in position on the display. Hence, if the diffraction pattern is bright enough over a relevant portion of the display, the readability of the information presented by that portion of the display will be impaired. Display readability degradation results in increased reading time, decreased reading accuracy, and viewer discomfort due to eyestrain, and even possibly headache and nausea. Diffraction patterns are particularly problematic in display applications where the positions of the display, viewer, and light source are not independently controllable such as in avionics or vehicle applications. In such situations, for example, in modern fighter jet cockpits with large transparent canopies, the operator (i.e., the pilot or driver) may experience performance degradations due to increased reading time or decreased reading accuracy due to the diffraction pattern. 
     Thus, there is a need for an apparatus and method for minimizing diffraction effects on flat panel displays. Moreover, given the complex structure of flat panel displays, there is a need for the apparatus and method to minimize such diffraction effects without adding undue weight to the displays or requiring extensive changes to their physical construction thereby negating desirable performance parameters of existing displays. Still further, there is a need for the apparatus and method to be easily modifiable for implementation in various types of flat panel displays. 
     SUMMARY OF THE INVENTION 
     One embodiment of the invention relates to a flat panel display system having a front viewing surface. The system includes a display layer, a first electrode, and a second electrode. At least one of the display layer, the first electrode, and the second electrode includes an edge having an edge texture, wherein the edge texture minimizes diffraction effects visible on the front viewing surface of the system caused by a point light source. 
     Another embodiment of the invention relates to a flat panel display system having a front viewing surface. The system includes a black matrix, a first electrode, a second electrode, a color filter, and an electrical structure. At least one of the black matrix, the first electrode, the second electrode, the color filter, and the electrical structure includes an edge having an edge texture, wherein the edge texture minimizes diffraction effects visible on the front viewing surface of the system caused by a point light source. 
     Another embodiment of the invention relates to a flat panel display including a plurality of display elements, each display element having a first structural feature with an edge and a second structural feature disposed on the first structural feature, wherein incident rays from a point light source incident on the edge of the first structural feature and then the boundary of the second structural feature generates undesirable diffraction effects. The display further includes means for minimizing the undesirable diffraction effects. In one preferred embodiment, the means for minimizing the undesirable diffraction effects includes an edge texture included on the edge of the first structural feature. In still another preferred embodiment, the edge texture includes a profile of the edge texture selected from a group including a fractal profile, a discontinuous curved profile, a continuous curved profile, a random profile, a piecewise linear profile, a non-linear profile, and a profile configured to decrease the diffraction effects caused by a smooth surface of the edge. 
     Still another embodiment of the invention relates to a method for minimizing diffraction effects on a flat panel display. The method includes generating an edge texture on an edge of a structural feature of a display element of the display, wherein the edge minimizes diffraction effects when exposed to a point light source. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will become more fully understood from the following detailed description, taken in conjunction with the accompanying drawings, wherein like reference numerals refer to like parts, in which: 
     FIG. 1 is a front view of a flat panel display showing the diffraction pattern generated by a point light source; 
     FIG. 2 is a partial cross-sectional view of the flat panel display of FIG. 1, showing light ray trajectories from the point light source; 
     FIG. 3 is a schematic diagram of a front viewing surface of a flat panel display in accordance with the present invention, the diagram showing an array of display elements; 
     FIG. 4 is a cross-sectional view of an embodiment of the flat panel display of FIG. 3, shown as a liquid crystal display; 
     FIG. 5 is a cross-sectional view of another embodiment of the flat panel display of FIG. 3, shown as an electroluminescent display; 
     FIG. 6 is a perspective view of a structural feature of the flat panel display of FIG. 3, showing an edge texture thereon; 
     FIG. 7 is a front view of another structural feature of the flat panel display of FIG. 3, showing an edge texture thereon; 
     FIG. 8 is a partial front view of the flat panel display of FIG. 3, showing edge textures included on a plurality of adjacent structural features of the display elements; 
     FIG. 9 is a partial front view of the structural feature of the flat panel display of FIG. 3, showing a first edge texture profile; 
     FIG. 10 is a partial front view of the structural feature of the flat panel display of FIG. 3, showing a second edge texture profile; 
     FIG. 11 is a partial front view of the structural feature of the flat panel display of FIG. 3, showing a third edge texture profile; 
     FIG. 12 is a partial front view of the structural feature of the flat panel display of FIG. 3, showing a fourth edge texture profile; and 
     FIG. 13 is a partial front view of the structural feature of the flat panel display of FIG. 3, showing a fifth edge texture profile. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 3, there is shown a flat panel display  30  of the present invention comprised of an array of display elements  32  arranged in an evenly spaced grid pattern for forming a composite image thereon. Alternatively, elements  32  may be arranged in a staggered grid pattern resembling a brick wall (not shown). Although elements  32  are shown square shaped, it should be understood that elements  32  may be any regular geometric shape, such as rectangles or any arbitrary shape as is convenient. Preferably the distance between adjacent display elements  32  is approximately 20 microns and each of the display elements  32  has dimensions of approximately 70 microns by 40 microns. 
     Each display element  32 , in turn, is comprised of numerous structural layers, or structural features, such as electrical conductors or devices, and emitting or transmitting layers (see FIGS.  4  and  5 ). These structural features are usually defined by a geometry having long, straight edges because such a geometry enhances yield in many fabrication processes and most efficiently uses the available display area. Thus, flat panel display  30 , when viewed from the front thereto, contains regularly spaced, parallel repetition of predominantly horizontal and vertical edges from one or more structural features of each element  32 . 
     Flat panel display  30  includes display devices having a relatively high mirror reflection component such as, but not limited to, a liquid crystal display (LCD), and electro luminescent (EL) display, a plasma display, a deformable—mirror display, or other light-transmissive or light-emitting displays. In one embodiment, FIG. 4 shows a partial cross-section of a LCD  40 , to be viewed from a viewing direction  58 , including the structural features of a front substrate  42 , a black matrix  44 , a color filter  46 , a common electrode  48 , a liquid crystal cell  50 , an electrical structure  52 , a pixel electrode  54 , and a rear substrate  56 . 
     Front substrate  42  is closest to the viewer and the rear substrate  56  is disposed furthest from the viewer. Black matrix  44  is disposed on a major side of front substrate  42  opposite the viewer&#39;s side. Color filter  46  is disposed on black matrix  44 , and common electrode  48  is disposed on color filter  46 . Liquid crystal cell  50  is disposed on the common electrode  48 , and electrical structure  52  and pixel electrode  54  are disposed on the liquid crystal cell  50 . Finally, rear substrate  56  is disposed on electrical structures  52  and pixel electrode  54 . 
     Front and rear substrates  42 ,  56  are preferably comprised of glass or other optically transparent and non-flexible material. Front and rear substrates  42 ,  56  may have dimensions approaching the size of the overall LCD  40 , since they act as a “housing” or protective layers. 
     Black matrix  44  comprises an optically opaque material such that light incident on black matrix  44  will be absorbed or reflected back with no transmission therethrough. Preferably, black matrix  44  is a metal, metal oxide, or a combination of metal and metal oxide such as chrome, chrome oxide, or a combination of chrome and chrome oxide. 
     Color filter  46  comprises an optically transparent material. Preferably color filter  46  is a dyed or pigmented polymer such as acrylic. 
     Common electrode  48  includes a first major side surface and a second major side surface substantially perpendicular to the viewing direction  58 , the first major side surface being closer to the black matrix  44  then the second major side surface. Common and pixel electrodes  48 ,  54  are comprised of a transparent conductive material such as indium tin oxide. 
     Liquid crystal cell  50 , also referred to as a modulator or display layer, comprises liquid crystal molecules capable of different molecular orientations to modulate the transmitted optical polarization state in accordance with the absence or presence of an electric field applied across thereon. 
     Electrical structures  52  can comprise numerous electrical devices, preferably thin film transistors or conductors. Furthermore, each electrical structure  52  aligns with each black matrix  44  (as shown in FIG. 3) so that electrical structures  52  are not visible from viewing direction  58 . 
     In another embodiment, there is also shown in FIG. 5 a partial cross-section of an electro luminescent (EL) display  60 , to be viewed from a viewing direction  78 , including the structural features of a front substrate  62 , a row electrode  64 , a first insulator layer  66 , an emitter layer  68 , a second insulator layer  70 , a column electrode  72 , an insulating layer  74 , and a rear substrate  76 . 
     Front substrate  62  is closest to the viewer and the rear substrate  76  is disposed farthest from the viewer. Row electrode  64  is disposed on a region side of front substrate  62  opposite the viewer&#39;s side. First insulator layer  66  is disposed on row electrode  64 , and the emitter layer  68  is disposed on the first insulator layer  66 . Second insulator layer  70  is disposed on emitter layer  68 , and the column electrode  72  is disposed on the second insulator layer  70 . Insulating layer  74  is disposed on column electrode  72 , and the rear substrate  76  is disposed on insulating layer  74 . 
     Front and rear substrates  62 ,  76  are preferably comprised of glass or other optically transparent material. Front and rear substrates  62 ,  76  may have dimensions approaching the size of the overall EL display  60 , since they act as a “housing” or protective layers. 
     Row electrodes  64  are comprised of a transparent conductive material such as indium tin oxide. Column electrodes  72  are comprised of a conductive material such as aluminum. First and second insulator layers  66 ,  70  are comprised of an amorphous or crystalline inorganic material such as yttrium oxide, aluminum oxide, or silicon nitride. 
     Emitter layer  68 , also referred to as a display layer, includes a first major side surface and a second major side surface substantially perpendicular to the viewing direction  78 . Emitter layer  68  comprises a light emittive material responsive to an applied voltage. Preferably emitter layer  68  is a solid inorganic phosphor or organic light-emitting polymer. More preferably, emitter layer  68  is a thin film and includes patterning on at least one major side thereof. Insulating layer  74  comprises a moisture impermeable material, preferably a fluorocarbon-based polymer or silicone-based oil. 
     It should be understood that the present invention is not limited to flat panel displays configured as in FIGS. 4 and 5. Instead, for example, LCD  40  may include additional structural features such as compensators, polarizers, more than one liquid crystal layers, backlight, etc. Similarly, EL display  60  may include additional structural features such as thin film transistors and other electrical structures, and multiple rows and/or columns of electrodes for each display element. Moreover, the ordering of the structural features within each display may be modified to facilitate certain performance parameters or fabrication processes. 
     When a viewer and a small, intense light source (i.e., a point light source), such as the sun, are located on the same side of the flat panel display  30 , i.e., the front of the display, a diffraction pattern is generated on that same side of the display  30  from certain edges of the structural features. This diffraction pattern, more specifically a reflection-mode diffraction effect, is caused by abrupt changes in the Fresnel reflection coefficients. Reflection-mode diffraction effect, such as shown in FIG. 1, occurs at every material boundary where the Fresnel reflectance is not identical on both sides of the boundary. 
     Thus, each edge of each structural feature visible or “exposed” when flat panel display  30  is viewed from its front viewing surface and which has this material boundary condition can cause reflection-mode diffraction rays similar to rays  96 ,  106  (see FIG.  2 ), and thus the diffraction pattern. For LCD  40  in FIG. 4, it is the edges corresponding to black matrix  44  and/or color filter  46  that is the primary source of reflection-mode diffraction. For EL display  60  in FIG. 5, it is the edges corresponding to the column electrodes  72 , the emitter layer  68 , and the row electrodes  64  that are the primary sources of reflection-mode diffraction. 
     It should be understood that if additional structural features are included in LCD  40  or EL display  60 , those structural features having visible edges and material boundary where the Fresnel reflectance is not identical on both sides of the boundary can also be sources of diffraction patterns. Similarly, although not shown, for other types of flat panel displays having edges of one or more structural features visible from the viewing direction, and these structural features having a material boundary where the Fresnel reflectance is not identical on both sides of the boundary, will be sources of diffraction patterns. 
     Undesirable diffraction patterns on flat panel displays can be minimized by decreasing the brightness of the diffraction rays, such as diffraction rays  96 ,  106  (FIG.  2 ), and/or by changing the azimuthal orientation of the diffraction rays. The brightness of the diffraction rays are determined by the reflectance differences and lengths of the structural feature edges responsible for the diffraction rays. The azimuths of the diffraction rays are determined by the edge orientation, wherein diffraction rays from predominantly horizontal and vertical edges have their azimuthal orientation in planes perpendicular to these edges. Therefore, flat panel display  30  includes very small or microscopic facets or curves, i.e., textures, to these edges within the flat panel display  30 . 
     The textures transform each diffraction-ray causing edge from a single, long diffraction source into multiple, short, discontinuous diffraction sources. Since the diffraction pattern brightness is partially determined by the edge length, each shorter edge becomes a weaker diffraction source. Moreover, each facet or curve diffracts rays into many different azimuthal planes rather than one diffraction azimuthal plane as in the case of the long straight edge. 
     For example, in the case of LCD  40  in FIG. 4, one or more edges of black matrix  44  and/or color filter  46  include the edge texture. In the case of EL display  60  in FIG. 5, one or more edges of row electrode  64 , column electrode  72 , and/or emitter layer  68  include the edge texture. It should be understood, however, that edge textures can be included on any structural feature within flat panel display  30  to control diffraction effects. 
     In addition to including the edge texture on any structural feature within flat panel display  30 , the edge texture can be applied to any one or more edge of the diffraction causing structural feature. This edge can be horizontal, vertical, diagonal, straight, curved, etc. Moreover, the edge texture may be applied to an entire edge length, a portion of the edge length, on a corner intersecting two edge lengths, a portion of a major side surface of the structural feature, etc. Moreover, a structural feature may include one or more types of edge textures, perhaps a first edge texture along its vertical edges and a second edge texture (different from the first edge texture) along its horizontal edges. Alternatively, adjacent structural features may include different edge textures. 
     FIG. 6 shows a perspective view of a diffraction causing structural feature  110  of the flat panel display  30  including a first major side surface  112 , a second major side surface  114 , and a first, a second, a third, and a fourth edge  116 ,  118 ,  120 ,  122 , respectively. Structural feature  110  includes two of its edge lengths textured, the first and third edges  116 ,  120 . Structural feature  110  also includes a portion  124  of the first major side surface  112  textured. FIG. 7 shows a front view of an alternate diffraction causing structural feature  130  including a first, a second, a third, a fourth, a fifth, a sixth, and a seventh edge  132 ,  134 ,  136 ,  138 ,  140 ,  142 ,  144 , respectively. Structural feature  130  includes an edge texture along the length of the first, sixth, and seventh edges  132 ,  142 ,  144 , respectively. Structural feature  130  also includes an edge texture along a portion of the fourth edge  138 . FIG. 8 shows a front view of a portion of a flat panel display  150  including visible structural features  152 - 162 . As shown in FIG. 8, an edge texture is formed along alternating structural features, such as structural features  152 ,  156  (skipping structural feature  154 ); or along the same edge length of each adjacent structural features  158 ,  160 ,  162 . 
     Many edge textures are possible, each with its own specific diffraction distribution. Diffraction distributions can be tailored to suit the diffraction effect minimization desired for a specific application, e.g., a certain type of flat panel display or certain display applications, by implementing the appropriate edge texture on the problematic structural features of the display. For example, when the edge texture is a smooth curve, the diffraction distribution will be a broad fan-like distribution. When the edge texture is a series of piecewise linear facets, the diffraction distribution will be distributed into a number of discrete planes at different azimuthal orientations. When the edge texture is a random pattern, the diffraction distribution will likewise be a random distribution, rather than the horizontal and/or vertical bands. Shown in FIGS. 9-13 are portions of structural features, each including a distinct edge texture. Profiles of edge textures, viewed from a direction substantially perpendicular to the major surface of the structural feature, include a piecewise linear profile  164 , a fractal profile  166 , a discontinuous curved profile  168 , a random profile  170 , and a continuous curved profile  172 . Although not shown, the edge textures can also include a variety of other shapes, as long as the edge texture results in some decrease of the diffraction effect otherwise caused by a smooth edge of the structural feature. 
     Texture application on diffraction-causing edges of structural features can be accomplished without significantly changing the display&#39;s geometry, processing methods, yield, or percentage of active display area. During the typical course of display fabrication, in which structural features are created by successive deposition, masking, etching, cleaning, growing, etc. process steps similar to integrated circuit fabrication, the process step responsible for the edge geometry of the problematic structural feature can be slightly modified to include the edge texture. For example, in the case of LCD  40  in FIG. 4, black matrix  44  is typically formed by depositing the layer of metal and/or metal oxide over color filter  46 ; photo masking the layer with a mask having a pattern of the desired geometry for black matrix  44  (i.e., a grid-like pattern throughout the viewing surface of LCD  40 ); and then selectively etching this layer based on the mask pattern, achieving the desired geometry for black matrix  44 . In order to include one or more edge textures to the edges of black matrix  44 , the mask used for patterning black matrix  44  above would include a specific pattern of these desired edge texture geometries. Therefore, diffraction effect minimization can be implemented by substituting a different mask during typical display fabrication processes. 
     It should be apparent that there has been provided in accordance with one embodiment of the present invention minimization of diffraction effects in flat panel displays. While the embodiments illustrated in the FIGs. and described above are presently preferred, it should be understood that these embodiments are offered by way of example only. Accordingly, the invention is not limited to a particular embodiment, but extends to alternatives, modifications, and variations that nevertheless fall within the spirit and scope of the appended claims.