Patent Publication Number: US-2010118397-A1

Title: Reduced Laser Speckle Projection Screen

Description:
BACKGROUND  
     Laser light beams are finding uses in a wide variety of applications. For example, scanning laser light beams may be used in display applications such as mobile microprojectors, automotive head-up displays, and head-worn displays. The laser light is typically projected on a display surface. Depending on the type of display surface, the reflected laser light may include an observable phenomenon commonly referred to as “speckle.” 
     Speckle is caused by interference patterns in the reflected laser light. Laser light typically has high spatial and temporal coherence. After being scattered off a display surface, the reflected light displays an interference pattern that appears as bright spots (speckle). 
     Speckle can be reduced by adding diffusive material to the display surface. Diffusive material on the display surface causes light to be scattered at much greater angles, thereby reducing the deterministic interference pattern and the resulting speckle. Although diffusive material on a display surface can be useful for speckle reduction, the greater scattering angles also cause the reflected laser spot to appear much larger. This phenomenon is referred to as “blooming” and can result in a reduction in image resolution. 
     Accordingly, the use of diffusive display surfaces for speckle reduction typically involves a trade-off between speckle reduction and image resolution. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         FIG. 1  shows a laser projector projecting light on a reduced laser speckle projection screen in accordance with various embodiments of the present invention; 
         FIG. 2  shows a cross-section of a reduced laser speckle projection screen with light scattering media; 
         FIG. 3  shows a cross-section of a reduced laser speckle projection screen with light scattering media with a dimpled surface; 
         FIG. 4  shows a cross-section of a reduced laser speckle projection screen with light scattering media and a laminated diffuser; 
         FIG. 5  shows a cross-section of a reduced laser speckle projection screen with a wire mesh, light scattering media, and a laminated diffuser; 
         FIG. 6  shows a cross-section of a reduced laser speckle projection screen having parabolic cells partially filled with light scattering media; 
         FIG. 7  shows a cross-section of a reduced laser speckle projection screen having collimated cells filled with light scattering media; 
         FIG. 8  shows a cross-section of a transmissive reduced laser speckle projection screen having collimated holes filled with light scattering media; and 
         FIG. 9  shows various projection screen cell layouts in accordance with various embodiments of the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     In the following detailed description, reference is made to the accompanying drawings that show, by way of illustration, specific embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. It is to be understood that the various embodiments of the invention, although different, are not necessarily mutually exclusive. For example, a particular feature, structure, or characteristic described herein in connection with one embodiment may be implemented within other embodiments without departing from the spirit and scope of the invention. In addition, it is to be understood that the location or arrangement of individual elements within each disclosed embodiment may be modified without departing from the scope of the invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims, appropriately interpreted, along with the full range of equivalents to which the claims are entitled. In the drawings, like numerals refer to the same or similar functionality throughout the several views. 
       FIG. 1  shows a laser projector projecting light on a reduced laser speckle projection screen in accordance with various embodiments of the present invention. Laser projector  100  projects laser light  102  on surface  112  of screen  110 . Laser projector  100  may be any type of laser projector, including for example, a scanned-beam laser projector, a scanned one-dimensional (ID) array laser projector, or an imaged two-dimensional (2D) panel laser projector. Any type of laser projection system can be used to project laser light on screen  110  without departing from the scope of the present invention. 
     Surface  112  includes cavities or “cells” that include a light scattering media. The light scattering media causes the laser light to scatter randomly within the cells and to emerge with many different phase fronts at various angles. As the phase fronts combine, apparent speckle is reduced. Each of the cells includes cell walls that keep light from propagating to adjacent cells, and this reduces blooming. The cell walls may be reflective or absorptive. Further, the cells may be only partially or completely filled with the light scattering media. 
     The cells may be any shape, and may be arranged in any pattern. For example, when viewed from above the surface, the cells may be hexagonal, circular, triangular, or any other shape. When viewed in cross-section, the cells may be formed as cups, parabaloids, hemispheres, cylinders, or any other shape. Various embodiments having different cell shapes are described further below. 
       FIG. 2  shows a cross-section of a reduced laser speckle projection screen with light scattering media. Reduced laser speckle projection screen  210  is shown having laser light  102  projected thereon. Projection screen  210  includes cells  212  formed on one surface. Cells  212  have non-zero volume, and are at least partially filled with light scattering media  214 . 
     Laser light entering light scattering media  214  is spatially scattered to create different randomly oriented and polarized phase fronts such that no clean cut wavefront is associated with the output light from any given cell. This is shown in more detail in view  220 , where the light rays are shown being randomly scattered in cell  226 . 
     Cells  212  are separated by cell walls to keep light from propagating to adjacent cells. In some embodiments the cell walls may be black absorbing (less efficient) or highly reflective (most efficient). Also, cells  212  may be any size and shape. For example, cells  212  in  FIG. 2  are formed as indentations, or “cups”, in one surface of screen  210 . These cups may be arranged in rows and columns to form a regular array, or may be arranged in a random or pseudo-random pattern. Reduction of apparent speckle may be achieved when cell structure profile, cell layout, sag depth, and sparseness and density of light scattering media are adjusted by design to allow light to scatter into and illuminate all portions of a cell&#39;s volume before exiting toward a viewer. 
     Light scattering media  214  may be any type of media that scatters light to reduce laser speckle. For example, in some embodiments, silicone based materials are used as light scattering media  214 . White room temperature vulcanizing (RTV) silicone rubber has been found to function well as a light scattering media due to its fineness and homogeneity. Also for example, in some embodiments, fine grain particulate having a particle size significantly small compared to cell size may be used. The particulate may be mixed with a transparent epoxy carrier to form light scattering media  214 . Also for example, colloidal suspensions and nanoparticles may also make good candidates for the light scattering media. In some embodiments, light scattering media  214  includes some amount of attenuating dye for trading brightness with some degree of apparent ambient rejection. 
     The size of cells  212  may be chosen based on the size of the laser spot size expected to be incident upon screen  210 . For example, in some embodiments, the cell diameter is chosen to be smaller than the laser spot size. This case is illustrated in detail view  220 . The laser spot distribution is shown at  224 . This laser spot illuminates cell  226  and to a lesser extent, the adjacent cells. The light emerging from cell  226  occupies cone  222 . In this example, the laser spot is able to span a few cells at once. Light exiting each cell will bloom to a full cell size, similar to that shown at  220 , effectively causing the spot size to grow by a factor of three. In some embodiments, the cell diameter is much smaller than the laser spot. In these embodiments, the blooming is reduced because each adjacent cell is smaller. In the limit, with an infinitely small cell size, blooming would be eliminated, although infinitely small cell size is not practical. In a practical system, the cells may be sized so that a few to tens or hundreds of cells can be illuminated by the laser spot at once. 
     By creating an array of cells with volumetric light scattering qualities across a surface of the screen, speckle reduction is achieved while maintaining apparent resolution of the projection display on the screen as seen by the viewer. For simplicity of illustration, the profile of projection screen  210  is shown having just a few cells. In practice, projection screens may have many millions of cells. Further, in some alternate embodiments, “cells” may be formed using reflective faceted granules on the order of cell size and slightly smaller. These granules may be mixed with the light scattering media to achieve a similar effect of limiting blooming without a fixed array structure, depending on mix density and faceted reflective grain shape. 
       FIG. 3  shows a cross-section of a reduced laser speckle projection screen with light scattering media with a dimpled surface. In embodiments represented by  FIG. 3 , light scattering media  214  includes a dimpled surface  316 . The dimpled surface may be formed during manufacture when the light scattering media is deposited. 
     If the surface  316  is left flat and specular (e.g., not dimpled), Fresnel reflections (a few percent of input beam light) off the surface results in a hot-spot appearing in the display field of view (FOV) to the viewer, depending on the viewer&#39;s position. The pixel location within the FOV is defined by the scanned-beam angular position which exhibits an angle-of-incidence (AOI) such that that beams&#39; light reflects at an angle of reflection so as to enter the viewer&#39;s eye. Anti-reflective (AR) coating may help diminish the effect, but may still allow some percentage of light reflection. By molding a dimpled surface onto the light scattering media with no extent beyond the cell extent, even this Fresnel reflected light can be scattered so as to mitigate hot-spot reflections. Further, this molded dimpled-relief surface can be AR-coated, as needed. 
     The dimpled surface  316  further reduces speckle that would otherwise result from specular reflection off a smooth surface. Although much of the incident laser light enters the light scattering media as described above with reference to  FIG. 1 , some of the light will also reflect off the surface of the light scattering media. By creating a non-specular (dimpled) surface, speckle is further reduced. 
       FIG. 4  shows a cross-section of a reduced laser speckle projection screen with light scattering media and a laminated diffuser. Laminated diffuser sheet  416  is included to encase the light scattering media  214 . In embodiments without a laminated diffuser sheet  416 , some light scattering media may protrude beyond the cell structures that are designed to limit blooming. Light scattering will then occur outside the cells, and blooming will increase. 
     Lamination can help achieve consistent fill of the light scattering media; however, an optically smooth sheet will cause specular reflection of the projected light causing a ‘hot spot’ to appear at some location on the screen due to position of the viewer and the position of the projector. AR-coating can diminish this specular reflection of the projected light, but use of a random surface relief diffuser can help scatter this initial reflected light. Further, in some embodiments, the diffuser outer surface can also be AR-coated. 
     Laminated diffuser sheet  416  is shown having a specular flat surface and a randomly dimpled surface. The specular flat back surface is in contact with light scattering media  214 , and the randomly dimpled surface is on the side opposite the light scattering media. The randomly dimpled surface reduces specular reflection, and further reduces speckle. In some embodiments, the randomly dimpled surface is also AR-coated. 
       FIG. 5  shows a cross-section of a reduced laser speckle projection screen with a wire mesh, light scattering media, and a laminated diffuser. Projection screen  500  includes backing  510 , wire mesh  530 , light scattering media  214 , and laminated diffuser sheet  416 . The light scattering media  214  envelops the wire mesh and is encased by the laminated diffuser sheet. Cells are created by the holes in wire mesh  530 . 
     In some embodiments, wire mesh  530  is a commonly available sieve mesh. In these embodiments, wire mesh  530  includes a non-zero displacement on the axis orthogonal to backing  510  (the z axis). The wire mesh displacement on the z-axis causes some degradation of performance due to loss in ideal containment of blooming, but such a screen has been fabricated and exhibits improved sharpness and reduced speckle as compared with paper. 
     In other embodiments, wire mesh  530  is formed by perforating a metal sheet. In these embodiments, wire mesh  530  does not include a displacement in the z-axis, and blooming is more effectively controlled. The perforated mesh may provide optical advantages over a sieve mesh, but may also be more difficult to fabricate in extremely small cell sizes. 
     In some embodiments, projection screen  500  is flexible enough to be rolled up. In these embodiments, the wire mesh  530  and light scattering media  214  are encased between an extremely thin reflective backing  510  and laminated diffuser sheet  416 . 
       FIG. 6  shows a cross-section of a reduced laser speckle projection screen having parabolic cells partially filled with light scattering media. Projection screen  610  includes cells  612  having parabolic cross-sections. In some embodiments, cells  612  have parabolic cross-sections regardless of the cross-section taken. In other embodiments, cells  612  have a parabolic cross-section on one axis, and a non-parabolic cross-section on a different axis. 
     Cells  612  are partially filled with light scattering media  214 . The remaining volume of cells  612  is filled with transparent filler media  614 , and then diffuser sheet  416  is laminated over the cells and the media. When laser light enters at low angles, the light spreads all around in diffuser, bounces off the cell walls and leaves at high angles of scatter. This will cause angular spread on the exit cone. Higher exit angles may be acceptable based on the application. 
     In some embodiments, cells  612  are completely filled with light scattering media  214 , and no transparent fill media is used. In other embodiments, the transparent fill media  614  includes a randomly dimpled surface, and diffuser sheet  416  is omitted. In still further embodiments, cells  612  are partially filled with light scattering media  214 , and transparent fill media  614  and diffuser sheet  416  are omitted. Any of the embodiments described herein may have cells partially filled with media or completely filled with media. Further, any of the embodiments described herein may include a laminated diffuser sheet or may omit a laminated diffuser sheet. Still further, any of the embodiments described herein may include a non-specular or dimpled surface, either formed in media or on a diffuser sheet. 
       FIG. 7  shows a cross-section of a reduced laser speckle projection screen having collimated cells filled with light scattering media. Screen  700  is formed by backing  510 , cell walls  712 , and light scattering media  214 . In some embodiments, the cells are cylindrical, however, this is not a limitation of the present invention. For example, the cells may also be hexagonal, square, rectangular, linear (for effect in ID), etc. Further, the cell layout grid may be hexagonal, square, rectangular, linear, etc. 
     Backing  510  and cell walls  712  may be made of any suitable material. For example, a molded polymer material, plastic, glass, or a fibrous paper material may be used. Screen  700  may be manufactured on a rolled process in which a grooved drum forms cell walls  712  through extrusion. The various embodiments of the present invention are not limited by the method of fabrication. 
     The cells of screen  700  may be completely filled or partially filled with light scattering media  214 . In some embodiments, ambient light rejection is increased by partially filling non-reflective cells. In these embodiments, ambient light arriving at high angles is absorbed prior to entering the light scattering media, whereas projected laser light arriving at lower angles enters the light scattering media and is scattered in accordance with the descriptions provided above. 
       FIG. 8  shows a cross-section of a transmissive reduced laser speckle projection screen having collimated holes filled with light scattering media. Screen  800  corresponds to screen  700  ( FIG. 7 ) without backing  510 . Without the backing, the cells of screen  700  become holes in screen  800 . 
     The holes in screen  800  are partially or completely filled with light scattering media  214 . Screen  800  is a transmissive version of screen  700 , which is reflective. Any of the screen embodiments described herein may be transmissive screens or reflective screens. For example, transmissive screens may include dimpled surfaces, either on media or diffusers. Further, transmissive screens may include diffusers on one or two sides of the screens. 
       FIG. 9  shows various projection screen cell layouts in accordance with various embodiments of the present invention. The cell layouts shown in  FIG. 9  correspond to cell layout patterns on a screen surface such as screen surface  112  ( FIG. 1 ). 
     Cells may be laid out in a rectangular grid  910 , a square grid  950 , or a triangular grid  960 . Cells may also be laid out in a hexagonal pattern  920  or as circles  930 . Cells may also be one dimensional as shown at  940 . The layout patterns shown in  FIG. 9  are examples, and the various embodiments of the invention are not limited to the patterns shown. For example in some embodiments, cells are laid out in a random pattern, and in other embodiments, cells are laid out in a pseudo-random pattern. 
     Any of the possible layout patterns may be combined with any of the cell cross-sections. For example, cells laid out in accordance with rectangular grid  910  may be shaped as cups, hemispheres, parabolas, or any other shape. Also for example, any of the possible cell layouts may be used with reflective screens or transmissive screens. 
     Linear cell arrays ( 940 ) may be formed by linear extension of any of the disclosed cell profiles, including rows of wires or micro-rods. For example, cells may be formed by walls in only one dimension such that each cell forms a strip either horizontally or vertically on the screen. Blooming still occurs in one dimension, but this may be outweighed by manufacturing benefits. For example, the horizontal direction may be left free to bloom, but an inexpensive rolled manufacturing process could be used to build the screen. 
     In some embodiments, a linear array of triangular facets (similar to brightness enhancement film) may also be used. Array of hollow corner-cube cavities may also be used as a cell structure, possibly in combination with triangular cells with alternating orientation. 
     Although the present invention has been described in conjunction with certain embodiments, it is to be understood that modifications and variations may be resorted to without departing from the scope of the invention as those skilled in the art readily understand. Such modifications and variations are considered to be within the scope of the invention and the appended claims.