Abstract:
The projection screen assembly minimizes speckle while preserving high ambient viewability. The screen assembly, having a light-source side and a viewing side, includes a screen layer, a polarizing contrast filter, and a speckle contrast reducing layer. The screen layer disperses light passing through the screen assembly from the light-source side. The polarizing contrast filter is disposed on the viewing side of the screen layer and reduces ambient glare. The speckle contrast reducing layer is disposed on the light-source side of the screen layer and reduces speckle formed in projected images on the viewing side of the screen assembly.

Description:
FIELD OF THE INVENTION 
   The present invention relates generally to projection screens. More particularly, the present invention relates to projection screens with reduced speckle. 
   BACKGROUND OF THE INVENTION 
   Rear projection screens are increasingly being used for projection displays in televisions, computer monitors, and other types of displays, for example. In general, rear projection display screens need to have high transmittance while minimizing ambient reflections to ensure adequate brightness and contrast. These characteristics are particularly necessary for displays viewed in high ambients, such as in avionics applications. 
   Beaded screens provide high transmittance with minimal ambient reflections. Beaded screen suffer, however, from a variety of visual artifacts, including sparkle or speckle. Such defects can occur as a result of local bead defects, or when light transmitted by a particular portion of the screen is mutually coherent with light transmitted by a neighboring portion of the screen. The mutually coherent light from neighboring portions of the screen interferes as it propagates away from the screen. A viewer&#39;s eye integrates such interference over the whole screen, with the result being that the viewer sees a number of bright spots across the screen. Both causes are referred to herein as speckle. The speckle decreases the viewability of the image projected from the screen. 
   A measure of speckle is the speckle contrast, which is defined as the ratio of the standard deviation of the pixel brightness over the average pixel brightness. If the speckle contrast of a screen is above a certain level, the speckle in the viewed image can be significantly distracting to the viewer. Accordingly, it is important to reduce the speckle contrast to a level acceptable to the viewer while substantially preserving other screen characteristics. 
   When screens are designed to enhance one or more particular characteristic, it is often found that other characteristics are degraded, or the cost of the screen assembly, or its complexity, is increased. For example, the introduction a component to the screen for reducing glare may adversely affect one of the other screen characteristics, such as gain, resolution or speckle. Ideally, measures taken to reduce speckle should affect the other screen characteristics as little as possible. 
   One proposed solution to the problem of speckle is described in U.S. Pat. No. 6,466,368 entitled “Rear Projection Screen with Reduced Speckle.” However, this proposed solution has disadvantages. For example, it describes use of a diffuser layer on the front of the screen from the viewer side. A diffuser layer on the front of the screen affects the high ambient contrast of the display because backscatter of incident ambient light from the diffuser layer significantly increases the reflectivity of the screen assembly. 
   Thus, there is a need to reduce speckle in projection screens while substantially maintaining the other screen characteristics. Further, there is a need to reduce speckle without adversely affecting contrast. Even further, there is a need for projection screens configured for the unique aspects of high ambients experienced in avionics applications. 
   SUMMARY OF THE INVENTION 
   The present invention minimizes speckle in projection screens while preserving excellent high ambient viewability. In particular, one exemplary embodiment relates to a screen assembly having a light-source side and a viewing side. The screen assembly includes a screen layer, a polarizing contrast filter, and a speckle contrast reducing layer. The screen layer disperses light passing through the screen assembly from the light-source side. The polarizing contrast filter is disposed on the viewing side of the screen layer and reduces ambient glare. The speckle contrast reducing layer is disposed on the light-source side of the screen layer and reduces speckle formed in projected images on the viewing side of the screen assembly. 
   Another exemplary embodiment relates to a projection screen including a substrate, a polarizing contrast filter, and a diffuser layer. The substrate has a plurality of beads on a light-source side. The polarizing contrast filter is located on a viewing side of the substrate. The diffuser layer is located proximate the plurality of beads on the light-source side of the substrate. 
   Still another exemplary embodiment relates to a rear projection screen assembly, having a light-source side and a image-viewing side. The screen assembly includes a means for dispersing light passing through the screen assembly from the light-source side, a means for filtering and polarizing incident ambient light disposed on the image-viewing side and reducing ambient glare, and a means for reducing speckle contrast disposed on the light-source side of the means for dispersing light and reducing speckle formed in projected images on the image-viewing side of the screen assembly. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The exemplary embodiments will be described with reference to the accompanying drawings, wherein like numerals denote like elements; and 
       FIG. 1  is a diagram depicting a cross-sectional side view of a projection screen assembly with speckle resulting from light interference patterns on the screen surface; 
       FIG. 2  is a diagram depicting a cross-sectional side view of a beaded screen assembly projecting an image with high efficiency according to conventional techniques; 
       FIG. 3  is a diagram depicting a cross-sectional side view of a beaded screen assembly in accordance with an exemplary embodiment; and 
       FIG. 4  is a diagram depicting a cross-sectional side view of the beaded screen assembly of  FIG. 3  having a quarter wave plate (QWP) in accordance with another exemplary embodiment. 
   

   DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     FIG. 1  illustrates a projection screen assembly  10  including a screen layer  12  that disperses light. The screen layer  12  can be a bulk diffuser layer or a refractively dispersing layer (e.g., a beaded layer) supported on a substrate layer  14 . The substrate layer  14  can be made of glass and can have a matte surface or antireflective-coated surface  16  located on the surface opposite the screen layer to reduce glare. 
   Light passing a region  22  in the screen layer  12  diverges and overlaps with light passing a region  24  in the screen layer  12 . An interference pattern results in an overlapping region  28 . The matte surface  16  on the substrate layer  14  acts as a screen to display the interference pattern. Points of constructive interference in the overlapping region  28  appear brighter on the matte surface  16  and points of destructive interference in the overlapping region  28  appear darker on the matter surface  16 , resulting in the appearance of speckle to the viewer. 
   Speckle also arises using other types of screen layers, such as bulk diffusers and lenticular dispersing layers. Speckle is often quantified by calculating the speckle contrast, defined as the standard deviation of the brightness measured across the screen divided by the average screen brightness. Thus, a given deviation in screen brightness measured in absolute terms produces a higher speckle contrast when the average screen brightness is low. 
   Speckle is an increasing problem for display screens because as light projector devices become smaller, speckle is more of a concern. Light coming from a projection lens with a finite pupil diameter is partially coherent, with the coherence length being dependent on the diameter of the projection lens. Everything else being equal, the smaller the lens diameter, the longer the coherence length. Accordingly, the coherence length of the light reaching a screen in an advanced display is longer, increasing the appearance of speckle. 
     FIG. 2  illustrates a projection screen assembly  30  having a beaded film  32 , a black resin  34 , a glass substrate  36 , a contrast filter  38 , and a lens  40 . Beaded film  32  includes pressed glass micro-beads located on the black resin  34  which is laminated onto the glass substrate  36 . The micro-beads of the beaded film  32  gather light from the lens  40  and channel it through the beads to exit at the bead-substrate junction. Ambient light is largely absorbed by the black resin  34 , providing reflectivity of less than 1.0%, for example. Beaded screens are particularly efficient due to high transmittance of glass beads. Advantageously, this combination of high transmittance and low reflectivity ensures excellent sunlight readability. 
   In certain applications (e.g., avionics), further reduction of screen reflectance is needed. Thus, the contrast filter  38  is added to the front of the screen. The contrast filter  38  is typically an absorptive element. Accordingly, display luminance is reduced by Ta (the transmittance of the absorptive contrast filer), but reflectance is reduced to a greater extent (by a factor of Ta 2 ), for a net gain in high ambient contrast. The lens  40 , which can be a Fresnel lens, can be placed behind the viewing screen, with the purpose of collimating the light from the projection lens and ensuring good luminance uniformity. 
   Beaded screens exhibit viewing artifacts that can be traced to variables in the screen manufacturing process, including cracked and missing beads that permit collimated light to reach the observer without being scattered into the defined viewing cone by the screen. Since different defects channel this light in different directions, the observer can see a sort of sparkle which moves at his position with respect to the screen changes. Graininess is visible on these screens, due to non-uniformities in bead packing density and depth of bead penetration into the black resin  34 . 
   In applications where a lens is located proximate screen layer  12 , the collimated light from the lens impinges on the beads and is refracted to pass through the front of the beads, which are not covered by the black resin  34 . If the beads are round and clear, the light is spread into the viewing cone, but if a bead is missing or cracked then collimated light incident on that bead is not scattered, but emits in a particular direction, with high intensity. Such points appear as bright spots, and different spots are apparent to the viewer in different positions. 
     FIG. 3  illustrates a exemplary projection screen assembly  50  having a beaded film  52 , a black resin  54 , a substrate  56 , a polarizing contrast filter  58 , a lens  60 , and a diffuser  62 . The diffuser  62  can be a light diffuser, such as a hologram, that improves artifacts in the screen assembly  50 . The diffuser  62  provides a degree of scatter to the collimated light exiting the lens  60 , sufficient to mask the viewing artifacts but not sufficient to noticeably reduce system resolution. 
   The diffuser  62  can result in a reduction of screen transmittance (lowering brightness) and an increase in ambient reflectance (lowering contrast). The polarizing contrast filter  58  can be a linear polarizer that increases transmittance and reduces reflectance. Accordingly, deficiencies introduced as a result of the diffuser  62  can be overcome. 
   Assuming that the polarizing contrast filter  58  has a transmittance Tp, with respect to polarized light, its transmittance with respect to unpolarized light is essentially Tp/2. If the projection system uses LCD microdisplay imagers, then light from the projection optics is polarized. Since the beaded screen is highly polarization-preserving, that means that the projected image is well-polarized when it enters the contrast filter. In terms of luminance, if Tp is the same is Ta (the transmittance of the absorptive contrast filter  38  in  FIG. 2 ), the effects of the two filters are equivalent. In terms of ambient reflections, however, the reduction factor for the absorptive filter  38  ( FIG. 2 ) is Ta 2 , while that for the polarizing contrast filter  58  ( FIG. 3 ) is (Tp/2) 2 , since the ambient light is unpolarized. 
   For a typical avionics display, Ta (=Tp) is approximately 70% (0.7). In this example, the reduction in ambient light reflected from the display when using the polarizing contrast filter is 1/(0.7/2) 2 , or more than 8 times the reduction due to the absorptive filter. This improvement in reflectance more than compensates for any negative effects of the diffuser, and permits a high contrast, high brightness projector with reduced speckle. 
     FIG. 4  illustrates the exemplary projection screen assembly  50  described with reference to  FIG. 3  with the addition of a quarter wave plate (QWP)  66  located between the linear polarizer  58  and the beaded screen  52 . The QWP  66  has the effect of removing any components of the reflected ambient that were preferentially polarized. It does not interfere with filter transmittance, nor does it affect the ability of the linear polarizer  58  to attenuate the non-polarizing components of the reflected ambient. The addition of the QWP  66  effectively results in the replacement of the linear polarizer in the polarizing contrast filter  58  with a circular polarizer. Advantageously, the polarizing contrast filter  58  and the QWP  66  provide improved contrast with or without the diffuser  62  described with reference to  FIG. 3 . 
   The diffuser  62  described with reference to  FIGS. 3 and 4  maintains the black appearance of the screen assembly  50 . Accordingly, the contrast in the screen assembly  50  is preserved. Diffuser layers located on the front (viewer side) of the screen reduce ambient contrast because incident ambient light backscatters from the diffuser layer. 
   Avionics applications generally are subject to direct sunlight, requiring projection screens with lower reflectivity to achieve desired contrast. As such, the polarizing contrast filter  58  described with reference to  FIGS. 3 and 4  is particularly advantageous in avionics applications. Replacing a conventional absorptive contrast enhancement filter by a polarizing filter offers significant increases in performance (both luminous efficiency and contrast). 
   It is understood that although the detailed drawings and specific examples describe exemplary embodiments of projection screens with reduced sparkle, they are for purposes of illustration only. The exemplary embodiments are not limited to the precise details and descriptions described herein. For example, although particular lenses, materials, and structures are described, other lenses, materials, and structures could be utilized according to the principles of the present invention. Various modifications may be made and the details disclosed without departing from the spirit of the invention as defined in the following claims.