Traditional light scattering elements such as those used in projection screens have progressed over the years with the current designs still suffering from design tradeoffs. Some of the key merits for describing a projection screen include, gain, angle of view, resolution, transmission, screen contrast, speckle contrast, and cost. The ideal projection screen for typical applications should have the following merits:                tailored viewing angles in the horizontal and vertical directions, so that the image may be seen from many directions;        high gain, to increase the image brightness, improve contrast, and reduce lamp requirements;        high resolution, such that high resolution images may be projected and displayed with full detail on the screen;        high optical transmission (high optical efficiency) for the light incident on the screen, such that the projection system has reduced lamp requirements or a brighter display;        high image contrast, where light from within the projection system or the ambient surroundings has a low or negligible impact on the perceived image contrast on the screen;        low speckle contrast, allowing the use of smaller filament thus higher coherence length light sources to achieve higher optical efficiency in the projection system; and        low manufacturing cost.        
The underlying mechanism by which projection screens work relies on a material which can scatter the projected image into the viewer's eye to produce what is called a real image. Each pixel on the screen effectively acts as a point source of light, and all together they produce the images we see. A surface, such as a projection screen, that scatters the light in all directions so the intensity follows the cosine of the angle and the screen brightness (luminance) is constant over the range of angles, is termed a “Lambertian Surface.” Thus, the brightness of a Lambertian screen appears constant, independent of the viewing angle. Projection screens are typically designed to scatter light more in the commonly viewed directions in order to increase the screen luminance in those directions to reduce constraints on the light source. In typical viewing situations, it is not necessary to project the image to the ceiling nor to the floor, since it is unlikely that a viewer will be in those positions. In principle, the brightness of an image on a projection screen from any particular viewpoint can be increased by having the screen preferentially direct light towards those viewing regions rather than scattering light equally in all directions. Therefore, if one could take all the light that would otherwise be wasted in non-viewing regions and re-direct it to the viewing region, i.e., in front of the screen at eye level, this would make the viewing region have a much higher luminance since the extra light is directed there at the expense of the non-viewing region. The increase in luminance relative to that of a perfect Lambertian surface is termed “gain”. Thus, a 2× gain screen has a luminance that is twice that of a Lambertian screen for a given angle (typically measured at an angle normal to the surface). For a non-Lambertian projection screen, the extent to which it can be viewed in the horizontal and vertical planes is described in terms of angle of view (AOV) which is typically measured as the full angular width at half the maximum of the luminance curve (FWHM).
Early projection screens were made using ground glass diffusers. Later, technology evolved to the use of particle-containing sheet polymers to diffuse the light uniformly, resulting in—potentially—high gain screens, where the AOV could be as large as desired and controllable by adding more particles to the polymer. These screens suffered a major drawback due to the loss in contrast due to the backscatter of ambient light. Over the years, several methods for increasing the contrast have been developed, including adding tint to a medium between or within the diffuser and the viewer. This increases the contrast, since the ambient light that is scattered back (i.e., “backscatter”) toward the viewer has passed through this tinted region twice and is significantly reduced in intensity relative to the light from the projection engine that has only passed through the tinted region once. Other methods for increasing display contrast in ambient light settings include focusing light by using lenticular lenses, beads, or reflecting surfaces (e.g., total internal reflection screens) through transparent apertures in-between light absorbing regions. By focusing the light through small holes, lines, or apertures, the fill factor for the black apertures can be very high, thus absorbing a significant amount of ambient light while passing the light from the projection engine. (These and other contrast enhancement methods are explained in “Projection Displays,” Stupp and Brennesholtz, John Wiley & Sons, 1999, pp. 153-175, incorporated herein by reference.) However, with the industry moving to higher resolution HDTV displays and higher resolution monitors, the requirement for the feature sizes of the reflective or refractive methods such as beaded screens or lenticular screens has become more difficult and more costly. With lenticular lens pitches now at less than 100 μm, the cost and complexity of manufacturing are becoming larger issues. With higher resolutions bead screens, the size of the beads needed has also been reduced and the cost and size tolerance required has also increased.
The demand for higher resolution displays has also increased the visibility of speckle. “Speckle” is the optical interference effect resulting from the interference of light rays emerging from a scattering element—such as a screen—that are mutually coherent. The viewers' eye integrates this optical effect and sees a visible pattern. Speckle is typically measured by looking at the variation in intensity across a uniformly illuminated screen. “Speckle contrast” is defined as the ratio of the standard deviation of the intensity to the average intensity. A projection system with “high” speckle contrast means that the speckle pattern is more visible than a system with “low” speckle contrast.
Historically, speckle has not been a problem in low resolution displays. With low resolution displays, if a high coherence length source is used, one can use thicker scattering elements or two scattering regions spaced apart by a non-scattering region in order to reduce the speckle without reducing the resolution to unacceptable levels. More recently, manufacturers have begun using small filament (or arc) light sources to improve optical efficiency and the resolutions of the displays have increased significantly. However, small arc light sources increase the coherence length and when these are combined with many high resolution projection sources, the resulting image quality suffers from speckle (or scintillation). It is expected that in the future, the manufacturers will continue to reduce the lamp filament size to increase the optical efficiency and that the projection display resolution will continue to increase, thus speckle will become more of a problem. With laser-based systems, speckle contrast is even higher than that with incoherent or partially coherent light sources. Projection screen materials using a single layer of a scattering element have a tradeoff of speckle versus resolution. A thick single layer reduces the appearance of speckle, but lowers the resolution of the screen. A single thin scattering layer has a high resolution, but speckle is more visible. Two scattering regions spaced apart can reduce the speckle contrast, but also typically decreases the resolution. The effect of the speckle and resolution tradeoff is discussed by Stupp et al., and also in Goldenberg et al., “Rear Projection Screens for Light Valve Projection Systems,” SPIE Vol. 3013, 1997, pp. 49-59, incorporated herein by reference. In “Rear Projection Screens for Light Valve Projection Systems,” Goldenberg, et. al., SPIE Proceedings, Vol. 3013, 1997 discusses the use of a thick bulk diffuser or scattering elements spaced apart, the conclusion is that using these techniques will reduce the speckle contrast while also reducing the resolution. As a result, manufacturers have simply added spherical particles or surface relief to one of the elements, such as the Fresnel lens or lenticular lens, to add a small amount of diffusion to reduce speckle. This causes a significant decrease in resolution. When diffusive qualities are added to the optical system before contrast enhancing spatial filtering apertures (e.g., a transparent stripe in a black stripe lenticular screen), less of the light is focused through the aperture by the lenses. This reduces the optical efficiency of the system. For example, in a Fresnel lens, lenticular lens, or transparent-black stripe region diffuser screen configuration, addition of particles to the Fresnel or lenticular lens causes less of the light to be focused through the apertures, because the lenticular lens can not focus many of these oblique rays through the clear apertures. Lenticular lenses are typically designed to work with substantially collimated light from a Fresnel lens. Adding the spherical particles to the Fresnel lens, lenticular lens, or other region to reduce speckle in high resolution displays is a tradeoff relative to the reduction in optical efficiency and the added high cost of the particles.
When symmetric scattering particles are added in a region on the viewer side of the contrast enhancing features, the backscatter is increased, usually resulting in reduced contrast.
Screens that use more than one scattering layer typically use optical adhesives to combine the screen components; or, spherical light scattering particles are added to Fresnel lenses, lenticular lenses, substrates or other elements. This often introduces spurious interfacial reflections at the element interfaces, that reduces the contrast of the screen and adds to the production cost. When the interfaces are slanted or curved, such as the case with Fresnel lenses or lenticular lenses, respectively, the spurious reflections are more significant, and reduces the optical efficiency, and, possibly, reduce image contrast. For instance, a small amount of spherical particles added to a Fresnel lens to reduce speckle contrast can cause a significant amount of the scattered light to totally internal reflect within the Fresnel lens because of the large slant angles on the features of the Fresnel lens. This reduces the speckle contrast at the expense of reducing image contrast and reducing optical efficiency by lowering the screen transmission.
Many commercial rear projection televisions have reduced horizontal and vertical viewing angles in order to reduce intensity requirements of the projection lamp. For example, some commercial rear projection televisions have wide horizontal FWHM (full-width half maximum) of 50 degrees and a more narrow vertical FWHM of 20 degrees. This allows for longer lasting or lower intensity lamps to be used, and also reduces the heat and electrical power consumption. These viewing angles are usually achieved as a benefit of using a lenticular lens with transparent-black striped region to increase the contrast. The lenticular lenses spread the light in the horizontal direction after focusing the light through transparent stripes in between opaque black stripes and onto a weakly scattering symmetric diffuser for the light scattering element. A weak diffuser (light scattering diffuser with small FWHM) typically has a high gain, but can also introduce speckle. The combination of the lenticular lens and the diffuser dictate the viewing angles (and resulting gain), and this restricts the design of the lenticular-black stripe system. Similar restrictions also occur in beaded and TIR-based systems such that the angles of view can not be adjusted independently of other elements in the system. Other systems with asymmetric viewing angles typically have high costs, reduced image contrast or high speckle contrast when applied to high resolution projection displays.