Rugged screen is no miser on brightness
Light is fascinating not only in how well it serves as a technological tool, but also in how pretty it can be. For those who entered the field of photonics at least partially as a result of the latter, advances in full-color imaging technology are of special interest.
Light is fascinating not only in how well it serves as a technological tool, but also in how pretty it can be. For those who entered the field of photonics at least partially as a result of the latter, advances in full-color imaging technology are of special interest. Better CCD and CMOS cameras, better projection light sources, and higher-resolution, brighter-hued displays all contribute to the more seamless integration of virtual with physical realities. Deeply in this mode, engineers at Nitto Jushi Kogyo (NJK; Tokyo, Japan) have developed a rugged high-brightness rear-projection screen that is simple to incorporate into its surroundings in the form either of a traditional hanging screen, a shaped surface such as a dome or curved wall, or an integrated structure (see figure).
A conventional high-brightness rear-projection screen is double-lenticular, with layers of molded-plastic optical elements. Its make-up is complex, containing a Fresnel lens to direct the incoming light, a double array of lenses to channel the light into the proper angular distribution, a diffusing plate to soften its characteristics, black stripes to block unwanted light, and an antireflective coating. Although it usually serves its purpose well, a double-lenticular screen can only be fabricated in flat shapes and, when made in sizes of more than 100 in., must be made in seamed arrays. In addition, a lenticular screen sometimes produces Moiré interference effects when a pixelated image is projected onto it.
The screen developed by NJK drops all the lenticular and Fresnel optics, instead incorporating all its function into a single scattering layer. Although diffusing screens are not new ("opal" glass, with its Lambertian characteristics, is one example), the NJK engineers tuned the size of particles suspended in the screen's scattering layer to most closely meet the angular requirements of a rear-projection screen; they also added a dopant to the layer to absorb ambient light and increase contrast.
The right properties produce a sharp, intense image with a broad-enough viewing angle. A balance of properties must be achieved: the micron-order particles must be just the right size, in the right concentration, and residing in a layer of the right thickness to correctly dictate the average number of times a photon is scattered. Small particles are better—but if they are made too small, the resulting wavelength-dependent Rayleigh scattering ruins color uniformity.
The molecular dopant, carbon black, attenuates both the desired transmitted light and the undesired reflected light, but tends to attenuate the latter more because of its longer path length in the absorbing layer. A 0.004% to 0.006% weight concentration of carbon black maximizes the contrast level. An antireflective coating reduces specular reflection.
The screens produced by NJK (and distributed in North America by Astra Products, Baldwin, NY) can be fabricated in sizes up to 300 in. and can be used in full-time outdoor conditions. Because the acrylic substrate is structurally strong, the screens can be made into structures as extreme as the walls of large-scale aquariums.