Directional Diffusers
Much of the energy projected onto a diffuse surface is scattered at high angles as it is either reflected or transmitted. A volume type diffuser also depolarizes light as it is scattered. The gain of a projection screen is the ratio of energy reflected into a specific angular zone to the energy that would be reflected from a perfect diffuser. Small glass beads on a screen will return projected light to its source, yielding a high gain in an angular zone around the source. Other shapes of refracting and reflecting material can be molded into screens to produce high gains in retroreflecting geometries and a few anamorphic or single plane scattering surfaces that may be made of lenticular optics. The available directions for high gain and the angular shape of the gain zone are limited by refractive optics but become nearly arbitrary for diffractive optics. Dispersion in diffractive optics prevent them from being used over very broad bandwidths but if the application is insensitive to this dispersion then diffuse energy may be directed and concentrated in some very unusual ways using diffractive screens.
The simplest form of directional diffuser is made by interfering a plane or diverging wave with a diffuse wave in either a reflection or transmission format. The resulting hologram will reconstruct the diffuse source when illuminated at the original angle and wavelength. If the same hologram is used as a projection screen, the information being projected onto it will only be visible when the diffuse source is visible. All other viewing positions and illumination geometries will see a diminished projection or nothing at all. The position and size of the original diffuse source will determine the viewing zone and the gain of the screen. A small diffuse source recorded a meter away from the holographic plate will form a very high gain screen that is visible only when the viewer is in the angular zone subtended by the small source at 1 meter. The same screen may be made to reflect only a narrow band of wavelengths centered around a particular phosphor or laser line, making it practically invisible for all other directions and colors. This sort of screen can be useful in conserving projected CRT or laser light in a cockpit or simulator. The angles chosen for construction and reconstruction are somewhat arbitrary so that specular reflection from the projector can be avoided in the viewing zone. We have constructed green screens of this variety with gains of 25 and efficiencies greater than 70%. When photons are precious and a small viewing zone or box is acceptable then this is a very good solution.
The viewing zone cannot always be constructed with real sized sources at end use distances. The viewing zone can always be specified as a set of angles or a specific pattern and scaled with lenses during construction. Perhaps the most useful and general method of creating a specified viewing zone is to project the pattern to infinity during construction. This is done by placing a correctly scaled diffuse pattern 1 focal length away from a lens placed close to the recording plane. The scaling is most easily done by ray tracing from the film plane through the lens to a plane one focal length away. This method also cancels aberrations introduced by the lens during construction. The lens limits the size of the hologram and thus the size of the screen that can be recorded this way. Practical lenses for this purpose are usually only 6 or 8 inches in diameter although some are available up to about 20 inches. Large screens must then be made some other way.
One of the applications we have dealt with recently required that we make a screen several times larger than we were able to expose in one shot. The angular viewing window was large and the surface area of the final screen was also large. We were limited by available laser light and lenses to a screen about 5 inches square. In order to make a larger seamless screen we multiplexed many overlapping exposures of the small screen on a much larger glass plate. One screen consisted of 144 separate exposures. The reason we could do this and maintain a constant angular viewing zone is because we had recorded a diffuse surface from infinity. In use the screen is visible only when the surface is reconstructed at infinity behind the viewer. Since it is at infinity, the angular zone remains constant for any size extended surface. This trick would be incredibly difficult to duplicate with conventional optical elements.
Another important property of these screens is their nearly transparent appearance to anyone not in the viewing zone. These screens may be applied to windows in observation and control towers where they remain transparent while reflecting just like an opaque beaded reflecting screen for those in the viewing zone. Simple small LCD projectors can easily be seen in a lighted room due to the high gain and efficiency of the screen and the view of the outside world is only slightly colored and obstructed. Backlit displays or control panels may also be overlaid with this type of screen to project information as needed without obscuring the backlit displays. This property is even more difficult to duplicate in refractive optics. Any refractive or reflective surface that diffuses incident radiation at one angle usually does so at all angles. Diffractive optics on the other hand do very little to incident radiation at the wrong angle or the wrong wavelength.
We refer to the second type of screen as an infinite conjugate directional diffuser because of its unique scaling property. Other directional diffusers have an ideal viewing zone that includes a finite distance and cannot be extended in size without enlarging and moving the viewing zone. Either variety may be constructed as a transmission or reflection hologram but all of our work has been with reflecting structures which are also naturally wavelength selective. Transmission screens can be used as diffuse illuminators of objects or selected areas of a plane. Patterns with unique properties in scale, focus and intensity distributions may be produced with these methods. Holographic directional diffusers are a unique class of optical elements that in general cannot be duplicated with conventional optical components.
Last modified on 8/2/97
