Intended for general indoor or outdoor illumination, a luminaire both emits and shapes light. Many luminaires contain a diffuser to aid in light shaping. An ideal diffuser for a luminaire would hide the appearance of the light source behind it and scatter all light within a specified beam distribution with no backscatter or absorption. Another ideal property for such a diffuser would be controllable anisotropic scattering; a designer could tailor the beam distribution to suit particular needs. Conventional diffusers all fall short in at least one of these areas. Holographic diffusers, though able to scatter anisotropically at low loss, have chromatic dispersion; the resulting rainbow halos may be pretty, but they do not make for high-quality illumination.
A diffuser developed by engineers at Ledalite Architectural Products (Langley, B.C., Canada) is a variant of the so-called kinoform holographic diffuser and promises to satisfy all the needs of the luminaire designer, including elimination of chromatic problems. The kinoform diffuser was invented in 1971 and is a phase-only, surface-relief hologram of a conventional diffuser. Though highly efficient, the original kinoform diffuser not only suffered chromatic dispersion, but also transmitted a considerable portion of the zero-order beam, making the light source visible through it.
Many beam shapes possible
The engineers at Ledalite have developed a class of kinoform diffusers with desirable non-Gaussian beam distributions that permit customized light shaping. The diffusers transmit up to 95%, have no chromatic dispersion, and completely eliminate the zero-order beam. Their distributions can be controllably varied from Gaussian through uniform to a batwing shape, and also can be shifted off-axis. Because no mathematical tools yet exist that can automatically design kinoform diffusers, the development was partially empirical.
Beam shapes can range from circular to elliptical and even linear. The engineers have developed versions of the diffuser that produce X-shaped anisotropic diffusion patterns in which the diffusion width of each arm can be varied from elliptical to linear. The diffusers can also be metallized to form reflective diffusers; one version of this reflector has the interesting property that it both diffuses and retroreflects light.
The kinoform surface profiles are produced in photoresist using holographic techniques, according to Ian Ashdown, head of research at Ledalite. The surfaces are then replicated using standard techniques. The diffusers can be replicated not only in discrete units, but also on plastic sheets and films, with suitable materials including acrylics, polycarbonates, and polyesters. For high-temperature applications, it is possible to replicate these diffusers in fused silica using sol-gel techniques.
The beam distribution of a kinoform diffuser depends on the distance between it and the light source. Varying this distance has the effect of changing the angle of distribution. In one design, a diffuser movable over a distance of 1.5 in. provides a beam spread that varies from a wide flood to a narrow spot, said Ashdown.
A diffuser that is a linear scatterer appears transparent in one direction but not the other. Such a diffuser has advantages when combined with a fluorescent lamp. As an observer walks away from the spot directly beneath the lamp, the single lamp image splits into two, raising the off-axis intensity. The diffuser also cuts off sharply above a certain angle, acting like a specular parabolic louver.
Because the diffuser properties are so wide-ranging, there are many luminaire geometries and arrangements yet to explore. In addition, use of the Ledalite diffusers is not restricted to luminaires. The company is looking at applications ranging from fiberoptics to solar cells, and has already developed a novel skylight that also serves as a solar collector, noted Ashdown.
