To simplify the task, engineers at Reynard Corp., San Clemente, Calif. (reynardcorp. com), designed a line of Circular Variable Neutral Density Filters. These round devices have a metallic coating that varies smoothly across the surface. Depending on where light is shining through, the filter changes its intensity from 100 to 0.1%.

The gradient coating encompass 45 or 360° of the circle, and filters can be from 1 to 8 in. in diameter. The gradients can also be chosen to work with light from UV to far IR. The coatings can be put on filter substrates that include fused silica, BK-7, ZnSe, and Ge. The coating is usually a nickel variant, which is well suited to low-power, room-temperature applications.

The filters render light densities neutral for lasers and wideband applications that use the entire spectrum of white light. And setting up two of the same filters back-to-back can give imaging systems large-aperture neutrality with constant density. Then counterrotating the two filters varies density in both rotational and radial directions. Such a setup would let highresolution cameras operate with full aperture while still being able to attenuate light.