Circular Variable Neutral Density (CVND) filters are used in high quality optical systems to change the intensity of light from 100 percent to less than 0.1 percent. In a system that uses this technology, the beam intensity changes as the filter rotates due to the density variation of a gradient metallic coating around the filter. To meet specific application needs, it is possible to fully customize these filters in terms of optical density gradient function, transmission gradient function, substrate type, coating materials, and size.
Reynard Corporation, a leading global supplier of optical components and thin film coatings, is one supplier of completely customized CVND filters. Used from the UV to the Far Infrared, density neutrality can be achieved in various applications from lasers in the narrow band, to wide band applications such as the spectrum of white light.
Densities can be supplied as a standard linear or customized gradient function. The gradient function can be supplied from 45 to 360 degrees of rotation. Substrates can be supplied with diameters from 1.0″ (~25mm) to 8″ (~200mm).
In imaging systems, large aperture neutrality with constant density can be achieved by configuring two back-to-back filters of equal design. Counter-rotating the two filters allows for a variable setting of the density or transmission requirement in both the rotational and radial directions. For large aperture imaging, the two-filter solution offers adjustment flexibility that cannot be physically achieved by any other means. This enables high-resolution cameras to operate with full aperture while still attenuating light.
These CVND filters are typically specified in linear optical density or transmission. However since all optical systems are not linear in operation, Reynard Corporation has the ability to customize the density gradient based on user-defined functions.
The filters can be manufactured on a variety of substrate materials including BK-7, fused silica, ZnSe, and Ge, and many more. A nickel variant is a typical coating for room temperature low-power applications. The ND coating can be enhanced to increase survivability in hostile environments by the use of other common and precious metals to match the absorption, spectral, and environmental characteristics of a customized system.