Neutral Density Filter Coatings & Dark Mirrors

There are some applications for which the light used is too intense. In those cases a filter is often needed to apply a reduction in the intensity of the light without influencing the spectral profile. These wavelength neutral filters are often specified in terms of their optical density. Optical density can be calculated by taking the negative of the log (base 10) of the transmittance. For example, an optical density of 0.5 would be equivalent to approximately 32% transmittance. An optical density of 1.0 would be equivalent to a transmittance of 10%. Standard neutral density filter designs are available for the UV, Visible, and IR bands and for a variety of optical density values.

A similar optically significant case involves surfaces for which reflectance and transmittance are required to both be minimized over a narrow or broadband spectrum. This type of coating is commonly called a Dark Mirror due to the black non-reflective appearance. Dark mirror coatings are often used to reduce stray light reflections at surfaces intended to be opaque. A dark mirror coating can be applied to a variety of glass and metal substrate materials.