Designing Metallic Neutral Density Filters with Constant Optical Density Versus Wavelength

Abstract

Ronald R. Willey

Observations in space and on earth can be subject to wide dynamic ranges of light levels which may need to be controlled by neutral density filters. The optical density (OD) of neutral density (ND) filters made with single metal layers is often not very neutral or constant in OD versus wavelength over the wavelength band of interest. Photographers are the major users of quantities of ND filters, but there are many other applications for smaller numbers of ND filters. There are a limited number of publications on how to design metallic ND filters. It is shown here that designs of metal-dielectric-metal (MDM) can be designed to have more constant OD over a wavelength band. Designs are shown employing software which accommodates the fact that the indices of thin metal films usually vary with film thickness.

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