Ultrathin film neutral density filter set for the infrared

A set of neutral density filters covering the optical density range of OD 1 through OD 10 has been designed and constructed for use as calibration and reference filters in the infrared. The filters are optically flat, with a variation of at most one OD within the spectral range of 2 micrometer to 25 micrometer. Many filters exhibit better than 0.5 OD variation. They each are constructed of a metallic film supported by an ultrathin (100 nm) polyimide film substrate that minimizes the chance of etaloning within the filter. The polyimide substrate is sufficiently robust to withstand handling in a laboratory environment and provide long-lived performance. This paper details the theoretical model used to determine appropriate materials for these filters, as well as the vacuum deposition processes developed to construct them.