Metallic film thickness requirements for mirrors in the infrared

The thickness requirements for mirrors used at wavelengths up to 100 microns and, possibly, up to 1 mm are given. The experimental and calculated reflectance data for aluminum, gold, and nickel coatings in the infrared region are shown. The penetration depth given by d = lambda/4 pi k2 is considered a satisfactory criterion for determining the thickness required for evaporated metallic films. Aluminum coating thicknesses of 1000A give penetration depths sufficient for wavelengths up to 100 microns. Reflectance for three UKIRT mirrors in series is 97% of maximum overall reflectance. Nickel requires thicker coatings than gold, and gives lower maximum reflectances than aluminum in the 0.5 to 10 micron band. The maximum reflectance from gold is slightly higher than aluminum from 2 to 10 microns and thicker coatings are required. At 1 micron the maximum reflectance from gold is significantly higher than of aluminum, but lower at 0.5 microns.