Reflection-Reducing Coatings of Magnesium Fluoride and Lithium Fluoride in the Near Infrared

A brief derivation of formulas for the transmittance of flat plates, uncoated and coated, is presented. Data showing the increased near infrared transmittance of seven flat samples resulting from quarter-wave coatings of MgF2 and LiF on their surfaces is presented. Five were coated to maximize transmittance for a wavelength of 2 microns, and two had coatings to maximize transmittance at 3.6 microns wavelength. Thick coatings of MgF2 do not adhere well. The coatings had absorption bands, possibly due to H2O. Estimates are made of the refractive indices of MgF2 and synthetic sapphire in the near infrared.