An infra-red optical filter comprising a coating of germanium on a transparent base is produced by heating the base to 200 DEG C. or more while the germanium is being deposited by evaporation-deposition. In this way a thick coating (e.g. 0.001 inch) is produced with less tendency to flake. Preferably the deposition is in two or more stages and the germanium is evaporated from an electrically conductive boat of tantalum, molybdenum or carbon. Carbon is preferred as the germanium alloys with the metals if the boat is used a few times. The base may be of glass plate, rock salt, synthetic sapphire or periclase or of "Kodial" or "Pyrex" (Registered Trade Marks) glass. The latter are particularly suitable with a thermal expansion coefficient similar to germanium. If the germanium is evaporated upwards the base may be heated with an electrically heated copper slug. After formation of the coating, blooming may be effected with zinc sulphide or metallic selenium.
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