Abstract: A method for preventing clouding of a lens employed in a high-temperature oxidizing environment, an example of which is a lens of a pyrometer used to sense exhaust gas temperature (EGT) of a gas turbine engine. Clouding is prevented by inhibiting the generation of volatile oxide species that react with high-temperature lens materials, forming deposits including oxides of chromium, molybdenum and other elements having volatile oxide species. The method is particularly directed to a pyrometer whose lens is formed of sapphire (alumina) or silica, and is mounted within a structure formed of a material containing chromium and/or molybdenum, such as a superalloy or stainless steel. The method entails forming an alumina scale-forming barrier coating such as a diffusion aluminide on surfaces of the structure that are subject to oxidation and high temperatures.