Abstract: A thorium-free low absorption coating for infrared CO.sub.2 laser optics comprises an interior BaF.sub.2 layer formed on a substrate and an exterior layer formed over the interior layer to a predetermined thickness sufficient to substantially prevent water adsorption by the interior layer. Generally, the predetermined thickness is greater than about 11000 .ANG., and in one embodiment of a two-layer low absorption coating designed for antireflection at 10.6 microns, the exterior layer has a physical thickness in a range between about 24600 .ANG. and 22800 .ANG., and preferably about 24000 .ANG.. In the preferred two-layer AR coating embodiment, the exterior layer defines a cover layer and an absentee layer, and the interior layer is relatively thin. The preferred exterior layer and the preferred substrate consist essentially of ZnSe. Multilayer stacked coatings of more than two layer can also be formed to provide low absorption partially reflective or totally reflective coatings as well as AR coatings.