Moisture resistant optical films: their production and characterization

We have used an accelerated humidity test consisting of a 24-h cycle between 27-50 degrees C at 95% relative humidity to rate the moisture resistance of typical protective films such as CaF(2), LaF(3), and fluorinated polymers (TFE, FEP) deposited on BaF(2) and NaCl substrates. The test life of BaF(2) specimens was increased from about 4 cycles to over 9, 26, and 42 cycles when protected by CaF(2), TFE, and LaF(3), respectively. Heat treating the as-deposited CaF(2) films further improved specimen life by a factor of 2. The lifetime of NaCl specimens was improved from a fraction of a cycle to 1 and 3 cycles, respectively, by CaF(2) and TFE coatings. No adherent LaF(3) films could be deposited on the salt. Microhardness measurements of the CaF(2) films showed a direct correlation with humidity resistance. The effect of heat treatment was apparently to sinter the fine CaF(2) particles composing the film, thus increasing film density, refractive index,and hardness while reducing moisture permeability. Caf(2) and TFE films function as antireflective, as well as protective layers, on BaF(2) and NaCl.