Nonradioactive infrared coatings for single- and multispectral applications

Infrared (IR) optical systems tend to make use of IR lens and window materials with high values of refractive index, such as germanium, zinc selenide and zinc sulphide. Consequent Fresnel reflection is sufficient to significantly degrade effective transmission and IR imaging capability.
To maximize IR optical performance in such systems, Barr and Stroud has for over 20 years produced high efficiency IR anti-reflection (AR) coatings for lenses and windows. Typically these coatings provide average reflectance below 0.25 percent per surface with sufficient durability to meet internal surface environmental conditions. Design constraints on these AR coatings require a low refractive index outer material, with recent technology and practice relying on thorium fluoride to fulfill this role.
The main disadvantage of thorium fluoride usage is its associated radioactivity and high toxicity. Moreover, present IR coating requirements insist upon replacement of thorium fluoride with a non-radioactive, low toxicity, low index film material. A research program at Pilkington Optronics (Barr and Stroud) has resulted in the identification and characterization of yttrium fluoride as a suitable non- radioactive replacement film material with reduced toxicity for use in the 3 to 5 micrometer and the 8 to 12 micrometer IR spectral regions. As described in this paper, this material has been successfully incorporated into high efficiency single and dual band AR coatings for germanium, zinc selenide and zinc sulphide, without compromising optical and durability performance.