In general, transmissive optics introduce aberrations into transmitted wavefronts whenever temperature
gradients exist within or across the optic. The aberrations result from thermally induced optical path length (OPL)
changes, and materials that minimize the effect are often labeled as "athermal". Several groups of materials which
appear to meet the criteria for "athermal" behavior are glasses pssessing highly negative thermo-optic coefficients
(dn/dT). Heavy metal fluoride (HMF), phosphate (PP), fluoro-zirco-aluminate (FZA), and fluorophosphate (FP)
glasses exhibit this somewhat unusual property.
A program to investigate HMFGs, PPs, FZAs, FPs and other temperature insensitive materials has been
underway at the USAF Weapons Laboratory since May of 1988. Of particular interest is the effect of glass
composition Ofl the refractive index, index temperature dependence, thermal expansion coefficient, and heat
capacity. Analyses have shown that the materials under investigation are highly superior to fused silica from a
thermally induced optical distortion standpoint.
The WL program has produced a wide variety of glass samples that are now undergoing optical, thermal, and
mechanical evaluation. This paper discusses the analyses thus far accomplished.