Optical systems which are used over a wide temperature range, or at temperatures and pressures different from STP require consideration of thermal and pressure effects on the index of refraction and material expansion. Here, for example, is the variation of the spot size of an optical system as temperature changes from 0 to 100°C:

The glass catalogues supported by ZEMAX contain thermal expansion and index variation with temperature and pressure data which are used to compute the effects of temperature on individual elements and the optical system as a whole. The thermal data is used to create mutiple configurations which reflect performance at various user defined temperatures and pressures. Since ZEMAX can optimise across multiple configurations simultaneously, this feature can be used to design athermalised lenses, as well as estimate performance changes with temperature.

ZEMAX also supports specification (and optimisation) of the thermal coefficient of expansion (TCE) of spacers and mounts between lens elements or groups.