Optical systems which are used over a wide temperature ranges, or at different temperature and pressure than standard, require consideration of thermal effects on the index of refraction and material expansion. Zemax uses an accurate nonlinear thermal model, not a simple dn/dt approximation.
Zemax supports specification (and optimization) of the thermal coefficient of expansion (TCE) for spacers between lens elements or groups. The TCE data is used to create multiple configurations which reflect performance at various user defined temperatures.
The glass catalogs supported by Zemax contain thermal expansion and index variation with temperature and pressure data which are used to compute the effects on individual elements and the optical system as a whole. Since Zemax can optimize across multiple configurations simultaneously, this feature can be used to design athermalized lenses, as well as estimate performance changes with temperature.
For example, here is the spot diagram of a simple doublet lens operating over the temperature range 0 to 100° C. Each configuration is a specific temperature, e.g. 0°, 20°, etc:
It can be seen that the lens goes out of focus somewhat as the temperature changes. After re-optimizing specifically for thermal performance, and allowing better choices of both optical glass and mounting material TCE, this performance
Thermal expansion properties of all Zemax sequential surfaces, including aspheric surfaces, are accurately modeled.
Related Knowledge Base articles:
How to Model Thermal Effects Using Zemax