Comparison of lenses' thermal expansion formulation in Zemax versus ANSYS with SigFit post processing

The goal of this paper is to study in which extent the use of Zemax is suited for athermalization purposes. The research questions targeted in this paper are: what are the differences in the formulation of materials’ thermal expansion between Zemax and Ansys; what is the impact on optical quality between both approaches; quantification of the differences between the two methodologies in terms of back focal length, spot radius and modulation transfer function (MTF). To quantify the differences between both approaches, it is used an objective working between -40°C and 110°C. Initially, only Zemax was used to evaluate the objective. Zemax considers a linear geometric expansion of every optical surface, which is here proved to not be the best approach to find a deformed geometry after a thermal load. The second approach is to create a 3D model and perform a finite element simulation in Ansys software. The input data is the thermal variation and the output is the deformed geometry of the lenses. Using SigFit software, it was possible to generate new mathematical equations of the deformed lenses and import this data into Zemax to start a new ray tracing. The new shape and location of lenses differs for both scenarios, and the difference in the focal plane shift is around 12%. The maximum spot radius difference is 27% and MTF relative error goes up to 16%. Zemax as a standalone software is valid if used as an initial guess for the optical designer. However, as a final stage for validation and detailed design, the approach containing Ansys and SigFit should be preferable.