As presented by Zemax Senior Computational Physicist Shawn C. Gay at this year’s Photonics West, TrueFreeform is a grid-based surface optimization of freeform surfaces in optical design.

Grid sag versus parametric freeform surfaces

Some lens design applications utilize parametric functions for representing freeform surfaces. Most common, perhaps, is a linear combination of Zernike functions. There is a downside to parametric approaches, though, namely that optimization of parameters typically forces the entire surface to change with each variable. In his talk, Gay spoke about a grid-based surface where sag contributions are specified on a rectangular grid. Optimizing on sag described in this fashion allows local influence on a specific region of the surface that is only as wide as twice the grid spacing.

Flexibility in designing freeform optical components

Grid-based surface optimization in OpticStudio allows flexibility in designing freeform optical components that is not possible with traditional surface types. In his paper, Gay focuses on a corrector plate with the rear surface specified by a variable grid sag surface. The goal of the corrector plate is to correct for Wavefront error introduced by a Zernike phase surface. Gay described the tests and reported on findings, including the importance of constraints on the grid variables and techniques that improve the calculation efficiency.