Lens systems used in thermal imaging systems frequently employ aspheric surfaces in order to obtain aberration correction with a minimum number of lenses. The use of infrared-transmitting gradient-index materials is a theoretical alternative method for achieving this aim. Moreover, gradient-index materials can be used in some cases to correct chromatic aberrations, which aspherics cannot correct. A design study has been conducted in which it is shown how the polychromatic optical performance of particular lens systems could be improved by utilizing ZnSe/ZnS axial gradient lenses, and how the performance over the field of view could be improved by utilizing a germanium/ silicon spherical gradient.
James W. Howard,
Danette P. Ryan-Howard,
"Optical Design Of Thermal Imaging Systems Utilizing Gradient-Index Optical Materials," Optical Engineering 24(2), 242263 (1 April 1985). https://doi.org/10.1117/12.7973467