Influence of substrate material and geometric features of measurement slits on the measurement results of infrared optical system quality characteristics

Subject of study. The influence of geometric characteristics and manufacturing technology, as well as the optical characteristics of their substrate material on the results of the line spread functions and modulation transfer measurements of long-wave infrared spectral range lenses, is studied. The aim of the study is to minimize the measurement error of these functions arising from measurement slit characteristics. Method. The theoretical computation method is used to determine the measurement error of the modulation transfer function arising from the geometrical characteristics of the measurement slits and pinhole. A practical computation method consisting of multiple measurements of the reference lens and a comparison between the measurement results and optical calcu-lations is applied to determine the measurement error of the line spread function and modulation transfer function of the reference lens, arising from the effect of the slit substrate material. Main results. The influence of the geo-metric features of the measurement slits and pinholes on the results of the line spread function and the modulation transfer function measurements is considered: the concept of the apparatus line spread function as a convolution of the true point spread function or line spread function with spatial functions of the measurement and object slits is defined. This study presents the construction of the calculation formulas of the absolute error to define the cor-rection coefficient for the finite size of the slits and pinholes used for evaluating the modulation transfer function according to the measured line spread functions. A reasonable error of the slit or pinhole size for the minimization of the modulation transfer function error is defined. The influence of the slit substrate materials (zinc selenide, calcium fluoride, and barium fluoride) as well as the metallic slit on the measurement results of the line spread function and the reference lens modulation transfer function is studied. The studies deduce that the results depend not only on the geometric features of the slit but also on the manufacturing technology and optical characteristics of their substrate material. Practical significance. Possible errors of the measurement slit manufacturing technology are considered. The recommendations on the optimization of the slit manufacturing technology on an infrared transparent substrate are presented. In the future, the above conclusions and recommendations will provide trace-ability of the measurements of the line spread function and the modulation transfer function of infrared optical systems in the optical industry. They will be useful to both measurement specialists and measurement equipment developers and manufacturers. (c) 2023 Optica Publishing Group