f/1.6 diffraction-limited air-spaced Cooke triplet photographic lens designs for MWIR and LWIR imaging applications: geometrical optics performance comparison between Ge–ZnSe–Ge and Si–Ge–Si triplet designs using Zemax

In this study Ge–ZnSe–Ge (spectral range 8–14 μm) and Si–Ge–Si (spectral range 3–11 μm) air-spaced photographic objective triplet lens designs are discussed. The two designs have focal length of 90 mm, f/1.6 and full field-of-view (FOV) of ± 4.5°. Geometrical optics calculations obtained with Zemax® for each lens design are provided for the modulation transfer function (MTF), point spread function (PSF), encircled energy, transverse ray fan plots, root-mean-square wave front error (RMS WFE), chromatic focal shift, field curvature, distortion, and RMS WFE versus wavelength. Furthermore, the MTF at 50% contrast modulation for both designs is evaluated over the temperature range 20–40 °C. The results indicate that the Ge–ZnSe–Ge triplet design is more susceptible to temperature variations, which lead to loss in image quality with the rise in ambient temperature