High performance thermal imaging with a singlet and pupil plane encoding

Pupil plane encoding enables extended depth of field and greatly reduced sensitivity to aberrations in an imaging system (field curvature, thermally induced defocus, astigmatism, etc.). The application of pupil plane encoding has potential in thermal imaging where it can enable the use of simple, low-cost, light-weight lens systems. We present numerical and modelling studies of the application of this technique to an uncooled LWIR imaging system, F/1, 75mm focal length, germanium singlet with a detector array size of 240×320 with 50 micron pixel. The initial singlet is corrected from coma and spherical aberration, but its performance across the field of view is greatly limited by astigmatism. The introduction of an encoding asymmetrical germanium phase mask at the aperture stop of the system, combined with digital image processing, allows the removal of astigmatism and improved imaging performance across the field of view. This improvement is subject to a noise amplification in the digitally restore image. There is as a tradeoff between the maximum correction to astigmatism and reduced signal-to-noise ratio in the recovered image.