Optimized thermal imaging with a singlet and pupil plane encoding: experimental realization

Pupil plane encoding has shown to be a useful technique to extend the depth of field of optical systems. Recently, further studies have demonstrated its potential in reducing the impact of other common focus-related aberrations (such as thermally induced defocus, field curvature, etc) which enables to employ simple and low-cost optical systems while maintaining good optical performance. In this paper, we present for the first time an experimental application where pupil plane encoding alleviates aberrations across the field of view of an uncooled LWIR optical system formed by F/1, 75mm focal length germanium singlet and a 320x240 detector array with 38-micron pixel. The singlet was corrected from coma and spherical aberration but exhibited large amounts of astigmatism and field curvature even for small fields of view. A manufactured asymmetrical germanium phase mask was placed at the front of the singlet, which in combination with digital image processing enabled to increase significantly the performance across the entire field of view. This improvement is subject to the exceptionally challenging manufacturing of the asymmetrical phase mask and noise amplification in the digitally restored image. Future research will consider manufacturing of the phase mask in the front surface of the singlet and a real-time implementation of the image processing algorithms.