A practical problem concerning the loss of image quality due to lack of sharp focus arising out of ambient temperature variation or by human error in mecha nical focussing has been haunting Thermal Imaging System designers. The defocuss effect drops down modulus transfer function (MTF) of optical system considerably which in turn drops minimum resolvable temperature difference (MRTD) , which is a vital parameter to assess the total system performance of thermal imagers. Theoritical studies have been carried out to see the effect of defocuss on MRTD for different detector sizes and different focal lengths keeping the F/No. of optics system same. It is found that per centage of deterioration in MRTD for the depth of focus i.e. 2 >(f/no.)2 governed by optics is not same for different systems though f/no.2is constant. Hence it is understood that the depth of focus formula 2 A(f/no.) governed by optics does not hold good in case of Thermal Imaging Systems. An empirical formula for allowable defocuss for 10% of telerance in MRTD which is a function of not only f/no. but also detector size Ad , focal length of optics 'f' and spatial frequency is derived.

Date Published: 1 March 1991
PDF: 12 pages
Proc. SPIE 1399, Optical Systems in Adverse Environments, (1 March 1991); doi: 10.1117/12.26090

Published in SPIE Proceedings Vol. 1399:
Optical Systems in Adverse Environments
Soon Fatt Yoon; M. H. Kuok; Donald E. Silva, Editor(s)