1 March 1991 Studies on defocus in thermal imaging systems
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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.
© (1991) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
B. Venkateswara Rao, B. Venkateswara Rao, } "Studies on defocus in thermal imaging systems", Proc. SPIE 1399, Optical Systems in Adverse Environments, (1 March 1991); doi: 10.1117/12.26090; https://doi.org/10.1117/12.26090

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