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
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.