Computer Modelling Technologies of Optical System of Polarizing Thermal Imager

Abstract

The energetic resolution is a main parameter of any thermal imager depending on the transmittance of its optical system. The optical system of a polarizing thermal imager (PTI) consists of a polarizer, a phase shifter and a lens arranged in series on the optical axis. This paper proposes a method for calculation of the energic transmittance of the PTI’s optical system for partially polarized radiation as a function of angular orientation of the polarizer and the phase retarder. The physical-mathematical model of transformation of partially polarized radiation within the optical system which depends on parameters of optical elements and their orientation in space is investigated. This model allowed us to determine the transmittance of the system ''polarizer - phase shifter'' system depending on the angle α between the optical axes of the polarizer and the phase retarder. The analysis of this method has shown that, for the natural radiation, the normalized transmittance of the optical system does not depend on the angular orientation of the phase retarder and is equal to 0.5. For the partially polarized radiation, the transmittance depends on the angle α: the maximum transmittance value will be achieved in the case when the optical axis of the phase retarder lies in the transmittance plane of the polarizer (α = 0°). For an arbitrary degree of polarization, the transmittance decreases with increasing angle α . At an angle (α = 45°), the transmittance is equal to 0.5 and does not depend on the degree of polarization of the examined radiation. To calculate the characteristics of the partially polarized radiation using Stokes parameters, the intensity is to be measured at the output of the optical system for angles α equal to 0°, 90°, 45°, and 135°. For such angles, the normalized transmittance for the degree of polarization of 0.5 is equal to 0.75, 0.25, 0.5, and 0.5, respectively. This feature of the PTI’s optical system must be taken into account when calculating the temperature resolution and the maximum range of the thermal imager.