Simulating radiation thermometer temperature measurement error from the performance change of an interference filter due to polarization effect

A radiation thermometer is an optical instrument in which the dielectric optical filter is a critical component. Dielectric multilayer optical filters are favored in most radiation thermometer designs for selecting a certain wavelength range of the flux. As a component those filters are affected by different conditions to which they are exposed, which might lead to error in temperature measurement using radiation thermometer. The radiation thermometer is mostly used at normal to the flux source where the interference filter have the possibility of receiving collimated radiant flux (angle of incidence 0°) after passing through a lens, but there are optical set up situation where the lens involved converge part of the flux beam. In a converged beam the incidence angle can be within a range of approximately 0-20°. In this situation the measured temperature will be affected, as polarization introduces some error specifically from the interference filter. The error might be very small for most industrial applications, but quantifying it is beneficial specifically for high accuracy measurement required in high temperature measurement. It will be demonstrated through simulation how the polarization effect can be quantified and be related to temperature error by considering different design conditions of a multilayer interference filter. The paper also demonstrates how a polarization consideration in a multilayer dielectric filter design can minimize this error. It is practically demonstrated that wavelength shift, broadening of a full width at half maximum (FWHM) and reduced transmittance at central wavelength can be determined by comparing the performance of the designed optical filters.