Effects of the variation of angle of incidence and temperature on infrared filter characteristics

The effects of the variation of angle of incidence and temperature on the characteristics of some typical ir filters are discussed. Collimated radiation, Lambertian radiation, and focused radiation, from a point source at infinity are considered and theoretical calculations of the wavelength shifts and bandwidth changes of three narrow bandpass filter designs are presented. Measurements were made of other filters irradiated by collimated radiation at various angles of incidence and good correlation exists between theoretical and measured data. Measured variation of the characteristics of some filters with temperature are also presented. Results indicate that the filters shift toward shorter wavelengths if either the angle of incidence is increased or temperature is decreased.