Effects of temperature on the properties of glass colour filters used in signalling equipment

Abstract

Glass colour filters of the selenium and copper oxide types used in aviation luminaires are affected by changes in temperature. For red and yellow selenium filters an increase in the working temperature causes an increase in the absorption of radiation at the blue end of the spectrum, resulting in a decrease in transmittance and a shift in the chromaticity towards the red. Both effects are reversible. For green copper oxide glasses similar reversible changes in transmittance and colour occur but are less significant. However, at elevated temperatures in enclosed luminaires, where there is normally a reducing atmosphere, copper oxide on the surface of these filters is reduced to metallic copper. This contaminating film can seriously reduce the transmittance and alter the colour. The effect is irreversible.