Detection of changes in transparency of optical elements

Abstract

In a radiation pyrometer in a gas turbine engine, non-uniformly distributed changes in transparency of an optical element in the system, e.g. contamination of pyrometer objective lens L.sub.1 by combustion products from the turbine can be automatically detected. Lens L.sub.1 focusses radiation from target T onto an additional lens L.sub.2, which in turn focusses it onto the end E of the fibre-optic bundle F. Considered reciprocally, lens L.sub.2 images the end E of fibre-optic F into L.sub.1, so that although each optical fibre in end E receives radiation from all parts of target T, each fibre is imaged into a corresponding discrete area of L.sub.1. Fibre-optic bundle F comprises two (or more) sub-bundles F.sub.1 and F.sub.2 forming complementary sub-areas of end E and each sub-bundle is provided with its own photodetector P.sub.1, P.sub.2 and associated pre-amplifier A.sub.1,A.sub.2 adjusted so that their outputs A.sub.1 S.sub.1 and A.sub.2,S.sub.2 are matched at the non-contamination condition. When one part of lens L.sub.1 is contaminated during service more than the other parts, this will show as an unacceptable mis-match between the outputs A.sub.1 S.sub.1 and A.sub.2 S.sub.2. This is automatically detected by differential amplifier D whose output gives a warning indication and can also be used to trigger a selector circuit X to select the strongest signal for onward transmission.