Abstract: A radiation thermometer comprises two radiation detectors for detecting radiation in wavelength bands centered on two different wavelengths .lambda..sub.1,.lambda..sub.2 (where .lambda..sub.1 >.lambda..sub.2) from a rotating body such as a gas turbine motor and for generating first and second signals S.sub.1,<S.sub.2 >respectively related to the radiance profile around said body and the average radiance around said body. A time averaging circuit generates a signal <S.sub.1 >representing an average value of said first signal. A dividing circuit generates a signal related to the ratio of <S.sub.1 >to <S.sub.2 >; and a monitor monitors at least one of said signals, predetermined arithmetic combinations of said signals, and quantities related thereto and provides corresponding output signals.