RADIATION PYROMETER

A pyrometer for viewing the blades of an engine has a forward lens which produces a first image of the blades in an image plane P1. Rearwardly of the image plane P1 is an assymetric faceted lens assembly having a central region with a long focal length, and an outer annular region of shorter focal length. The outer region of the rear lens assembly forms a second image of the first image in a second image plane P2. The inner region forms a third image of the forward surface of the first lens in the second image plane P2. A disc mask is fixed centrally to the forward surface of the first lens, and an annular mask is located in the first image plane P1. The forward end of a fibre-optic cable is located in the second image plane P2 and comprises a central bundle of fibres which receives the second image, and two outer bundles which receive different parts of the third image. By monitoring the radiation from the forward surface of the first lens, the amount of contamination on the surface can be monitored.