Reflecting means for infrared detectors

In a thermal imaging system for passive imaging of infra-red rays, a converging imaging cone is incident on a detector 2. To shield the detector from rays outside the imaging cone which would cause noise, a thin plate 10 is provided and, since the detector is cooled but the plate is not and therefore generates infra-red radiation, the plate is formed as a Fresnel reflector to reflect radiation emanating from the detector back onto itself. As compared with the conventional "cold shield" reflector which has a spherical reflecting surface, the use of a plate formed as a Fresnel reflector makes the imaging system thinner in the axial direction in the region of the detector.