Variable temperature test target

Apparatus for assessing the thermal sensitivity of optical systems comprises a test target 4 in the form of a plate in which slits 11 are cut at a predetermined spatial frequency. Radiation from a heated bar 1 is reflected from two separate portions thereof on opposite faces A,B of the bar to the test target 4 by concave mirrors 2, 3, radiation from one mirror 3 being reflected by the test target 4 and radiation from the other mirror 2 being transmitted through the slits 11 of test target 4 so that the resulting radiation passes along a common path 9 to a collimator 8 and hence to the optical system under test. The bar 1 may be heated at one end and cooled at the other end such that a temperature gradient is produced along the length of the bar. The above mentioned other mirror 2 is mounted in a movable holder 10 which, by means of a micrometer device 7 can be adjusted along the length of the bar 1 so that the radiation at the test target 4 emerges from portions of the bar 1 which are at different temperatures.