Abstract: Method and apparatus for measuring thermal radiation emitted by a sample, and for determining emissivity. The apparatus comprises a chamber in which the sample may be positioned, the chamber comprising a plurality of walls including a first wall containing an observation port. The chamber is located in a cold environment, and the walls of the chamber are brought to a selected temperature by circulating a temperature conditioned fluid through the walls. The sample is moved along a movement axis within the chamber past the observation port. As the sample is so moved, radiation emitted outwards through the observation port by the sample is measured.