Radiation thermometers measure the thermal energy emitted by a source and relate this to its temperature by means of the Planck law of radiation. They consist of optics (generally lenses) to collect and focus the emitted energy onto a detector. The signal from the detector can either be measured directly, or it can be converted to a temperature using a system of electronics. Filters are usually used to define the wavelength or wavelength band over which the emitted energy is measured.

Many types of radiation thermometer are available for different applications. For measuring high temperatures a thermometer should be chosen that operates at a short wavelength, where the rate of change of emitted radiation with temperature is very high. However, for low temperature applications where the amount of emitted radiation is low, a broad-band device operating at longer wavelengths is required. NPL can calibrate infrared thermometers between -40 °C to 3000 °C.