A blackbody source is an ideal, 'Planckian', radiator, i.e it emits thermal (visible and infrared) energy whose intensity at a given wavelength and temperature is given by the Planck Law of radiation. Blackbody sources, whose temperatures are known or can be measured, are therefore extremely useful standards for the calibration of radiation thermometers.

An ideal blackbody source is a completely enclosed cavity held at a uniform temperature. The radiation field inside the cavity depends only on the temperature, and not on any physical property (size, shape, construction material). It completely absorbs and emits all radiation and has an emissivity of 1. For practical purposes, in order to view the radiation field inside the cavity, it is necessary to introduce a hole or aperture. Since this means a departure from the 'ideal' situation it is not possible to have a practical blackbody cavity with an emissivity of 1. However, by careful design cavities can be made with emissivities that closely approach 1. At the NPL, all our cavities are designed to have an emissivity of at least 0.999, and some have an emissivity of 0.9999 or better. NPL can supply and calibrate blackbody sources between -40 °C and 3000 °C.