A black body is an idealised physical body that absorbs all incident electromagnetic radiation, regardless of frequency or angle of incidence

Blackbody radiation” or “cavity radiation” refers to an object or system which absorbs all radiation incident upon it and re-radiates energy which is characteristic of this radiating system only, not dependent upon the type of radiation which is incident on it. The radiated energy can be considered to be produced by standing wave or resonant modes of the cavity which is radiating.

A simple example of a black body radiator is the furnace. If there is a small hole in the door of the furnace heat energy can enter from the outside. Inside the furnace, this is absorbed by the inside walls. The walls are very hot and are also emitting thermal radiation.

The amount of radiation emitted in a given frequency range should be proportional to the number of modes in that range. The best of classical physics suggested that all modes had an equal chance of being produced and that the number of modes went up proportional to the square of the frequency.

Stars approximate blackbody radiators and their visible colour depends upon the temperature of the radiator. The curves show blue, white, and red stars. The white star is adjusted to 5270K so that the peak of its blackbody curve is at the peak wavelength of the sun, 550 nm. From the wavelength at the peak, the temperature can be deduced from the Wien displacement law.

# Wien’s Displacement Law

When the temperature of a blackbody radiator increases, the overall radiated energy increases and the peak of the radiation curve moves to shorter wavelengths. When the maximum is evaluated from the Planck radiation formula, the product of the peak wavelength and the temperature is found to be a constant.

For a blackbody radiator, the temperature can be found from the wavelength at which the radiation curve peaks.

# How does a blackbody look like?

A blackbody is usually made of two parts linked through a cable:

• emissive head: including an emissive surface coated with a highly emissive coating or a cavity. An accurate temperature sensor is inserted into the emissive surface or cavity measuring the temperature of the source in real time. Depending on the temperature range the emissive head is also equipped with heating and/or cooling means.
• electronic controller: which brings power to the heating/cooling meanwhile acquiring and displaying the temperature of the emissive surface in real time. The electronic controller is equipped with accurate servo control loop for high-stability temperature regulation.