What is a blackbody?
A blackbody is an object which absorbs all incident radiations it receives whatever the wavelength or direction and re-emits all these absorbed radiations. An important property of blackbodies is that the re-emitted energy level only depends on the temperature of the blackbody.
For a prescribed temperature and wavelength, no surface can emit more energy than a blackbody.
A blackbody is then an optical reference source, though a theoretical device.
What are the wavelength boundaries of infrared radiation?
The infrared radiation is the electromagnetic radiation where wavelengths are between 700 nanometres and 1 millimetre. Thus, it is located between the red limit of visible spectrum and the shortest microwaves.
However, taking into account the major applications of thermal sensors, the main considered spectral range is between 1 Āµm and 50 Āµm including 3 major sub spectral ranges corresponding to the atmosphere transmission windows:
- 1 Āµm to 3 Āµm or Short Wave Infrared (SWIR) or band I
- 3 Āµm to 5 Āµm or Middle Wave Infrared (MWIR) or band II
- 8 Āµm to 14 Āµm or Long Wave Infrared (LWIR) or band III.
What is emissivity?
Usual objects are not blackbodies. They do not absorb 100% of the incident energy and usually select the absorbed wavelengths.
Consequently, they cannot re-emit all the incident energy. The ratio between the re-emitted energy of a usual object and the re-emitted energy of a blackbody at the same temperature of the object is called emissivity and noted Īµ. This ratio depends on wavelength and is comprised between 0 and 1. Of course, the emissivity of a true blackbody equals 1.
However, such bodies do not exist and manufacturing āblackbodiesā consists in creating optical sources with emissivity value as high and as constant as possible over the widest spectral range. These sources are called grey bodies but practically sources with emissivity higher than 0.9 are also called blackbodies.
What is the Planckās law?
As mentioned in FAQ section, the quantity of energy emitted by a true blackbody only depends on its temperature. This radiation level, named Radiant Emittance R, is defined by the following distribution discovered in 1900 by the German scientist, Max Planck:
- h is the Planckās constant, h=6.626 x 1034 Js
- K is the Boltzmannās constant, K=1.381 x 10-23 J/K
- c is the speed of light, c=2.998 x 108 m/s
- Ī» is the wavelength (in meters)
- T is the temperature of the blackbody in Kelvin: T (Kelvin) = 273,16 + t (Celsius degree).
Download our calculation sheet to determine the energy emitted by a true blackbody at selected wavelengths, depending on the temperature of the blackbody.
Is it worth learning the Planckās law to use a blackbody?
- For any given wavelength, the Radiance level is an increasing function of the temperature,
- For any given temperature, the Spectral Radiance curve reaches a maximum which wavelength can be easily calculated from the easy-to-remember Wienās law
Example: a blackbody at 800K (i.e. 527Ā°C approx.) emits its maximum radiation at about 3.6 Āµm, i.e. in the MWIR spectral range.