Black Body Radiation | Definition, History, Applications

Definition of Black Body Radiation

Black Body Radiation: When radiant energy falls on the surface of anybody, a part of it is reflected, a part is absorbed, and the rest is transmitted. The whole energy is not absorbed because generally, the surfaces of ordinary bodies are not perfect absorbs of radiation.

Black Body Radiation

However, if the radiant energy is allowed to fall on the blackened metallic surface or carbon black, it is found that the energy is almost completely absorbed. A body that completely absorbs the radiant energy falling on it is called a perfectly black body.

The absorption is more perfect if we take a hollow sphere blackened on the inside and having a small hole for the entry of radiation. This is so because any radiation that enters through the hole is reflected repeatedly by the walls of the sphere till finally it is completely absorbed.

A black body is a perfect absorber of radiant energy and a perfect radiator. In fact, of all the bodies, the black body radiates the maximum amount of energy for the given temperature. The radiation thus emitted is called black body radiation.


When solids are heated, they emit radiations over a wide range of wavelengths. For example, when an iron bar is heated in a furnace, it emits radiation and becomes dull red and then progressively becomes redder and redder as the temperature increases. When heating is continued, it becomes brighter orange, then yellow, then white, and finally, it becomes blue at very high temperatures.

As we know, red light has a higher wavelength, and blue light has a lower wavelength, so the radiation emitted goes from a higher wavelength to a lower wavelength.

Since the longer wavelengths (red) have lower intensity and shorter wavelengths have higher intensity. If this trend were to continue, the intensity would keep increasing indefinitely as the wavelength becomes shorter and shorter and may enter the ultraviolet region.

However, the intensity of black body radiation does not show this expected behavior. An ideal body that emits and absorbs radiations of all wavelengths or frequencies is called a black body. And the radiation emitted by this body is called black body radiation.

Further Explanation of Black Body Radiation

At a given temperature, the intensity of radiation emitted from a black body increases with wavelength decrease. It reaches a maximum value at a given wavelength and then decreases with a further reduction of wavelength.

According to this theory, energy is emitted or absorbed continuously. Therefore, the energy of any electromagnetic radiation is proportional to its intensity and independent of its frequency or wavelength.

Thus, the radiation emitted by the body being heated should have the same color (wavelength or frequency) throughout. Although the intensity of the color might change with temperature variation.

History of Black Body Radiation

History of Black Body Radiation

In late 1859, Kirchhoff had defined a black body as an object that is a perfect emitter and absorber of radiation. He guessed, that he should recombine these two expressions in the simplest possible way. And thus transform the result into a formula relating the energy of the radiation to its frequency.

Sources of Black Body Radiation

Sources of Black Body Radiation

Blackbody radiation sources are utilized in thermal imagers testing and calibrating. When the source has been incorporated as an optical test bench automated testing using CI Systems Computerized Test Executive Software. It may be employed to determine the thermal imager operational characteristics or Unit Under Test. 

The amount of radiation emitted should be proportional to the number of modes in that particular range in any given frequency range. The blackbody radiation level only depends on the temperature, well-known via Planck’s theory. In most cases, black bodies are employed as optical sensors reference sources. In most cases, a source with a lower emissivity level independent of its frequency is identified as a gray body.

Essentially, as the blackbodies’ temperatures values emitting over the observable range are extremely high (consequently being very expensive sources in contrast to classical lamps). The black bodies are mainly preferred for usage over the infrared spectral range beginning from 1 µm. This is the reason why they are also referred to as Infrared Reference Sources.

Applications of Black Body Radiation

Applications of Black Body Radiation

These are some applications of black body radiation:

1. The black bodies are used for lighting, heating, security, thermal imaging, and testing and measurement applications.

2. A blackbody radiation source with a known temperature, or whose temperature can be measured. It is usually used for calibrating and testing the radiation thermometers.

Also Read


Leave a Comment