Black/grey/white body

So far we have assumed that the radiation being studied originates from an ideal body. In this case, ideal means that the maximum physically possible is emitted as heat radiation. The ideal body is known as a black body.

The sun behaves like a black body because its emitted spectrum comes very close to a black body. Similarly white radiator paint has a very high emission capacity for radiation in the range 300-350K.

The opposite of a black body is the white body. This is an object that does not emit any radiation.

Between white and black is the mixture of both, the grey body. These are closest to reality and feature behaviour between the two extremes.

The bodies can be categorised according to the radiation behaviour. The emission coefficient describes the behaviour of a grey body. The emission coefficient "epsilon" of a black body is equal to "one", which corresponds to the maximum radiation at a certain temperature.

In white bodies the emission coefficient "epsilon" is equal to "zero", which corresponds to no radiation.

Accordingly, grey bodies have an emission coefficient "epsilon" between "zero" and "one".

The image shows the curves at 500 K. This time the axes are linear, unlike the illustration before.
The black curve corresponds to the ideal black body and the red corresponds to the ideal white body. The blue curve represents a grey body with an epsilon of 0,6. This curve then assumes 60% of the maximum emission or absorption intensity. The curve is only compressed in its intensity, on the x-axis, there is no change in the wavelength range, see previous page.

The differentiation between the various bodies is very useful. Less useful, that is to say less obvious at first sight, is the naming convention.

Black has nothing to do with the colour black, but rather the property of black objects to absorb all light. So far we have only looked at emitted radiation. However, the properties of emitting and absorbing are directly related, see next page. Black bodies have the property of absorbing radiation in the entire radiation spectrum.

The same applies to white bodies that reflect the light (the difference to a mirror is the diffuse, undirected reflection. A mirror is not a white body).

The grey body is somewhere between the two.

Black/grey/white bodies describe and assume the properties of visible light for the entire radiation spectrum!

Coloured bodies:

In addition to the described black/grey/white bodies are the so-called coloured bodies. In these bodies the properties of radiation are selective. This means that the behaviour cannot be determined from an ideal curve as before. In this case the emission coefficient varies depending on the wavelength.

A known example of this behaviour is window glass. The intensity of 100% corresponds to the maximum radiation. This is the curve of a black body.

Window glass allows the majority of visible light to pass through, while longer wavelength radiation is absorbed. This means that the glass absorbs this radiation energy.