Radiation properties of real bodies

A black body with this emissive power has, according to the Stefan-Boltzmann law, a temperature of 

As a first approach, extraterrestrial solar radiation can be taken to be radiation from a black body at this temperature, see also section 5.3.5. 

The proportion of the radiation emitted by this black body within the wavelength interval Ai = 0.38/zm to A2 = 0.78 fxa1, according to (5.61) and (5.62), is 

In the following sections we will look at the radiation properties of real bodies, which, with respect to the directional dependence and the spectral distribution of the radiated energy, are vastly different from the properties of the black body. In order to record these deviations the emissivity of a real radiator is defined. Kirch-hoff s law links the emissivity with the absorptivity and suggests the introduction of a semi-ideal radiator, the diffuse radiating grey body, that is frequently used as an approximation in radiative transfer calculations. In the treatment of the emissivities of real radiators we will use the results from the classical electromagnetic theory of radiation. In the last section the properties of transparent bodies, (e.g. glass) will be dealt with. 

According to Kirchhoff s law a black body emits the maximum radiation energy at every wavelength in every direction in the hemisphere. It therefore suggests itself to relate the four radiation quantities, used to characterise the emission of any 

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The concept of hlackhody is determining the basis for describing the radiation properties of real surfaces. The black body denotes an ideal radiative surface which absorb all incident radiation, being a diffuse emitter and emit a maximum amount of energy as thermal radiation for a given wavelength and temperature. The black body can be considered as a perfect absorber and emitter. 

This says that one single material function is sufficient for the description of the emission, absorption and reflective capabilities of an opaque body. Table 5.4 shows that it is possible to calculate the emissivities ex, s and from s x. Correspondingly, with known incident spectral intensity Kx of the incident radiation, this also holds for the calculation of ax, a and a from a x as well as of rx, r and r from r x, cf. Tables 5.1 and 5.2. So, only one single material function, e.g. e x = s x(X, f3,ip,T), is actually necessary to record all the radiation properties of a real body6. This is an example of how the laws of thermodynamics limit the number of possible material functions (equations of state) of a system. 

So far, we have studied the radiation energy associated with black bodies. However, real surfaces do not behave like a black body. The next section is devoted to the surface properties of real surfaces. 

The heat flux radiated from a real surface is less than that from an ideal black body surface at the same temperature. The ratio of real to black body flux is the normal total emissivity. Emissivity, like thermal conductivity, is a property which must be determined experimentally. 

Of the next eleven books listed, " comments will be passed on only five. The text by Eyring et fl/. includes consideration of the thermodynamic properties of crystals, black-body radiation, dielectric, diamagnetic, and paramagnetic properties of matter, real gases, equilibrium properties of liquids, liquid mixtures, and surface chemistry. 

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