Emissivity

Each body whose temperature is above the absolute zero emit heat radiation. The emissivity of a body indicates how much radiation he compared to an ideal thermal emitter, a black body emits.

Black body radiation as a reference

A black body is a hypothetical idealized body which completely absorbs any incident on it at any frequency of electromagnetic radiation. According to Kirchhoff's law of radiation absorption and emissivity of a body is always proportional. Since the black body having the maximum absorbance at any frequency (namely, 100%), so it has to give the largest possible physical thermal radiation performance, which is possible at the given temperature and at each frequency. In other words, he is standing next to an equally hot other body with lower emissivity, it releases its energy more quickly and also lights up brighter than the other body.

Since he radiates equally maximum in each direction, the radiation emitted by it is equally strong in all directions; he radiates perfectly diffuse. In addition, intensity and frequency distribution of the emitted by a black body radiation do not depend on its material properties or of its history, but only on its temperature; they are described by Planck's radiation law.

The universal character of the votes of a black body thermal radiation and the fact that at any frequency, no real body can radiate more than a black body, it suggest the emissivity of a real object to the specified from the black body maximum possible value to relate. The ratio of the votes of a body radiation intensity to the radiation intensity of a black body of the same temperature is called the emissivity of the body. The emissivity can take values ​​between 0 and 1. Depending on whether the frequency and directional distribution of the radiation should be taken into account, four different emissivities can specify.

The emissivity of a body must be known so that from the intensity of the emitted thermal radiation its temperature can be determined with a pyrometer or a thermal imaging camera.

Emissivities

Directional spectral emissivity

The spectral radiance (unit: W · m -2 · Hz -1 sr -1) a body temperature indicates that the radiation performance of the body at a frequency in represented by the polar angle and the azimuth angle direction per unit area, per unit emits frequency interval and per unit solid angle. The spectral radiance of a black body is independent of direction and is given by Planck's radiation law.

The directional spectral emissivity of a body is the ratio of a surface element of the body at the frequency in the signal emitted by the angle and given direction of spectral radiance to that of a black body of the same temperature emitted at the same frequency in the same direction the spectral radiance:

Hemispherical spectral emissivity

The spectral exitance (unit: W · m -2 · 1 Hz ) of a body of the temperature indicates, that the radiation power emitted by the body at a frequency in the entire half-space per unit area and per unit frequency interval. The spectral emittance of a black body is given by Planck's radiation law.

The hemispherical spectral emissivity of an object is the ratio of a surface element of the body radiated at the frequency in the half-space spectral emittance to that of a black body of the same temperature at the same frequency emitted into the half space the spectral emittance:

Facing total emissivity

The total radiance or radiance ( unit: W · m -2 · sr -1 ) of a body temperature indicates which radiation power of the body emits at all frequencies in the given by the polar angle and the azimuth angle direction per unit area and per unit solid angle. The radiance of a black body is independent of direction and is given by Planck's radiation law.

The targeted total emissivity of the body is the ratio of a surface element of the body on all frequencies in the signal emitted by the angle and given direction of the beam density of the radiated by a black body of the same temperature at all frequencies in the same direction radiance:

Hemispherical total emissivity

The emissivity (unit: W · m -2 ) of a body of the temperature indicates that radiation power per unit area of the body to emit at all frequencies in the half-space. The emittance of a black body is given by the Stefan- Boltzmann law.

The hemispherical total emissivity of an object is the ratio of the area of an element of the body on all frequencies radiated into the half-space emittance to that of a black body of the same temperature at all frequencies radiated in the halfspace radiance:

All beam sizes and emissivities can naturally be formulated as a function of wavelength instead of frequency.

Properties

All four emissivities are described material properties of the body under consideration (this is true in the case of analog -defined absorption coefficients for the directional spectral absorptance ). The directional spectral emissivity describes the direction and frequency dependence of the emitted radiation by comparison with the light emitted from a black body radiation. The hemispherical spectral emissivity describes only the frequency dependence of the directional total emissivity only the directional dependence and the hemispherical total emissivity only the total emitted radiation output. For many materials is only the latter known.

A body having a directional spectral emissivity is not dependent on direction, a Lambert radiator; he gives off completely diffuse radiation. A body having a directional spectral emissivity does not depend on the frequency, is a gray body. In both cases, there are often considerable simplifications for radiation calculations, so that real bodies often - be approximately regarded as diffuse emitters and gray body - as far as possible.

According to the Kirchhoff's radiation law is for every body the directional spectral emissivity is equal to the directional spectral absorptance. For the other emission and absorption coefficients, the equality holds only under additional assumptions.

Tables

Basically, the information is to enjoy emissivity in the many -to-find tables with caution. Due to the many possible variations that are rarely all specified, there may well be greater differences.