Radiation from electrical nonconductors

Semiconductors operate on a different principle. When radiation falls on them, they change from a nonconductor to a conductor. No temperature change is involved in the process only the change in electrical resistance is important. This takes place over an extremely short period of time. Response times of the order of nanoseconds are common. This enables instruments to be designed with very short scanning times. It is possible to complete the scan in a few seconds using such detectors. These kinds of instruments are very valuable when put onto the end of a GC and used to obtain the IR spectra of the effluents. Such scans must be made in a few seconds and be completely recorded before the next component emerges from the GC column. 

A few comments about the validity of tlie diffuse approximation are in order. Although real surfaces do not emit radiation in a perfectly diffuse manner as a blackbody does, they often come close. The variation of emissivity with direction for both electrical conductors and nonconductors is given in Fig. 12 26. Here 0 is tlie angle measured from the normal of the surface, and thus 0 = 0 for radiation emitted in a direction normal to the surface. Note that Sg remains nearly constant for about 0 < d0° for conductors such as metals and for 6 < 70° for nonconductors such as plastics. Therefore, the directional emissivity of a sur face in the normal direction is representative of the hemispherical emissivity of the surface. In radiatioit analysis, it is common practice to assume the surfaces to be diffuse emitters with an emissivity equal to the value in the normal (6 = 0) direction. 

For practical purposes, only a mean value of the emissivity or absorptivity over the direction is required. Sieber (1941) obtained experimental data on total emissivity of opaque materials depending on the temperature of the source. Many authors (Ginzburg, 1969 Kreith, 1965) have reproduced these results graphically (Figure 18.1). The behaviour of electrical conductors and nonconductors with temperature of the radiator can be approximately interpreted from the dependency of the monochromatic emissivity on wavelength and the relationship between temperature of the radiator and the wavelength. 

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