TABLE OF CONTENTS The ability of a surface to radiate energy is governed by the material of which the surface is composed and its physical condition. Any attempt, therefore, to place a numerical value on its radiating ability must be related to a definition of the surface condition. It is usual to choose smooth polished surfaces for this purpose and thus arrive at values which are comparable from one metal to another.
A perfect radiator (blackbody) provides a standard of comparison for defining the radiating ability of any other body or surface by determining the ratio of the emission of the surface to that of a blackbody when they are at the same temperature. An examination of the ratios thus obtained shows that the radiating ability of a metal surface varies with wavelength, temperature and angle of emission. The definition of the emittance, as this ratio is called, must therefore take into account these variations.
Spectral, directional emittance, ? ?, of a surface is the ratio of the energy emitted over an infinitesimally small wavelength range at wavelength ? into a specified direction, per unit area of the surface, to the energy emitted by a unit area of a black surface at the same temperature. The emittance in a direction normal to the surface, called the normal, spectral emittance, ? n ?, is most commonly employed.
The spectral, hemispherical emittance, ? ?, is a directional average of the spectral, directional emittance, and gives the ratio...
