Temperature Radiation Emissivities and Emittances

Standardization of thermal emittance measurements :

“…The specific objectives include (1) the selection of suitable materials and the preparation and calibration of specimens as working standards of normal spectral emittance, to be used for calibration of spectral emittance equipment in other laboratories, and (2) the design or selection of suitable data-processing equipment (a) to automatically correct the emittance data for calibration errors of the equipment, (b) to record the corrected data in form suitable for direct entry into an electronic digital computer, and (3) to compute, concurrently with a spectral emittance measurement, a single value of total emittance, solar absorptance or absorptance for radiant energy having any other known spectral distribution of flux.…”

“…Different meanings are ascribed to the same words by different individuals, and some of the conflicting definitions have been adopted by different technical and scientific organizations of national scope. The terminology used in this investigation follows that advocated by Jones [l], Worthing [2] and others, including American Standards Association [3], in that the word-ending "ivity" is reserved for the properties of materials, and "ance" for the properties of specimens. The word-ending "ion", as in reflection, absorption and transmission, is applied to acts or processes; radiation is a process, and sometimes denotes that which has been radiated.…”

“…WADC TR 59-510 Pt IV 2 Absorptance is a property of a specimen; it is the ratio of absorbed radiant flux to incident radiant flux.…”

“…In addition to these considerations involving the angular distribution of radiant flux, there are certain aspects of the spectral distribution that must not be overlooked in applying equations (2) t© (5).…”

“…For extremely narrow wavelength intervals the spectral distribution -of flux from any source that would be used is sufficiently close to blackbody distribution (just as a sufficiently short segment of a circle approximates a straight line).-!/ Furthermore, in considering applicable spectral conditions for equations (2) to (5) a clear distinction must be made between emittance (®r emissivity), on the one hand, and the rate of emission of radiant energy, on the other. For the latter case the counterpart of equation (2) is , e + p + t = (jo (6) in which: e = rate of emission of radiant energy by the specimen p = rate at which radiant energy is reflected from the specimen t = rate at which radiant energy is transmitted through the specimen uo = rate at which radiant energy, of any wavelength distribution, is _____ impinging upon the specimen.…”