Symbols - Units And Nomenclature For Radiometry And Photometry - An Update

SYMBOLS - UNITS AND NOMENCLATURE FOR RADIOMETRY AND PHOTOMETRY - AN UPDATEIrving J . SpiroThe Aerospace CorporationEl Segundo, CaliforniaThis paper is an update of part of an article in the SPIE Journal for February- March 1966, entitled ff Symbols, Units, and Nomenclature for Atmospheric Transmis­ sion, ff by Irving J. Spiro, R. Clark Jones, and David Q. Wark. In the interim 1966- 1974, Fred Nicodemus, lately of the Naval Weapons Laboratory at China Lake, Cali­ fornia and now with the National Bureau of Standards (NBS) at Gaithersburg, Maryland, has done an outstanding job in solidifying the concepts proposed (as a committee action) in 1966.Together with associates, Nicodemus has expanded the logic and codified the con­ cepts, both in the Military Standard on Infrared Terms and Definitions, AD-784 341 (December 1971) and in the Reference Book of Radiometric Nomenclature soon to be pub- lished by John Wiley and Sons. Tables 1, 2, and 3 are adapted from these two sources. They represent a complete geometrical and logical collection of all possible radiometric and photometric concepts.ALTERNATE NAMESRadiant incidance is also called ir radiance.Luminous incidance was also called illuminance, but the CIE has objected to this term and recommended that it no longer be used.Radiant sterance is also called radiance.Luminous sterance is also called luminance.Despite the acceptable alternates, there are good reasons why the term sterance should be retained above all others.The reader is reminded that all other concepts can be derived from the spectral radiant sterance. There are two excellent articles which describe all of these deriva­ tions. One is by R. Clark Jones in "Radiometry, " IR Physics, Vol. 5, pp 19- 36, where the sterance is treated as a field quantity; the other is by Fred Nicodemus in"Radiometry, " Applied Optics and Optical Engineering, Vol. 4, Academic Press, N.Y. (19o2), where the sterance is based on the power from a source.NEW NAMESAs stated earlier, all other concepts can be derived from the sterance. By inte­ grating with respect to area or solid angle, the definition can be found for intensity, exitance, and incidance. There have been objections raised to all of these terms. "Intensity" should be used for flux per unit solid angle (from a point) but has frequently been misused because of other definitions for intensity. "Exitance" and "incidance" are coined terms for flux per unit area, although why separate terms are needed for the flux arriving at or leaving a surface has never been made clear. "Incidance" is frequently corrected to "incidence" by secretaries, proofreaders, or typesetters, who cannot find the word in the dictionary.While discussing alternate terms for "intensity," Ed Zalewski and Jon Geist of NBS suggested the term "pointance. " This is terse, has a Latin root, is a clean conno­ tation of the geometrical meaning (from a point), and pairs well with sterance.The term "areance" was coined origi­ nally in 1973 by Professor Jurgen Meyer- Arendt. Later, Nicodemus proposed that areance could be similarly used for the flux per unit area. Again it is short, has a clear geometrical meaning, a Latin root, and pairs well with sterance. Also, if there is the need for separate terms for incident and emergent flux, in- and ex- could be used to get in-areance and ex-areance.The introduction of pointance and areance solves the problem of having unde­ sirable terms, and provides three well- matched, easily understood, purely geomet­ rical terms. Please consider these terms in relation to your work; give them a fair comparison with your more familiar terms. If there is favorable response, the words34