A. Introduction to surface characteristics

To the thermal designer, surface radiation characteristics are numbers that must be entered into a computer in order to get answers to the questions put to the computer about the size, orientation, or spacing of elements; material selection; and so forth. The designer hopes that values of these characteristics can be found in a handbook or data compilation. At times, however, the handbook leaves the designer unsatisfied. It lists no value for a given material or such a large range of values that the designer realizes that a test measurement is in order. Thereupon it becomes apparent that many different kinds of tests can be conducted; many different numbers can be obtained. Which test or set of tests is to be run? How is a single number to be extracted from the test results? What do the numbers that are listed in the handbook mean? Are they really applicable?

It is apparent that two categories of radiation characteristics must he distinguished: (1) model characteristics used to model surface behavior in a computer program or calculation scheme, and (2) real characteristics that describe how the surface really docs behave. This section begins with a review of real characteristics and their measurements, reviews the electromagnetic theory of reflection, and concludes by examining the use of surface characteristics in thermal design.

The nomenclature used for surface radiation characteristics varies considerably in the literature. One question that often occurs is the meaning of an “ivity” ending versus an “ance” ending. Worthing, of the U.S. National Bureau of Standards, at one time suggested using “ivity” for an intrinsic property of a pure, perfectly polished and annealed specimen, and “ance” for surfaces having roughness, surface oxide layers, or other nonideal features. Various workers have followed the suggestion at one time or another, and others have not. The “ivity” ending used here has no special meaning and may be regarded as interchangeable with the “ance” ending.

B. Absorption and emission characteristics