Downloaded from https://www.everyspec.com MIL-STD-81OC b. Albedo. Albedo is the ratio of the amount of electromagnetic’radiation reflected by a body to the amount incident upon it. Th”isfraction is often expressed as a’percentage, such as 34 percent for earth albedo. The spectrum of reflected ener~ is generally different from that of the incident radiation. The definition of albedo, while identical with reflectance, is commonly used in astronomy and meteorology to describe the reflectivityof planetary bodies and their atmospheres to solar radiation. c. Biaek body. A black body is defined to be a body which absorbs all incident energy.
Therefore its absorptance a , is equal to 1 for all wavelengths. According to Kirchoff1s law, such an object will be a perfect emitter of radiation at all wavelengths. d. Collimation angle. See .Solarsubtense angle e. Emissivity.(S). Emissivity is the ratio of the radiant flux emitted by a specimen to that emitted by a black b,odyradiator at the same temperature and wavelength. Generally refers to a specific sample or measurement of a specific sample. Total hemispherical emissivity is the energy emitted over the hemisphere above the emitting e.1ement for al1 wave1engths. Normal emissivity refers to the emissivity normal to the surface to the emitting ) body. f. ‘Radiance.
Radiance is radiant flux emitted in a specified direction per unit projected area of surface, per unit solid angle. The preferred symbol for this quantity is L, and is usually expressed in units of Watts cm -2 -1 sr . The irradiance (radiant flux density) produced by radiation from the source upon a unit surface area oriented normal to the line between source and receiver, divided by the solid angle subtended by the source at the receiving surface, is equal to source radiance if the receiver to source distance is large relative to the source size.
If the radiant source is a perfectly diffuse radiator (that is, emits exactly according to Lambertts Law), then its radiance is equal to its emittance per unit solid angle. g. Radiant-enmgy (Q) . Radiant energy is energy emitted, transferred or received as radiation. 517.2-il METHOD 517.2 B /