17-1059

reflectance factor (at a surface element, for the part of the reflected radiation contained in a given cone with apex at the surface element, and for incident radiation of given spectral composition, polarisation and geometric distribution) [R]

ratio of the radiant flux or luminous flux reflected in the directions delimited by the given cone to that reflected in the same directions by a perfect reflecting diffuser identically irradiated or illuminated

Unit: 1

NOTE 1 For regularly reflecting surfaces that are irradiated or illuminated by a beam of small solid angle, the reflectance factor may be much larger than 1 if the cone includes the mirror image of the source.

NOTE 2 If the solid angle of the cone approaches 2π sr, the reflectance factor approaches the reflectance for the same conditions of irradiation.

NOTE 3 If the solid angle of the cone approaches 0, the reflectance factor approaches the radiance factor or luminance factor for the same conditions of irradiation.

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