diffusion factor (of a diffusing surface, by reflection or by transmission) [σ]

ratio of the mean of the values of luminance, L, measured at 20° and 70° (0,35 rad and 1,22 rad) to the luminance measured at 5° (0,09 rad) from the normal, when the surface considered is illuminated normally


Unit: 1

NOTE 1 The diffusion factor is intended to give an indication of the spatial distribution of the
diffused flux. It is equal to 1 for every isotropic diffuser, whatever the value of the
diffuse reflectance or diffuse transmittance.

NOTE 2 This way of defining the diffusion factor can be applied only to materials for which
the indicatrix of diffusion does not differ appreciably from that of ordinary opal

NOTE 3 See NOTE to “half-value angle”.

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