diffusion factor, <of a diffusing surface, by reflection or by transmission>

σ

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

Note 1 to entry: 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 to entry: This way of defining the diffusion factor can be applied only to materials for which the scattering indicatrix does not differ appreciably from that of ordinary opal glasses.

Note 3 to entry: For indicating the shape of the scattering indicatrix it is recommended that the diffusion factor, σ, be used for strongly diffusing materials, and the half-value angle, γ, for poorly diffusing materials.

Note 4 to entry: The diffusion factor has unit one.

Note 5 to entry: This entry was numbered 845-04-89 in IEC 60050-845:1987.

Note 6 to entry: This entry was numbered 17-312 in CIE S 017:2011.