ratio of the luminance of the surface element in the given direction to that of a perfect reflecting or transmitting diffuser identically illuminated
unit : 1
NOTE 1 – For photoluminescent media, the luminance factor is the sum of two portions, the reflected luminance factor βs and the luminescent luminance factor βL : βv = βS + βL.
NOTE 2 – In Germany, instead of the luminance factor β (or transmittance τ) it is usual in colorimetry to use the quantity "Hellbezugswert" A = 100 β (or 100 τ).
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