The principle of scattering of light involves the comparison measurement of the intensity of the reflected light at two different angles, the roughness factor can then be calculated.  On the other hand, color can also be measured with this method, but only the light perpendicular to the surface is used, and instead of a photo-detector, three detectors are used, one for green, one for red, and one for blue.
Technical Description

K  = scaling coefficient ( = 100 )

Js = intensity of the scattered component        

Jr = intensity of the reflected component

R  = relative roughness size                    

R = K 2 Js / ( Js + Jr )

Example estimates: On mirror Jr = max  Js = 0  R = 0

                              On paper Jr = Js                R = 100

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