This article describes how the colour of translucent sheets (with various transparencies) can be properly evaluated starting from a transmission measurement. Special attention must be paid to the position of the sample, which must be tangent to the integrating sphere. The paper also refers to the evolution of the CIELAB colour difference equation, with particular reference to the CMC (1:1) colour tolerance system, and to the new LABmg colour space, which has been regulated by UNI (the Italian authority). The LABmg space has a tolerance ratio from 1 to 1.5, which is much more advantageous than the 1 to 5 tolerance ratio of the corresponding CMC system. This means that, by using the LABmg space, it is possible to fix numerical tolerances for all colours which are practically equal to 1.
