Abstract: The invention is an optical measuring unit (10) for carrying out a reflective measurement, the unit comprising a light source (11) suitable for illuminating a surface to be measured, a measuring sensor (12a, 12b) for detecting a light reflected by the surface to be measured, and a light blocking optical element (13) separating the measuring sensor (12a, 12b) from the direct light of the light source (11) and having an internal space that comprises a straight centreline light tube (14) extending from the light source (11) to the surface to be measured. The inner space of the light blocking optical element (13) comprises a shaded space (19) extending from the centreline of the light tube (14) farther than the light tube (14), in which a monitoring sensor (15) is arranged, said monitoring sensor (15) being subjected to a part of the direct light of the light source (11) and enabling compensation of a change of the light emission of the light source (11).