Abstract: A cylindrical block is provided with two ducts, forming a V whose legs terminate in exit windows in one axial end face of the block. A light source is disposed in the bottom of the V to direct light into both ducts. The block is further traversed by a bore, terminating as to one end in between the windows, avoiding the ducts and terminating in an offset position relative to the V-bottom and the light source at the other end of the block. A shaft traverses this bore for rotating a shutter in front of the windows. A complementary, light receiving unit is disposed with misaligned entrance windows in front of the unit windows, but their centers are traversed respectively by the optical axes of the ducts.