Abstract: The intensity 2 of radiation coming from an object 1 to be measured and illuminated by collimated radiation is measured at several wavelengths by focusing the radiation, via a lens 4 and a planar mirror 6, on a detector group 3 having several detector elements (3a to 3d). The radiation coming from the object is directed by the lens and the mirror such that, by rotating the mirror, tilted at a small angle with respect to its axis 5, around this axis and by keeping the radiation on the surface of the mirror, the focus F of radiation is moved in a detector plane D along an uninterrupted circular path R crossing each detector element positioned substantially in the same plane and excited at a different wavelength.