Abstract: The present invention relates to an optical detector device, in particular for analyzing substances, mixtures of substances or chemical reactions and for determining refractive indices, the device comprising a light source, a measurement path and a reference path and an optoelectronic detection means, with an optical divider being provided downstream of the light source for forming two radiation sources for divergently radiated light beams, of which one is assigned to the measurement path and one to the reference path, and the phase velocity of the light being influenced at least in the measurement path, and the divergent light beams being superimposed in the area of the optoelectronic detection means to form a characteristic intensity distribution.