NOVELTY - The detection device has a light source, a light divider for providing 2 sources for measuring and reference light paths (3,4) with adjustment of the phase velocity of the light in the measuring path and an optoelectronic detector (5), at which the divergent light beams obtained from the 2 light paths are superimposed for obtaining a characteristic light distribution. USE - For analysis of substance, chemical or biochemical reaction, or for refractive index measurement. ADVANTAGE - The device allows direct evaluation of chemical or biochemical reaction without using fluorescent or radioactive substance.