Abstract: A device for detecting fluorescence generated in a sample medium in a cuvette tube irradiated with light for analysis purposes. The device includes a main element comprising a light source, an optical element mounted on the cuvette tube in optical contact therewith, and a photodetector. The light source is placed on the outside of the cuvette tube so as to send light into the tube in a direction essentially transversely to the longitudinal axis of the tube. The refractive index of the optical element is essentially the same as the refractive index of the wall of the cuvette tube. The photodetector functions to detect fluorescence light which falls on the detector after having passed in the cuvette tube via the optical element.