A device for determining a concentration-related quantity of a fluorescent contrast agent applied to an object (2), in particular a turbid medium. Said device generally comprises a source (4) of electromagnetic radiation for irradiating the object (2) at an excitation wavelength and at least one first detecting means (6, 7.1, 7.2, . . . , 8) for detecting fluorescent electromagnetic radiation emitted by the contrast agent at a fluorescence wavelength, said first detecting means producing fluorescence intensity data (F). The proposed device further comprises at least one second detecting means (6, 7.1, 7.2, . . . ) for detecting electromagnetic radiation transmitted by the object (2) at the excitation wavelength, said second detecting means producing transmission intensity data (T), and evaluating means (10) adapted to receive the transmission intensity data and the fluorescence intensity data and to determine said concentration-related quantity of the contrast agent from a ratio (R) of fluorescence intensity data and transmission intensity data.
