Optical diagnosis system for small animal imaging, in which the animal which has been arranged on a bearing plate and treated with an activatable optical contrast medium is irradiated by an excitation source and the resulting fluorescent radiation that is radiated back is detected by a detector system, in which case the bearing plate is designed as a radiation-transparent window (13) for a reference radiation—generated by a second marker of the contrast medium—for the detection of the initial concentration of the inert contrast medium.