A system visually distinguishes diseased tissue from healthy tissue after a treatment is administered to a patient to provide different concentrations of a fluorescent marking substance between the diseased tissue and the healthy tissue. A light source illuminates the tissue with excitation light. A detector detects light returning from the tissue, and a controller characterizes the returning light according to a measured property indicative of the different concentrations. A light projector projects light having a predetermined cue in response to the characterization of the returning light.
