Abstract: A diagnostic apparatus comprises a source (1) of probing electromagnetic radiation and means (2) for transmitting the output from the probing radiation source (1) to biological tissue (3) to be examined. The apparatus also comprises means (4, 41) for detecting probing radiation reflected from the tissue (3) and stimulated radiation resulting from excitation of the tissue (3) by the probing radiation. A processing means (5-9) responsive to the reflected and stimulated radiations to produce a signal for diagnosis of the condition of the tissue and means (15, 16) for regulating the intensity of the probing radiation including a feedback circuit (16, 17, 18) for controlling the regulating means (15) and responsive to the intensity of the probing, reflected and/or stimulated radiation are also included.