Abstract: The output of a light source preferably of a fluorescent lamp is filtered to provide light energy in a predetermined spectrum and this light energy used to excite a biological specimen which may comprise antigens and antibodies which are tagged with flurorescent material. Light energy at a predetermined frequency is emitted by the fluorescent material in response to this excitation, the magnitude of this light energy representing the population of the biological specimen of interest. This light energy is used to excite a photovoltaic cell which produces an electrical output in accordance with the excitation thereof. Compensation for temperature drift and other offset voltages which might be developed in the photovoltaic cell is provided by means of a second mating photovoltaic cell from which light excitation is eliminated, the output of this second photovoltaic cell being used as a compensating bias voltage to eliminate voltage outputs due to temperature drift and other offset voltages.