Abstract: An apparatus is disclosed which comprises a photoelectric probe for introducing light pulses into a measurement area and for producing a measurement signal representative of the light as modulated in the measurement area, such modulation including modulations characteristic of the circulatory system blood volume variations of the area. Demodulation circuitry is disclosed which demodulates the meaningful from the non-meaningful data of the measurement signal, and utilization devices responsive to various characteristics of the meaningful data are disclosed, which utilization devices include a pulse wave meter, pulse amplitude meter, pulse rate meter, and tissue perfusion meter.