Abstract: A highly stable and sensitive photometric apparatus is obtained by combining a sensitive, stable, unchopped, dual-beam optical photometer with an accurate electronic sensing circuit. The circuit includes PIN silicon diode detectors on which the two beams of the photometer are imaged. One beam is a reference to detect light source variations and the other a signal beam passing through the sample cell of the photometer. The PIN diode outputs are inputs to parametric amplifiers. The amplifier outputs are then provided to log taking means which take the logarithms of the two signals and then take the difference of the two logarithms to provide a final output proportional to absorbtion in the cell.