Abstract: An optical oil monitor that measures particle contamination in oil by passing light through an oil sample and picking up the light that is scattered at 90.degree. by the particle contamination and measures chemical breakdown by the attenuation of the light passing substantially straight through the oil with a second sensor. Alternately a sample and a reference are passed between the light responsive sensors for error correction and calibration so that each sensor will have an output signal alternating between a sample signal and a reference signal. A control signal of the same frequency is provided in combination with a plurality of gates to separate the sample and reference signals. The attenuation reference signal is compared to a standard signal to produce an error output for regulating the intensity of the light source to compensate for power source fluctuations, lamp characteristic changes, window soiling and the like.