Abstract: Both junctions of a thin film thermocouple are formed on one side of a glass substrate. That side of the substrate is exposed to the diesel engine oil the soot content of which is to be measured. A light source on the other side of the substrate has its radiation directed through the substrate to a location at one of the junctions so that the light is absorbed by the oil adjacent that junction to a degree determined by its soot content. The temperature of the oil and the adjacent junction are increased by an amount dependent on the soot concentration and the thermocouple output is therefore a measure of the soot concentration in the engine oil.