Abstract: An apparatus for measuring a blood parameter comprising a red light source and an infrared light source for directing light toward a blood-receiving location and a detector, for receiving light from the first and second light sources which has been back scattered by the blood. The intensity of the first light source is adjusted so that the intensity of light at the detector from the first light source remains substantially constant over a range of values of the blood parameter and the intensity of the second light source is adjusted so that the intensity of light at the detector from the second light source is substantially constant over a range of values of the blood parameter. A signal is provided which is related to the intensity of one or both of the light sources and which provides an indication of the blood parameter. A circuit is provided to compensate for errors resulting from the light sources.