A non-invasive blood constituent measuring device and method are disclosed for measuring changes in blood thickness of predetermined blood constituents relative to total change in blood thickness at a test area to thereby determine the concentration of such constituents in the blood in a living body, which measured constituents may be, for example, hemoglobin and oxyhemoglobin to enable determination of oxygen saturation of blood. The device includes a plurality of light emitting diodes operationally controlled by timing circuitry for sequentially emitting light at different predetermined wavelengths toward a blood containing tissue sample, such as an ear lobe. A linear sensor receives emitted light passing through the sample and a train of AC modulated pulses indicative thereof is formed and then the signal representative of the light received from each emitter is scaled so that the DC components of each are normalized to a predetermined reference level with the pulse train being divided into channels at a decoder where remaining DC offset is removed and the DC component in each channel is then removed at a low pass filter, after which the AC signals in each channel are multiplexed and converted to a digital signal indicative of changes in the thickness of blood constituents for processing in a digital processor to determine therefrom the saturation of the measured blood constituents. A test unit is also included for testing operation of the device by introducing known AC modulated test signals into the circuitry.