A device for non-invasively measuring concentration of one or more analytes in a living subject or a biological sample, wherein the device includes several light sources, a system for controlling the timing and intensity of the light source outputs, a system for passing the light through the subject or sample, a system for measuring the amount of light transmitted, and a system for relating the measurement to the concentration of the analyte in question. The light sources are narrow band sources at different wavelengths, and are capable of being rapidly switched between two levels of intensity. The actual number of light sources required and the wavelengths of the sources are dependent upon the specific analyte being measured.