Abstract: A device for non-invasively measuring an analyte within the body uses a source of light to illuminate a tissue component, then detects and measures the light during more than one selected time period. The light source may be tuned or coupled with one or more filters to provide a selected light spectrum. The tissue component may be any extremity which contains blood and can conduct light, preferably a finger or ear lobe. The detector selects a wavelength or group of wavelengths. The light source may be varied, the source filters may be modulated, or the detector may be sampled at selected times. In general, it is desirable to take one measurement during maximum blood flow in the tissue component and a second measurement during minimum blood flow. The blood flow may be modified by pressure or temperature. The minimum blood flow measurement provides a reference to be subtracted from the signal for the analyte of interest in blood.