Abstract: A non-invasive and an in-vitro method for determining a person's hemoglobin concentration are described. The methods employ substantially simultaneous measurements of absorbance of near-infrared and long wavelength visible light. The measurement consists of a combination of ordinary absorbance data at multiple wavelengths in the form of a ratio or of a ratio of derivative absorbance data. The method minimizes the need for path-length measurement or extinction coefficient determination or estimation of scattering losses. In the in-vitro setting, the method minimizes the need for cell lysis or for reagents.