Abstract: A specialized spectrometer comprises a light source to generate a monochromatic light beam to generate resonance Raman peaks or resonance near infrared absorption peaks from a region of the subject and a reference light source to illuminate the region with a reference light beam. The light energy from the resonance Raman peaks can be measured with a detector coupled to an optical wavelength separator. A portion of the reference beam scattered from the region can be measured with a reference detector. An amount of the constituent can be determined with a processor in response to the resonance Raman peaks measured with the detector and the portion of the reference beam measured with the reference detector. The illumination of the region with reference beam with the monochromatic beam allows accurate determination of the amount of the constituent, and the amount may comprise a concentration, such as an amount per unit volume.