A method for measuring a chemical concentration in tissue has two measurement steps. A portion of tissue is illuminated with a first generated light and a second generated light. A first Raman scattered light, corresponding to the first generated light, and a second Raman scattered light, corresponding to the second generated light, are directed to a plurality of light sensors, each light sensor measuring light at a different wavelength, that wavelength being proximate to a wavelength of an expected Raman shift wavelength for the chemical in the tissue. A measurement is obtained from each light sensor, each measurement being specific to the first scattered light and/or the second reflected light to that light sensor. The measurements of the first scattered light and the measurements of the second scattered light are used to calculate a concentration of the chemical in the tissue.