an improved method and apparatus for determining noninvasively and in vivo one or more unknown values of a known characteristic , particularly the concentration of an analyte in human tissue . the method includes : irradiating the tissue with infrared energy having at least several wavelengths in a given range of wavelengths so that there is differential absorption of at least some of the wavelengths by the tissue as a function of the wavelengths and the known characteristic , the differential absorption causeing intensity variations of the wavelengths incident from the tissue ; providing a first path through the tissue ; optimizing the first path for a first sub - region of the range of wavelengths to maximize the differential absorption by at least some of the wavelengths in the first sub - region ; providing a second path through the tissue ; and optimizing the second path for a second sub - region of the range , to maximize the differential absorption by at least some of the wavelengths in the second sub - region . in the preferred embodiment a third path through the tissue is provided for , which path is optimized for a third sub - region of the range . with this arrangement , spectral variations which are the result of tissue differences can be reduced . at least one of the paths represents a partial transmission path through the tissue . this partial transmission path may pass through the nail of a finger once and , preferably , twice . also included are apparatus for : reducing the arterial pulsations within the tissue ; and maximizing the blood content in the tissue .