A method and apparatus for noninvasive measurement of a human body component, such as glucose, in vivo, include near infrared spectroscopy based laser sources driven at a carrier frequency lying within the characteristic absorption of the component. The apparatus also drives the laser diodes with a modulation frequency to generate a frequency difference between the measuring light and reference beam, and the interference of the two beams results in a beat frequency signal, which frequency is proportional to the optical path difference between the measuring light and the reference beam. The scattering lights from human tissue with different optical lengths are simultaneously detected and selected based on the beat frequency. The method is a convenient embodiment of the floating reference principle, which takes advantage of optical length selection.