A spectrophotometric instrument including a source of measurement light signals having measurement light wavelengths, and a probe having a tissue-engaging surface, a plurality of send fibers coupled to the measurement light signal source for transmitting the measurement light signals to the tissue-engaging surface, and a plurality of receive fibers for receiving light including the measurement light signals after the measurement light has been transmitted through the tissue. The instrument also has reference signal optics coupled to the measurement light signal between the measurement light signal source and the tissue-engaging surface for transmitting a reference light signal portion of the measurement light signal, a detector for generating electrical signals representative of the measurement light signals and the reference light signals, optical paths for coupling the measurement light signal from the receive fibers and the reference light signal portion from the reference signal optics to the detector, and an optical path control for selectively allowing either the measurement light signal portion or the reference light signal portion to the detector. The optical path control enables the detector to output a reference light sample value when the reference light signal portion is coupled to the detector, and to output a measurement light sample value when the measurement light signal portion is coupled to the detector.