An embodiment of the disclosure provides a system for determining information on one or more constituents in a medium. The system includes N light emitters L1 . . . LN, wherein each light emitter Lx provides an amplitude modulated (AM) light at modulation frequency fx into a flow path of the medium from one side of a containment vessel for the medium. The system further includes a photodetector, for receiving the AM light from each light emitter after it passes through the flow path of the medium, and converting the AM light to an electrical signal characterized by a summation of frequency components from each modulation frequency fx. The system further includes one or more measuring circuits providing information about a concentration of one or more constituents in the medium determined from log ratios of a pair of amplitudes of fy and fz frequency components in the electrical signal.