A system for non-invasive measurement of parameters relating to a biological tissue comprising: a plurality of light sources, each operable to emit a light signal with one or more predetermined wavelengths a light detector for detecting light from the tissue as a result of illumination by the sources, and means for applying modulation functions to the light emitted to implement code division multiplexing, each source being associated with a different function, wherein the functions are selected so that there is substantially no cross correlation between them, and wherein the sources have overlapping spectra.
