An optical assembly and method for the non-invasive determination of a concentration of an analyte in a tissue sample is disclosed. The assembly comprises an optical arrangement comprising a first optical interface for reflecting light incident thereon and a second optical interface for reflecting light incident thereon. The second interface comprises an interface between an optical element of the arrangement and the tissue sample under investigation. The light reflected from the first and second interfaces is arranged to combine to generate an interference pattern characteristic of a difference in phase between light reflected from the first interface with the light reflected from the second interface.