The present invention is related to optical non-invasive methods and instruments to detect the level of analyte concentrations in the tissue of a subject. The spectra of mid-infrared radiation emitted from a subject's body are altered corresponding to the concentration of various compounds within the radiating tissue. In one aspect of the invention, an instrument floods a body surface of the subject, such as the subject's eye, with radiation in the mid-infrared range and measures analyte concentrations based on mid-infrared radiation reflected back to the instrument.