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 measures the level of mid-infrared radiation from the subject's body surface, such as the eye, and determines a specific analyte's concentration based on said analyte's distinctive mid-infrared radiation signature.