A method and an instrument for the non-invasive detection of an analyte (e.g. glucose) concentration in human body tissue such as blood is disclosed. The instrument measures analyte (e.g. glucose) concentration by detecting the infrared radiation naturally emitted by a human body using infrared detector with combination of adequate filters such as for example negative correlation filter or narrow band filters or other detector-filter assemblies. The method and instrument is based on the discovery that natural infrared emission from the human body, especially from the tympanic membrane, is modulated by the state of the emitting tissue. Spectral emissivity of human infrared radiation from the tympanic membrane consists of spectral information of the tissue (e.g. blood) analyte (e.g. glucose). This can be directly correlated with the blood analyte concentration, for example, the blood glucose concentration.