Abstract: An optical sensor for non-invasively measuring the content or composition of one or more chemical components within especially a living tissue of a body portion. Radiation sources (38), emit measuring radiation with at least two wavelengths to a portion (6) of the body of a patient. A detector (37) receives the measuring radiation having passed through said body portion of a patient and converts same to electric form. At least one radiation transfer section is located between either the radiation sources or respectively the detectors and the external surface (50) of the body portion of a patient. The radiation transfer section includes measuring-radiation transmitting ends. An outer end (16, 26) facing the external body surface (50) has a surface area which is generally larger than the surface area of an inner end (13a; 19a) facing any given radiation sources or respectively detectors. A measuring-radiation diffusely reflecting surface (14, 17) is positioned between the ends.