Abstract: A spectrophotometric instrument for conducting in vivo patient examinations has a sensor which is applied to the patient target area, e.g. the forehead, which includes a source for emitting electromagnetic energy e.g. selected wavelengths in the near infrared range, such that the energy passes through the underlying tissue and is emitted at other locations spaced from the point of entry. The sensor also includes detectors for receiving the resulting light energy at two or more such other locations and sending corresponding signals to a processor for analysis, by which characteristics of the tissue transmissed by the examination wavelengths may be determined.