Abstract: An apparatus for non-invasive measurement within a human or animal body with an emitter for emitting electromagnetic radiation into the human or animal body and a detector for detecting the scattered and attenuated radiation. The detector comprises a first principal detector and at least one second reference detector. The second detector is disposed a distance from the emitter which is less than the distance of the first principal detector from the emitter. The second detector is disposed on a line of radius from the emitter which forms an angle with the line of the radius from the emitter on which the first principal detector lies.

Abstract: A surface sensor device (1) with an electromagnetic radiation emitter (3) and a radiation detector (2) disposed on opposite sides of an instrument outlet (6), which instrument outlet (6) is required to be placed in close proximity to a surface (30). The surface sensor (1) also having a member (8) disposed in the path of the emitted radiation and the radiation reflected from the surface (30). The member (8) allows a portion of the emitted radiation to pass within it from the emitter (3) to the detector (2) while providing a path for a further portion of the radiation to pass through the member (8) towards the surface (30) for scattering and reflection by which means light is returned to the member (8) back towards the detector (2).