Abstract: An optical measurement assembly includes a processor for providing a light source, for receiving light from a sample, and for analyzing the received light for detecting and measuring a target substance, and an optical head including a housing, a source light mirror mounted in the housing for directing a light beam onto the sample, the beam having a central axis extending from the source light mirror, a collector mirror mounted in the housing and having a central axis coinciding with the light beam central axis, and configured to receive light reflected from the sample, and a return mirror disposed in the housing for receiving light from the collector mirror. A source light guide extends from the processor light source to adjacent the optical head source light mirror, and a return light guide extends from adjacent the optical head return mirror to the light receiving means in the processor.