Abstract: A method and apparatus for making an optical measurement through a tissue such as skin of a person includes an implantable optical system having an entry window, an exit window, a reference path between the windows and a measurement path between the windows. The reference path includes a reference element with known optical characteristics, and the measurement path is in fluid communication with the body fluids of the person. A first optical signal is directed through the tissue of the person, the entry window, the reference path, the exit window and the tissue of the person to an external optical sensor, and a second optical signal is directed through the same portion of the tissue of the person, the entry window, the measurement path, the exit window and the same portion of the tissue of the person to the external optical sensor. Variations in the optical effects of the tissue on the measurement can be measured with the second optical signal and used to improve measurement accuracy.