According to the present invention, a method and apparatus for non-invasively determining the blood oxygen saturation level within a subjects tissue is provided. The method comprises the steps of: a) providing a spectrophotometric sensor operable to transmit light into the subjects tissue, and to sense the light b) detecting light after passage through the subjects tissue using the sensor, and producing initial signal data from the light sensed c) calibrating the sensor to that particular subject using the initial signal data, thereby accounting for the specific physical characteristics of the particular subjects tissue being sensed and d) using the calibrated sensor to determine the blood oxygen parameter value within the subjects tissue.
