A method for using a medical device comprising an optical sensor to measure calibrated oxygen saturation in a body tissue uses a standard spectral response of blood established for multiple of oxygen saturations and a standard spectral response of a reference material. The standard responses are established using a spectrometer. The spectral power output of the optical sensor is measured using a spectrometer. The optical sensor output signal response to the reference material is obtained. A processor computes a device-specific calibration curve for the medical device using the measured spectral power output and the standard spectral response of blood and computes an optical gain using the standard spectral response of the reference material and the measured spectral power output of the optical sensor. The device-specific calibration curve and optical gain of the optical sensor are stored in a memory of the medical device.
