A measurement light detector detects light transmitted by a light source (104,106,108) of an implantable system that is scattered back into an implantable housing (804), and produces a measurement signal indicative of the intensity of the light detected by the measurement light detector (806). A calibration light detector (1106) detects a portion of the transmitted light that has not exited the housing (804), and produces a calibration signal (1132) that is indicative of the intensity of the light detected by the calibration light detector (1106), which is indicative of the intensity of the light transmitted by the light source. Changes in the intensity of the transmitted light are compensated for based on the calibration signal (1132) produced by the calibration light detector (106).
