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A measurement light detector detects light transmitted by a light source of an implantable system that is scattered back into an implantable housing, and produces a measurement signal indicative of the intensity of the light detected by the measurement light detector. A calibration light detector de

A measurement light detector detects light transmitted by a light source of an implantable system that is scattered back into an implantable housing, and produces a measurement signal indicative of the intensity of the light detected by the measurement light detector. A calibration light detector detects a portion of the transmitted light that has not exited the housing, and produces a calibration signal that is indicative of the intensity of the light detected by the calibration light detector, 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 produced by the calibration light detector. This description is not intended to be a complete description of, or limit the scope of, the invention. Other features, aspects, and objects of the invention can be obtained from a review of the specification, the figures, and the claims.

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What is claimed is: 1. A method for compensating for changes in intensity of light transmitted by a light source of an implantable system that includes an implantable housing including a window through which light can pass; a light source, within the housing, that transmits light, wherein a portion

What is claimed is: 1. A method for compensating for changes in intensity of light transmitted by a light source of an implantable system that includes an implantable housing including a window through which light can pass; a light source, within the housing, that transmits light, wherein a portion of the light exits the housing through the window; and a measurement light detector, within the housing, to detect light scattered back into the housing through the window, and to produce a measurement signal that is indicative of the intensity of the light detected by the measurement light detector; the method comprising: (a) providing a calibration light detector within the housing to detect a portion of the light that has not exited the housing, and to produce a calibration signal indicative of the intensity of the light detected by the calibration light detector, which is indicative of the intensity of the light transmitted by the light source; (b) producing calibration measurements based on the calibration signal; (c) using the calibration measurements to perform calibrations that compensate for changes in the intensity of light transmitted by the light source; and (d) between consecutive calibration measurements, predicting a metric indicative of a change in the intensity of the light since a most recent calibration measurement, and using the predicted metric to perform a calibration that compensates for predicted depletion in the intensity of light transmitted by the light source. 2. The method of claim 1, wherein step (c) includes: detecting changes in the intensity of the light transmitted by the light source based on the calibration measurements; and taking into account the detected changes in the intensity of the transmitted light when using the measurement signal for a diagnostic and/or therapeutic purpose. 3. The method of claim 2, further comprising using the measurement signal to determine levels of blood oxygen saturation and/or levels of hematocrit, wherein the detected changes in the intensity of the transmitted light are taken into account when using the measurement signal to determine levels of blood oxygen saturation and/or levels of hematocrit. 4. The method of claim 1, wherein step (c) includes: adjusting the measurement signal, based on the calibration measurements, to compensate for changes in the intensity of the light transmitted by the light source. 5. The method of claim 1, wherein step (c) includes: adjusting a drive signal that drives the light source, based on the calibration measurements, to keep the intensity of the light transmitted by the light source substantially constant. 6. The method of claim 1, wherein calibrations performed at step (c) are performed periodically. 7. The method of claim 1, wherein calibrations measurements produced at step (b) are produced periodically. 8. The method of claim 7, wherein: calibrations performed at step (c) are performed periodically; calibrations measurements produced at step (b) are produced periodically; and calibrations are performed at step (c) more often than calibration measurements are produced at step (b). 9. The method of claim 1, wherein calibration measurements are only taken at step (b) when it is determined or presumed that the patient is at rest. 10. The method of claim 1, wherein step (b) includes using the calibration light detector to obtain calibration data for a period of time, and averaging or otherwise combining the calibration data to produce a said calibration measurement. 11. A method for compensating for changes in intensity of light transmitted by a light source of an implantable system that includes an implantable housing including a window through which light can pass; a light source, within the housing, that transmits light, wherein a portion of the light exits the housing through the window; and a measurement light detector, within the housing, to detect light scattered back into the housing through the window, and to produce a measurement signal that is indicative of the intensity of the light detected by the measurement light detector; the method comprising: (a) providing a calibration light detector within the housing to detect a portion of the light that has not exited the housing, and to produce a calibration signal indicative of the intensity of the light detected by the calibration light detector, which is indicative of the intensity of the light transmitted by the light source; (b) producing calibration measurements based on the calibration signal; (c) using the calibration measurements to perform calibrations that compensate for changes in the intensity of light transmitted by the light source; (d) estimating a depletion curve for the light source based on calibration measurements; and (e) between consecutive calibration measurements, using the estimated depletion curve to perform a calibration that compensates for predicted depletion in the intensity of light transmitted by the light source. 12. A method for compensating for changes in intensity of light transmitted by a light source of an implantable system that includes an implantable housing including a window through which light can pass; a light source, within the housing, that transmits light, wherein a portion of the light exits the housing through the window; and a measurement light detector, within the housing, to detect light scattered back into the housing through the window, and to produce a measurement signal that is indicative of the intensity of the light detected by the measurement light detector; the method comprising: (a) providing a calibration light detector within the housing to detect a portion of the light that has not exited the housing, and to produce a calibration signal indicative of the intensity of the light detected by the calibration light detector, which is indicative of the intensity of the light transmitted by the light source; (b) producing calibration measurements based on the calibration signal; and (c) using the calibration measurements to perform calibrations that compensate for changes in the intensity of light transmitted by the light source, wherein: calibrations measurements produced at step (b) are produced periodically; the implantable system includes multiple said light sources, each of which transmits light of a corresponding wavelength; the measurement light detector detects light from each said light source scattered back into the housing through the window, and produces a said measurement signal for each said light source; the calibration light detector produced calibration measurements for each said light source at step (b); and calibrations are separately performed for each said light source at step (c). 13. The method of claim 12, wherein calibrations are performed at step (c) more often than calibration measurements are produced at step (b). 14. An implantable system, comprising: an implantable housing including a window through which light can pass; a light source, within the housing, that transmits light, wherein a portion of the light exits the housing through the window; a measurement light detector, within the housing, to detect light scattered back into the housing through the window, and to produce a measurement signal that is indicative of the intensity of the light detected by the measurement light detector; a calibration light detector within the housing to detect a portion of the light that has not exited the housing, and to produce calibration measurements based on the calibration signal, the calibration measurements indicative of the intensity of the light detected by the calibration light detector, which are indicative of the intensity of the light transmitted by the light source; and a processor to use the calibration measurements to perform calibrations that compensate for changes in the intensity of light transmitted by the light source, wherein the processor, between consecutive calibration measurements, predicts a metric indicative of a change in the intensity of the light since a most recent calibration measurement, and uses the predicted metric to perform a calibration that compensates for predicted depletion in the intensity of light transmitted by the light source. 15. An implantable system, comprising: an implantable housing including a window through which light can pass; a light source, within the housing, that transmits light, wherein a portion of the light exits the housing through the window; a measurement light detector, within the housing, to detect light scattered back into the housing through the window, and to produce a measurement signal that is indicative of the intensity of the light detected by the measurement light detector; a calibration light detector within the housing to detect a portion of the light that has not exited the housing, and to produce calibration measurements based on the calibration signal, the calibration measurements indicative of the intensity of the light detected by the calibration light detector, which are indicative of the intensity of the light transmitted by the light source; and a processor to use the calibration measurements to perform calibrations that compensate for changes in the intensity of light transmitted by the light source, wherein the processor, between consecutive calibration measurements, uses a depletion curve for the light source to perform a calibration that compensates for predicted depletion in the intensity of light transmitted by the light source. 16. The system of claim 15, wherein the processor estimates the depletion curve for the light source based on calibration measurements. 17. The system of claim 15, wherein the depletion curve is provided by a manufacturer of the light source.