Apparatus and methods for detecting foreign material obstructing a blood content sensor window to facilitate higher data accuracy from such sensors, in accordance with one embodiment of the invention for detecting foreign matter with the blood content sensor when the blood content sensor is not contacted with living tissue, an illuminator for blood content detection and the blood content sensor share a common outer window. When the window is at a distance from tissue and in absence of foreign matter on the window, all or most of the emitted light will be dispersed and little or no light will be reflected back into the sensor. Conversely, when the window is at a distance from tissue and foreign matter is present on the window, a substantial portion of the emitted light will be reflected back into the sensor indicating the presence of the foreign matter.
