An optical sensor system (20) has an optical detector (28) with an active detector area (38) and a detector output signal (40). A test-signal optical source (44) has a controllable optical source (48) having an output of a wavelength detectable by the optical detector (28), and a test-signal director (50) that directs the output of the optical source (48) to the active detector area (38) of the optical detector (28). A housing (22) encloses the optical detector (28) and the test-signal optical source (44). A test instrumentation controller (52) controls the operation of the optical source (48), and receives the detector output signal (40) for evaluation.