How can you uncouple changing extraneous light of the measurement signals?
 
You can do this by using pulsed light sources and the difference calculation between the measurement with and measurement without target illumination. The difference calculation can be conducted by software after corresponding ADC accesses. Or it can be realized in the hardware by an AC coupling, that will evaluate the changing signal parts, which are generated by the frequency of the illuminating source. But note, that for pulsed illuminating source there will be realized a multiple of possible frequency of the foreign illuminating source.

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