The first and second photodiodes are operably coupled to means for subtracting the total currents flowing through each photodiode to produce an output electrical signal substantially independent of dark current contributions, and proportional to the level of {HEAD ~ illumination incident} on {TAIL ~ the photodiodes} 
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The first and second photodiodes are operably coupled to means for subtracting the total currents flowing through each photodiode to produce an output electrical signal substantially independent of dark current contributions , and proportional to the level of { HEAD ~ illumination incident } on { TAIL ~ the photodiodes }
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