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| == Different sensitivities of sensors ==
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| Why are there differences in sensitivities between individual sensors?
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| Differences in sensitivity between sensors may be caused by different luminous intensities of LEDs, slightly different geometrical alignments of the LED in respect to measuring chamber and photodiode, different photo sensitivities of photodiodes, and different absorption characteristics of filters. We observed strong effects on the observed signal when the orientation (rotation, angle) of an LED was slightly changed. Further, we measured the light intensities recoded by the photodiode of each sensor (n = 16) using either the sensors integrated LED as light source (experiment A) and using an external (always the same) LED as light source (experiment B). Although the geometric reproducibility of experiment B was problematic, the standard deviation was reduced from 7% in experiment A to 2% in experiment B. This indicates that the variation of LED luminosity and small variations in LED alignment may be the main contributing factors to the total variance in sensor sensitivity. In fact, the variance of LED luminosity is a well known problem, partially addressed by LED manufacturers in a pre-sorting process ("binning" or "ranking"). Even within such pre-sorted "bins" (that are not always available) manufacturers state typical luminosity variations of +/-15 to +/- 20%, if any statement is given at all.
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| Different sensitivities do not have adverse effects after calibration.
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