the signal processing unit being configured to calculate time averages of the weighted first signal component and the weighted second signal component from a time when the object to be measured begins to enter the region to a time when the object to be measured begins to exit the region, to determine {HEAD ~ a phase difference} between {TAIL ~ the modulation signal} and the signal of the fluorescence by using the calculated time averages 
the signal processing unit being configured to calculate time averages of the weighted first signal component and the weighted second signal component from a time when the object to be measured begins to enter the region to a time when the object to be measured begins to exit the region , to determine { HEAD ~ a phase difference } between { TAIL ~ the modulation signal } and the signal of the fluorescence by using the calculated time averages
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