During each scan line, dark reference cells on the CCD are sampled and {HEAD ~ the voltage} from {TAIL ~ the sampling} is subtracted from the CCD signal 
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During each scan line , dark reference cells on the CCD are sampled and { HEAD ~ the voltage } from { TAIL ~ the sampling } is subtracted from the CCD signal
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH