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