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