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