The maximum brightness accommodated depends on the sensor's capacity to hold charge. When more charge is generated than the sensor can hold, the excess charge overflows, just like pouring too much water into a cup. When overflow is too great, it can spill over into the shift registers used to transport charges, causing vertical smear. Smear can be minimized by careful design and the use of overflow drains. There can be a considerable difference in smear resistance among different CCDs, even of the same type.
Pixel pushing Once the charge is accumulated, it needs to be serialized as a video signal.
CCDs act as bucket brigade shift registers-each CCD element can store a charge and pass it along to its neighbor. All CCD chips share a common operational pattern. Charges accumulate on pixels during the field time. At the end of the field, the charges are rapidly moved from the active pixels into some type of storage. During the following field, the stored charges are marched out, one after the other, to form the field's video signal.