Manufacturers of CCDs grade devices according to the number and type of
defective pixels. The manufacturing yield of each sensor grade strongly affects
the CCD cost; the more perfect the sensor, the higher its cost. Because the CCD
is a large-cost component in the overall camera system, the choice of sensor
grade is an important consideration when purchasing a camera. Unfortunately,
each CCD manufacturer uses a different scheme to grade devices.
Grading schemes typically run from a grade 0 device, designating the highest
quality available (nominally defect-free), to grade 1, 2, or 3, with the number
of defects increasing with the grade number. Defect amounts are defined by the
manufacturer in terms of number of pixels, columns, or clusters whose response
differs by ±N%. The deviation, N, is defined independently by each CCD
manufacturer and definitions vary widely.
The central zone is an area in the middle
of the CCD array. The exact location and size varies with the manufacturer.
Defects in this region are usually specified separately from the overall number
This is the group of pixels surrounding the
defect in question, usually 10,000 pixels or less. Again, the exact
specification is manufacturer-specific.
A point defect is a pixel whose response
differs by ±N% compared to the mean values of all pixels in the neighborhood.
"N" can be as low as 6% or as high as 20%, depending on the manufacturer.
This is a group of adjacent point
defects. The maximum allowable number of defective pixels in a cluster varies
between 3 and 9, depending on the manufacturer.
Column or Row Defect:
A column or row defect refers to a
column or row, or partial column or row, whose response varies by at least ±N%
from the neighborhood mean value. "N" is usually the same number reported for
A trap is a pixel that traps charge during
the charge-transfer process. Charge transfers out of the trap at a lower rate,
leading to charge being left behind. Once a trap is filled, a steady state is
reached and it no longer consumes signal electrons. Some manufacturers give
specifications for both the number of low-level traps (filled with typically
<2000 e-) and high-level traps (filled with typically <10,000 e-). The
physical location of the trap is also important, particularly for low-light
applications. Traps in the serial register of the CCD can affect signal from
nearly the entire sensor. Traps in a column only affect that columns signal.
Traps are often quite dependent on the CCDs operating temperature.
Some defects (pixel, cluster, or column) are
substantially brighter than adjacent regions. Often, this is due to
higher-than-average dark current. These defects tend to disappear as the device
is cooled. Because their location and dark current rate are constant, they can
often be compensated for by dark current