A CCD imager is composed of a two dimensional array
of light sensitive detectors or pixels. The CCD array
is mechanically quite stable with the pixels retaining
a rigidly fixed geometric relationship. Each pixel within
the array, however, has its own unique light sensitivity
characteristics. As these characteristics affect camera
performance, they must be removed through calibration.
The process by which a CCD camera is calibrated is known
as "Flat Fielding" or "Shading Correction".
Flat fielding can be illustrated in the following
IC = [(IR - IB) * M] / (IF - IB)
Where IC is the calibrated image; IR is the non-calibrated
object exposure; IB is the bias or dark frame; M is
the average pixel value of the corrected flat field
frame; and IF is the flat field frame.
IB Flat fielding requires the acquisition of two
calibration frames. First, a bias frame or a dark frame
should be taken. Bias clears the camera of any accumulated
charge and reads out the cleared CCD. The resulting
image is a low signal value image. In this image, all
of the pixels have approximately the same value, which
consists of the electronic offset of the system of the
inherent structure of the CCD. Dark clears the CCD of
charge, allows charge to accumulate for a specified
amount of time with the shutter closed and then reads
out that charge (dark current). A dark frame contains
the standard bias component as well as the dark signal.
The dark command is most useful when taking long exposures
with low light levels.
IF The second calibation image, the flat field frame,
measures the response of each pixel in the CCD array
to illumination and is used to correct for any variation
in illumination over the field of the array. The optical
system most likely introduces some variation in the
illumination pattern over the field of the array. The
flat fielding process corrects for uneven illumination,
if that illumination is a stable characteristic of each
object exposure. Thus, it is necessary to illuminate
the CCD with a light pattern that is as representative
of the background illumination as possible. This illumination
should be bright enough, or the exposure made long enough,
so that the CCD pixels signals are at least 25 percent
full scale or preferably higher. For a Photometrics
camera equipped with a 12 bit analog processing card,
the level should be at least 1000 ADUs.
IR An exposure of the object of interest is acquired.
(IR - IB) The object frame must be corrected for
electronic offset by subtraction of the bias/dark frame
(IF - IB) The flat field frame must also be corrected
for electronic offset by subtraction of the bias/dark
frame from it. The average pixel value of the bias/dark
corrected flat field frame must then be ascertained