A CCD array is mechanically quite stable; the pixels
have a fixed geometric relationship. Each pixel within
the array, however, has unique light sensitivity characteristics.
Because these characteristics affect camera performance,
they must be removed through calibration. The process
by which a CCD image is calibrated is known as fat felding
or shading correction.
fielding requires the acquisition of two calibration
images. Both images must be obtained close to the time
of the object exposures to be calibrated, because these
images are used to provide quantitative calibration
of pixel signals. If you are going to compare corrected
images, all must be corrected using the same calibration
bias or dark image is acquired. In a bias image, all
of the pixels have approximately the same value, which
consists of the electronic offset of the system and
any inherent structure of the CCD. A dark image contains
the dark signal as well as the standard bias component.
A dark image is most useful for correcting long exposures
with low light levels.
The flat field image measures the response of each
pixel in the CCD array to illumination. The optical
system introduces some variation in response. The reason
being that the flat fielding process corrects for uneven
illumination only if that illumination is a stable characteristic
of each object exposure. The flat field image should
be as representative of the background illumination
as possible. The illumination should be bright enough
or the exposure long enough, that the CCD pixel signals
are at least 25 percent of full scale.
When a raw image is created the flat fielding technique
can be applied. The flat field image is corrected for
electronic offset by subtracting the bias/dark image
from it. The mean pixel value of the resulting corrected
flat field image is ascertained. The mean pixel value
is unique to each combination of bias/dark and flat
The bias/dark image is then subtracted from the exposed
image. The resulting image is multiplied by the mean
pixel value of the corrected flat field image and finally
divided by the corrected flat field image itself. (If
the corrected flat field image contains a pixel with
zero value, that pixel in the corrected image is assigned
a full scale value.)
The mean pixel value of the corrected image is not
exactly the same as that of the raw image. However,
multiplying by the mean pixel value of the corrected
flat field image keeps the number more or less the same.