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CAMERAS FRAME GRABBERS IMAGING SOLUTIONS
BOBCAT Hardware User’s Manual
IMPERX
Rev. 2.0.7
6421 Congress Ave.
4/8/2014
Boca Raton, FL 33487
www.imperx.com
+1 (561) 989-0006
194 of 329
2.17 FLAT FIELD CORRECTION
A CCD imager is composed of a two dimensional array of light sensitive pixels. Each pixel within
the array, however, has its own unique light sensitivity characteristics. Most of the deviation is
due to the difference in the angle of incidence and to charge transport artifacts. This artifact is
called ‘Shading’ and in normal camera operation should be removed. The process by which a
CCD camera is calibrated for shading is known as ‘Flat Field Correction’. Refer to Figures 2.33a
and 2.33b for images acquired before and after Flat Field Correction.
This feature is available as a
standard feature only for cameras with 1.0” optical format or bigger.
The BOBCAT series of cameras incorporate a Flat Field Correction mechanism. The Flat Field
Correction mechanism 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 field correction process compensates for uneven illumination, if that illumination is a stable
characteristic of each object exposure.
During factory final testing, our manufacturing engineers run a program specially designed to
identify the shading characteristics of the camera. The program creates a Flat Field Correction
file, which contains coefficients describing these shading characteristics. This file is then
downloaded into the camera’s non-volatile memory. When Flat Field Correction is enabled, the
camera will use the Flat Field Correction coefficients to compensate for the shading effect.
Each IMPERX camera is shipped with the Flat Field Correction file that was created for that
camera during factory final testing. Users may wish, however, to create their own Flat Field
Correction file because of the uniqueness of their operating environment (i.e. lens, F-stop,
lighting, etc.). Therefore, IMPERX provides a Flat Field Correction utility that allows users to
generate a Flat Field Correction file. This file can then be downloaded into the camera. While
creating the Flat Field Correction file, 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 of full scale (for 12 bit mode the level should be at least 1000 ADUs).