BOBCAT Hardware User’s Manual
Imperx, Inc.
Rev. 2.0.2
6421 Congress Ave.
11/20/2012
Boca Raton, FL 33487
+1 (561) 989-0006
138 of 265
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).