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Cognex CDC Series Camera Description
Cognex MVS-8100D and CDC Cameras Hardware Manual
47
Darker Initial Images
The information in this section applies to the CDC-50 and CDC-100, but not the
CDC-200.
Under certain operating conditions, you may notice a small difference in the overall
brightness of initial images, compared to succeeding images. When the conditions are
met, the brightness of the initial image may be darker than that of succeeding images
by no more than 5%. The conditions under which this might occur include:
•
The first image acquired following system initialization
•
The first image after a delay, if the acquire cycle is non-periodic and the delay
between any two successive acquisitions exceeds two seconds
This brightness difference between the first and succeeding images should not affect
the performance of Cognex software vision tools, but should be tested in your
application. If your application is sensitive to brightness differences, you can write your
application so as to discard the darker initial images.
Image Shading Gradient: CDC-50 Only
When using the CDC-50 camera in global shutter mode, it is possible for the image to
exhibit a shading gradient: normal at the top and getting progressively brighter towards
the bottom. The operating conditions required to bring about this condition include:
•
Operating in global shutter mode
•
Very high contrast image (for example, one containing a combination of very dark
regions with very bright, almost saturated regions)
•
In this formula, the minimum shutter time,
T
shutter(min)
, is in milliseconds. ROI stands
for region of interest, a rectangular subset of the overall image, settable in software.
ROI
width
is the width of the ROI in pixels, and is the number of columns in the ROI
image.
ROI
height
is the height of the ROI, and represents the number of rows in the
ROI image.
The greater the difference between your choice of exposure and
T
shutter(min)
, the more
likely it is that you will observe the image shading gradient. If the gradient does occur
and presents a problem in your application, the possible remedies include the following:
•
Use rolling shutter mode if your application does not require capturing objects in
motion
•
Increase the exposure time to the maximum tolerable by your application
•
Lower the contrast of the image by reducing the illumination and/or by increasing
the lens’ F-stop setting
T
shutter min
(
)
ROI
width
59
+
(
)
ROI
height
×
44700
gain
×
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