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The following sections detail the causes of possible image quality problems with respect to optical
misalignment and how to identify them.
Image Will Not Focus Well over the Entire FOV
If the image is skewed with respect to the sensor pixel array, then a sharp image perpendicular to
object travel will not be achievable regardless of attempts to adjust focus. This is due to a
component of the image motion traveling in the long axis of the sensing array.
TDI pixel array skewed
with respect to image
Image
TDI Pixel Summing
Image feature has sideways component
Image Motion
as it moves across the pixels array
Digital image will
appear smeared in
long axis of the sensor
Figure 21 Effects of Camera Skew
The camera mounting assembly should have a mechanism to minimize any skew between the
image motion and TDI summing direction.
For example, for 12 rows and no more than ¼ pixel movement along the long axis, skew will need
-1
to be less than tan
(0.25pixel/12rows +11spaces) = 0.6°.
Image Will Not Focus at Edges of Field of View.
If sharp focus can be achieved at the centre of the image but not at the edges, this may be due to
the camera's optical axis not being perpendicular to the material surface in the scan direction. If
the optical axis is not perpendicular, then this will cause a parallax issue at the edges of the field of
view resulting in a component of image motion traveling in the long axis of the sensing array.
68 •
The Piranha XL XDR Camera
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