Tech Briefs Magazine - May 2021 - PIT-10

Machine Vision

A heat source near the optical path
can introduce image warping that locally
shifts the position of the image content.
For line-scan cameras, there exist image

Grayvalue

Test data shift

Pixel

Sensor direction x

Figure 1. Sketch of the measuring scheme for the image shift

0.1
0.08
1

0.06
0.04

0.02
0
-0.02

-0.04
-0.06

-0.08
-0.1

Suppression of Image Warping

Conclusion

Correlation

Grayvalue

Scan direction y

Pixel

Test

The image warping effect can be suppressed by preventing the turbulences.
For this purpose, a fan can be utilized to
create a laminar airflow. As the pressure
in a laminar flow is constant, no gradient
lens effects will occur. The airflow either
needs to cover the whole surface of the
heat source, or the whole volume of the
optical path where turbulences are likely
to appear.
The second option is typically easier to
realize for line scan cameras as the optical path is constrained to a plane. The
fan can therefore be installed to the side
of the camera with the flow direction
along the sensor line. In the example
above, the air in the volume directly
below the viewing slit of the tubelight
needs to be perfused by a laminar flow.
As the sidewalls of the illumination block
a direct side access for the air stream, the
fan was installed on top of the tubelight
and blowing into the viewing slit at a tilted angle as shown in Figure 3. This
method reduced the magnitude of the
distortion to an indiscernible small size.

Correlation between
Reference and Test

Reference

Elapsed time /s

interpolation. The shift with the highest
correlation between test and reference is
recorded for each pixel and can be plotted
in a color scale image to visualize the data.
Figure 2 shows this visualization from
a static line pattern image captured with
a Chromasens allPIXA camera with
5 μm optical resolution. The heat source
in this case is the Chromasens Corona II
tubelight illumination that was operated
at maximum LED current. In this setup
the magnitude of the optical distortion
is in the range of <0.15 pixel. The turbulences are visible in the shift image as
areas of similar distortion. They extend
over a size of 10mm - 30mm and persist
for 200ms - 800ms.
This measurement shows that the
magnitude of the image disturbance due
to heat convection from a standard illumination will not have a noticeable
impact for standard inspection tasks.
However, specialized image processing
tasks that depend on subpixel accuracy
(e.g. sub-pixel accurate feature extraction or sub-pixel based image correlation) will likely show decreased measurement accuracy due to these distortions.

-15

-10

-5

Position along sensor direction / mm

Figure 2. Color image representation of the measured image shift. The color scale indicates the shift in
sub-pixel units.

Side

Front
Camera

Camera

Fan
Fan
Tubelight

Tubelight

Figure 3. Recommended suppression setup for a Chromasens tubelight setup. For illustration, the sensor is depicted as a red point in the left, and as a red line in the right image.

regions of similar shift that cover several
mm of the field of view that emerge and
vanish in the timeframe of about one
second. Sub-pixel analysis of image content will be influenced negatively by this
shift. To suppress this effect, it is recommended to cover the whole volume
along the optical path that is affected by
turbulences with a laminar air current
from a fan.

This article was written by Timo Eckhard,
Team Leader, Research & Innovation,
Innovation & IP Management; and
Sebastian Georgi, Research & Innovation
Manager, Chromasens GmbH (Konstanz,
Germany). For more information, contact Mr.
Eckhard at timo.eckhard@chromasens.de, Mr.
Georgi at sebastian.georgi@chromasens.de, or
visit http://info.hotims.com/79413-221.

Photonics & Imaging Technology, May 2021

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Tech Briefs Magazine - May 2021

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