Tech Briefs Magazine - May 2021 - PIT-8

Preventing Heat-Induced Image Warping in

Machine Vision Line Scan
Cameras

I

t is well known from astronomical
imaging that various atmospheric
conditions and weather effects have
an impact on image quality. This is
due to local changes of the refractive
index of the air in the optical path.
These changes are wavelength-dependent and vary with changes in the atmospheric pressure and its humidity.
In the field of machine vision, both
pressure and humidity can usually be
assumed to be constant over the whole
optical path. However, there is an exception to this rule if there are turbulent air
currents in the optical path that may
cause local pressure changes. A typical
cause for turbulences is heat convection
between parts at different temperatures.
Common heat sources in machine vision
can be either the illumination, high-load
electronics, or high temperature samples such as poured metal.

turbulent air act as a gradient lens that
warps the image content in the affected
areas. The magnitude and extension of
the warping is both too complex and too
dependent on the setup to model mathematically. Instead, we show an example
measurement of this effect to represent
its typical magnitude and to guide you in
measuring it in your own setup.
In a line scan camera, the optical distortion is constant for each line along the
scanning direction and is therefore not visible in the image. This fact reduces the
problem to one spatial dimension - perpendicular to the scan direction - and the
time dimension. Both dimensions can be
observed at once by acquiring an image of
a static target with the line scan camera.
The optical distortion will shift the
position of the image content
in x-direction
while

the extension of the warping in y-direction
represents the time information.
This shift can easily be measured by
imaging a static line pattern. The basic
principle of this measurement is depicted
in Figure 1. A reference image intensity
profile is taken by averaging a region of
interest containing an area of a few
columns around each x-position (depicted in green). A square test block (depicted
in red) is passed along the whole column
and for each position the test
data is shifted in
subpixel
steps
via

Influence on Imaging

asharkyu/Shutterstock.com

The different refractive indices in
the

8

Photonics & Imaging Technology, May 2021

Cov

ToC


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Tech Briefs Magazine - May 2021

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