It 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.
Influence on imaging with line scan cameras
The different refractive indices in the 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.
Figure 1: Sketch of the measuring scheme for the image shift
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 interpolation. The shift with the highest correlation between test and reference is recorded for each pixel and can be plotted in a colour scale image to visualise the data.
Figure 2: Colour image representation of the measured image shift. The colour scale indicates the shift in sub-pixel units.
Figure 2 shows this visualisation from a static line pattern image captured with a Chromasens allPIXA camera with 5μm 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 specialised 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.
Continue reading article…