20.3 Improving the acquisition / image quality 223
No oscillation occurs, regardless of DC ambient vs. AC indoor light.
This operation mode is known as flicker-free or flicker-less operation.
What it mainly does is to adjust the frame frequency to precisely the frequency of the power line. Usually the
line frequency is very stable and therefore is the harmonic frequency difference of the two signals are very slow;
probably in the range of < 0.1 Hz.
The fact that we do not know the phase relation between the two frequencies means that we scan the alternating
ambient light source with our camera. The shorter the exposure time, the more we see a slow change in brightness.
Using AutoExposure/AutoGain can completely eliminate this change because the frequency of change is very low.
That means it will be legal if we calculate a brightness difference in one picture and apply it for the next one, because
the change is also valid in the next one; as we fulfil the Nyquist theorem.
If we use an arbitrary scanning frequency like 20 fps or whatever your algorithm and data flow is accepting, is wrong
in this aspect and leads to oscillations and undesired flicker.
Pointing to a 60 Hz display with flashing backlight an oscillation of 10 Hz can be seen of course.
Figure 3: wxPropView - Intensity plot while pointing the camera to a 60 Hz display
20.3.4.3 Conclusion
To avoid oscillations, it is necessary to adapt the camera frequency to the frequency of AC light. When using auto
expose a flicker-free mode (timer based external trigger) is needed. If the camera is used throughout the world it
is necessary that the frequency of AC light can be set in the software and the software adapts the camera to this
specific environment.
20.3.5 Working with binning
With binning it is possible to combine adjacent pixels vertically and/or horizontically. Depending on the sensor, up
to 16 adjacent pixels can be combined.
See also
https://www.matrix-vision.com/manuals/SDK_CPP/Binning_modes.png
Binning will lighten the image at the expense of the resolution. This is a neat solution for applications with low light
and low noise.
The following results were achieved with the mvBlueFOX3-2124G, however, binning is also available with the mv←-
BlueCOUGAR-X camera family.
MATRIX VISION GmbH
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