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Changing the Exposure Time During Grabbing Causes an Overtrigger Error#

Assume you're using a line or area scan camera and change the exposure time during grabbing. Depending on the exposure time's initial and target values, you may receive an overtrigger error from the camera. As a result, image loss occurs. This article explains why reducing the exposure time can cause an overtrigger.

The problem is that the readout phases from two successive images can't overlap. The following diagram shows the normal operation of a Basler camera in triggered mode.

Normal Operation in Triggered Mode

After an external trigger, the first exposure will start. When this is finished, the first readout will begin. To optimize performance, it's desired to start the next exposure already while the first image is still read out. As long as the second exposure ends after the first readout has finished, everything will work fine.

The next diagram shows what happens when the exposure time is suddenly decreased.

Sudden Decrease of Exposure Time

So if you want to make sure to not overtrigger your camera, implement an algorithm in your exposure time transitions which takes the above into account. A good solution to change the exposure from A to B involves dividing the large decrease into several smaller steps, while the very first step is the most critical one.

For each step, the exposure time must meet the following guideline:

Formula Exposure Time

The Resulting_line_periodA can be queried from each camera like this (C#):

double resulting_line_period = camera.Parameters[PLCamera.ResultingLinePeriodAbs].GetValue();

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