Dear Witchball,
the difference between free running mode and external trigger mode using a line scan camera is the following.
Free running mode:The camera generates its own internal trigger at the selected line scan speed. For example 2000 lines/sec.
This mode will only work, if the object under the camera is moving at a constant velocity. The slightest deviation from a constant speed
will cause image distortion.
Externally triggered mode:This is usually the preferred mode for any line scan application. Triggers are usually generated by an encoder which is directly linked to either the motion of the object or the camera. This way you can guarantee that your images are geometrically correct. It allows allows for variations in speed of your object (or camera) movement. This mode directly synchronizes the "firing" of the camera to the motion of the object.
You need to calculate the trigger impulses ideally, so that you'll end up with perfectly adjacent lines. If you have too few triggers, the image will be compressed and information will be missing (undersampling), if you have too many trigger pulses, the image will be longer or stretched (oversampling). However, usually it is not possible to arrive at an exact encoder or trigger count, so oversampling is quite normal. The scan lines will overlapp somewhat. You need to perform a linear image correction calibration by selecting two points with a known distance. LabVIEW has "canned" VIs for that.
Another thing to consider is the exposure mode. I am not familiar with your particular camera ( we are using DALSA). With DALSA cameras you can choose a couple of different exposure modes. The first dynamically adjusts the exposure time to the time interval between triggers, the second sets a fixed exposure mode. The latter is usually more common.
If you use the frame grabber to do the triggering, it will basically start collecting available lines from the camera based on the trigger event and start filling up the image buffer that you have prepared. For instance, if you set the image size to camera resolution x 1000 lines, it will collect 1000 lines after the trigger has been received. However, this does not synchronize each line to the movement of the object or camera, since the camera is in a free running or asynchroneous mode. Even if you would set up the frame grabber to acquire individual lines, this would cause "jitter" between frame grabber trigger and when the actual scanned line becomes available.
Best,
Markus Tarin
