A trigger to begin the exposure usually comes from an external source such as a PLC or optical sensor.
The images on the left show what happens when a free running camera is used where the frame start is not synchronised to the bottle position. This results in the position of the bottle varying in each captured frame, and even being completely absent from some frames
The images on the right show asynchronous triggering, which delivers an image acquisition at the correct time, thus simplifying the processing and analysis.
Using edge preselect mode, one edge of the trigger pulse is selected to initiate the capture, and the exposure time (shutter) is preselected. Pulse width control is very similar to the edge preselect mode, with one edge of the trigger signal initiating the capture, but in this case the exposure (shutter) time is governed by the width of the trigger pulse.
Frame delay readout mode allows the readout of the image to be delayed for a period of time. This is achieved by storing the image in the vertical shift register of the CCD sensor or in a frame buffer inside the camera. The image is then read out when the corresponding external signal is sent to the camera. This allows multiple cameras to be triggered simultaneously and read out in sequence reducing the demands of the camera interface.