Triggering is the part of a vision system that everyone designs last and nobody debugs confidently. It is also the single largest source of "the image is sometimes blurry" and "the part is half out of frame" complaints, because trigger timing errors are intermittent by nature.
Software triggering: convenient and imprecise
A software trigger is an API call from the host application. The camera receives it after the command crosses the USB or Ethernet stack, is scheduled by the operating system, and is processed by the camera firmware - a delay that typically runs from a few hundred microseconds to several milliseconds, with a spread that can exceed 1 ms under load. On a line moving 500 mm/s, a 1 ms spread moves the part 0.5 mm. That is often invisible and sometimes fatal.
Software triggering is perfectly adequate when the part is stationary during exposure - an indexing table that dwells, a manual load station, a robot that presents and waits. It is the wrong tool for a continuously moving web or a high-speed conveyor.
Hardware triggering: the timing is set by physics
A hardware trigger drives a dedicated input line on the camera. The sensor begins exposure on a clock edge, with jitter measured in microseconds, independent of what the operating system is doing. Typical sources are a photoelectric or fibre sensor detecting the leading edge of the part, a PLC output, a shaft encoder index pulse, or a proximity switch on a mechanical cam.
Three details decide whether it works:
Edge polarity and debounce. A mechanical sensor that chatters on the part edge will generate three triggers and three exposures. Configure the camera for rising-edge only and, where the hardware allows, add a debounce window in the sensor or PLC rather than in software.
Trigger delay versus exposure time. Vision cameras expose for a fixed duration after the trigger; if the part is still moving during that window, you get motion blur regardless of how precise the trigger was. Short, bright strobes are the standard answer - freeze the motion optically rather than shortening exposure to the point where you lose signal.
Signal integrity. Long unshielded trigger cables pick up drive and solenoid noise, producing sporadic phantom triggers. Use shielded twisted pair, keep the run short, and reference the camera's trigger ground properly.
Encoder-driven triggering for moving material
On a web, a roll or a continuously fed strip, the correct trigger source is not a part sensor but a shaft encoder. The encoder emits a fixed number of pulses per millimetre of travel, so triggering every N pulses produces images at exactly constant spatial intervals regardless of line speed. This is mandatory for line scan cameras and extremely useful for area scan systems inspecting a continuous surface, because it makes the images directly comparable and lets you stitch them without resampling.
Choosing the camera is easier after this decision
Once you know you need hardware triggering, you also know you need a camera with proper opto-isolated trigger inputs and, ideally, a hardware trigger output to fire a strobe. Most machine vision families in the industrial range provide this, from the compact MV-CU013-A0GM to the 1.6 MP MV-CS016-10GM and the higher-rate MV-CA013-20GM. Compact consumer-style USB cameras frequently do not, and that limitation only becomes visible during commissioning.
The rule of thumb worth remembering: if the part moves while you are taking the picture, the trigger belongs in hardware. If the part is stationary, software triggering removes an entire class of wiring problems and is the correct choice.