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Using strobed lighting to get more from machine vision

Pulsing the lighting is a powerful technique that can improve machine vision systems in many different ways. A dedicated lighting controller can help reduce costs, improve performance and open up new imaging possibilities. This is particularly the case if you use a lighting controller that will regulate the current drive to the LED. A quality LED controller will use current control to maintain precise, stable and highly-repeatable light intensity.

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It is common to use pulsed light to ‘freeze’ motion for high-speed inspection. But when the light is lit only in short bursts it’s also possible to increase the light output beyond the manufacturer’s specified maximum, using a technique called “overdrive”. There are cost benefits whenever an LED is pulsed instead of running continuously. Using the light only when it is needed to capture an image can extend the LED lifetime, for example, if the LED is on for only 10% of the time, the lifetime will be increased by a factor of 10.

Once a light is being strobed, many useful applications become possible. A short pulse of bright light is often utilised to overcome interference from ambient light, and this setup can improve the depth of field and enable computational imaging. Strobed light can also be used to set up multi-lighting and multi-intensity schemes at a single camera station, or even sophisticated line scan applications.

High speed imaging and improved depth of field

Motion blur in images from fast-moving objects, for example on a high-speed production line, can be eliminated with appropriate pulsing of the light. In some cases a defined camera exposure will be good enough to freeze motion, but the resolution is likely to be far better by using a strobed light instead. The controller will enable precise control of pulse width, frequency and intensity, and also allows the light to be "overdriven". Overdrive can boost the light output up to 10x its brightness rating in short pulses – a very useful feature in applications that benefit from high brightness. Increased brightness could allow the whole system to be run faster because of the reduced exposure times. Higher light output may also allow the aperture to be reduced to give better depth of field. Gardasoft controllers include our patented SafePower and SafeSense technology which prevents overdriving from damaging the light.

Ambient light control

Ambient light conditions frequently interfere with machine vision measurements and these issues can be solved by pulsing and overdriving the system’s LEDs. For example, overdriving the LED by 200% doubles the light intensity and enables the camera exposure to be halved, so reducing the effects of ambient light by a factor of 4.

Multi-lighting schemes

Lighting controllers are frequently used to reduce the number of camera stations. Several lights are set up at a single camera station and pulsed at different intensities and durations in a predefined sequence. Each different lighting can highlight particular features in the image. Multiple measurements can be made at a single camera station instead of needing multiple stations, and this reduces the mechanical complexity and saves money. For example, sequentially triggering 3 different types of lighting could allow a single camera to acquire specific images for barcode reading, surface defect inspection and a dimensional check in rapid succession. Pulsing can also be used for computational imaging, where a component is illuminated sequentially by 4 different lights from different directions. The resultant images would be combined to exclude the effect of random reflections from the component surface. Pulsed multiple lighting schemes can also benefit line scan imaging by using different illumination sources to capture alternate lines. Individual images for each illumination source are then easily extracted using image processing software.