Image quality is of paramount importance in machine vision because all subsequent processing and measurements are performed on the image data captured by the camera sensor. Even the most sophisticated software cannot deliver information that is not available in the original image. Many factors can influence image quality and system illumination is high on the list. Here, Jools Hudson of Gardasoft Vision looks at the six essential considerations for enhancing machine vision lighting for image optimisation.
1. Optimise the image at source
The image contrast and signal-to-noise ratio are key to producing an optimum image and these two measures are determined by the intensity and quality of light reaching the camera. To obtain a good, consistent image the light must be both sufficiently bright and stable, so both the lighting used and the way it is controlled should be carefully considered. Inadequate lighting control is likely to lead to significant underperformance from the machine vision system. Variations in light intensity can result in poor measurement repeatability with major repercussions in a manufacturing environment, leading to excessive waste, defective product reaching the end user or even line stoppages.
2. Get the most from your LEDs
LED lighting is often specified by voltage but the brightness is actually determined by the current through the LEDs and not the supply voltage. This means that accurate control of light intensity requires good current control. LEDs are remarkably reliable devices but the light output reduces as the device ages and, in shorter timescale, the increase in temperature after the light is switched on will change the brightness. Switching the light on only when an image is being acquired by pulsing the light minimises the amount of heat produced and significantly improves LED lifetime. Strobing an LED light also brings the major benefit of allowing the light to be ‘overdriven’ in short pulses to intensities much higher than the manufacturer’s light rating. Overdriving is a very powerful technique, but it is important to ensure the LED is not driven at too high a current for too long which may damage the light. Gardasoft’s SafeSense™ technology for lighting controllers imposes safe working limits on overdrive, based on manufacturers’ specifications, pulse width and duty cycle.
3. Build in flexibility
It is common for machine vision requirements to change. To guard against the unexpected, system flexibility should be built in so that existing systems can be adapted to changing requirements and environment. The inclusion of variable lighting control allows the intensity to be adjusted to suit an altered application so that the same vision system can be used with different light settings for different product batches. Making use of overdrive offers even more flexibility because, with an appropriate lighting controller, LEDs can safely be overdriven at up to 10x the published maximum. A common occurrence is where an established system becomes adversely affected by extraneous light sources. If the light is already running at maximum rating in continuous mode, then overdrive enables increased brightness to significantly reduce ambient light interference, without having to change to a higher intensity LED.
4. Maintain stable illumination
LED brightness is proportional to the current running through the device. In fact, a very small change in LED drive voltage will result in a much larger change in light output intensity. This is critical since even for simple vision calliper measurements a 10% change in light level can cause a change of 0.5% in the measured values. All Gardasoft LED controllers regulate the LED current to produce a stable, tightly controlled and highly repeatable light output.
5. Make setup easy
A well-designed lighting controller can bring significant benefits to the setup and use of machine vision systems. Opto-isolated trigger inputs can connect to all common signal sources and a digital, button front panel allows easy and instant configuration of lighting and a controller should incur minimal delay between the trigger signal and the light pulse. Ethernet compatibility with inbuilt web pages can provide live, measured values of current and voltage to determine the performance of the light.
6. Utilise network connectivity
GigE Vision and GenICam compatibility bring enhanced connectivity between components from different manufacturers and enable the controller to be discoverable on the network. Ethernet communications provide remote access to all the lighting within a system to indicate whether a light is connected, disconnected, or short-circuit. It also offers the possibility of remote troubleshooting of a vision system and allowing the configuration of a lighting controller to be changed to suit different batches of product. Adjustments can be made to increase the current into a light whose intensity has reduced due to ageing. An application such as the Gardasoft Vision Utility allows the machine vision control software to select a batch type and directly communicate with the controller to set the configuration required for that batch.
Find out more
Getting machine vision lighting right is a critical step to the successful implementation of a machine vision system. More information is available in a freely downloadable white paper: www.gardasoft.com/Six-Essential-Considerations.