Modern industrial processes are becoming ever more sophisticated: components are getting smaller, production lines are getting faster, and tolerance for inferior product is lowering day by day. Vision systems are having to respond to these changes. Cameras themselves are offering higher resolutions and at faster frame rates, but alongside this improved performance is the critical need for sufficient illumination.
‘Generally speaking, the market for illumination is mainly governed by camera technology,’ says Simon Stanley, managing director of StockerYale Ireland. ‘In the past few years, camera resolution has been constantly improving, and linescan cameras are now integrated in production lines operating at quite high speeds. However, without sufficient illumination, the cameras are rendered useless. Therefore, the illumination particularly needs to offer high brightness for such high speed and resolution – this makes it very much a specialist area when it comes to lighting for machine vision.
‘Illumination is vital in machine vision applications for providing contrast so that the camera can see the image clearly. To be effective, the illumination needs to provide an adequate amount of uniform light and be from the correct angle, so as to avoid the effects of shadows etc.
‘It’s important to consider illumination at as early a stage as possible when designing a machine vision system. We like to work with the machine system integrator so that the camera and lighting technology are built into the application. It is of course possible to add illumination retrospectively, but this can make the job more difficult. Some camera manufacturers are actually building illumination into the camera itself, which can provide a suitable solution in some circumstances.’
It’s clear, then, that illumination is a critical element of a vision system, but there is a choice of light sources. In the past, there were many more options to consider, but in recent years the majority of applications are covered by just one light source – the LED.
‘LEDs are by far the technology of choice when it comes to illumination for machine vision purposes,’ continues Stanley. ‘Within the past 10 years or so, LEDs have become a credible light source, with blue LED also becoming widely available over this time. LEDs are becoming more efficient, costing less and there is a greater range of colours available – all of which makes them very attractive to vision system integrators.
‘LEDs also offer a radically better lifetime than conventional light sources – this means, obviously, that they last longer, but also that there is no downtime or maintenance cost associated with changing lamps etc. For systems that are often designed to run 24/7, eliminating or reducing downtime is a significant factor.
‘LEDs are more easily controlled than many other lighting options (they can be turned on and off very quickly, so can be set to strobe with the camera, for example), and generally offer a more uniform performance level.
‘In around 60 per cent of applications, red LEDs provide sufficient performance. However, many different colours are available which further extend the range of applications to which LEDs are suited. These include applications outside the range of visible white light. One example would be in pharmaceutical packaging, where suppliers often mark the packaging with coding that can only be read under UV light. Thanks to recent advances in efficiency, LEDs can now be utilised in such applications.’
Look in any vision distributor’s catalogue and the popularity of LED-based illumination systems is borne out in the range of products available. LEDs still have a way to go though in satisfying every need.
‘There are still some areas where LEDs can improve,’ says Stanley. ‘They still struggle in the flat spectrum (e.g. daylight), as there is a gap in the typical blue LED/phospor generated spectrum, and in short wavelengths, such as those used in curing applications. Also, when very high intensity point sources are required, LEDs cannot easily compete with the brightness offered by more conventional light sources, for example, a metal halide bulb.’
Chris Baldwin, optics and lighting product manager with distributor Firstsight Vision, agrees that LEDs don’t yet quite rule the market across the board.
‘For applications requiring exceptional brightness,’ says Baldwin, ‘we still sell a significant amount of non-LED products, such as gas discharge systems. It’s true that LEDs have an exceptionally quick rise and fall time (in the order of nanoseconds), but with limited power throughput, they cannot provide the instantaneous peak energies of discharge lamps. This means LEDs are not suited to freezing very high speed motion, for example.
‘Another area where LEDs are yet to gain a stranglehold is in large-scale applications, such as a wood mill, for example. In such examples, huge areas may need to be lit and to do so with LEDs would be just far too expensive. In such instances, you still find fluorescent light with a high frequency ballast, which has the advantage of a long lifetime too (subject to suitable maintenance).’
Laser lighting used for 3D measurement and positioning, courtesy of Firstsight Vision
A slightly different interpretation of the term ‘illumination’ sees the use of lasers in machine vision for profiling and positioning.
‘Lasers, in conjunction with a vision system, can be used to ensure a fixed geometrical structure is exactly the right size, width, height and so on,’ says Baldwin. ‘A grid of lasers, or laser generated spots, can help with 3D profiling. A laser can also provide an extremely high degree of accuracy. For example, lasers we offer can produce a line that is just 5.5µm thick. By beaming a laser line over a PCB, for example, users can measure the offset to determine a highly accurate representation of thickness changes.’
Regardless of your choice of lighting source, it’s clear that illumination is a major factor in ensuring the effective and efficient operation of a vision system.
Click here for a table that describes which lighting techniques will be most useful when used to examine different features of a target object - reproduced from the Vision Elements machine vision handbook, with kind permission of Firstsight Vision.
Tackling the LED power problem
One of the restrictions on LEDs being suitable for very high brightness applications is that they require very high power in order to generate that brightness. This high power means they generate too much heat, and can’t be cooled sufficiently in order to provide lifetime or efficiency of any use. One-time vision system integrator Andy Falconer recognised this problem and set up V-Cubed, a company that has developed water-cooled high-power LED lighting solutions for machine vision.
‘Our previous experience as users of machine vision helped us in developing our range of high-power LED-based illumination sources for use in high-speed applications,’ says Falconer. ‘Our main product is an LED line light that goes head to head with other high power sources, such as fibre optics with metal halide lamps.
‘The problem with pushing the brightness of LEDs ever higher has always been cooling management. Our VLX2 line light range is water-cooled, rather than air-cooled, and that allows them to operate more efficiently and cope with a much wider range of temperatures. Air-cooled options are fine for lower power units, but they can’t be used effectively in an environment that has a high ambient temperature, as they can never cool to a level below that ambient temperature. Being water-cooled, if required by the application, our line light can be cooled further using “off-the-shelf” recirculating chiller units.
‘We have also built in LED failure detection – something that is hopefully never required, but electronic devices can fail. The failure of one or two LEDs on a line light might not be noticeable to the naked eye, but the loss of light level can have a significant impact on the image. Our systems have built-in communications, which can report a single LED failure on the array, meaning it can be identified and addressed quickly. The VLX2 also has intensity profile control, which means the line light can be set to compensate for optical geometry on line scan cameras.
‘It’s true that the water-cooled nature of the product means it is a little more expensive than its air-cooled alternatives, but if an application demands high brightness – and particularly if it’s in a high temperature environment – customers quickly realise the value of the product.’ 
One of VCubed's range of high-power LED line lights
The art of filters
Just as important in achieving maximum clarity of images are filters for the cameras. While filters should be used in conjunction with – rather than in place of – illumination, they can help to solve many common problems caused by, for example, ambient light.
Bernard Parry of Special Application Products, which distributes the Spectrum Illumination range of products in Europe, says: ‘Filters can make an enormous difference. If there is a factory with skylights, for example, sunshine can be a major problem. Using a bandpass filter, however, can negate much of the sunlight effect, and allow the camera to pick up the essential information required. In some cases, users have gone to the trouble of creating an entirely closed system just to avoid problems with ambient light, whereas in truth they could have most likely solved the problem with a simple filter.
‘Filters are also useful in UV lighting applications for fluorescent materials. If, for example, the contaminant in question were green, then a green filter would eliminate just about every other colour or reflection, leaving the camera to pick out just the green contaminant.’
