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Figure 1: Machine vision, which increasingly takes advantage of illumination by LED light sources, often replaces human inspection in automated production lines (Courtesy of Schott).
In machine-vision systems, the typical low power requirements and long lifetime of LED-based lighting have made solid state lighting (SSL) the preferred choice for illuminating automated manufacturing systems.
A variety of LED-based ring-lights, dome lights, backlights, and spotlights based on LED technology are available. Due to a combination of the reliability of this technology and the phasing out of traditional incandescent or fluorescent light sources, the marketplace and deployment of LED systems continues to expand. Lighting vendors are building on these developments to offer different configurations of LED products to meet the needs of specialized machine-vision applications.
Historically, fluorescent and quartz halogen lighting sources have been used most commonly. In recent years, LED technology has improved in stability, intensity, and cost-effectiveness. Although it is still not as cost-effective for large area lighting deployment, particularly compared with fluorescent sources, LED lighting may be more appropriate if application flexibility, output stability, and longevity are important parameters.
LEDs are highly reliable, long-lasting illuminants with low maintenance requirements. Also, as DC voltage-powered components, they do not exhibit 100 Hz flickering – unwanted beats or overlapping when using CCD cameras. LEDs can be continuously dimmed down to zero percent light. Their instant start property facilitates strobe operation as well as externally triggered pulse operation.
LEDs’ relative insensitivity to motion and vibration and their low weight make it possible to use LEDs in moving machine vision applications (for example in robotic systems). Because of their small size, LEDs can be used to produce special illumination components that provide optimum contrasting for specimens in machine vision.
Because of their compact modular design, it is easy to develop LED components, for example ring-lights, which can be controlled in segments. In this way, a significant contrast enhancement (of images) is achieved compared with full circle illumination. When steering several illumination components via a common control unit, it is easy to generate mixed light and to quickly switch over between different types of illumination. Modern “intelligent” control units make it possible to completely control the LED illumination of subjects by PC, which facilitates exact specification of illumination settings
Machine vision needs
LED-based lighting is used in many industrial machine-vision inspection systems to help determine / confirm the shape of parts in manufacture – by defining edge geometry, or enabling the size and position of drilled holes to be imaged and verified. Manufacturers often want to change the product they are inspecting on a given production line, so the LED lighting types, power, wavelength and associated optics and cameras need to be flexible enough or interchangeable to allow adjustment.
Figure 2: Diverse effects of various LED-based lighting techniques on a ball bearing (Photo courtesy of Microscan).
A key consideration when choosing any type of lighting is the configuration of the light source and its spectral properties. Besides considering the uses of different cluster designs – whether backlights, ring lights, and other designs of illumination units – the types of the fitted LEDs must also be chosen carefully. Variables such as output wavelength, power and geometry must suit the application. The choice will depend on the color and/or reflectance of the parts under inspection.
Figure 3: Cree’s XLamp CXA2011 LED array is suited to Machine vision (Courtesy of Cree).
Advanced Illumination (Rochester, VT, US) is a machine vision manufacturer of LED lighting products. John Merva, Vice President of Sales & Marketing, said, “We make lighting units of different sizes and shapes, wavelengths, and power for applications in just about any manufacturing operation that is automated.
Traditional lighting sources, such as incandescent or fluorescent, had problems of relatively short lifetime and inconsistent output – especially near the end of life – wavelengths and brightness tend to change, which is why LEDs are much more reliable and preferred today. The LED manufacturers we tend to favor are the main three: Nichia, Cree and Phillips-Lumileds, among others.”
Figure 4: Light source relative intensity vs. spectral content. Bar at bottom denotes approximate human visible wavelength range (Source: Advanced Illumination).
It might appear that the ever-growing selection of LED light sources would make finding the ideal sources easier but the increasing diversity actually means that systems developers need to do more R&D. Considering new sorts of SSL sources, such as OLEDs, Merva said he considers these alternative sources but added, “the changes that happen in this industry sector tend to happen rather slowly.”
Commenting on trends in the use of LEDs for machine vision systems, Merva said, “The types of the LEDs we normally use are split between the different colors of visible light and the higher power white LED sources. In the early days, machine vision used red LEDs because of their relative high power, low cost and wide availability but more recently red has been superseded by white, in conjunction with full color cameras.”
Machine vision and its reliance on solid-state lighting will continue to grow. We are seeing the number of camera deployment points growing across industry and elsewhere, especially considering that a “smart” camera can now be purchased for under USD 2,000. Historically, the vision problem needed to be significant to justify the cost of deploying cameras but nowadays it can be relatively minor and the savings will still justify the cost of the LED-based vision system. Furthermore there is a multitude of more complex manufacturing techniques going on, so there is room for growth, Merva believes, “possibly by a factor of ten greater than today’s penetration because these technologies are solving customers’ problems at the right price.”
Back and edge lighting
To achieve ideal illumination, many systems builders use an array of surface-mounted LEDs across the backlight with a diffuser positioned to produce even lighting. This results in a high-brightness backlight with a relatively thick display. To reduce this thickness – space is always at a premium in the manufacturing environment – some manufacturers also offer edge-lit backlights in which the LEDs are placed around the outside of the display and light transferred across the face of the display by a diffusion plate.
Moritex USA, based in San Jose, CA, offers both types of backlights. Its MDBL series provides a higher luminance than its edge-type MDBC series, whereas the MDBC series’ thinner design is more useful in applications with space constraints. Phlox, Aix en Provence, France, and Lumitex, Strongsville, Ohio, US, offer different variations of this technique in their range of machine-vision lights. Phlox uses a PMMA (polymethylmethacrylate) pipe lit by a linear LED source in its range of backlights; Lumitex uses an optical fiber mounted on a back reflector to create lighting panels as thin as 0.33 mm.
Christophe Blanc, Phlox director (and patent holder) said, “At Phlox, we only make a small part of the machine vision lighting system; we make backlights for which we use LEDs in conjunction with light pipe technology. We use PMMA and we manufacture a groove on top of the acrylic plate. The groove extracts the light, so we effectively inject light from the side of the plate.”
For its visible spectra applications Phlox deploys only Nichia LEDs. Nichia was the original manufacturer of white LEDs (invented by Shuji Nakamura, who nowadays works for Cree). The company uses only Nichia white/blue gallium nitride LEDs except for infrared applications. For its infrared machine vision applications, Phlox previously used Osram-OS LEDs but more recently the company has been working more with infrared LEDs from the Taiwanese supplier Lite-On.
As well as developing novel configurations such as flat dome lights, LED lighting manufacturers typically offer a range of different wavelengths for different applications. Aside from the more traditional product offerings using white, red, green, and blue LEDs, UV and IR lights are becoming increasingly popular to perform more complex vision inspection applications. These include systems where materials to be inspected may exhibit fluorescence or where operating personnel must be shielded from bright LED illumination. In other cases, illumination products that combine LEDs with multiple wavelengths are being used to reduce the number of inspection stations required to inspect multiple facets of complex parts.
The LED-based illumination systems that Banner Engineering (Minneapolis, MN, US) supplies for automated manufacture range from ring lights to back lights, spot lights to linear array lights. Those are the vision-specific light designs.
Figure 5: Banner Engineering’s High Intensity Area Lights are available in sealed IP69K versions so are suited to machine vision applications in food and beverage manufacture.
Figure 6: Osram-OS’s Ostar Observation infrared LED, model SFH4750 (Source: Osram-OS).