LEDs support multiple vision applications

July 1, 2007
With efficiencies and power improving, high-brightness LEDs find general and niche machine-vision applications.

With efficiencies and power improving, high-brightness LEDs find general and niche machine-vision applications.

By Winn Hardin, Contributing Editor

The unique properties of high-brightness light-emitting diodes (HBLEDs) compared to incandescent, fluorescent, and gas-discharge lamps has pushed solid-state lighting (SSL) deep into the machine-vision market. According to analysts, such as Robert Steele at Strategies Unlimited, approximately 50% of all machine-vision illuminators are solid-state, and machine vision accounts for approximately $13 million in global sales of HBLEDs.

SSL offers lifetime expectances of 60,000 hours, compared to 2000 hours for an incandescent bulb. LEDs can be made very small and are resistant to shock and vibration. They are slightly less efficient at converting electrons to photons, but generate less heat and consume less overall power as a result. Compared to other machine-vision lighting sources, LEDs enable light to be put in small places without as many environmental concerns and with greater efficiency through narrow spectral illumination bands and smarter control electronics.

The quest for a new light bulb is driving the development of white HBLEDs. Although SSL still lags slightly behind traditional lamps in lumens per watt, the gap is quickly closing. Recent white-light HBLED designs have generated more than 70 to 80 lm/W, compared to 80 lm/W for a fluorescent lamp (see table).

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“Xenon remains the standard for white-light strobing applications,” explains William Biederman, director of Vision Light Tech USA. “However, with LED performance improvements, they can replace xenon in some machine-vision applications. Xenon’s high-brightness performance maintains the stronghold in applications such as license-plate recognition, where stand-off distances and high output are the most important parameters.”

The search for a better white light also has led to an expanding number of colored HBLEDs. While red LEDs still account for more than of 60% of the HBLEDs sold for machine vision-mainly because of the improved spectral response of CCD sensors in the red and near-infrared (NIR)-new colors with machine-vision utility are emerging.

“The rate increase in efficiency for blue HBLEDs is by far greater than other types,” notes Simon Stanley, managing director of StockerYale LED manufacturing. “Excluding automotive applications, the biggest illumination markets are white lights, and for the moment that technology is based on blue LEDs with a phosphor, and that’s driving blue LED development.” White LEDs are generated either by mixing a red, green, and blue LED for a bright, three-chip solution, or by adding a phosphor layer to the blue LED to convert blue to white light.

Brighter white lights are also driving the development of HBLEDs that emit in the ultraviolet (UV). The UV is more difficult to achieve with SSL; however, the phosphors for converting UV light to white-light continuums are more stable and efficient. This is being weighed against new manufacturing methods that reduce the optical losses in blue LEDs. Stanley says, “The quantum efficiency of converting blue to longer wavelengths is pretty good. But because of the way the phosphors are applied-they’re blobbed on the LED in an epoxy-you get losses from absorption and scattering.”

Whichever way the industry pursues white-light generation, when combined with a growing number of niche applications for lot code reading using UV-fluorescent inks, sterilization, and microscopy applications, the UV LED market becomes more attractive.

LED Expertise

As the number and breadth of the spectra served by SSL increases, machine-vision integrators need to match that growth with education and experience in specifying and designing with HBLEDs. For instance, blue LEDs are useful for generating white light, but they are also helpful for improving contrast when inspecting copper, for instance, where a white light or red light would reduce contrast, explains Vision Light Tech’s Biederman (see Fig. 1).

FIGURE 1. To inspect copper, two LED spotlights are tested at the same angle and adjusted histograms so exposure between images is as close as possible. Under blue light, the center circle was not distinguished from the surrounding circle and the grid pattern is apparent (top). Under red light, the center circle is distinguished and the grid pattern no longer distracting (bottom). If the grid pattern needed to be detected, the blue light would be a better choice.
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“Some people look at an application and say, ‘I’ve always done this with metal halide lamps,’ and they think that means they need a lot of light,” Biederman says. “The truth is, they only need the right part of the spectrum. They don’t look at the spectral response of the camera and match it to the light.

“Plastics are another difficult inspection where it’s often better to use a single wavelength and filter out the ambient light rather than a full-spectrum lamp. The trick is you have a certain amount of software, processing power, and time to do your applications. To do it right, you only want the camera to see what it needs to see, and that’s the real beauty of knowing your application, from the spectral response of the camera, to the light, to the filters you use.”

Infrared HBLEDs are also growing in use as integrators become more aware of the unique benefits of this portion of the spectrum. For instance, the index of refraction for longer-wavelength IR light means the illumination can penetrate the surface of certain materials. Human tissue is one common example, but industrial scientists also know that IR penetrates cracks in metals, crystalline materials, and beneath oils, says Bob Proscal, engineer at LED supplier Optek Technology.

“There used to be a need for IR-sensitive inks for blind lot codes, but that application has trailed off as UV inks have grown,” Proscal explains. “But in the IR, you don’t have the health concerns of UV light. UV light is used to kill bacterial and germs, and in certain spectra bands poses a risk of cataracts to humans.”

Light Tech’s Biederman says, “The other great thing about IR is the strong response of most CCDs in that area, as well as the large assortment of filters that allow you to control what the camera sees. Most of the applications we see today in machine vision use either visible or IR HBLEDs.”

LEDs Get Smart

The growing maturity of the LED market has meant more than simply a wider selection of colors and brighter light sources. “A major development of the LED industry has been the range of LEDs with reproducible color temperatures,” explains Jon Chouinard, product manager at Siemens Energy & Automation NERLITE offices. “Manufacturing processes are better understood, leading to greater consistency, which is helping all aspects of the LED lighting industry and particularly machine vision, where uniformity is critical.”

StockerYale’s LED business centers heavily around machine-vision applications, according to Stanley, and therefore includes unique testing steps to guarantee uniformity for challenging machine-vision applications. StockerYale purchases finished wafers and packages the LEDs before adding driver electronics, optics, and housing, unlike most suppliers who purchase the packaged LEDs and add drivers and optics to create a finished light (see Fig. 2).

FIGURE 2. With chip-on-board methods, LEDs can be placed on circuit boards in high-density arrays, allowing for compact, extremely bright illuminators.
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“One of our advantages is our ability to work with diced LED wafers. We can take samples from various points around the wafer and produce a map of the color and intensity, which yields an intensity map based on individual LED chips on the wafer,” says Stanley. “We use automated pick-and-place machines with the map to create lights with uniform output.”

StockerYale’s approach from the wafer up gives it flexibility. The company is beginning manufacture of a 12-mm ringlight with 196 individual chips inside.

Spectrum Illumination, LED supplier to the automotive market, released a new white-light HBLED that produces 90 lm/LED, or twice that of the company’s existing products, at the International Robots & Vision Show (Rosemont, IL, USA) in June. Spectrum president Dave Muyskens expects that output to grow to 240 lm very soon. He notes, “Everything’s moving to HBLEDs, so the drivers [control electronics] are becoming more and more important.”

While some existing drivers for HBLEDs use closed-loop feedback and current monitoring to guarantee a specific output as the lights age-which limits the number of lighting-related machine-vision errors over time-HBLED vendors are adding more control features to their products. Spectrum Illumination plans to introduce a light based on a product developed for internal aircraft lighting that combines five LEDs to produce radiation between 470 nm (blue) and 630 nm (red) and any color of white in 10-nm increments.

Spectrum has introduced the “Monster Brain” Ethernet controller. The unit, which comes with an embedded Web server for automatic TCP/IP configuration, connects directly to a LAN and allows analog control of up to four channels. “You don’t have to load software,” explains Muyskens. “You open up the a browser, type in an IP address, and you can program up to four channels-three in analog or four strobe or any mode in-between.”

“Consolidation is already happening in the wider LED industry and will continue,” says Chouinard. “I think OEMs will take LED integration in-house as these products become more commoditized, but there will still be room for companies to be successful serving niche industries.”

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