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LED Brightness, Uniformity Improvements Open New Applications
by Winn Hardin, Contributing Editor - AIA Posted 09/14/2009
Light emitting diodes (LEDs) produce more lumens per Watt of electrical power than incandescent lamps, while lasting longer. And since they are semiconductor products, LED band gaps that produce the light can be engineered to produce specific ranges of ‘light,’ ranging from the ultraviolet (UV) through the infrared (IR). Each of these benefits makes LEDs attractive to the machine vision industry.
Despite the fact that the low-volume machine vision industry isn’t large enough to drive LED development, the vision industry is leveraging LED developments to its own end to improve brightness, uniformity, and color selection, while adding smart optical and electronics designs to make these miniature lights more effective for machine vision applications. “Higher brightness with better uniformity is opening up new application areas in biomedical devices, solar and other application areas,” says Dr. Scott Kittelberger, Chief Operating Officer of Volpi USA (Auburn, New York).
Brightness and Uniformity
The first LEDs were leaded devices, hermetically sealed without much attention to heat dissipation, and as a result, these devices produced a few mill watts. Surface mount devices (SMD) soon followed but also lacked a means for heat dissipation. These devices were mainly used as indicator lights for electronic devices. High brightness LEDs came along, delivering enough light for backlighting electronic displays, automobile tail lights, signage, and other applications. These high brightness devices included a heat dissipation channel to their host PCB board; however, they were quite large, measuring up to 1-inch on a side. This limited the manufacturer’s ability to pack the LEDs closely together, which reduces the uniformity of the packaged illuminator. Light produced with these devices has ‘hot spots’ or areas where the fan-shaped light from the LED is brighter than in other places.
And as all machine vision engineers know, inconsistent light sources play havoc on image processing algorithms. “It’s very hard to make uniform light fields with those devices,” explains John Merva, Executive Vice President of Advanced illumination (Rochester, Vermont). “It’s like looking at a bunch of spot lights. New, high brightness SMD packages allow you to pack six, twelve or more surface mount devices in the same area.”
An alternate packaging method, called chip-on-board (COB), places dozens of LED chips very close together on aluminum-based PCBs, rather than encapsulating each chip or a few chips in its own hermetic package, however, this exponentially increases heat generation. “Unfortunately, you have to buy very large volumes to get a price break on bare LED dies and few companies outside of illumination, displays, and signage need that kind of volume,” explains Advanced illumination’s Merva. “Costs for bare LEDs die are similar to SMDs, and they still must be packaged and customized. This raises costs and limits the use of COB to fewer, demanding applications.”
LED light manufacturers for machine vision are further using special films and low-cost custom optics to improve brightness and uniformity. Advanced illumination, for example, applies dispersive and collimating films originally developed for flat panel displays directly to the LED window to improve uniformity while increasing light output. SCHOTT North America (Southbridge, Massachusetts) is now able to use simple polycarbonate lenses instead of complex collimating lenses to collect and direct LED light from its new LLS fiber optic light sources. As a result, combined with the on-going advancements with LED packaging, the LLS is now two to three times brighter than units produced just 18 months ago, according to SCHOTT’s Business Manager for Machine Vision, Carl VanDommelen. In the meantime, Volpi’s Kittelberger spends much of his time customizing optics for OEM LED illumination applications using acrylics and other cost-effective optic materials. “You don’t need imaging grade optics to direct light, and the applications couldn’t afford the extra cost if you did use them,” Kittelberger explains.
Demand ultimately reshapes markets, however, and Navitar’s (Rochester, New York) Director of New Product Development, Bill Bridson says that plastic optic manufacturers are exploring LEDs for catalog solutions. “Certain plastic lens makers are being sponsored to come out with unique, economical shapes that permit capturing the LED outputs,” says Bridson.
Finally, machine vision lighting companies are developing new, cost-effective LED driver electronics to improve LED brightness, stability and longevity. “Leading manufacturers have always provided current drive capability rather than voltage drive capability because an LED is a variable forward voltage device,” explains Advanced illumination’s Merva. “As you drive it harder, the junction temperature changes, and so does the forward voltage drop. In the past, it was inexpensive to add the resistors to control voltage, but that led to brightness instability and you’re adding to your heat dissipation problem. Current drive sources get the resistors out of the LED circuitry, and that makes the light run cooler and with increased stability. Today, we’re making $35 current sources that are equivalent to sources that used to cost hundreds of dollars.”
White Trumps Red, UV Wins
The brightest, most common LED lights used to produce red light. Red LEDs dominated early because manufacturers had more experience working with gallium arsenide (GaAs), which can be engineered to produce most of the visible and some of the infrared spectrum, than the nitrides, which produce blue visible light down into the ultraviolet (UV). Red LEDs also were useful for pumping fiber lasers used in telecommunications networks. “Way back when, IR LEDs were the brightest, but now they’re more difficult to get because most of the development is going into white LEDs for more pedestrian applications than high-technology and machine vision,” explains SCHOTT’s VanDommelen.
With the advent of blue LEDs, manufacturers were able to combine multiple chips or add phosphor coatings to blue LEDs to produce white LEDs from a single illuminator, opening up the world of consumer applications in lighting, displays, signage and automotive. This development has been a two-edge sword to the machine vision industry.
“We’ve seen a lot more interest recently in UV and IR LEDs for very specific solar cell inspection applications,” explains VanDommelen. “Customers often ask us if we can build an IR or UV LED array, and of course the answer is, ‘yes’, but the output intensity and lifetime aren’t comparable to what we’re getting out of visible LEDs, and it’s going to be more expensive. Unfortunately, the machine vision market doesn’t drive the LED market due to the comparatively small volumes we purchase.”
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