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Save That Light Bulb: LEDs May Make It a Collector's Item Before Too Long
by Winn Hardin, Contributing Editor - AIA Posted 12/16/2003
Like discussions of machine vision and image processors, cameras or optics, any discussion of machine vision lighting systems must take its lead from the lighting industry at large. After all, machine vision did not drive the development of the microprocessor, CCD camera or 35-mm optics, but it sure has benefited from development of those technologies.
Lighting is no different. Within the lighting industry, it is not unfair to say that solid state lighting, or light emitting diodes (LED), are the greatest thing since the light bulb. In fact, in many low-voltage lighting applications -- including traffic lights, automobiles, signage and consumer electronic devices – solid state has replaced filament bulbs. Advances in high-brightness LEDs and new electronic controls coupled with vendor-based efforts to make the highly customized LED lighting industry as 'customer friendly' as possible are changing the way vision integrators are designing their applications.
Solid state basics
Unlike light bulbs that heat a filament until it glows or fluorescent lamps that use charged particles and phosphor coatings to produce light – both of which can waste considerable energy through heat production – solid state LEDs are semiconductors, similar to microprocessors. LEDs produce light by combining electrons and electron holes in a semiconductor material, which in turn generates photons or light. The material properties of the semiconductor determine the light 'color' or spectral response of the LED.
LEDs are not just fashionable, but functional. At heavy duty cycles, LEDs can last 10 times longer than a standard light bulb, are more efficient at converting electrical energy into light energy compared to filament-based bulbs, can generate specific light colors without the use of inefficient filters or coatings.
According to Joe DiRuzza, director of sales and product management at Stocker Yale (Salem, NH), LEDs that emit in the UV are becoming of particular interest to machine vision suppliers. ‘‘The problem is that there’s still not enough output, and hopefully we can get deeper into the UV. Right now the most efficient UV LEDs emit around 405 nm, and we’ve looked at 395 nm, but 365 nm would be the best. Suppliers are developing LEDs at 375 nm, which is pretty close.
LEDs also can provide directional, or structured, light that enhances the vision system designers ability to fully illuminate complex parts. The directional control of LEDs allows a vision system integrator to inspect the inside of containers, such as glass bottles, by using a ring light package and crossing the individual light beams at the bottle neck and expanding the light as it travels deeper inside the bottle. This ability to carefully aim LEDs has led to a host of lighting configurations, including ring, axial, spot, line, and darkfield lighting.
Vision system lighting still a 'black art'
The variety and number of LED light configurations speaks directly to the unique needs of most machine vision applications. ‘‘Basically, every part presented to the camera is going to be a different part and reflect light differently,’‘ explains Marcel Laflamme, vice president of sales and marketing at RVSI/NER (aka Northeast Robotics, Weare, NH), ‘‘But LEDS are still made based on the semiconductor production model, and that means that suppliers are not necessarily flexible. So you have to be flexible in your packaging and design. We've designed 300 different lights, and it usually takes a new design every time someone inspects a new part. Customers complain about the cost of the light, but we end up with a lot of models with small volumes. Luckily, all the [LED light makers] are in the same boat.’‘
To help customers select an LED light, lighting providers make their optical engineers available to potential end users, and are developing 'flexible' lighting manufacturing and design processes.
Advanced Illumination (Rochester, VT) President John Thrailkill explains: ‘‘We've always offered variations on standard products – you can have a ring light with different voltage type, 12 V versus 24 V; or current versus voltage drive; lights with various fields of view or stand offs. That's something we've done for quite a while. Today, we're building lights using extrusions, utilizing industrial fabrication equipment so we can process the extrusions in house and make custom lights faster and with less engineering effort. Customer Service can use a matrix that takes the number of LEDs, type, etc., and determine a price without having to wait on a lengthy quote process. The matrix includes a drawing system that ties into our model numbering system. Given a part number created from the matrix, the people on the floor can quickly find the pre-generated LED wiring diagrams and other characteristics to build that light’‘.
When budgets and volumes preclude such customization, companies such as Edmund Industrial Optics (Barrington, NJ) offer application services using off the shelf components, whenever possible. ‘‘Is everything custom? Not necessarily. A lot of customization comes from using off the shelf parts in new ways – pushing the envelope. People that are building high level processing machines…have the resources to go custom, but individuals in the second tier may not. Many of these people are very knowledgeable about straightforward applications work with off the shelf components. However, when you're working with multiple lights, for instance, that's where people can struggle,’‘ said Greg Hollows, Product Line Manager for industrial lighting at Edmund Industrial.
Additionally, in very complicated or highly demanding cases, Hollows says that Edmund is one of the few companies that uses non-sequential ray tracing modeling software. This software is different from standard ray tracing software, requiring a precise understanding of the optical properties of all materials and surfaces in the image chain, including the part itself, mirrors, etc. A modeling simulation using this software can take up to 2 weeks or more to complete, but can result in a system that really dials in a custom lighting and optical solution, which can lead to lower costs in the long run, Hollows said.
Bright lights, new applications
While design methodologies are impacting every LED light used in machine vision to some degree, high-current, high brightness LEDs are expanding the application base of machine vision systems.
‘‘LEDs used to be basically discreet, that is one chip one wire, and encapsulated in epoxy of some kind. What's coming out now is a multitude of chips in one cavity. These LEDs are called 'chip on a board' and can generate up to 5 W per LED,’‘ said RVSI/NER's Laflamme. ‘‘The only problem is they can be hard to use because of the additional heat created by these high-current LEDs.’‘
‘‘These LEDs were not originally designed to be used in large arrays with dense packaging,’‘ explains James O’Hanley, vice president of CCS America (Waltham, MA). ‘‘So in order to accomplish this, we needed to address the heat issue.’‘
CCS America (Waltham, MA) and Schott Fiber Optics (Auburn, NY) have created intelligent controllers to deal with heat dissipation and extend the lifetime and stability of LED lights. According to Tony Ruffini, director of sales and marketing for Schott Fiber Optics, the company’s Ultra LED universal LED controller with matrix plate uses a microprocessor to automatically recognize the type of LED light head, and monitor the temperature of the light head to guard against early failures due to excessive heat. The Pro Ultra also provides an overdrive intensity mode and asynchronous trigger that provides up to 150 percent of standard light output.
According to CCS America’s O’Hanley, the company’s new LDR2 LED design accelerates heat dissipation by adding a layer of conductive material between the PCB supporting the LEDs and the aluminum housing, eliminating the normal gap between the two. This cuts heat generation by 50 percent, O’Hanley explained, while stabilizing the operating temperature at lower levels, and stabilizing light output as temperature fluctuations are reduced.
In addition to the flexible design methodologies and intelligent controllers discussed above, LED light manufacturers are creating new electronics that can operate multiple LED arrays from a single controller and surface-mount/flex-circuit designs that keep costs to a minimum. These and other improvements are helping to simplify machine vision lighting while improving the performance of these systems in a wider variety of applications.
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