The boom in LED lighting has surged through many consumer and business sectors, showing us the advantages of long-lived and robust solid-state illumination over incandescent bulbs. Yet all the attention associated with LED lighting in applications such as architecture, automobiles, and consumer electronics has obscured its effect on smaller markets such as machine vision, where the impact is at least as profound.
Machine-vision illumination technologies have varied widely, from LEDs and fluorescent lamps, to xenon flash lamps and quartz halogen lamps. Incandescent lamps have not normally been used because of their heat-generation properties and marked variation in brightness with power supply frequency. The goal of all this lighting is to enhance the contrast of an object under inspection so that the desired features are more easily identified.
Selecting the right type of lighting largely depends on the size, surface features, and geometry of the part to be inspected. Although fluorescent tubes are probably the single most common light source for machine vision, LEDs provide the greatest variety of forms. LEDs can be found in one of several configurations, including ringlights, spotlights, backlights, and diffuse lights, all of which provide end-users with considerable design flexibility. LEDs last far longer than competing lighting technologies, which is a crucial consideration in a manufacturing environment. In addition, LEDs can be strobed without wear, and operate in the red and near-IR, where the charge-coupled devices (CCDs) commonly found in machine-vision cameras have the highest sensitivity.
Robert Steele, director of optoelectronics at Strategies Unlimited (Mountain View, CA), thinks that high-brightness LEDs are capable of servicing 70% to 80% of the lighting requirements of the machine vision market. Although high-brightness LEDs for imaging applications make up only a small fraction of the total 2003 LED component market—estimated at $2.7 billion—Steele thinks that growth is doubling each year, making the 2003 LED market in machine vision around $11 million.
The real value of course comes when OEMs and systems integrators design LED lighting into complete vision systems. For example, Ford Motor Company (Detroit, MI) sees machine vision as an increasingly integral part of many of its production processes, including robot guidance, inspection, traceability and identification, and process control. Valerie Bolhouse, a staff technical specialist at Ford's Advanced Manufacturing Group, says that LED lighting is one of the key factors increasing the robustness of vision systems and making them viable for the automotive factory floor.
In addition to robustness, Ford can take advantage of the spectral response of CCDs in the near-IR and the monochromatic output of LEDs to mitigate the effects of ambient light. A bandpass filter matched to a near-IR LED will block out much of the ambient light in the factory and pass only the reflected illumination from the light source, enhancing contrast in a part under inspection.
Since LEDs can be tailored to produce almost any wavelength of light without use of a color filter, system integrators have also been getting interested in UV LEDs for detection, identification, and excitation applications. UV light can be used to inspect dye penetrants, labels, and UV-sensitive ink and glue. Current devices emit around a peak wavelength of 405 nm, but 395 nm is available, 375 nm is under development, and 365 nm will be desirable for many applications.
Structured light applications are another good use of LEDs in machine vision. A high-speed bottle inspection system, for example, can rely on an LED array to look inside the bottle.
One hurdle that must be overcome is heat management. The new generation of high-current or high-flux LEDs use multiple or large dies to provide up to 500 mW per LED. Such power has advantages in automotive applications and robot guidance, but creating adequate heat dissipation is a challenge. Given the rapidity with which LEDs have penetrated the machine vision illumination market, such a relatively mundane obstacle is a welcome sign that LED lighting has arrived here as well.
CONARD HOLTON is editor in chief of Vision Systems Design; e-mail: [email protected].