LEDs brighten inspections
Jon Titus, Contributing Technical Editor -- Test & Measurement World, 10/1/2004
As LED manufacturers seek to replace lamps in many general-illumination applications, they produce larger LEDs that provide more light at higher efficiencies and at lower cost. Recently, those LEDs have come to the fore as light sources in machine-vision applications. Most system designers employ high-power LEDs to produce short pulses of intense light, although some LEDs can operate continuously at lower power. Traditionally, vendors and designers have used xenon flash lamps and high-intensity incandescent lamps to provide bright light. But flash lamps are expensive, and precise control of flash characteristics proves elusive. In addition, as incandescent and flash lamps age, their light-output characteristics change, which can degrade image quality.
Suppliers of light sources for machine-vision systems often use high-power LEDs to produce repetitive light pulses that may last only a few microseconds. That type of lighting forms a critical part of an inspection system that must acquire images of products that move rapidly down a production line at, say, 20 items/s. A short pulse of light would "freeze" any motion so a camera could capture an image without blur.
Setting the camera for a 20-µs exposure time and coupling it to a bank of high-energy LEDs does the trick.
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| A line of red LEDs provides light for a line-scan camera. Models of the Cobra 500 illuminator can operate as an always-on or as a pulsed-light source. Courtesy of StockerYale. |
In addition to helping cameras capture images of fast-moving components, high-energy pulsed LEDs can let a lens provide an extended depth of field. When a product undergoing inspection reflects additional light, system designers can specify a smaller lens aperture for the same exposure. A smaller aperture increases the depth of field so an inspection system can read information from the top of tall components on a PCB as well as from labels farther away on the PCB itself.
Designers can choose one or more colored or white lights to illuminate a product, depending on the application. Some inspections may require a color to enhance contrast of certain components or to match light-source characteristics to a camera's specifications. The spectral sensitivity of monochrome CCD and CMOS cameras, for example, peaks in the red and infrared (IR) portions of the spectrum. Thus, inspection systems in which black-and-white images suffice can take advantage of high-output red or IR LEDs.
If you plan to design a new vision system or need to replace existing lights, LEDs offer additional characteristics that make them good choices for automated inspection tasks.
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