New LED light arrays improve lighting for machine vision
Lighting is the most critical element in a machine-vision system, allowing the system to measure and interpret the energy reflected from objects. Recently lighting manufacturers have become more adept at creating LED-based products for sophisticated applications, as well as standard factory-inspection systems. Leveraging the latest developments in semiconductor and optics technology, these products often differ considerably from established designs, providing system developers with greater flexibility when choosing illumination systems.
While the typical luminous efficiency of white LEDs is currently 40 lumens/W, the target, according to the Optoelectronics Industry Association (Washington, D.C.), is to reach 150 lumens/W by 2012. Two approaches exist to increasing the light output from LED lighting sources. The first, favored by companies such as Lumileds (San Jose, CA), Cree (Durham, NC), and Osram (Danvers, MA) is to produce single-power LED packages that can be attached to a printed-circuit board in tight clusters.
The second approach, favored by companies such as Opto Diode (Newbury Park, CA), Lamina Ceramics (Westampton, NJ), Enfis (Swansea, England), and CCS (Kyoto, Japan), is to package a number of LED chips onto thermally efficient tiles that provide high power density and efficient light-emitting arrays. The advantage of this approach is that multiple colors can be combined within the array to produce white light.
Light arrays produced by Opto Diode, for example, have 99 LEDs in a nearly square 1-in. format. With a power consumption of 1.2 W, the light array produces a luminous output of 250 lumens at a wavelength of 525 nm over a 100° beam angle. Using a multilayer cofired, ceramic-on-metal package, Lamina Ceramics builds light engines that incorporate multiple high-brightness LEDs packaged into arrays. One product line of red LED arrays, for example, is configured with 39 cavities, each populated with multiple LEDs. Enfis says it customizes the design of LED light engines by using in-house and out-sourcing facilities.
Flat Domes
Cloudy day illuminators (CDI) use an integrating sphere to produce a self-contained continuous diffuse lighting environment. CDIs can be used in applications involving objects inside clear packaging, such as blister-packed pharmaceuticals, by eliminating reflections from the clear cell coverings, making them virtually disappear while providing a high-contrast view of the cell contents.
“While these approaches are useful in many applications,” says Dirk Vermeersch, president of CCS Europe (Sint-Pieters-Leeuw, Belgium), “they suffer in applications that demand smaller, more flexible lighting configurations that can be more easily integrated into a machine-vision system.” To overcome these problems, CCS has developed a novel “flat-dome light” that integrates the benefits of coaxial and dome lighting into one compact lightweight flat design. Each of the CCS flat-dome panels consists of four arrays of linear LEDs mounted at the periphery. Light shining across the flat panel is then reflected downward by hundreds of hemispherical arrays. The result provides uniform illumination over glossy surfaces while at the same time providing shadowless diffused illumination on objects with curved or uneven surfaces.
To compare the effects of its new flat-dome lighting with coaxial and domed lighting, CCS engineers used the flat-dome panels to illuminate pharmaceutical blister packs. “While coaxial lighting eliminates the red lettering on the surface of the object,” says Vermeersch, “it cannot eliminate the specular reflection associated with the uneven surface of the film.” When the same part is illuminated with the flat-dome lighting, however, the red lettering on the surface of the blister pack and the specular reflections from the film are eliminated. The pinhole defects can then be detected more easily.
In this way, improving the performance of LEDs and the illumination products that incorporate them are resulting in brighter, more stable lights with greater MTBFs. For the machine-vision system integrator, the advances now being made in semiconductors, packaging, and intelligent control systems will allow low-cost LED lighting to be even more effectively controlled and maintained in critical imaging applications.
CONARD HOLTON is editor in chief of Vision Systems Design; e-mail: [email protected].