If you need absolute color accuracy of less than 1%, then we usually look at a three-CCD prism camera solution. If spatial accuracy over a wide inspection area is more important, then a very-high-resolution single-chip Bayer camera may be better. If you need high speed, CMOS offers higher frame rates and multi-line sensors with NIR capability and is very effective for high-speed printing applications where colorimetry measurements are very important because NIR can help you judge between true black ink and black made by combining cyan-magenta-yellow inks. And for some printing applications, knowing the difference is important for quality purposes.”
This means that an optic for a more expensive three-chip solution may also be more expensive because it requires an optic that uses more layers of optical materials to correct for the chromatic aberration, prism, and other sources of optical distortion. And while some might think you can correct this sort of distortion in software, it may not be that easy. “If you knew the exact part dimensions and colors, maybe you could correct the image using look-up tables, but any change in color, shape, or lighting could throw that correction off,” Hollows adds.
For imaging applications that need to measure multiple colors, white LED lights have supplanted halogen and fluorescent lights for a number of reasons, including LEDs’ ability to offer a wider range of colors and varieties of “white” light, also referred to as the color temperature.
“We offer white LEDs that run from cool blue-whites around 6500 Kelvin to neutral whites around 4000 K to warm whites at 2200 K that have more red in them,” says Oliver Szeto, President of Metaphase Technologies Inc. (Philadelphia, Pennsylvania). “While we see a lot of machine vision applications use the 6000 K cool whites because of high output, there’s also a trend toward lights that more closely mimic natural sunlight, around 3500 K.” For color machine vision applications that require accurate color measurements, a light’s color rendering index (CRI) is important because it quantifies how closely the light will reproduce colors compared to a reference light source, usually daylight.