Advanced Imaging

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Advanced Imaging Magazine

Updated: July 8th, 2008 05:26 PM CDT

Practical Color Machine Vision

Monochrome for Most, but Color Can be Essential
Basler
Basler color line scan cameras at work in a machine vision application.
Basler
Basler color line scan cameras at work in a machine vision application.
Toshiba
Image of a defective disc with defect highlighted using a Toshiba IK-TF5 camera integrated into the Xiris PI-2000C CD label inspection system. The camera used to check print labels operates at VGA resolution, processing 500 labels per minute.
Toshiba
Toshiba's IK-TF5 camera is integrated into the Xiris PI-2000C CD label inspection system.
Prosilica
A Prosilica GC-1380C high performance color machine vision cameras with Gigabit Ethernet interface (GigE Vision).
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By Keith Reid

In the world at large, monochrome is largely passé. Even the cheapest TVs today feature color, and have for many years. A variety of military applications are still in monochrome, but that is typically a function of the limited wavelengths used in night vision applications. Monochrome is still common in the CCTV universe, but a shift is under way there toward both color and higher resolutions. Then, there is machine vision. For some of the most advanced imaging applications in common use, machine vision is still, largely, a monochrome world—and with good reason.

Imaging in industrial production is typically used for such tasks as quality control. The general goal is to identify a feature or features on an object, such as a circuit board, to determine if something is missing or has been improperly aligned during the manufacturing process. Common machine vision requirements like edge detection and blob (lighter vs. darker) analysis often do not require color, and can benefit in many cases from monochrome. These processes typically work to maximize the data relevant to the specific task at hand, while minimizing unnecessary background data. Color generally generates a higher data volume, requires more data processing, has reduced sensitivity and a somewhat reduce spatial resolution which can work against specialized machine vision functions. There are a variety of additional color issues that come in to play, including white balance and lighting, that can affect the performance of a color camera but have less impact on the monochrome camera.

Monochrome can even handle various color-related machine vision tasks more effectively than a color camera. "Quite often, if somebody is just looking for a 'green mark' or a 'red mark,' rather than using a color camera, this can better be handled using a monochrome camera with single color filters or a combination of lighting to better discriminate the color of interest from the background," said Marty Furse, CEO, Prosilica Inc. (Burnaby, Can.). "If you need to align a green stripe with a triangle shape, you can sometimes achieve that better by enhancing the green with a green light and using a filter to subtract the background."

From a bandwidth and processing standpoint, the differences between color and monochrome can be a toss up. "When you have Bayer output, which is not interpolated data, you have the same amount of data as monochrome so it doesn't really impact the bandwidth, but in this case, the host computer usually needs to perform color interpolation before image processing begins," said Furse. "There are the various color interpolation modes: YUV 411, YUV 422, YUV 444 and RGB 24, which is an interpolated 24-bit color. Bandwidth can become an issue when using interpolated modes. Generally speaking, RGB 24 pushes at least three times more data than monochrome, but the host computer does not need to interpolate the resulting data prior to processing. Thus, there is a trade-off between interpolated and un-interpolated modes because you're trading off host computer loading versus interface load. It is not always a clear decision as to which is better. It tends to be application-specific and interface specific choice." Furse added that with GigE, the bandwidth is high enough that to run the camera at full speeds even with interpolated color.

With the cameras themselves, there is the option to go with a one-chip or three-chip solution. A one-chip camera is typically much simpler in design, featuring a single sensor element and a Bayer pattern filter that provides 50 percent green, 25 percent and 25 percent blue. The green value is doubled to reflect how the human eye resolves green to provide for a similar result. Bayer output does have some negative issues, such as a loss of data. The answer is to use interpolated color which relies on algorithms to fill in the missing data, but a cost in bandwidth and processing power. Both monochrome and color single-chip systems are roughly comparable in price, and in fact a single-chip color system can be cheaper give the high-volume production of this format in consumer markets.

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