Will CMOS sensors catch up?
Jon Titus, Contributing Technical Editor -- Test & Measurement World, 11/1/2005 2:00:00 AM
Engineers who design inspection systems may wonder why cameras based on popular CMOS sensors haven't appeared in large quantities in the machine-vision market. After all, CMOS area-array sensors serve well in consumer devices such as cell phones and digital still cameras. In addition, CMOS technology offers several advantages over charge-coupled devices (CCDs). The CMOS fabrication technologies lend themselves to the inclusion of timing circuits, analog-to-digital converters (ADCs), and other functional blocks within an area image sensor. Cameras that employ CCD sensors must include discrete circuits that provide those functions.
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| Image-sensor manufacturers offer CCD and CMOS detectors in a variety of formats, most of which aim to satisfy the needs of consumer products. Courtesy of Dalsa. |
Unfortunately, most commercial CMOS sensors lack a critical feature—a "global" shutter—inherent in CCD sensors. A CCD sensor will simultaneously reset all its detectors (pixels) and, after an exposure, will transfer the accumulated charges to a shielded area. The CCD sensor then shifts out the charge for each pixel. This type of technology goes by the name interline transfer, or ILT. The simultaneous action at all detectors prevents image distortion during an exposure and readout.
Most CMOS sensors, though, use an electronic "rolling" shutter that can lead to distorted images when significant motion occurs in an image during an exposure. A CMOS sensor sequentially charges rows of detectors, waits a short exposure time, and then reads the exposure information row by row. Think of how a flat-bed scanner operates and you'll get the general idea. Move a scanned paper during a sweep, and you see a skewed image. So, images acquired with a CMOS-based camera on a fast-moving production line may appear distorted.
But don't count out CMOS sensors. Terry Guy, a product marketing manager in the Image Sensor Solution Group at Eastman Kodak, explained that some designers employ a strobe light to illuminate an object. They produce a light pulse that lasts only as long as the CMOS camera's exposure time. Thus, the camera "sees" the object only during the strobe period, which eliminates distortions. Guy also noted that generally, CMOS sensors offer less sensitivity and introduce more noise than CCD sensors. He expects, though, that technology advances will eventually let CMOS sensors catch up with CCDs.
According to Dave Litwiller, VP at Dalsa, a supplier of sensors and cameras, many CMOS-sensor manufacturers have concentrated on building devices for consumer products that don't require a fast shutter. After all, a slight skew in a picture-phone image doesn't matter. So far, these manufacturers haven't seen enough of a machine-vision camera market to offer CMOS devices with better shutter characteristics. Litwiller thinks the emergence of global shutters in CMOS sensors, the advantages of integrating functions within the sensors, and CMOS sensors' low power demands make these sensors increasingly attractive to machine-vision camera designers.
For an animated description of the ILT technique, visit: micro.magnet.fsu.edu/primer/digitalimaging/concepts/interline.html.
Imager scans 1-D and 2-D codes
To operate the Quadrus Mini code reader, you simply plug it in, point it at a symbol, and push the EZ button. The unit automatically focuses and sets exposure time, gain, and illumination intensity before it reads linear bar codes and 2-D codes. The Mini can decode 2-D marks that move at up to 100 ft/s on a production line. www.microscan.com.
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