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How Everything Works How Everything Works

Question 509

How does a UPC scanner work?
UPC labels are the bar codes placed on consumer goods to identify them as they pass over a glass window containing a UPC scanner. Although UPC labels were first conceived by Norman Joseph Woodland in the late 1940's, the scheme to read those codes required a very bright and narrow beam of light that could be scanned rapidly across the bars in order to measure their widths. Conventional light sources barely worked and the idea didn't catch on until lasers became available. A modern UPC scanner begins with a laser that emits a tightly collimated beam of light. Early scanners used helium-neon lasers, but new scanners use cheaper and more reliable solid-state or diode lasers. In a typical scanner, the red beam from a laser is directed toward a spinning object—either a carefully faceted and mirrored disk or a flat disk containing a carefully designed hologram. Laser light that reflects from the spinning object emerges from the glass window above the scanner and sweeps rapidly through the space like a tiny searchlight. When this light beam encounters a UPC label, each dark bars absorbs the beam while each light bar reflects it. Thus as the beam scans across the UPC label, the amount of light the product reflects fluctuates up and down in a characteristic manner. When a photodetector in the UPC scanner detects such a fluctuating reflected light signal, it determines that the laser beam is hitting a UPC label. A computer studies the sequence of the light and dark bars to determine exactly what UPC label is being hit and identifies the product to the store's computers.

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