Bar Code Scanning

Bar code scanners are electro-optical systems that include a means of illuminating the symbol and measuring reflected light. The light waveform data is converted from analog to digital, in order to be processed by a decoder (which is either built into the scanner, or a separate plug-in device), and then transmitted to the computer-based application software.

Scanners are either handheld or fixed-mount. Handheld scanners are used to read bar codes on stationary items. With fixed-mount scanners, items having a bar code are passed by the scanner - by hand as in retail scanning applications, or by conveyor belt in many industrial applications.

Handheld scanners offer three different technology choices: contact wands, CCD (charge-coupled device), and laser. A wand is a pen-shaped device with a light aperture tip that the user draws across the bar code. Contact wands require some practice to achieve the proper degree of tilt (typically 30 degrees) and correct motion speed for a successful read, but they are the least expensive bar code scanning device.

At the next price level are CCD scanners that use a stationary flood of light [usually Light Emitting Diodes (LEDs)] to reflect the symbol image back to an array of photosensors. Depth of field (DOF), the optimal distance for the scanner to read the bar code, ranges from contact to six inches, though greater DOF has been achieved. Because CCD scanners contain no moving parts, they tend to be more rugged than laser scanners. CCD-based handheld image readers (see machine vision systems below), lately coming to market, are capable of reading 2D matrix as well as stacked and 1D codes.

Laser scanners employ a beam created by a laser diode that is spread into a horizontal arc by means of a rapidly moving mirror. Though the light sweeps at about 40 scans per second, it appears (if it is in the visible light spectrum) as a single line. Laser scanners that operate in the invisible infrared spectrum use some means of auxiliary lighting that enables users to aim the laser beam. Revolving polygons or oscillating mirrors may also be employed to produce a more sophisticated moving-beam rastered, cross hatched, or starburst pattern for improved readability and omnidirectional laser scanning.

The advantages of laser scanning include a larger field of view and also DOF, which averages 6 to 12 inches but can achieve distances of 35 feet (with special reflective long-range labels). Laser scanners can best tolerate symbol skew and are perceived by some as the easiest to use because of their DOF and broader field of view. The tradeoff, however, is a cost which is somewhat more expensive than CCDs. The latest development in handheld laser scanning technology is rastering scanners that read 2D stacked codes.

Fixed-mount scanners use either moving-beam laser or CCD technology (often referred to as "machine vision" or "vision-based" technology in the fixed-mount configuration). Laser fixed-mount scanners are most familiar at grocery checkout. They are also used widely in work-in-process (WIP) manufacturing applications and in warehousing and distribution sortation and shipping applications. Very small fixed-mount scanners are commonly used in laboratory and process control applications. Overhead or side-mounted laser scanners are most commonly used across all industries, but fixed-mount vision-based scanners are beginning to gain favor, especially in high-speed sortation.

Links
Linear Symbologies
2D Symbologies

Reprinted with permission from AIM, Inc.
www.aimglobal.org

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