The bar code message is carried, via a label, to many data collection locations where it will be "read". Now this message, together with time, place, and additional information recorded at a the reading point, will combine to become a record in a database. The collective information in the database will enable an organization to operate a department or function more efficiently through timely and accurate detailed reports of activities. The purpose of this section is to explain how bar code scanners "read" the data messages on labels, and what characteristics scanners should have to perform bar code reading effectively.

A bar code scanner is a device which projects a tiny spot of light crossing the bars and spaces of a label, and then precisely measures the reflectance back from that spot. Scanners then create an electrical signal (voltage) proportionate to the strength of the reflected light, and amplify and shape this signal for reliable transmission to a data entry terminal.

To achieve the most dependable results from bar code data collection you must match the "X" dimension of the symbol with a scan spot of proportionate diameter. In other words, your scanner can project a spot that is too large, too small, or ideally matched for your bar code. As the scan spot moves across the bar code, reflections from each white space and light absorption from each black bar, in combination, produce a voltage variation through the scanner's electronic sensor.

Since bar code does not normally misread, this spot size mismatch will result in a poor "first read rate". This type of problem is more noticeable with a wand scanner than a laser scanner, which scans tens or hundreds of times per second, and often finds a good read path relatively quickly. A gross mismatch between the bar code "X" dimension and the scan spot will produce a "no read" at that scan station.

There is a wide choice of scanning instruments available in today's marketplace. Different scanners have been developed to respond to the variety of bar code applications which have emerged over twenty years of practice. The scan rate may be as low as one scan per second using a wand scanner, or as high as 1000 scans per second with a fixed mount laser scanner. To decode a bar code message properly, the speed of the scan spot over the bars and spaces of a symbol must be relatively constant across the entire symbol. A speed variation by the scan spot of more than two to one will challenge the best decoding logic, since the shape of the signal produced by the spot reflection will be distorted.

Bar code technology "won" the competition for industrial data collection technology twenty years ago, in large part due to the distance bar code could be read from a fixed mounted scanning station adjacent to an automatic conveyor. The variation in distance that a bar code can be read from a scanner is called depth-of-field. The scanner marketplace offers scanners with "zero" depth-of-field, called contact scanners, as well as laser scanners with depths-of-field of several feet.

The most cost effective method of bar code scanning is the wand scanner, or light pen as it is sometimes called.

Using a wand scanner requires attention to certain details for greatest effectiveness. As in all applications, there should be great emphasis on the printing excellence of the bar code message presentation. The wand user must be trained to properly scan a label. This involves starting with the pen tip lightly touching the label well to one side of the symbol and moving across all the bars to a position well to the other side of the bar code in one continuous motion. The wand angle is important, too, as the wand should be held at an angle of about 60° above the label surface. This is about the angle one would use to write with a pen, so it does not typically present a training problem. In addition to making sure that the bar code has been printed to exact tolerance, there should be a good match between the wand spot resolution and the "X" dimension of the symbology. Wand scanners can be purchased with several choices of spot resolution.

When contact with the bar code label is awkward or if the nature of the application makes contact impossible, the hand held laser gun is an alternative scanning device to consider. The general design of a laser gun incorporates a solid state laser diode, creating a scan line by projecting a beam of energy off a rotating prism or oscillating mirror. The scan beam then exits through a scanner window to trace across the bar code symbol. While a device of this sophistication is about 6 times more costly than a wand scanner it is useful and worth the premium under certain circumstances. Scanning a bar code on a curved or irregular surface, or through multiple layers of stretch wrapping would require a non contact device such as the laser gun.

If the label is damaged or poorly printed the multiple scans generated by a laser gun will more rapidly seek and find an acceptable read path through the bar code, reducing employee frustration and accelerating data capture, when compared to wand scanning.

When bar code labels can be brought to near-contact with the scanning device the user has one more choice to consider, the charge coupled device (CCD) scanning method. CCD technology has been used commercially for about ten years and is a technique whereby the bar code is entirely flooded with light (typically by a cluster of LEDs) and the image of the bar code is transferred to an array of very small photo detectors. Instead, the characteristics of the bar code are determined by electronically sampling each individual photodetector, which interprets each bar and space measurement by the number of adjacent detectors sensing black, in comparison to white.

The cost of these devices lies about halfway between the wand scanner and the hand held laser gun. They are more rugged than the laser gun, being lighter and without moving parts. However, CCD scanners are required to be in near contact (an inch or so) with the bar code. They would not be suitable for an application where labels are set back several inches behind stretch wrap, as is common in many warehouse operations. A bigger drawback to their use stems from the need to read a wide variety of label lengths and formats, where at least one bar code message is long, or expressed in a large "X" dimension. Here the length of the bar code may well exceed the width of the scan head which would make the label unreadable. Where it is practical to use CCD scanners, you will find that the training requirement is the easiest to fulfill. The concept of "taking a picture" of the label by covering it completely with the end of the scanner and tripping a switch is easy for anyone to grasp.

Today, Macintosh and Newton users have all the scanning tools necessary to create application specific scanning stations, choosing and intermixing devices to adapt any retail or industrial data entry location to this technology. If clerks or inspectors are required in a combined production and data collection task, contact or proximate hand scanners are quick and reliable.

Copyright ©2001 Data Capture Institute, Inc.