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How Bar Codes Are Read

Bar codes are read by sweeping a small spot of light across the printed bar code symbol. The sweep starts at the white space before the first bar and continues passed the last bar and ends in the white space which follows the last bar. Because a bar code cannot be read if the sweep wanders outside the symbol area, bar heights are chosen to make it easy to keep the sweep within the bar code area. The longer the information to be coded, the longer the bar code. As the length increases, the height of the bars must be increased to allow for more wandering during reading.

The words "reader" and "scanner" are often used interchangeable to describe the device which is used to read a bar code. Strictly speaking, a bar code scanner only designates the optoelectronic part of the device which transforms the optical image of the bar code into electrical signals. A bar code reader, on the other hand, includes the bar code scanner and the decoder which transforms the electrical signal from the scanner into ASCII (American Standard Code for Information Interchange) representions of the data. The preferred term is "bar code reader."

Types Of Bar Code Scanners

There are three basic types of bar code scanning devices. The first type of scanner is the contact wand. These scanners resemble a "wand" or "pen". Light from a light source is focused at the tip of the wand. A photodetector senses changes in the reflected light as the wand is drawn across the bar code symbol. These scanners are inexpensive, and rugged, but must be kept in contact with the symbol throughout the scan. If the symbol is on a curved surface or in a difficult to reach space, contact wands will be difficult to use to successfully read the symbol.

The second type is the active non-contact scanner which generally uses a beam of focused light to read the bar code. The most common form of active non-contact scanner uses a laser beam that is automatically scanned back and forth across the symbol at a high rate. Active non-contact scanners may be stationary as are found in grocery stores or they may be hand held. Industrial versions of active non-contact scanners, however, often keep the beam of light fixed on a single spot and are arranged so that the symbol is moved across the field of view of the scanner.

The hand-held laser scanner is an active non-contact type which is very popular for some applications. The major advantage is that the scanner can read bar code from several feet away. If the symbol is printed large enough, the laser scanner can read the symbol from as far away as 30 feet. In a warehouse this ability can be a definite advantage. But hand-held laser scanners are more expensive than other bar code readers. They also have moving parts which can be sensitive to rough use.

The third type is the passive non-contact scanner which uses a small video camera or photoelectric strip to convert an optical image of the symbol to a video signal which is then decoded. Commonly called a "CCD scanner", these devices are priced midway between the contact wand and the non-contact laser scanners, are rugged, and do not require contact with the symbol for successful reading. However, these scanners have a limited depth of field and generally must be held within a few inches of the symbol.

Contacts Wands

The least expensive way to read bar code symbols is with a contact wand scanner. Contact wands are hand held bar code scanners which must be placed in contact with the bar code symbol and moved across the entire symbol in order to correctly read the symbol. Since the operator manually performs the scanning function, wand scanners do not need any moving parts to scan the symbol. This makes the wand reader a low cost, light weight, and rugged way to read bar code.

A wand reader has both a light source and a light detector in the same pen-like container. In general, the light emitted from the source is projected through an opening in the tip. Usually this opening or aperture will be covered with a transparent element to protect the tip. When the tip is placed in contact with a bar code symbol and the wand moved to scan the symbol, the projected light is reflected back by the light colored spaces and absorbed by the dark bars. The reflected light enters the tip of the wand. The light then enters a tubular metal shield containing lens elements to focus the image of the bar code on to a photocell. The photocell converts the varying reflected light level to a varying electrical current. The current is passed to signal conditioning circuitry which converts the analog signal to a digital or on-off signal. This conditioned signal is sent to decoding circuitry either in the body of the wand or to an external decoder.

The distance between the tip of the wand and the bar code is critical. The reflected light will only strike the photocell accurately from a precise distance. This distance is the nominal focal length of the wand. The size of the opening or aperture determines how precise this distance must be.

Early wands used nylon or plastic tips, and these tips often were worn down from hard use. The shortened length of the tip made the wand "out of focus." New wands use glass tip crystals and steel tips to resist wear. Some wand scanners have tips which may be unscrewed to replace the tip crystal. If the tip is not tightened, the wand may also become out of focus. Finally, if the bar code symbol is covered by a thick clear laminate to protect the symbol, the spacing caused by the thickness of the laminate may be too great a distance for the focal length of the wand.

The aperture of the wand refers to the diameter of the opening through which the reflected light passes. The aperture determined how much of the symbol the photocell will see. The wand aperture is given in thousandths of a inch (abbreviated"mils"). A scanner with a 5 mil aperture will see an area with a 5 mil diameter. This sets the resolution of the wand and determines the maximum density of the bar code symbol that will reliably read. If the selected aperture of the scanner is too large and the bar width too small, the wand will not recognize the bar. For example, if the smallest bar of the symbol is 4 mils wide and the aperture is 10 mils, 60 percent of the aperture will see reflected light from the spaces on each side of the bar. The photocell will see more reflected light than would be expected from a black bar, and the decoder will not recognize the bar as a bar. On the other hand, if the aperture is too small for the size bar code being scanned, then a printing flaw such as an ink spot or void may be erroneously recognized as a narrow bar or space. The aperture must be sized to be small enough to recognize a small bar, but be large enough not to be adversely affected by printing imperfections.

As a rule of thumb, the aperture should be of a size equal to the x-dimension (smallest element width) of the bar code symbology most frequently read by the wand. This aspect of wand selection is one of the most frequently overlooked considerations, and is often the source of bar code symbol reading problems.

In order to decode the information in the bar code symbol, the widths of the bars and spaces must be recognized and measured. If the speed of scanning were constant, the distance the wand traveled could be measured with the use of a clock which started to run from the instant the edge of the first bar was encountered. However, since the wand is scanned over the symbol by hand, the speed of scanning will vary from moment to moment and from person to person using the wand. Since the movement of the wand is variable, the widths of the bars are determined by measuring the relative time between edges of symbol elements (going from a bar to a space or a space to a bar.) This is usually done by the decoder by comparing the rate of change between light and dark elements to a constant clock. Since each decoder uses proprietary software to do the comparison, the speed tolerance will vary from manufacturer to manufacturer.

The type of light source used in the wand is also a consideration when selecting a wand. Most wands now use light emitting diodes (LED) rather than incandescent lamps. These LED's emit light with a wavelength peaking at 630 to 720 nanometers (visible light ranging from bright to deep red) or 720 to 900 nanometers (infrared). Wands using LED's producing visible light will read bar code symbols printed with both carbon and dye based inks. Wand which use near infrared LED's (820 nanometers) usually will only reliable read bar code symbols printed with carbon based inks. Wands using LED's that produce light with a wavelength greater than 820 nanometers should only be used to read carbon based ink printed bar code. Wand specification generally list the wavelength of light used and often proceed the wavelength with the letter "B" (e.g. B633 means the light has a wavelength of 633 nanometers).

Active Non-Contact Readers

The use of helium-neon gas lasers as a light source revolutionized non-contact bar code reading. Prior to the introduction of He-Ne lasers in the early 1970's, scanning bar code labels at a distance required labels made from retroreflective material consisting of thousands of microscopic glass beads. This special label construction made non-contact labeling costs quite high, and made it impractical for most applications. In 1987, several Japanese electronics companies introduced visible light laser diodes, and laser diode bar code scanners quickly captured the market over the He-Ne laser scanner.

Noncontact laser scanners may be found in fixed or handheld units. The beams produced may be stationary or moved to automatically scan the symbol. The advantage that laser light has over other forms of light is that it can be focused and collimated to a very small beam. Because the light is coherent (a single frequency), the beam will not spread much over a given distance. Therefore, the diameter of the beam will remain small enough to resolve the wide and narrow bars of the bar code even if the reading distance varies. That property allows laser scanners to read bar code on curved surfaces.

Laser scanners may be self scanning or they may produce a stationary beam. In self scanning units, the collimated beam of laser light is moved back and forth between 40 and 800 times a second. Handheld units generally operate at the lower end of these scanning speeds because the symbol being scanned is usually stationary. If the symbol is on a box moving down a conveyor, the scanning speed must be fast enough to read the label before it moves passed the scanning area.

Because of the rapid scanning speed, the narrow beam of a laser scanner appears as a continuous line or geometric figure. In applications where the orientation of the symbol is not known, the pattern traces by the beam will be a figure-eight or starburst. These patterns insure that at least one scan will pass over all the bars and spaces in the symbol. Handheld self scanning laser readers generally produce a thin horizontal line which is aimed at a symbol. Often this visible line is only used for aiming an invisible laser bar code reading beam.

An accurate scan of any bar code requires at least one successful pass. Since self scanning laser readers scan at a high rate, they are able to read poorly printed bar code (those which may take several scanning attempts) without the user noticing.

Some laser readers focus a single stationary beam of light over a region. Either the unit itself or the symbol must be moved to scan the beam across the bar code symbol. These readers are often used for industrial applications like reading a bar code on an assembly moving down a conveyor. Since the symbol is moving at a constant speed, and is at a controllable position, there is no need for the complex optics to produce a self scanning beam.

Passive Non-contacts Readers

Active non-contact bar code readers use a light source and a single photodectector. The scanning movement comes from the operator moving the scanner, the symbol moving relative to a fixed scanner, or the light source moving through scanning optics. Passive non-contacts readers, on the other hand, operates like a video camera. The bar code label is illuminated by a photoflash or by a light source. The image of the bar code is focused on to an array of photodetectors called a charge coupled device (CCD). The image of the dark bars of the symbol will fall on some of these photodetectors, while the light spaces will fall on other of the detectors. An electrical signal is applied to the CCD array and the light value at each photodetector is sequentially read out. This sequential light level value "looks" the same as the varying voltage produced when an active scanner scans a beam of light across a bar code symbol. The signal from the CCD array can be processed and decoded in the same way as in an active scanner.

These systems use photoflash or bright LED arrays to increase the depth of focus. In photography, the brighter the scene, the smaller the aperture which may be used and still have a proper exposure. However, as the aperture gets smaller, the range over which the scene will be in focus gets greater. By using bright light, passive non-contact scanners can maintain a depth of focus of several inches.

The field of view of passive non-contact scanners is also limited. This means that they can not read long bar codes. Resolution, the density of symbol which can be read, is limited by the number of photodetectors in the CCD array.

These scanners are available in fix and handheld forms. With their limited field of view, they operate best with bar code formats of fixed length like UPC version A. Because of the smaller depth of field, passive non-contact scanners must be moved to close proximity to the symbol to be scanned. The passive non-contact scanner offers a benefit over contact wands since they are able to scan the entire symbol without moving the reader. Hand held CCD array scanners have an effect scanning rate of 3 to 5 scans per second, while fix passive non-contact scanners operate at 7 to 10 scans per second. The major advantage over active, moving beam non-contact scanners is that they are generally less expensive than active non-contact scanners. These characteristics make CCD array scanners fit between the contact wand and the active non-contact scanner.

An explanation of wedge readers can be found here.

A "Rap" video on YouTube that explains how a barcode reader works can be found here.

A very interesting video on YouTube from a company the integrated art work into a package's barcode can be viewed here.

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