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Visible Laser Diode
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A bar code reader is a device that optically retrieves the image of a bar code and converts it to its digital equivalent. In order for a bar code reader to obtain a "photograph" of the bar code, it requires a source of light to illuminate it. Generally, the source used is a laser. A laser is chosen because it can easily be focused over a long depth of field. Depth of field refers to the distance throughout which a scanner is able to read codes.
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Where CCD (Charge Coupled Device) readers using LED's as a light source traditionally need to come in contact or very close to the code, laser scanners, having several inches of depth of field, do not. This results in no scratching or wearing of the surface of the item or bar code on the item.
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Another instance for the need of depth of field is when the bar code reader cannot possibly contact the code due to protective material, such as glass, or simply because of the proximity of the item itself.
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When a laser is used in a bar code reader, typically the beam is swept though the code, hence the term bar code scanner. The types of lasers used in bar code scanners are infrared laser diodes, visible laser diodes and helium-neon laser tubes.
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For many reasons, the most popular of these is the visible laser diode (VLD). Laser diodes are very reliable, with lifetimes several times greater than helium-neon tubes. Laser diodes operate at very low voltage (typically just over two volts) and do not require special power supplies. They are also packaged in a very compact (see figure 1), lightweight and rugged housing. Since the laser diode consumes very little power, it can be used in bar code readers that may be powered directly by a computer or even batteries.
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Visible lasers are preferred to infrared lasers because it has been found that codes printed on some substrates and materials cannot be read when using an IR type scanner. Although it may appear to the eye that the code has dark bars printed on a light background, the light emitted from an infrared laser is absorbed differently than visible light.
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Another reason they are not commonly used in bar code scanners is that infrared lasers require a visible aiming beam to assist the operator. Since infrared radiation is not visible to the human eye (see figure 2), the scanner must provide an additional source to illuminate the code to assist the operator.
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Another feature designed into visible laser diodes is a monitoring photodiode. Besides light emitted from the front facet of the laser chip (see figure 3), a portion of the light is emitted from the rear facet. This light is received by a photodiode located within the laser diode housing. The signal produced is then available on a lead located on the back of the diode housing. This signal may then be used by the VLD drive circuitry to control the output power of the laser.
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Lastly, the prices of visible laser diodes are becoming very competitive. At only a fraction of the cost of helium-neon tubes, VLD's provide both a highly efficient and cost-effective method of laser bar code scanning.
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