Laser Scanners

Laser scanners sweep a pinpoint beam across a barcode at high speed, giving them greater depth of field and faster, more reliable reads than CCD scanners — with safety and resolution trade-offs worth understanding.

Laser scanners usually take the characteristic "gun"-type shape, typically trigger-operated and apparently "firing" a slim beam of red laser light at the bar code. The beam effect is in fact an optical illusion. What is actually happening is that a pinpoint of laser light is scanned across the code at a rate of around 30 scans per second, with the speed and intensity of the scan creating the "line" effect. The very practical and useful result is that the user can see exactly where the laser is being targeted and "aim" the scanner at the bar code correctly.

Laser scanners have two major benefits: first, they generally have a much greater depth of field than CCD scanners, normally operating at around 15 to 20 cm from the bar code (although this can vary according to the code size). Some laser scanners are special long- range versions that can operate even further away, but these usually use the much higher-powered Class II lasers and are both more expensive and require higher power to drive them.

The second major benefit is speed and accuracy of reading. Because the laser is scanning the code at a high speed, it quickly has many "attempts" at reading the signal, so the decoding will often appear to the user to be virtually instantaneous. As a double check, scanners are often set to have a "redundant read" option, which means that the scanner will not confirm and process the scan until it has received the same reading two or three times; only then will it accept that the code has been correctly read. Redundant reading can be so fast with laser scanners that two or three times redundant scanning is often a default setting, and many users will be completely unaware that this additional error checking is even happening!

The relatively low amounts of energy used in laser scanners do lead to limitations of use in conditions of high ambient light, for example outdoors in bright daylight. In these cases, the background light energy can be higher than the reflected light from the laser, effectively "dazzling" the scanner. For this reason, laser scanners are generally not suitable for use outdoors, although specially designed outdoor models are available. CCDs, and even wands, can suffer from similar problems, although usually to a lesser extent.

Laser scanners also exist in other forms apart from the familiar gun style. The next most common form is the fixed-head laser scanner. Modern supermarket checkouts are generally equipped with fixed- head scanners. Instead of the side-to-side scan of the laser gun, fixed scanners fire the laser beam in special patterns, with the objective of intercepting and reading a passing bar code presented at any angle. The same approach is applied to the industrial use of fixed scanners, where, for example, a scanner may be mounted to read codes on items passing along a production line.

Output signals from laser scanners generally take the same form as those of CCD scanners, i.e., decoded serial, wand emulation, or keyboard wedge output. In addition, some scanners transmit a raw laser signal (laser output) and must be attached to suitable decoding equipment. Specialist EPOS (electronic point of sale) terminals sometimes require an output type known as OCIA.

The short answer is yes, in proper use they are quite safe. Certain international standards define the safety considerations for different power levels of laser light used in electronic products. The details are very technical; however, class 1 lasers, the lowest power, are very low power. The level of light energy that the human eye would receive from a class 1 laser is, as a rough guide, less than the light it would receive on a bright sunny day! In any event, the normal human blink reflex would provide more than adequate protection should a class 1 laser happen to hit the eye.

Class 2 lasers are higher power than class 1 and carry warning labels about the possible effects of looking directly into the laser beam. However, these products are still safe in general operation, for two reasons: first, the blink reaction will normally protect the eye in time to avoid any damage; second, with bar code scanners, the beam is constantly moving. As a rough guide, the intensity of a class 2 laser beam is roughly analogous to looking at the sun, so users are strongly advised to avoid doing so!

Resolution refers to the ability of a given scanner to "see" different sizes of bar code elements. If you have very small bar codes, where the size of each element (i.e., the narrowest bar) is very narrow, you will need a reader capable of seeing very small bars. This requires a high-resolution reader.

A larger bar code can, of course, be read with a high-resolution reader. The main risk here is that larger codes can be more susceptible to imperfections that a high-resolution scanner might detect. Hence, a bigger or poorly printed bar code is often best read with a lower- resolution reader.

For the vast majority of day-to-day applications, a medium-resolution scanner represents the best compromise between these two extremes. Most users normally only need to concern themselves with resolution when reading very small or very large bar codes.

Resolution is normally expressed as the width down to which a given scanner can resolve: medium resolution is usually viewed as around 0.15mm - 0.19mm.