Speeding Checkout – Lasers in Barcode Scanners

While you’re standing in the checkout line of a supermarket or department store, waiting for the clerk to finish checking out the order of the person in front of you, you’re probably not thinking about lasers. Yet without them, the checkout process, which can seem interminable at times, would take even longer.

That’s because just about everything in the store has a barcode printed on it, and a barcode scanner is able to not only read it electronically but also translate it into product descriptions and pricing information that will be printed on your receipt. This not only saves time but also eliminates the possibility of human error during data entry, and lasers play a key role in the process.

What is a barcode?

Although it may appear to the human eye to be just a meaningless series of bars and spaces, the barcode actually contains a wealth of information about the product, its size, color, and other properties, as well as its price. It is assigned to every product according to a standardized set of rules administered by a worldwide company called GS1 and its U.S. subsidiary, GS1US, to ensure that each barcode is unique and no two products ever share the same barcode.

The barcode scanner consists of a light source, a lens, and a light sensor that translate optical impulses into electrical ones.The barcode scanners found in most retail stores use a laser beam as the light source and typically employ either a reciprocating mirror or a rotating prism to scan the laser beam back and forth across the barcode.A photodiode, which converts light into electrical signals, is used to measure the intensity of the light reflected back from the barcode.The photodiode generates a waveform that is used to measure the widths of the bars and spaces in the barcode. Dark bars in the barcode absorb light, and white spaces reflect light, so that the voltage waveform generated by the photo diode is a representation of the bar and space pattern in the barcode. This waveform is decoded by the scanner in a manner similar to the way Morse code dots and dashes are decoded.

The scanner's resolution is measured by the size of the dot of light emitted by the reader. If this dot of light is wider than any bar or space in the barcode, then it will overlap two elements (two spaces or two bars), and it may produce the wrong output. On the other hand, if a too small dot of light is used, then it can misinterpret any spot on the barcode, making the final output wrong.
The most commonly used dimension is 0.33 mm, although some scanners can read codes with dimensions as small as 0.075 mm. Smaller barcodes must be printed at high resolution to be read accurately.

Omni-directional scanners

In order to speed checkout even more by eliminating the need for the checkout clerk to hold the package in a precise position over the scanner, many stores use omni-directional scanners.

Almost all omni-directional scanners use a laser. Unlike simpler single-line laser scanners, they produce a series of straight or curved scanning lines of varying directions in the form of a star burst or other multi-angle arrangement. When such a pattern is projected at the symbol, one or more of the angled lines will be able to cross all of the symbol’s bars and spaces, no matter what the orientation. Most omni-directional scanners use a single rotating polygonal mirror and an arrangement of several fixed mirrors to generate their complex scan patterns.

Perhaps the most familiar examples of omni-directional scanners are the horizontal scanners in supermarkets, where the checkout clerk slides packages over a glass or sapphire window to produce the familiar “beep” indicating that the item has been scanned. These omni-directional scanners work no matter how the clerk is holding the package. They are also better at reading poorly printed, wrinkled, or even torn barcodes.

Laser scanners need to produce precisely controlled and positioned laser beams, even in complex patterns. Precision optics such as lenses, mirrors, and prisms supplied by Esco Optics help assure accurate readings, even in adverse conditions.