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Simulated image of a typical barcode being scanned by a red laser.

Barcodes and barcode scanners

by Chris Woodford. Last updated: September 25, 2013.

Blip! Blip! Blip! Buying things at a grocery store has never been easier or quicker thanks to barcode technology. You must have seen the black-and-white zebra stripes on everything from cornflake packets to library books and the laser wands that are used to read them. But have you ever stopped to think how they work?

Photo: An electronic zebra? A barcode represents the line of numbers printed underneath it with a pattern of black and white bars. Barcodes are designed for computers to read quickly by scanning red LED or laser light across them.

What are barcodes used for?

If you run a busy store, you need to keep track of all the things you sell so you can make sure the ones your customers want to buy are always in stock. The simplest way of doing that is to walk around the shelves looking for empty spaces and simply refilling where you need to. Alternatively, you could write down what people buy at the checkout, compile a list of all the purchases, and then simply use that to reorder your stock. That's fine for a small store, but what if you're running a giant branch of Wal-Mart with thousands of items on sale? There are many other difficulties of running shops smoothly. If you mark all your items with their prices, and you need to change the prices before you sell the goods, you have to reprice everything. And what about shoplifting? If you see a lot of whisky bottles missing from the shelves, can you really be certain you've sold them all? How do you know if some have been stolen?

Using barcode technology in stores can help to solve all these problems. It lets you keep a centralized record on a computer system that tracks products, prices, and stock levels. You can change prices as often as you like, without having to put new price tags on all your bottles and boxes. You can instantly see when stock levels of certain items are running low and reorder. Because barcode technology is so accurate, you can be reasonably confident that any items that are missing (and don't appear to have been sold) have probably been stolen—and maybe move them to a more secure part of your store or protect them with RFID tags.

A barcode-based stock system like this has three main parts. First, there's a central computer running a database (record system) that keeps a tally of all the products you're selling, who makes it, what each one costs, and how many you have in stock. Second, there are the barcodes printed on all the products. Finally, there's one or more checkout scanners that can read the barcodes.

How barcodes represent the numbers 0-9

Artwork showing how different barcode stripes represent the numbers 0 through 9

A barcode is a really simple idea: give every item that you want to classify its own, unique number and then simply print the number on the item so an electronic scanning device can read it. We could simply print the number itself, but the trouble with decimal numbers is that they're easy to confuse (a misprinted eight could look like a three to a computer, while six is identical to nine if you turn it upside down—which could cause all sorts of chaos at the checkout if you scanned your cornflakes the wrong way up). What we really need is a completely reliable way of printing numbers so that they can be read very accurately at high speeds. That's the problem that barcodes solve.

If you look at a barcode, you probably can't make head or tail of it: you don't know where one number ends and another one begins. But it's simple really. Each digit in the product number is given the same amount of horizontal space: exactly 7 units. Then, to represent any of the numbers from zero through nine, we simply color those seven units with a different pattern of black and white stripes. Thus, the number one is represented by coloring in two white stripes, two black stripes, two white stripes, and one black stripe, while the number two is represented by two white stripes, one black stripe, two white stripes, and two final black stripes.

Photo: Each digit in a barcode is represented by seven equal-sized vertical blocks. These are colored in either black or white to represent the decimal numbers 0-9. Each block has been designed so that, even if you turn it upside down, it can't be confused with any other.

You've probably noticed that barcodes can be quite long and that's because they have to represent three different types of information. The first part of a barcode tells you the country where it was issued. The next part reveals the manufacturer of the product. The final part of the barcode identifies the product itself. Different types of the same basic product (for example, four-packs of Coca-Cola bottles and six-packs of Coca-Cola cans) have totally different barcode numbers.

Most products carry a simple barcode known as the UPC (universal product code)—a line of vertical stripes with a set of numbers printed underneath it (so someone can manually key in the product number if the barcode is misprinted or damaged in the store and won't scan through the barcode reader). There is another kind of barcode that is becoming increasingly common and its stores much more information. It's called a 2D (two-dimensional) barcode) and you sometimes see it on things like self-printed postage stamps.

How does a barcode scanner work?

A simple numbered diagram showing the parts of a UPC barcode scanning system and how they work.

It would be no good having barcodes if we didn't have the technology to read them. Barcode scanners have to be able to read the black-and-white zebra lines on products extremely quickly and feed that information to a computer or checkout terminal, which can identify them immediately using a product database. Here's how they do it.

For the sake of this simple example, let's assume that barcodes are simple on-off, binary patterns with each black line corresponding to a one and each white line a zero. (We've already seen that real barcodes are more sophisticated than this, but let's keep things simple.)

  1. Scanning head shines LED or laser light onto barcode.
  2. Light reflects back off barcode into a light-detecting electronic component called a photoelectric cell. White areas of the barcode reflect most light; black areas reflect least.
  3. As the scanner moves past the barcode, the cell generates a pattern of on-off pulses that correspond to the black and white stripes. So for the code shown here ("black black black white black white black black"), the cell would be "off off off on off on off off."
  4. An electronic circuit attached to the scanner converts these on-of pulses into binary digits (zeros and ones).
  5. The binary digits are sent to a computer attached to the scanner, which detects the code as 11101011.

In some scanners, there's a single photoelectric cell and, as you move the scanner head past the product (or the product past the scanner head), the cell detects each part of the black-white barcode in turn. In more sophisticated scanners, there's a whole line of photoelectric cells and the entire code is detected in one go.

In reality, scanners don't detect zeros and ones and produce binary numbers as their output: they detect sequences of black and white stripes, as we've shown here, but convert them directly into decimal numbers, giving a decimal number as their output.

Types of barcode scanner

Different types of barcode scanners are available for all kinds of applications. In small, convenience stores, you'll typically find a basic wand scanner. The simplest ones look like electronic pens or giant, oversized razors. They shine red LED light onto the black and white barcode pattern and then read the pattern of reflected light with a light-sensitive CCD or a string of photoelectric cells. If you have a pen scanner, you have to run it across the barcode so it can reach each block of black or white in turn; with a wand scanner, the CCD or photocells read the entire code at once.

A barcode scanning wand

Scanning a barcode with the Amazon app on an iPhone/iPod

In a busy superstore, you're more likely to see a very sophisticated laser scanner. It'll be built into the base of the checkout lane, under a piece of glass, and you may be able to see the laser beam being bounced around at high-speed by a spinning wheel so it reads products (literally) in a flash. Another technology uses a small video camera to take an instant digital photograph of the barcode. A computer then analyzes the photograph, picking out only the barcode part of it and converting the pattern of black and white bars into a number. (Barcode-scanning apps that run on cellphones work this way, using the phone's built-in camera to photograph the code.) Scanners like this can accurately read dozens of products waved past them each minute and are far more accurate than old-style checkouts (where you have to key in the price of every item by hand). The best barcode scanners are so accurate that they make only one mistake in something like 70 million pieces of scanned information! (Compare that to typing on a keypad, where you're typically likely to make one error in every 100 characters you type.)

Photo: Left: A typical wand-type barcode scanner (also called a barcode reader). Right: Scanning a barcode with Amazon's iPhone/iPod app. You find a product you like, scan the code, and the online store pops up with the product details automatically.

Barcode scanning technology has been around since the early 1970s but only really caught on in the 1980s and 1990s after stores started to invest in sophisticated, computerized electronic point-of-sale (EPOS) checkout terminals. Back then, store checkouts cost many thousands of dollars. Today, scanners are much more affordable. You can buy a simple, USB barcode scanner and software and hook it up to an ordinary laptop or computer for just a few dollars. Thanks to barcodes, even tiny convenience stores can run as smoothly as Wal-Mart these days!

Who invented barcodes?

How did we arrive at a point where virtually everything we buy is marked with a barcode? Here are some of the key moments in barcode history:

The original barcode scanner

I've dipped into the archives of the US Patent and Trademark Office and pulled out the records of the original barcode pattern scanner, invented by N. Joseph Woodland and Bernard Silver. I've colored and numbered it to quickly illustrate how it worked. In the top picture, you can see the entire apparatus, including the barcode scanner, which is shown in the center in blue; in the lower picture, you can see a more detailed view of the scanner itself:

Norman Woodland and Bernard Silver's original barcode pattern scanner from 1949/1952, drawing from US patent 2,612,944.

  1. Like modern packages in grocery stores, Woodland and Silver envisaged items would have barcodes printed on one face.
  2. You place the item to be scanned with its barcode face down on a conveyor made of some transparent material.
  3. A variety of lights shine up on the barcode.
  4. The scanner picks up light reflected off the barcode.
  5. The scanner sends a signal to a sorting mechanism that can push the item in different directions.
  6. The item is pushed onto different conveyors according to its particular barcode.
  7. Now looking in closeup at the scanner: It has a lens on top that spreads the light reflected off the barcode.
  8. The light from the lens spreads out onto a larger glass surface.
  9. An electric motor and axle (red) move a scanning head (green).
  10. Guided by the grooves in the axle, the scanning head moves from side to side.
  11. A photoelectric cell (orange) inside the scanning head picks up the pattern of light and dark areas from the barcode, sending corresponding signals to a detector circuit.

You can find a full description and more detailed drawings in US Patent #2,612,944: Classifying apparatus and method by Norman J Woodland and Bernard Silver.

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Text copyright © Chris Woodford 2008, 2012. All rights reserved. Full copyright notice and terms of use.

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Woodford, Chris. (2008) Barcodes and barcode scanners. Retrieved from http://www.explainthatstuff.com/barcodescanners.html. [Accessed (Insert date here)]

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