Scanners

Last updated: September 14, 2007.

If you've got a printed document and you want to get it into your computer, who you gonna call? Chances are, you're going to "call" your scanner! A scanner is a brilliantly useful piece of equipment that has some things in common with a photocopier and other things in common with a digital camera. Let's take a closer look at how it works.

Photo: A feed-through scanner (seen from directly above) looks much like an ordinary computer printer.

Processing pictures

They say a picture is worth a thousand words. What that really means is that pictures contain a huge amount of information. To describe a complex picture, you might need far more than a thousand words! Now just imagine if you're a computer and you want to make sense of a picture like the Mona Lisa. Where do you begin? People find this job easy because a huge part of our brain (something like 25 percent) is devoted to processing the things we can see with our eyes. But if you're a computer, you can't process a whole picture at once like a person can; instead, you have to go about trying to understand the information the picture contains in a much more systematic way.

A scanner's job is to turn a picture a human can understand into a digital picture that a computer can store and process—an image coded in the form of millions of numbers (zeros and ones) called binary code. A scanner does this by moving a scanning head backwards and forwards across the picture to recreate the image, one line at a time, inside the computer. This process is called scanning and that's how a scanner gets its name. (Televisions also use a process called scanning to build up the pictures on their screens, but they do it so quickly that we can't (usually) see it happening.)

What happens during scanning

There are several different types of scanner, but they all work in broadly the same way. The most popular ones are flat-bed scanners, which look a bit like photocopiers that have been on a diet. You open up a lid, place the document you want to scan face down on a piece of glass, and then close the lid again. When you tell the computer to scan, the scanner moves its scanning head from side to side and progessively down the page until it's covered the entire printed area. The scanning head contains a very bright light and a light-sensing unit called a CCD (charge-coupled display), side by side. As the unit moves back and forth, the bright light shines up onto the page and reflects the pattern of black and white characters back down into the CCD. The CCD is the same sort of light detector that you find in a digital camera (and you can find out what it does in our digital camera article). It detects the pattern of light being reflected into it off the printed page and produces a series of electrical signals. This effectively converts the light pattern on the page into a pattern of numbers that your computer can store.

Another type of scanner looks and works much like a computer printer and you have to feed documents through one page at a time. In a flatbed scanner, the scanner head moves down the page, which remains stationary. But in a feed-through scanner, the page moves through and the scanner head simply moves from side-to-side. My old Canon inkjet printer, pictured in the top photo, came with a scanner cartridge. To turn the printer into a feed-through scanner, I simply have to remove the print cartridge and put the scanning cartridge in instead. My computer figures out what I've done and it's smart enough to know that the printer is now, effectively, a scanner.

Photo: This clever Canon cartridge turns an inkjet printer into a scanner.

Left: You can see what the scanner cartridge looks like from above. On the right of this picture, you can see the gold-coloured connectors where the scanner cartridge sends its data, via the converted printer, to my computer.

Right: The same cartridge, viewed from underneath, has two slits in its base (circled in red). The white-colored one on the left is the bright light that shines onto the paper. The black slit right next to it is a plastic lens. Underneath it is the CCD that converts the reflected light into a string of digital information.

Black and white or color?

Like a photocopier, a scanner can make black and white images of a page (whether the page is in color or black and white) simply by scanning a light across it. But to make decent color images of a color page, it has to scan the red, green, and blue elements of the page separately. Some scanners do this using separate red, green, and blue lights. Instead of sending the light across the page once, they send it over three times—once to capture each of the three colors that combine to make up a color page. On the photo I've taken here, you can see how my scanner scans each line of the page three times using separate red, green, and blue lights. Scanning a whole page can take some time!

Photo: Each line is scanned three times, once for red information, once for green, and once again for blue.

Now what?

Once you've scanned a page, you end up with an image file on your computer (usually in a graphical format called TIFF, BMP, or JPG). If you've scanned a page from a book, what you have is effectively a photograph of a page from a book. You still don't have the words in your computer in a form that you can paste into a word-processor and edit. Indeed, your computer doesn't even know that the image you scanned is a page: for all it knows (or cares), you might have scanned a photograph.

To turn a scanned image into usable text, you have to use a type of computer program called OCR (optical character recognition). This works its way through the image file in a systematic way looking for recognizable patterns of letters and characters. When it finds them, it strings them together, in sequence, to build up a simple text file. After the OCR program has done its job, your original image of the printed page will have been turned into something much more useful: a digital file you can edit however you want.

Apart from flatbed and feed-through scanners, you can also get pen scanners, which look like fat, electronic pens. They have built in mini-scanning units and OCR programs. You simply run them across a line of printed text in a book or magazine. A tiny digital camera in the tip of the pen scans an image of the printed line and a built-in OCR unit turns it into processable text. When you plug the pen into your computer, the text file is automatically transferred across.

Go shop

• Compare prices and read customer reviews about scanners at Amazon.com
• UK: Compare prices and read customer reviews of scanners at Kelkoo.