A scanner is a device that converts printed matter into digital information that your computer can use. Using small electronic components they record how much light is reflected off the item being scanned, and report that information to the computer. To see a whole image they break it up into cells called pixels. There are four main types of scanners on the market.

Flatbed. The most common desktop scanners resemble photocopier machines, in that the item being scanned rests on a glass plate while the scanning head moves underneath it.

Sheetfed. These are like a fax machine and move the page being scanned past the scanning head. Sheetfed scanners tend to be less exact than their flatbed counterparts because of the difficulty of moving a sheet of paper without introducing distortions. It is a good choice for handling lots of paper or photographs unattended.

Slide. These need special attention in the scanning process because they need a scanner that passes light through the image rather than reflecting light off it. Because of their small size, slides also need to be scanned on a unit with very high resolution.

Drum. Drum scanners are the most effective and versatile type of scanner, but they are expensive and harder to operate. Their advantage lies in the fact that they typically use photo-multiplier tubes which are a better quality and the original is rotated past the scanner heads a large number of times making it much more accurate.

Things to look for:

Resolution

Generally the higher the resolution the better the results. This is the amount of pixels a scanner can see, usually given in dots per inch (dpi). However, there different ways of measuring this and you have to be careful.

Firstly, there is optical resolution. A scanner's optical resolution is determined by how many pixels it can actually see, but some machines scan several times as they move down the page. This gives you two different numbers such as 300 x 600 or 300 x 1200. In each case the 'real' resolution is always the smallest number - you don't get more resolution.

Next is interpolated resolution which does not measure how many pixels the scanner can see but how many pixels it can guess. Using a process called interpolation, the scanner turns a 300 x 300 dpi scan into a 600 x 600 dpi scan by inserting new pixels in between the old ones, and guessing at what light reading it would have sampled in that spot had it been there. This process almost always diminishes the quality of the scan, and should be avoided.

Unless you are going for fine edge photographic work, where you need all the resolution you can get, there is no need for much more than 1200 dpi. More than this takes up disk space and cannot be seen by the naked eye. If you are printing family snaps, 300 or 600 dpi is all your will ever need. Optical character recognition programs work at 300 to 400 dpi.

Bit depth

Scanners with higher bit depths tend to produce better colour images. A scanner looks at the image pixel by pixel and records what it sees. However, different scanners record more information about each pixel, and this is called its bit depth.

Most colour scanners today are at least 24-bit, meaning that they collect eight bits of information about each colour, i.e. red, blue and green. A 24-bit unit can theoretically capture over 16 million different colours. This is near-photographic quality, and is sometimes called 'true colour' scanning.

There are now 30-bit and 36-bit scanners that can handle much more colours, but few software packages can handle this much detail at the moment.

Dynamic range

Another important criteria for evaluating a scanner is the unit's dynamic range, which is somewhat similar to bit depth in that it measures how wide a range of tones the scanner can record. Dynamic range is measured on scale from 0.0 to 4.0 and the single number given for a particular scanner tells how much of that range the unit can distinguish. Most colour flatbeds have a dynamic range of about 2.4.

High-end scanners are usually capable of a dynamic range between 2.8 and 3.2, while a drum scanner can manage 3.0 to 3.8. Generally, it is a good idea to go for a scanner that offers the higher dynamic range.

Scanning speed

Unless you do a lot of scanning it is not a good idea to worry too much about speed. But if you are choosing a commercial scanner speed becomes very important. The best way to evaluate a scanner's speed is to try out a few sample scans.

Drivers

Scanners rely on software to operate. Most use a standard called Twain to control the interaction between different programs. Under Twain, applications send their scanning instructions in a standard format that any compatible driver software can understand. The downside is that some drivers are not very good and do not support all of a scanner's functions.

How much should I pay?

Home users would be happy with a scanner costing between £60 and £120. More commercial machines for use in photographic studios should be about double that.