Most scanners on the market today are flatbed scanners, usually for scanning documents of A4 though A3 paper sizes. There are two of types of flatbed scanner types available: reflective scanners and transparent scanners.

When you place an item such as a book or piece of paper on the glass plate of a flatbed scanner, the document is illuminated from underneath, and the scanner users mirrors to copy an image of the document on the plate to a digital file. This is known as reflective scanning.

Transparent scanning is used for items that are see-through, for example slides, and these need to be placed in a special box that clips onto the glass of a flatbed scanner. A special light then illuminates these items inside the lid, and the item can be slowly scanned.

These transparency adapters can either be built into the top of a flatbed scanner or come as separate attachment. This is not a standard feature on flatbed scanner models though, so if scanning transparent items is important to you, make sure you verify that your chosen scanner can accommodate this.

Flatbed scanner models will use either CCD (charge-coupled device) or CIS (contact image sensor) sensor technology. The former is slightly older technology, and CIS is becoming more prevalent. Even though CIS tends to produce a slightly lower quality scanned image, it allows for smaller devices and uses less power than a CCD scanner, and so CIS scanner models can be powered via a USB cable and do not need an additional power source.

There are other types of scanner available. Handheld scanner models can be useful if you travel and need to scan documents frequently, as these are portable and save images to a memory card rather than needing to be attached to a PC. These are often used by students, or professionals who frequently need to take many notes, as they can be used to scan books or periodicals easily.

Sheetfed scanner models, which used to be very popular, are still available. These require users to feed an item individually through a narrow feeder slot, and are thus are only suitable for flat documents.

Film scanner models are primarily designed for professional use; these are specifically designed to scan film negatives, slides, or APS film. Most will focus on one type of film, so if this is what you need, take care that the scanner you choose can cope with the film you are using.

You can also buy multi-feature or 'all-in-one' machines that combine a printer, scanner, and fax machine, which often appeals to small business owners, as they are a more efficient use of office space than a number of separate items.

Quality

The quality of an image produced by a scanner will be affected by the resolution of the scanner. The resolution of a scanner is normally expressed in dpi, or dots per inch. Generally, the higher the dpi, the clearer and crisper the final scanned image will be. Sometimes you will see manufacturers quoting both the actual and ‘interpolated’ dpi; the former is the more interesting, as the latter effectively illustrates what can be achieved using image-editing software. 'Interpolation' refers to a method of improving photos by using software to fill in more pixels in a photo by guessing what that pixel should look like based on the pixels around it.)

Scanner models suitable for the home may have maximum optical resolutions of up to around 4800 dpi (dots per inch), with the option for reducing this if you wish. For most home purposes, a resolution of 1200 dpi is usually fine.

For scanning photos for uploading to the web, you will not need anything close to 4800 dpi resolution, but it is good to have the capability in case you wish to do something that requires a better resolution, such as enlarging a small section of a picture or scanning 4" x 6" pictures to print as 7" x 10" prints. Prints from a scanner of this resolution would be largely indistinguishable from something you could have printed at a high street photography studio - as long as your printer is capable of producing prints of the same dpi as your scanner, of course.

Generally, the higher the resolution of the scan, the more memory the saved image will take up on your computer; scanning a photos at a few hundred dpi should result in a file of around 1Mb. Scanning at the higher resolutions will also take longer and increase the file size.

You will also normally see a measure called bit depth on most scanner models. This indicates the colour depth; the amount of information that the scanner can capture about each individual pixel of colour when scanning. The higher the bit rate, the better the scanner will be at producing nuanced colour. For most simple home use, a scanner with a bit rate of 24 should be fine, though if you plan to scan numerous slides, you may want to look for something more in the 48-bit range.

On the specifications of higher end scanner models, you may see a specification for Dmax, or dynamic range. The measures how well the scanner will reproduce the lights, darks and shades of a document or image. The higher the Dmax number, the better the scanner will perform on this scale. This is not of huge importance for a personal home scanner, but if you are looking for a business scanner to produce high-resolution documents or images, it is worth bearing in mind.

Specifications

Scanner models today are generally very versatile and flexible, allowing for easy scanning of individual documents as well as book pages or photographs.

Ready to choose a scanner? Picking the right scanner for you is a case of balancing the varying features of different scanner models: quality, speed, software, and hardware specifications.

Most scanners offer a maximum scan size of A4. If you need to occasionally scan something larger, look for a scanner with 'folio mode' which can scan two halves of an A3 document and then piece the digital images back together to give you one final image.

Most scanners will come with some software to let you edit and enhance your scans, if you want to crop an image, or remove redeye in photos, for example. If you wish to edit photos, make sure the scanner includes software that allows you to do more sophisticated adjustments such as changing the colour balance or removing blemishes, or to optimise a photo for sending via email. Scanners that are more expensive may come with reduced versions of established software such as Adobe's Photoshop Elements or Ulead's PhotoImpact software, both of which allow for extensive editing.

Other features to look for include one-touch scanning, which allows you to scan an item as a PDF and attached it directly to email via the scanner without having to use your computer, useful if you send many photos to relatives for example.

If you are going to scan documents frequently, the speed of the scanner will be important. It should be noted that the resolution at which you scan documents will affect the speed; higher resolution images take longer to scan.

For those of you who will need to scan unusually long items, say artwork or banners, an automatic document feeder tray may come in useful. A scanner can either be bought with one of these or sometimes it can be purchased as an accessory afterwards, though it is normally cheaper to buy a printer with this already installed.

If you need to scan text documents as editable files, then you will need a scanner with an optical character recognition program, or OCR. This can be useful, for example, for editing handwritten notes, or for keeping a safe copy of old typed or handwritten documents with your own notes added.

One other thing to keep in mind is the interface between your scanner and your printer. Scanners today usually come equipped with a USB 2.0 port, and your PC needs to be compatible with this. If your PC or Mac is older and only has a USB 1.1 port, the scanner should work (though double-check this) but it will be slower than it would be if plugged into a USB 2.0 port. Some scanners will also work with FireWire or Bluetooth, thought those are usually more expensive, and aimed at business users rather than the home user.

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