Not long ago, scanners were rare accessories for the home computer.  Costly and difficult to use, only professional digital images and document processors found them practical.  But developments in imaging technology have resulted in an abundance of low-cost, easy-to-use scanners, and in the past few years they have become all but standard on most home and small office computing systems.

Here is a quick guide to understanding the truth behind the numbers in scanners.

Types of Scanners

First, you need to know what kind of scanner will best suit your needs.

flatbed: The most popular type of scanner in homes today is the flatbed scanner.  Flatbed scanners look something like a small copy machine, with a pane of glass upon which you lay the object that you wish to scan.  Flatbed scanners are ideal for people who intend to scan a wide range of items, such as photos, books, documents, and even 3-dimensional objects.  An automatic document feeder (ADF) can be added to some models to ease scanning multiple pages.

sheet-fed: Also common are sheet-fed scanners, which are excellent for scanning large numbers of loose, individual sheets, but cannot handle bound documents or photographs.  If you want to scan multiple pages for document management (like invoices or receipts) or for editing in a word processor, then a sheet-fed scanner should be your first choice.

film scanners: If you need to scan slides, negatives, or transparencies, you should consider a film scanner.  These are specially designed with a much higher resolution than other scanners, and are not suitable for other media.  If you will only occasionally scan film, then you can get a transparency adaptor for your flatbed scanner.  Film scanners are only practical for those who will scan film regularly.

handheld: Handheld and pen scanners are capable of scanning two to five inches at a time.  While useful for scanning small images or passages of text, they are difficult to manipulate when capturing whole pages.  People who do a lot of research might find them useful for note taking.

specifications

When you begin comparing scanners, you'll notice a lot of attention paid to resolution and bit depth.  While these numbers should not be the ultimate factors in your choice of scanners, you should have an understanding of what they mean and how they might affect your scanning success.

optical resolution: Resolution is a measurement of the numbers in dots per inch (dpi, also referred to as ppi, or pixels per inch) of the sharpness and clarity of an image.  A scanner with 300 dpi resolution can capture an image with 90,000 dots (the smallest identifiable part of a picture) per square inch.   Three hundred dpi is suitable for scanning most graphics and text, especially if it's for the Web.  But if you are going to print what you scan, look for something higher, like 600 or 1200 dpi.

hardware resolution: This number is usually given in two numbers, the horizontal and the vertical.  The first number is the same as the optical resolution.  The second number tells you how small the steps are that a scan head's motor can take while scanning down the page.  In general, you can ignore this number.

interpolated resolution: This is sometimes referred to as maximum or enhanced resolution, and is usually a very high number.  Using interpolation, the scanner can generate intermediate values based on known values.  For example,  a scanner with an optical resolution of 300 dpi and an enhanced resolution of 4800 dpi will actually capture 90,000 dots, and then based on the value of each of those dots, add 15 dots between each of those known values, yielding a higher resolution.  In general, you can ignore this number: it tells you nothing about how well the scanner can capture an image, only how well it can fix it up.

bit depth: Bit depth refers to the number of bits used to capture each dot.  The higher the number, the more color gradations will be visible.  In general, a 24-bit scanner will be suitable for most tasks such as scanning photographs, drawings, and text, particularly for the Web. A 30-bit or 36-bit scanner is best for scanning film or transparences and for professional photographic work. 

speed: Most manufacturers list speed among their desirable specifications, but in actuality--unless you are going to scan hundreds of pages at a time--speed should not be a major consideration.  It is well worth a few extra seconds if you can get a quality scan the first time.

Manufacturers and retailers place a lot of emphasis on these numbers, but in fact they can be quite misleading.  Because quality optics and image optimization algorithms contribute significantly to the output of a scan, it is possible to get a higher quality image from a 300 dpi scanner than a 600 dpi scanner.  It's like projecting a film onto a screen: it doesn't matter how high the resolution is if the lens is out of focus.  Since there is no single standard that measures the overall quality of a scanner, the best way to ensure you are getting your money's worth is to buy from a trusted and reliable manufacturer with a commitment to superior products.

ease of use

Perhaps more important than the numbers describing a scanner's capabilities is the ease with which you can use it.  It doesn't matter how high a resolution your scanner can reach if you can't figure out how to make it scan, or how to optimize your scans for a particular use.  Today's scanners come with a variety of means to make scanning easy.  Here are some of the features you should look for.

software: Most scanners ship with software of some sort, but there are two things you should be sure of.  First, the software should be appropriate to your needs.  Some software is better for scanning images; others are better for scanning text.  Make sure you get one that does what you want it to do. If you are going to be scanning text that you want to edit in your word processor, make sure you get optical character recognition (OCR) software, which translates the scanned images of your text into individual characters.

Second, make sure you are comfortable using the software.  While some programs may be intended for novice users, others may be too complicated to figure out with a three-inch thick manual.   If possible, give the software a test drive to make sure it suits your level of expertise. Good software will make it easy for you to get your image from the scanner into your desired application in as few steps as possible.

interface: The type of interface you use to connect your scanner to your computer can have an effect upon performance.  SCSI interfaces, while the most difficult to install, are usually quicker than parallel interfaces, and have been the traditional first choice for most scanner users. In the past few years, however, USB ports have become more and more common, delivering similar speeds as SCSI ports but with much easier installation.  As Firewire ports become more common on home computers, there will be more and more scanners available with this super-fast interface as well.

LCD interface: An LCD menu allows you to change settings directly on your scanner instead of on your computer.

one-step buttons: More and more scanners are featuring buttons that allow you to complete routine tasks in one step.  Just put your photo or other object on the glass, press a button, and the scanner will automatically perform a scan at optimum settings for the desired destination, whether it is the Web, e-mail, or your printer. 

support: Be sure to examine the scanner's warranty, support, and return policy.  Many cheaper models have only a 90-day warranty, and since there are many features that you have to try out for yourself to see if they work properly for your needs, there is a greater chance you'll have to return it than with other hardware.  If you have problems, you'll want to know that there is someone there to help you solve them.

what to spend

When you're comparing scanner prices, remember the old axiom, "You get what you pay for."  While it's possible to get a 600 dpi flatbed scanner that makes decent scans for under $100, you might be sacrificing when it comes to speed, software, or support.  In general, the home user should plan on spending between $200 and $300 for a good quality scanner.  If you're thinking about getting an all-in-one machine--some combination of scanner, printer, fax, and copier--you will probably spend an additional $150 to $200, depending upon the quality of the printer and other accessories