The average person wants a colour scanner for photos or graphics. A small business owner might be looking for a portable scanner for business trips or a black & white scanner to read documents and translate them into editable text. Graphic artists or desktop publishers might be looking for a specialized photo or slide scanner. Armed with the proper information, you'll be able to find a scanner that does just what you ask.

How A Scanner Works
So what is a scanner anyway? A scanner is an imaging device with an array of photosensitive cells or "eyes" that detect light reflected off or transmitted through the object being scanned. This array of light receptors is a long thin line of charged-couple devices (CCD). A CCD measures light intensity then converts it into electrical charge.

Another bit of technology called an analogue to digital converter (ADC) is required to digitize the information, putting into a form your computer can understand. Each CCD in the array of thousands creates one pixel, and for each pixel a certain number of bits is stored. The more bits of information assigned to each pixel the better the image quality.

Most scanners use CCDs but some newer scanners use CIS (Contact Image Sensor) technology. With a CCD scanner, the light reflected from the original document passes through a system of mirrors and lenses which redirect the light to the CCD array. In a CIS scanner, the array of image sensors lies just under the document to be scanned so the sensors catch the reflected light directly. Since CIS scanners don't need a complex optical system they are cheaper to manufacture, smaller and more durable. They are well suited to cramped desktops and make useful portable scanners.

The sensors in a CIS array contain on-board logic for producing digital images and consume less power than CCDs. However, this on-board logic uses space that would normally be used for light sensing. This results in lower quality scans so most people forgo the slight savings of CIS and buy CCD scanners.

Types of Scanners

• Flatbed Scanners
  Flatbed scanners are constructed with a flat glass plate over the photosensitive array and a cover to go over the glass. You can place paper or objects like open books on top of the glass. Light emitted from under the glass reflects off the object being scanned and is picked up by the moving CCD array in the scan head. Flatbeds are a popular and flexible type of scanner that come in both monochrome and colour. Their only draw back is that they hog a lot of desktop space. Prices for general purpose models can start below $150, for higher image quality the range is $375-$1200.

• Sheetfed Scanners
  Unlike a flatbed scanner, the photosensitive array in a sheetfed scanner is stationary, and the page being scanned is moved over the array. This is a good type of scanner for someone who intends to scan a lot of documents to produce editable text. If you combine a sheetfed scanner with an automatic document feeder (ADF), you don't need to feed in each sheet by hand.

  Sheetfeds are available in both monochrome and colour. Although you can feed pictures or small pages into the scanner, the scanned images can get warped so flatbeds are still more flexible. Sheetfed scanners are nicely compact so they're good for limited spaces. You can pick up one of these scanners for under $150. Rapid, high volume business models can get into the thousands.

• Handheld Scanners
  Handheld scanners have scan heads 4-5 inches wide which makes them very portable. They plug into your parallel port so they're easy to share among people and can be a good addition to your laptop. The down side is that you will have to make several passes over a regular sized document to scan the whole thing. Usually, software is provided that will put the small scans back together, like stitching a patch-work quilt, but this is often time consuming. Moreover, unless you have the slow steady hands of a surgeon, your scans can turn out unevenly.

  You can get cheap handhelds for under $150, but their declining popularity might make them hard to find. High-end wireless models with superior image quality and stitching software are available for around $750. Another kind of handheld scanner is the pen scanner. You drag a pen scanner over a line of print like a highlighter. The built-in optical recognition software turns the scan into text which can be downloaded to a computer via serial or USB port. Pen scanners can hold hundreds of text pages and cost around $200.

  Portable scanners are like mini versions of sheet-fed or flatbed scanners that are compact enough to fit in a briefcase. Portable sheet-fed scanners are about the size of a three-hole punch while flatbed models are smaller than a hardcover book. These types of scanners plug into a PC Card or USB port and can scan in colour or black and white. Expect to pay about $250. Portable and handheld scanners have weaker light sources so their image quality isn't close to a full-fledged scanner.

• Slide, Film, and Transparency Scanners
  The typical person won't be scanning slides or transparencies, but people in the graphics or publishing industry often do. Slides, film, negatives and transparencies have to be scanned so that light transmitted through them is picked up by the photosensitive array. Most scanners have the light source and the scan head on the same side so they can't accommodate these types of images. Since slides and film are so small you need very high resolution scanners to do the job.

  There are a few options if you need to scan slides. Some flatbed scanners support a transparency adapter (TPA or TPU) which costs between $150-$525. This is a backlight that you attach instead of the scanner cover. These adaptable flatbeds sometimes have a special section on the glass bed that is magnified especially for slides.

  Other kinds of flatbeds come with a pull out drawer where you can place slides, but they are fairly pricey starting at about $1200. The last option is to get a dedicated slide scanner. These scanners have small scanning areas 4"x5", high resolution and high bit depth (see the criteria section) which makes them much more expensive. The cheapest start at $750 and can go as high as a few thousand.

What To Look For :

• Bit Depth
  As with any sophisticated piece of technology, there are a few technical aspects of scanners you'll want to understand before you go shopping. One term you're bound to bump into is bit depth. For each pixel in an image, the scanner can remember a certain number of bits which is known as bit depth. The higher the bit depth, the better the scanner is at distinguishing between really close shades of the same colour. This results in higher image quality.

  The minimum bit depth for decent image quality is 24. So for each pixel, the scanner remembers 8 bits of information for each of three colour levels (red, blue and green). Overall, this means the scanner can theoretically represent around 16.8 million colours.

  Inherent in the scanning process is a certain loss or distortion of information which is caused by a number of factors. This is commonly referred to as "noise." In practice, noise cuts 24 bit depth down to around 18 which decreases the number of colours represented. As a result, scanned photos will lose some image quality in the highlights and shadows. But the average user with a regular printer should find 24 bits adequate for most documents and web graphics.

  The next couple of steps up in bit depth are 30, 32, 36, 42 and 48. If you plan on getting a scanner for slides, negatives or transparencies you need at least 30 bits, and 36 bits would be preferable. Now you might be wondering what happens if you get a scanner with 30 bits, but your monitor or printer can only handle 24 bits of colour data. The extra information at higher bit depth will still produce smoother colour gradations and better overall image reproduction.

• Resolution
  There are two types of resolution, optical and interpolated; the first is the most important. The optical resolution of a scanner is measured in dots or pixels per inch (dpi or ppi). More pixels mean better resolution and sharper image quality. The number of pixels a scanner can generate is based on how many CCDs are arranged horizontally and vertically in the scan head. Remember one CCD produces one pixel. Resolution is quoted both horizontally and vertically (eg. 600x300). When comparing resolutions always use the smaller number. If you need to pick up fine details, small fonts or complex lines or edges you'll want more optical resolution.

  The interpolated resolution for a scanner is always higher than its optical resolution. To get interpolated resolution the scanner takes the information from two true pixels and using mathematical algorithms fills in a third pixel between them. Since more pixels are being mathematically generated the interpolated resolution is higher. Optical resolution is a more reliable indicator of image quality so don't be fooled into mistaking interpolated resolution for optical resolution. Higher interpolated resolution will be sufficient if you just need to enlarge images.

  An optical resolution of 300 dpi should be enough for the average user. It will produce an image that will look good on the web or printed out on an inkjet or laser printer. Graphic artists who want to scan line art or images with lots of detail will want to go up to 600 dpi. Only people who are looking to buy a scanner for slides, negatives or transparencies need 1200 dpi resolution. Such a high resolution is needed to compensate for the small size of the original.

  Even if your printer has lower resolution than your scanner, scanning at higher resolutions will produce better image quality in the finished document. You want to scan at higher resolution than what you'll print at because the image quality degrades when you process the image with editing software like PhotoShop. A scanner with a higher resolution will also produce better images even at resolutions below the maximum. For example, a 150 dpi scan from a 600 dpi scanner looks better than the same scan from a 300 dpi scanner.

• Dynamic Range or Optical Density
  For those of you who need a good graphics scanner or want really high image quality, you need to consider dynamic range, also called optical density (OD). It is a measure of how well the scanner can capture the tonal range of an image, the gradations from bright highlights all the way to dark shadows. Dynamic range is measured on a logarithmic scale (like the Richter scale) from 0-4 with zero as pure white and four as pure black. Dynamic range is the difference between the darkest and brightest optical densities a scanner can capture. The bigger the difference the larger the dynamic range.

  Most flatbed scanners have an OD around 2.8-3.0 which is fine for the tonal range of photographs. To scan slides, negatives and transparencies which have broader tonal ranges you need a higher OD. For slides and transparencies aim for 3.2, a bit higher at 3.4 for negatives. When comparing scanners with the same bit depth, choosing one with a higher dynamic range will give you better image reproduction. However, many scanners aimed at the average user do not report dynamic range so don't be surprised if you can't find it in the specs.

• Speed
  Most people buying a scanner for personal use won't be too concerned with speed. But you don't want to get stuck with a scanner that barely crawls along. Remember that speed depends on the scanning resolution, the higher the resolution the longer you'll wait. An efficient time for a 600 dpi colour scan is about 100 seconds but only 30 seconds for a monochrome 300 dpi scan. Reviews will usually rate a scanner for speed.

• Software
  When you buy a scanner you're also paying for the software that's bundled with it. The bundle will probably contain a driver program, colour calibration software, image-editing software and optical recognition software (OCR). For Windows the driver program is usually TWAIN compatible.

TWAIN isn't an acronym and doesn't have anything to do with new country. A TWAIN driver allows imaging devices like scanners to be compatible with TWAIN-supported software. Since TWAIN is an industry standard most bundled software will operate off TWAIN. Colour calibration software tries to maintain the original colours of the image throughout its processing. This means that what you see on the monitor and what you print out should be reasonably close to the colours of the original.

• Image-editing Software
  For image-editing software, more often than not, you'll be getting watered down or "lite" versions of graphics programs (ie. Adobe PhotoDeluxe or Ulead PhotoImpact). Upgrading to the full version of a program will be worth the money if you really want all the cool effects that let you twiddle with images. More expensive scanners sometimes include full versions of software. Decide what you'll want to do with your scans and choose a software package that fits your needs.

• Optical Recognition Software (OCR)
  To produce editable text from a scanned document you need OCR software (ie. Caere OmniPage and Xerox TextBridge). While a scanned document can be read on a monitor, the computer sees it as a picture, not as editable text. You can't move the words around or type in new ones. Optical recognition programs examine the scanned image and convert it into a format that can be read by a word processor, spreadsheet or database.

  Many scanners come with limited OCR software, but you're not going to get the sophisticated programs that can translate tables, charts and spreadsheets. If scanning for OCR is your main purpose for the scanner, you can get away with a black and white scanner.

• Interface
  Connecting a scanner to your computer can be done in a number of ways. Choose the one best suited to your needs. Plugging a scanner into the parallel port and configuring it is fairly easy. Most come with pass-through connectors to attach your printer to the scanner, just don't try to use both at once. Parallel ports tend to be the slowest way to connect, but the latest models can be almost as fast as the other two options, USB and SCSI.

  The Universal Serial Bus (USB) is quickly becoming popular, offering straight-forward installation, flexible configuration and faster data transfer. Most high-end scanners, however, connect via a SCSI port. SCSI stands for Small Computer System Interface, but everyone just says "scuzzy". This type of connection is fast but can require more work to configure and install. Macs support SCSI connection but many PC's do not.

  If you want to get a SCSI scanner then you may have to open up your computer to install a SCSI controller card. Most SCSI scanners come with controller cards and SCSI cables. Even if you already have a SCSI card for another device, it may not be compatible with the scanner or the provided cables. All this bother will pay off if speed is a primary concern for you. Speed demons should also consider high-end scanners that support FireWire which is akin to USB but even faster.

• Memory and Storage
  To put to plainly, images can devour available hard disk space, especially if you need to store high quality versions. The common formats for saving images like JPEG and GIF compress files but loose information in the process. Graphic artists who need high image quality use TIFF or other formats to save files, but these can get quite large. Keep in mind that storing all the photos from your European vacation may require additional hard-drive space or an external storage device.

  A general rule is that whatever the size of the image file, you require twice the amount of memory to load it. To be on the safe side and ensure your computer doesn't get sluggish you should have three times the memory. For images of 8 MB or less, 32 MB of RAM is just fine. Consider investing in more memory to work with larger image files.

Shopping Tips
The most convenient way to gather information is via the web. Once you've decided what you're looking for in a scanner, there are matching engines that will filter through a catalogue of scanners and pick the ones that match your criteria. For instance, some engines let you specify the type of scanner, price, interface, scanning area and manufacturer.

When you've narrowed the search to a handful of models you can start reading reviews. Articles usually comment on image quality, price, speed, usability and software bundles. It's always a good idea to check out feedback from people who own the product, or ask friends and family what they recommend. Many sites have editor's picks or top 10 lists.

Show me the plastic! Credit cards are a convenient way to buy plus some companies automatically insure your purchase against loss, theft or breakage. Before purchasing make sure you've read all the fine print about returns, restocking and shipping fees.

Warranty
Not all warranties are created equal. Most major manufacturers will stand behind their product but it's still a good idea to compare warranties before you buy. The best warranty will cover parts and labor on everything inside your scanner. If onsite service is available, find out what it will cost, who provides it and under what conditions it's offered. Also consider the length of a warranty. A more extensive warranty for a shorter term will serve you better than a watered down one that lasts longer.