Specifications

SCANNERS
Selecting a scanner—what you need to know
Although the concept can be confusing, resolution is just a measure-
ment of how many pixels a scanner can sample in a given image.
Resolution is measured by a grid. Think of a chessboard, with eight
squares along each side. The resolution of that chessboard would be 8
x 8. If the chessboard had 300 squares along each side, its resolution
would be 300 x 300 -- the typical resolution of an inexpensive desktop
scanner today. That scanner samples a grid of 300 x 300 pixels for
every square inch of the image, and sends a total of 90,000 readings
per square inch back to the computer. With a higher resolution, you
get more readings; with a lower resolution, fewer readings. Generally,
higher resolution scanners cost more and produce better results.
However, manufacturers will occasionally try to confuse you with
another type of resolution in the hope of selling more product. Here's
what you need to know about both:
Optical Resolution: A scanner's optical resolution is determined by
how many pixels it can actually see. For example, a typical flatbed
scanner will use a scanning head with 300 sensors per inch, so it can
sample 300 dots per inch (dpi) in one direction. To scan in the other
direction, it will move the scanning head along the page, stopping
300 times per inch, so it can scan 300 dpi in the other direction as
well. This scanner would have an optical resolution of 300 x 300 dpi.
Some manufacturers stop the scanning head more frequently as it
moves down the page, so their machines have resolutions of 300 x
600 dpi or 300x1200 dpi. Don't be fooled; what really counts is the
smallest number in the grid. You can't get more detail by scanning
more frequently in only one direction.
Interpolated Resolution: The other thing to watch out for is claims
about interpolated (or enhanced) resolution. Unlike optical resolu-
tion, which measures how many pixels the scanner can see, interpo-
lated resolution measures how many pixels the scanner can guess at.
Through 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 sam-
pled in that spot had it been there. This process almost always
diminishes the quality of the scan, and should therefore be avoided.
It can also be accomplished by almost any image editing software, so
it doesn't really add to the value of the scanner. Unless you plan to
scan line art at very high resolutions (more on that later), ignore
claims of interpolated resolution.
How much do you need? There are several useful rules of thumb to
help determine how much resolution to pay for. First, figure out how
the scanned image will be used. Scans that will appear in books or
magazines need higher resolution than those that will appear in desk-
top printouts or in web pages.
• Photos printed on a traditional printing press should be scanned at
approximately twice the line screen of the printed piece. For exam-
ple, most magazines are printed at 133 lines per inch, so an image
destined for their pages would be best scanned at 266 dpi.
• Line art illustrations that will be printed on a traditional press
should be scanned at the highest resolution possible, or at least
1200 dpi. These images are not converted to halftones before print-
ing, the way photos are, so the higher the resolution used for scan-
ning, the smoother these illustrations will be.
• Images that will be printed on desktop laser or color printers are
less demanding, because the output resolution of those devices is
usually only 300 dpi or 600 dpi (compared to the 2540 dpi output
resolution used by magazines and books). These printers use a line
screen of 50-100 lines per inch so images should be scanned at 100-
200 dpi for best results.
• Photos and line art being scanned for on-screen viewing (including
web pages) require even less in the way of resolution, because
most screens can only display 72-80 dpi. Scanning at a higher reso-
lution only creates extra information that the screen display will
ignore, and slows down the displaying of the images.
• Images that will be converted to text should be scanned at 300 or
400 dpi. Most optical character recognition (OCR) programs will
accept either. At 300 dpi, your scanning will be done faster, but at
400 dpi, you may get better results, especially on small type. If you
will be primarily using text from your scanned images, a 300 dpi
scanner will suit you fine.
Resizing & Resolution.
All these rules of thumb assume that you will be printing an image at
its original size. It's important to remember that you will need more
resolution if you will be enlarging an image, and less if you will be
reducing it. For example, if you are going to print your image at 200%
of its size on a desktop color printer, and want it to have 200 dpi res-
olution, you will need to scan at 400 dpi. Alternatively, if you will be
printing at 50% of its size, you only need to scan at 100 dpi. Take this
into account when deciding how much resolution you will need. But
the bottom line is, if you do the math, you'll end up with the follow-
ing conclusions:
If you will be printing on a traditional printing press, and will be
working with originals in a variety of sizes, get the highest resolution
scanner you can afford.
If you will be printing on a desktop laser or
color printer, decide whether you will need to enlarge items frequent-
ly. Usually a 600 dpi scanner is the right choice. If you will be display-
ing images primarily on screen, chances are a 300 dpi scanner will
give you all the resolution you require.
SCANNERS
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EQUIPMENT LEASING AVAILABLE
Within each type of scanner, individual models differ from each other in a number of ways. Though each manufacturer’s
product will have a different “look”, what really counts is how a particular unit measures up on the following counts:
• Resolution • Bit-depth • Dynamic range • Scanning method • Scanning area • Speed
These six criteria together determine why one scanner performs technically better than another. In addition you might want
to check out the case design, the quality of construction, how much desk space is needed (footprint), bundled software,
connection type (SCSI, parallel, USB, FireWire), and of course, price.
Resolution