Specifications

SCANNERS
Bit Depth
When a scanner converts something into digital form, it looks at the
image pixel by pixel and records what it sees. That part of the
process is simple enough, but different scanners record different
amounts of information about each pixel. How much information a
given scanner records is measured by its bit depth.
The simplest kind of scanner only records black and white, and is
sometimes known as a 1-bit scanner because each bit can only
express two values, on and off. In order to see the many tones in
between black and white, a scanner needs to be at least 4-bit (for up
to 16 tones) or 8-bit (for up to 256 tones). The higher the scanner's
bit depth, the more accurately it can describe what it sees when it
looks at a given pixel. This, in turn, makes for a higher quality scan.
Most color scanners today are at least 24-bit, meaning that they col-
lect 8 bits of information about each of the primary scanning colors:
red, blue, and green. A 24-bit unit can theoretically capture over 16
million different colors, though in practice the number is usually
quite smaller. This is near-photographic quality, and is therefore
commonly referred to as "true color" scanning.
An increasing number of scanners can capture 30 and 36-bits or bil-
lions of colors (in theory). The only problem is that most software
doesn’t handle anything larger than a 24-bit scan. Still, those extra
bits are worth having. When a software program opens a 30-bit or
36-bit image, it can use the extra data to correct for noise in the
scanning process and other problems that hurt the quality of the
scan. As a result, scanners with higher bit depths tend to produce
better color images. One caveat though, not all monitors can display
a 24-bit, true-color image.
Dynamic Range
Another important criteria for evaluating a scanner is 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 (perfect white) to 4.0 (perfect black),
and the single number given for a particular scanner tells how much
of that range the unit can distinguish. Most color flatbeds have diffi-
culty perceiving the subtle differences between the dark and light
colors at either end of the range, and tend to have a dynamic range
of about 2.4. That's fairly limited, but it's usually sufficient for proj-
ects where perfect color isn't a concern. For greater dynamic range,
the next step up is a top-quality color flatbed scanner with extra bit
depth and improved optics. These scanners are usually capable of a
dynamic range between 3.0 and 3.6 and are well-suited to more
demanding tasks like standard color prepress.
For the ultimate in dynamic range, the only choice is a drum scan-
ner. These units frequently have a dynamic range of 3.4 to 4.2 and
deliver all the color quality one could ask of a desktop scanner.
Although they are overkill for most projects, drum scanners do offer
high quality in exchange for their high price. While a high dynamic
range is no guarantee of good scanning results (many other factors
come into play), it is generally an indication that the scanner manu-
facturer is striving to please educated buyers by producing a higher-
quality product. All other things being equal, go with the scanner
that offers the higher dynamic range.
Scanning Method
Part of what determines the overall quality of a scanned image is
the method by which a scanner collects data. Unlike other kinds of
scanners, flatbed scanners use two different approaches. One is
charge-coupled devices, or CCDs, and the other is contact image
sensors, or CIS.
Charge-coupled devices are tiny light-sensitive semiconductor chips
that consist of an array of light-sensitive photocells. CCD scanners
pass a light source over a document on the scanning bed and use a
system of mirrors and a lens to focus the image on the chip. CIS
scanners, on the other hand, replace the chips, mirror and lens with
a single row of sensors placed extremely close to the document. The
result is a scanner that is thinner and cheaper than traditional CCD
scanners but not yet as good in resolution, image quality and color
fidelity.
Scanning Area
On a less technical level, a further consideration to be weighed
when buying a scanner is the maximum size image a particular unit
can scan. Most sheetfed scanners can easily scan documents up to
8.5˝ wide and 14˝ long. Among flatbed scanners, particular units are
divided between those that can scan only a letter-size (8.5 x 11˝)
page and those that can handle a full legal-size (8.5 x 14˝) scan.
Letter-size scanners are also sometimes referred to as A4-sized, in
reference to the European paper sizing system. In choosing a scan-
ner, consider how important it is to you to have a full legal-sized
scanning bed. Most users have little need for this feature, but real
estate agents and others who work with documents in that format
will appreciate the flexibility.
One caveat on scanning area size: an increasing number of scanners
are using lenses to increase the resolution available in one part of
the scanning bed, by concentrating the light reflected off the image.
This is very useful for scanning slides and other materials that bene-
fit from being scanned at high resolution, but it can be misleading if
the high-resolution area is only a small part of the overall scanning
area. Be on the lookout for scanners offering dual resolutions, and
realize ahead of time that the highest resolution will probably only
be available for small images.
Scanning Speed
A final specification to check while shopping for a scanner is speed.
How much scanner speed depends on is how many times you intend
to use it. Those scanning once or twice a day, you won’t mind a slow
scanner than those who are constantly acquiring images.
Unfortunately, evaluating scanning speed is difficult, because there
is no single standard for evaluating the time it takes to complete a
scanning operation. Manufacturers frequently specify the raw speed
of their scanner motors, but that speed rarely correlates with real-
world performance. Other factors to consider including the speed of
the scanning software, the size of its memory cache, and the type of
connection between scanner and the computer— all weigh in on
overall performance. It's also generally true that scanners take
longer when scanning at higher resolutions, or when applying filters
to an image during the scanning process.
SCANNERS
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Selecting a scanner—what you need to know