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Scanning can be done in-house or contracted out. The cost-effectiveness
of each approach will depend on the volume, type, and fragility of
materials being scanned, the required quality of the resulting digital
objects, and the expertise and equipment available in-house. The
economics of this equation will change with market conditions and
technological advances. In making this decision, it can be helpful
to know something about the strengths and weaknesses of the various
types of scanners available, in order to assess their appropriateness
for any particular imaging project or strategy. However, be aware
that scanner selection is only one factor affecting the success of
a digitizing project. For instance, any area where high-quality scanning
will take place should have controlled lighting (no natural light)
and continuous gray-tone walls, ceiling, and floor and be free of
dust and vibrations. Service bureaus offering image capture vary
considerably in the quality levels they provide, and should the decision
be made to outsource digitization, a variety of sample images should
be sent to several vendors, and the quality of the resultant scans
compared before digitization is begun.
There are four general types of scanners: drum (fig. 9), flatbed (figs.
7, 10), film or transparency (fig. 11), and digital
camera (essentially a traditional still camera with scanner technology
attached to it, or a "scanback") (fig. 8). Most scanners use CCD (charge-coupled
device) light-sensitive image sensors, though the newer CMOS (complementary
metal oxide semiconductor) technology is making some inroads in lower-cost
and lower-quality mobile applications. Scanning is a process that
generally resembles photography or photocopying, and in fact it is
advisable to use the services of a professional photographer for
image capture, if possible, to ensure the highest possible quality
of reproduction. Depending on the type of capture device, the work
to be captured may be placed either in front of a digital camera
(on a stand or tripod) or on or in a scanner. A shot is then taken,
but instead of exposing the grains on a piece of negative film or
on a photocopying drum, light reflects off (or through) the image
onto a set of light-sensitive diodes.
Each diode responds like a grain of film, reading the level of light
to which it is exposed, except that it converts this reading to a
digital value, which it passes on to digital storage or directly
into computer memory for editing and other postcapture processing.
Rather than exposing the entire image at once, the diodes may sweep
across the source image, like the light sensors on a photocopying
machine. The number of distinct readings, taken vertically and horizontally,
determines the resolution of the scanned image. The possible range
of values that can be recognized by the digitizing hardware is the
dynamic range of the device, which helps determine the maximum sample
depth of the resultant images. (As of this writing, the chosen scanner
should have a minimum bit depth of 36, but a bit depth of 42 or 48
may be preferable.)
The hardware device (i.e., the scanner) functions together with
its driver (the program that allows a peripheral device such as a
scanner to interface with a computer's operating system) and its
managing application program or software, and each of these three
elements will have an impact upon image quality. It is possible to
use third-party scanning and editing software for postscanning image
manipulation rather than the program that comes bundled with a scanner.
Such software should be chosen on the basis of its ability to perform
required tasks, such as saving image files into the needed variety
of formats and compression schemes (for instance, TIFF, GIF, JPEG/JFIF,
PNG, JPEG2000, and LZW), or converting images from one format to
another. Another important capability is batch processing,
the ability to apply a given process—such as compression, the creation
of thumbnail images, or the addition of a watermark or copyright
notice—to multiple files. (Note that such capabilities may be needed
in-house even if a service bureau does the original image capture,
and also that some may be provided by DAM systems, perhaps linking
to third-party software.) A manual override for any automatic scanning
function is also essential, as any system will occasionally misjudge
material. Using hardware and software that support ICC color profiles
enables the scanner and monitor employed to be calibrated to the
same settings and helps ensure color fidelity and consistency. Be aware that such capabilities may be needed
in-house even if a service bureau does the original image capture,
and also that some may be provided by DAM systems, perhaps linking
to third-party software.) A manual override for any automatic scanning
function is also essential, as any system will occasionally misjudge
material. Using hardware and software that support ICC color profiles
enables the scanner and monitor employed to be calibrated to the
same settings and helps ensure color fidelity and consistency.
Digital cameras are the most versatile capture devices. Attached
to an adjustable copy stand (similar to a microfilming stand), the
camera can be moved up or down in order to fit the source material
within its field of view. This allows the scanning of materials larger
than most scanners can accommodate and does not require direct contact
with the original, which may be an important conservation concern.
Moreover, digital cameras allow more control over lighting and setup
than is possible with scanners, and can also capture images of three-dimensional
objects rather than being limited to documenting two-dimensional
originals or analog surrogates. However, high-quality digital copy-stand
cameras are expensive, and the more portable and affordable handheld
consumer cameras cannot offer the same quality of image capture.
The choice of a digital camera can eliminate the often-laborious
workflow step of creating analog photographic intermediaries such
as negatives or transparencies. This will save time but will not
provide an additional, robust surrogate of the originals.
Drum scanners resemble mimeograph stencil machines from the 1960s;
source material is placed on a drum that is rotated past a high-intensity
light source during image capture. Drum scanners use traditional
photomultiplier tube (PMT) technology rather than CCDs. They
tend to offer the highest image quality, up to 8000 samples per inch
(spi), but require flexible source material of limited size that
can be wrapped around a drum, which may be a serious conservation
concern and may require the use of analog photographic intermediaries.
Drum scanners are expensive as compared to most other scanner types.
("Virtual drum" scanners, which use CCD technology and a crafty arrangement
of mirrors, are more affordable but cannot approach the same resolution.)
Flatbed scanners are highly affordable and resemble photocopying
machines; source material is placed flat on the glass and captured
by CCD arrays that pass below it. Newer scanners generally have a
resolution of between 1200 and 5000 spi, depending on their price
and quality. Flatbed scanners require source material to be no larger
than the scanner's glass and to lay facedown and flat in direct contact
with the scanner, thus making them impractical for fragile or oversize
materials. Many flatbed scanners now come with built-in slide or
transparency holders, and transparency adapters can be easily purchased
if such devices are not included. These may allow multiple transparent
images to be scanned at a time, if each image in a strip has the
same color and resolution requirements. However, if one cannot invest
in an expensive, top-of-the-line flatbed scanner, transparency scanners
can generally achieve higher-quality image capture from transparent
material.
Transparency scanners generally resemble small boxes with a slot
in the side big enough to insert a 35mm slide, though multiformat
or 4-by-5-inch scanners are also available. Inside the box, light
passes through the transparency to hit a CCD array. Transparency
scanners are designed to scan small areas at high resolution. They
can offer resolution comparable to that of a mid- to high-end flatbed
scanner and are highly affordable.
The nature and characteristics of the source material should be
examined to determine what limitations they impose upon scanner selection.
Will capture be from the original work or from a photographic reproduction?
How fragile or robust is the source material? Is it transparent or
reflective, two- or three-dimensional, or pliable enough to wrap
around a large drum? Once the range of scanner types has been narrowed,
a choice must be made among the features and capabilities of various
models, noting such characteristics as ability to support ICC color
profiles, maximum possible resolution, and sample depth.
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