Image Scanner Types
The first image scanner
ever developed was built in 1957, at the US National Bureau
of Standards, by a team led by Russel Kirsch, and it was a
drum scanner. The first image ever scanned on this machine
was a 5 cm square photograph of Kirsch's
then-three-month-old son, Walden. The black and white image
had a resolution of 176 pixels.
The drum scanners'
particularity is that they use photomultiplier tubes (PMT)
for the image capture, rather than charge-coupled device (CCD)
arrays found in flatbed scanners and inexpensive film
scanners. Reflective and transmissive originals are mounted
on an acrylic cylinder, the scanner drum, which rotates at
high speed while it passes the object being scanned in front
of precision optics that deliver image information to the
PMTs. Most modern color drum scanners use 3 matched PMTs,
which read red, blue, and green light respectively. Light
from the original artwork is split into separate red, blue,
and green beams in the optical bench of the scanner.
It's not called a drum scanner for nothing, in case someone
was wondering, but because of the large glass drum on which
the original artwork is mounted for scanning. One of the
unique features of drum scanners is the ability to control
sample area and aperture size independently.
The sample size is the area that the scanner encoder reads
to create an individual pixel. The aperture is the actual
opening that allows light into the optical bench of the
scanner. The ability to control aperture and sample size
separately is particularly useful for smoothing film grain
when scanning black-and white and color negative originals.
Only a few companies continue to manufacture drum scanners.
While prices of both new and used units have come down over
the last decade, they still require a considerable monetary
investment when compared to CCD flatbed and film scanners.
However, drum scanners remain in demand due to their
capacity to produce scans that are superior in resolution,
color gradation, and value structure. Also, since drum
scanners are capable of resolutions up to 12,000 PPI, their
use is generally recommended when a scanned image is going
to be enlarged.
Even though flatbed scanners are more and more used in
various operations, drum scanners continue to be used in
high-end applications, such as museum-quality archiving of
photographs and print production of high-quality books and
magazine advertisements.
Flatbed scanners, also called desktop scanners, are the most
versatile and commonly used scanners. They are usually
composed of a glass pane(or platen), under which there is a
bright light (xenon or cold cathode fluorescent), which
illuminates the pane, and a moving optical array. This
scanner allows the user to place a full piece of paper,
book, magazine, photo or any other object onto the bed of
the scanner and has the capability to scan that object.
Images to be scanned are placed face down on the glass, an
opaque cover is lowered over it to exclude ambient light,
and the sensor array and light source move across the pane,
reading the entire area. An image is therefore visible to
the detector only because of the light it reflects.
Transparent images do not work in this way, and require
special accessories that illuminate them from the upper
side. Many scanners offer this as an option.
Film scanners work quite simply. Thus, uncut film strips of
up to six frames or four mounted slides are inserted in a
carrier, which is moved by a stepper motor across a lens and
CCD sensor inside the scanner. Dedicated film scanners are
often better than flatbed scanners, regarding the
resolution, partly because they don't need to scan large
areas.
Handheld scanners have two variations, namely document and
3D scanners. They use the same basic technology as a flatbed
scanner, but rely on the user to move them instead of a
motorized belt. This type of scanner typically does not
provide good image quality. However, it can be useful for
quickly capturing text.
Most hand scanners were monochrome, and produced light from
an array of green LEDs to illuminate the image. A typical
hand scanner also had a small window through which the
document being scanned could be viewed. They were popular
during the early 1990s.
Handheld 3D scanners are popular for many applications, such
as industrial design, reverse engineering, inspection &
analysis, digital manufacturing and medical applications.
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