by January 12, 1998 0 comments
scan1.jpg (9888 bytes)
CCD technology is used in most desktop scanners

There was a time when
scanners used to be huge, heavy, and very expensive. Today they are much smaller, lighter,
and cheaper. They have moved out of the rarified environs of the graphics professional to
every day home and office use.

What is it that makes the
scanner that sits on your desktop do its magic? What is it that captures the images from
paper and brings it alive on your monitor?

Traditionally, flatbed
scanners use something known as a CCD–Charge Couple Device–to do their job.
These scanners use a light source, (usually a cold-cathode light source), to illuminate a
thin horizontal-strip of the object placed on the scanning surface. The light reflected is
captured by the CCD (a semi-conductor chip). Measure the intensity of the light that is
captured. This information is then passed on to an analog-to-digital converter to create
the digital replica of the image that has been scanned.

CCD scanners are heavy, use
cumbersome optical reduction techniques, and a single light source. CCD is used in most
flatbeds today, especially in the high-end scanners. But things are changing at the
low-end.

scan2.jpg (14901 bytes)

PMT
is used in drum scanners to produce high quality scan
(Visuals courtesy: HP’s reviewing and testing desktop scanners by Robert Gann of HP)

Heralding in the
change is the CIS–Contact Image Sensor, also known as LED-illuminated scanner. Here,
colored LEDs are used instead of lamps. The LEDs are turned on and off much more rapidly,
and are more stable than the lamps used in CCD scanners. Light produced by these scanners
is more evenly distributed than in CCD scanners, where the lamp is prone to variation in
light intensity across its surface, with the center being brighter than the ends.

LEDs used in CIS scanners
use less power, and are thinner than the lamp in CCD scanners. The cathode lamps in CCD
scanners need elaborate housing which the CIS scanners do away with. Thus, you end up with
a thinner, lighter and cheaper scanner.

But all is not rosy with
CIS. The light emitted by the LEDs is way less than that produced by the lamp in a CCD
scanner. So, the LEDs have to be placed only a millimeter or two below the glass surface,
making them prone to damage due to impact with the glass plate. It is also not possible to
scan bound-books on a CIS scanner, as the LEDs do not emit enough light to reach the
raised spine of the book. Also, CIS is as of now, limited to 300 or 600 pixels per inch.

On the other extreme are the drum scanners,
which produce very high-quality images and are highly priced. In drum scanners, as the
name indicates, there is a long cylindrical drum on which the image is placed. These drums
are usually transparent. The image is placed on the cylinder, face out. Like CCD scanners,
there is a single row of white light, which illuminates the image while the drum rotates
at a high speed. This light is reflected back from the image and is captured by Photo
Multiplier Tubes (PMT). PMTs are light-sensing vacuum tubes which are extremely sensitive
(much more sensitive than CCDs). PMTs are capable of amplifying the signal manifold giving
a wide dynamic range. Drum scanners are able to produce high-resolution images and are
able to enlarge the original image thousands of times without losing resolution, depth and
picture quality. 

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