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Technologies that run Scanners

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PCQ Bureau
New Update
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CCD technology is used in most desktop scanners
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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.

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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.

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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.

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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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