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Certain peripheral devices look like a “thing in itself” for the outsider, especially if the device includes both: electronics and mechanical parts. They are something that seems too hard to be grasped by a common person. Flatbed scanners are an example of such devices.

At first glance, construction of a scanner has nothing sophisticated about itself: a case with a few connectors and a couple of buttons, a lid and a glass where you place the original for scanning. That’s quite simple and you of course can use your scanner without bothering about technicalities. At second glance, when you try to find out how the stuff works and what those mysterious numbers in the specifications stand for, things become more complicated. So if you don’t have a scanner, but are planning to buy one, you should learn what real things stand behind the characteristics declared by the manufacturer. In this article I will try to take a scanner apart (literally) to show you what it is made of. Doing this exciting job, we will also learn certain facts about scanners in general.

I guess we should start out with the most important component of each scanner. Its eyes, which are a light-sensitive matrix.

Matrix

It is the matrix that mostly determines what a scanner is. It transforms color and brightness of the incoming light flux into analog electrical signals that would make sense for its only mate – analog-to-digital converter (ADC). The ADC acts like both an interpreter and a guide, always coming with the matrix. Without interpretation, no processor or controller would understand the analog signal from the matrix. But they do understand digital signals produced by the ADC – a string of zeroes and ones. The digital electronics of a scanner are blind – you can shine at them with your flash-light and never get a single reaction. The matrix is quite another thing: a stream of light falls onto its surface literally beating electrons out of its sensitive cells. The higher the intensity of light, the more electrons the matrix releases. And as a result, the stronger will these electrons be when they all rush to the exit. However, the electrons current is too weak to be heard by even the most sensitive ADC. That’s why there is an amplifier at the matrix exit, which can be compared with a megaphone making a siren wail out of a mosquito hum. The amplified signal (analog so far) will be “weighed” by the converter so that each electron could receive a digital value, according to its current strength. That’s where the matrix quits the game: we’ve got digital information now to be processed by other devices. Image reconstruction doesn’t require the matrix any more.

This is the general picture, let us now delve somewhat deeper into technical details. Many contemporary scanners for home of office use (Small Office Home Office – SOHO) come with a matrix of one of the two types: Charge-Coupled Device (CCD) and Contact Image Sensor (CIS). This fact alone produces two natural questions: “What is the difference and which one is better?” You can catch the difference with a naked eye: a CIS scanner comes in a thinner case compared to a similar CCD machine (its height is usually about 40-50mm). The second question is rather harder to answer, since both matrix types have advantages and shortcomings. They are listed in the following table:

 

Advantages

Drawbacks

CCD

Support high resolutions (contemporary low-cost CCD-scanners support up to 2400dpi optical resolution);
Long lamp service time;
High quality scanning;
High focal range (depth of field);
supports slide-adapters and automatic documents feeder.

Relatively high pricing (compared with CIS-scanners);
It takes the lamp too long to heat up before scanning;
Requires additional power supply source.

CIS

Small dimensions;
Fast start-up;
Low cost;
Low power consumption (many CIS-scanners are powered via USB);
Autonomous functioning.

Limited optical resolution (up to 1200dpi);
Not very high focal range;
Sensitive to side lighting;
Relatively low scanning quality.

 
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