Charge Coupled Devices (CCDs) Sensors

Characteristics
A Charge Coupled Device is an electronic mechanism than can convert an image to its electronic form by transforming a light pattern into an electric charge pattern. Individual elements in the CCD have the capability of collecting, storing and transporting electrical charge from one element to another which designs its image sensors. Each photo sensitive element will then represent a pixel. The maximum density range of CCD sensor may vary. The range is from 2.4 to 3.6.

Resolution
Scanners that are based on the CCD technology have a certain amount of CCDs built into them. For instance, a scanner with the capability to scan a total of 10500 pixels over a width of 12 inches would have a dpi (dots per inch) of 875 when scanning an image at full scale. As an image needs to get blown up beyond full scale, the resolution drops. When the dpi decreases to 300, the amount that the image can get blown up is at its maximum. In our example, if we divide 875 by 300, the maximum this scanner could blow up while still maintaining business printing standards is just under 300%. Another result of resolution being based on a fixed number of CCDs is there are just a few fixed at a few settings available when choosing resolution.

There is also a problem with trying to scan larger original images because resolution strips lie under the center of the mounting glass. If the dimensions of the original image are larger than the fixed strip, the image can not be scanned at the highest possible resolution. As the width of an object increases, the maximum resolution will decrease.

Recent technological improvements have helped CCD scanners get over this barrier. XY and Zoom technology aims to limit the width of the bed that the scanner must deal with without limiting physical width of scanning area. The scanning head moves forward and backward, left and right to pass the bed many times and scan a narrow strip at a high resolution. If the original image that is being scanned is wider than that strip, software stitching is used to assemble and put together multiple strip images. Zoom lenses focus the CCD chip entirely on the image so no elements are wasted on parts of the bed that contain no image.

Although XY technology solved the CCD scanner's problem with scanning large images, this technology requires a lot of moving mechanical parts. The addition of these parts have increased the cost of making CCD scanners, so the manufacturers that include this technology within their CCD scanners have to greatly increase the cost of the CCD scanner, making it a cost ineffective product.

CCD manufacturers have attempted to increase CCD scanners resolution by interpolating data. Mathematical processes estimate new pixels in between existing ones to artificially increase the resolution. The problem with this method is that the new pixels don't add any new detail, so they are just magnifying a low resolution scan.

CCD Scanners are very vulnerable to external influences because of the way they scan. Each CCD in a CCD scanner is assigned a strip of the image to collect pixel information. The optics focus on one horizontal strip of the image at a time, then go to the next line above it once finished. If one of these CCDs were to malfunction, it would not be able to collect pixel data for its entire row, which would ruin the scan.

One of the main claims of the proponents of CCD scanners is that they are very user friendly. You place the images on the screen and put down the cover, just like a photocopier. It is not that simple, because when you place the images on the screen without mounting them, they will shake from the vibrations of the scanner's motor.

A CCD scanner cannot distinguish light from two neighbouring points because it collects signals from an entire area all at once. Some CCD sensors are susceptible to flare from strong signals that can cause an imprecise measurement of nearby weak signals, which contributes to a higher overall background. CCD scanners are already quite susceptible to noise, which increases the chance of the scanner encountering one of these strong, noisy signals.

In an effort to combat the problems encountered when using CCD scanners, the manufacturers invented elaborate film holders, mounting mats, and transparency trays that require extra costs and special handling.