The Scanning Process

Here are the steps that a scanner goes through when it scans a document:

The document is placed on theglass plateand thecoveris closed. The inside of the cover in most scanners is flat white, although a few are black. The cover provides a uniform background that the scanner software can use as a reference point for determining the size of the document being scanned. Most flatbed scanners allow the cover to be removed for scanning a bulky object, such as a page in a thick book.

Alampis used to illuminate the document. The lamp in newer scanners is either acold cathode fluorescent lamp(CCFL) or axenon lamp, while older scanners may have a standardfluorescent lamp.
The entire mechanism (mirrors, lens, filter and CCD array) make up thescan head. The scan head is moved slowly across the document by abeltthat is attached to a stepper motor. The scan head is attached to astabilizer barto ensure that there is no wobble or deviation in thepass. Pass means that the scan head has completed a single complete scan of the document.

The image of the document is reflected by an angledmirrorto another mirror. In some scanners, there are only two mirrors while others use a three mirror approach. Each mirror is slightly curved to focus the image it reflects onto a smaller surface.
The last mirror reflects the image onto alens. The lens focuses the image through afilteron the CCD array.

The filter and lens arrangement vary based on the scanner. Some scanners use athree passscanning method. Each pass uses a different color filter (red, green or blue) between the lens and CCD array. After the three passes are completed, the scanner software assembles the three filtered images into a single full-color image.

The Scanning Process

Most scanners today use thesingle passmethod. The lens splits the image into three smaller versions of the original. Each smaller version passes through a color filter (either red, green or blue) onto a discrete section of the CCD array. The scanner combines the data from the three parts of the CCD array into a single full-color image.

Another imaging array technology that has become popular in inexpensive flatbed scanners iscontact image sensor(CIS)。独联体取代了CCD阵列、镜子、过滤器s, lamp and lens with rows of red, green and bluelight emitting diodes(LEDs). The image sensor mechanism, consisting of 300 to 600 sensors spanning the width of the scan area, is placed very close to the glass plate that the document rests upon. When the image is scanned, the LEDs combine to provide white light. The illuminated image is then captured by the row of sensors. CIS scanners are cheaper, lighter and thinner, but do not provide the same level of quality and resolution found in most CCD scanners.

We will take a look at what happens between the computer and scanner, but first let's talk about resolution.