Hardware Reference
In-Depth Information
Scanners
A
scanner
digitizes a hard-copy page, converting it into a graphic fi le on the PC. Most scanners
are fl atbed, like a copier. Figure 3.6 shows an example of a fl atbed scanner.
FIGURE 3.6
Flatbed scanner
You place the original face down on the glass and close the lid, and then either press a
button on the scanner or issue a command in a scanning program to start the scan. Scanners
can be purchased as stand-alone units or as part of a multifunction printer-scanner-copier.
Scanners can use parallel, network, or USB interfaces, but most use USB.
Inside a scanner is a fi xed linear array called a
charge-coupled device (CCD)
. It's
composed of an array of photosensitive cells, similar to the eye of an insect, which converts
light into an electrical charge. A light bar moves across the object being scanned. A system of
mirrors refl ects the light to a lens and then into the CCD. The mirrors are slightly curved to
compact the image as it refl ects. Each of the photosensitive cells produces an electrical signal
proportional to the strength of the refl ected light that hits it, and that signal is converted to
a binary number and sent to the computer. Dark areas have lower numbers; light ones have
higher numbers.
The CCD receives data for one line of the image at a time, sends it on to the computer,
and then tells the stepper motor to advance the lamp to the next line. The CCD doesn't
have to have a number of cells equivalent to the number of pixels in the entire page—only
the number of pixels in a single row. On a scanner that can accept an 8.5
11-inch sheet,
at 300 dpi, that's about 2,600 cells. Scanners with a higher dpi have more CCD cells—for
example, about 10,400 in a 1,200 dpi scanner. This is the scanner's
horizontal dpi
, or
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