Desktop Scanners
Usually flatbed devices which employ Charge coupled devices (CCD) to capture the photons. CCDs are used in camcorders, fax machines and digital cameras. A CCD consists of an array of silicon electrodes (see Fig. 6-4 for more details).
- Densities up to 4 can be achieved with the most sensitive CCDs.
- However, these are more expensive and the price advantage of desktop scanners over drum is decreased.
- As always there are trade-offs between price and performance, but because they are based on solid state circuitry as opposed to vacuum tubes, the CCDs will eventually win in both price and performance.
- Each photon which strikes an electrode knocks out an electron. An analog-to-digital (ADC) converts the electron signal to a digital value for each pixel.
- The bit-depth of the output of the ADC. (8-bit for 256 gray levels, 12-bit for 4096 gray levels). The deeper bit-depths make better statistics possible, if only for achieving a good sample of 256 gray levels.
- Photoshop 5.x has a 16-bit/color mode.
- PMTs must capture data serially, while CCD can capture data in parallel.
- Desktop scanners generally employ linear arrays of electrodes while camcorders, digital cameras and Linotype-Hell Topaz and Kodak Photo CD scanners scan in 2 dimensions.