Per Scanning - The Professional Way, by Sybil Ihric & Emil Ihric (1995 so it's dated):
"Charge_Coupled Devices (CCDs)
Flatbed, sheet_fed, and hand_held scanners, dedicated film and slide scanners, and still video and digital cameras use CCDs to sense light levels. A CCD is a solid_state electronic element composed of multiple tiny sensors, which can register ananalog electrical charge proportional to the intensity of light falling on it. CCDs have various configurations depending on the type of scanner. In flatbed scanners, CCD component sensors are arranged in one row (for three_pass
scanning) or three rows on a chip (for single_pass scanning), as shown in Figure 2_1. This arrangement allows the device to sample the entire width of an analog original image and record it as a complete line. Each time the CCD samples a line of the original image, it sends the charges (representing analog light levels) to the A/D converters to be translated into binary data. The CCD is now free to receive electrical charges from the next line. This process takes only a small fraction of a second in newer scanners.
Note: In slide scanners, digital and video
cameras, CCD sensors are typically arranged in
a rectangular array so that information can be
captured as a simultaneous block rather than
one scan line at a time.
CCD designs vary in their sensitivity to light levels and to extraneous electrical noise. These
factors are discussed more fully in the "Resolution" and "Dynamic Range, Density Range" sections of this chapter. They can affect the resolution of which the scanning device is capable, the range of color and gray tones the scanning device can register, and the accuracy of the color or grayscale information detected. Cheaper scanning devices tend to use lower quality CCDs than their higher_end counterparts. The more expensive CCDs found in professional_quality flatbeds, slide scanners, and higher_end digital cameras yield better quality image input.
Photomultiplier Tubes (PMTS)
Drum scanners of all types (see Chapter 3) use photomultiplier tubes (PMTs) instead of CCDs as
light_sensing devices. PMTS are based on an older vacuum_tube technology which, while more expensive to maintain than the newer CCD technology, has proven its quality and reliability.
In typical drum scanners that have three PMTS (one each for red, green, and blue), a xenon or tungesten_halogen light source focuses on an extremely small area of the original using fiber optics and condenser lenses. (Figure 2_2 shows a transparency lit from the inside of the drum; reflective materials are lit from the outside.) Light transmitted or reflected from the image is then projected onto angled, partially transparent mirrors as shown in Figure 2_2. Each mirror reflects some of the light and transmits some to the next mirror. The reflected portion of the mirrored light passes through the appropriate color filter and thence into the corresponding PMT, where a process known as optical amplification takes place. Electrons emitted when light strikes the cathode of the PMT travel through layers of dynodes, which emit additional electrons, thus amplifying them to the point at which the light can be converted into electrical signals. The anode of the PMT measures analog variations in these signals, which then travel to the A/D converters to be registered as digital data.
PMT technology permits capture of the broadest possible range of tones with high fidelity. Until very recently, in fact, all CCD_based devices were considered inherently inferior to PMT_based drum scanners in terms of the image quality that could be achieved. Continuing improvements in CCD and A/D converter technology have eliminated many perceived CCD shortcomings, and some experts maintain that higher_end CCD_based scanners can now reproduce images with fidelity similar to that of drum scanners. Chapter 3 addresses that ongoing
debate in more detail.
A/D Converters and Processors
Analog_to_digital converters play a major role in the quality of the digital signals that become the image on your monitor. Their task is to process the continuously variable analog voltage readings sensed by the CCDs or PMTs into numbers that represent the color or grayscale values. Both the number of colors and the level of detail that a scanner can capture are linked to the sensitivity of its A/D converters. As you might expect, lower the cost of the scanner, the less sensitive its A/D converters are likely to be.
Desktop drum scanners and many midrange and higher_end scanners use additional processors, such as digital signal processors (DSPs) to increase scanning speed and perform other image processing tasks on the fly."
Maris
| is the difference between the CCDs of a high end scanner and the
| photomultiplier tubes of a drum scanner. How do they compare, what
| are their differences? Why is a drum scanner such a high resolution
| device?
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| Dieder Bylsma |
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