6B. Technical Infrastructure: IMAGE CREATION

Key Concepts

introduction
how scanners work
scanner types
image processing

HOW SCANNERS WORK
Scanners operate by shining light at the object or document being digitized and directing the reflected light (usually through a series of mirrors and lenses) onto a photosensitive element. In most scanners, the sensing medium is an electronic, light-sensing integrated circuit known as a charged coupled device (CCD). Light-sensitive photosites arrayed along the CCD convert levels of brightness into electronic signals that are then processed into a digital image.

CCD is by far the most common light-sensing technology used in modern scanners. Two other technologies, CIS (Contact Image Sensor), and PMT (photomultiplier tube) are found in the low and high ends of the scanner market, respectively. CIS is a newer technology that allows scanners to be smaller and lighter, but sacrifices dynamic range, depth-of-field, and resolution. PMT-based drum scanners produce very high-quality images, but have limited application in library and archives scanning for reasons we'll discuss shortly.

Another sensing technology, CMOS (Complementary Metal Oxide Semiconductor), appears primarily in low-end, hand-held digital cameras where its low cost, low power consumption and easier component integration permits smaller, less expensive designs. Traditionally, high-end and professional digital cameras employ CCD sensors, despite their expense and the complexity of their design, because they exhibit much superior noise characteristics. Although some innovative designs that render low-noise CMOS-based images are emerging, CCD still dominates the high end of the market. Click here for more details on scanner operation. Further technical details on digital cameras can be found here.

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