Digital images are "electronic photographs" scanned from original documents. A digital image can accurately render the information, layout, and presentation of the original, including typefaces, annotations, and illustrations. Although digital-imaging technology has been available for some time, its widespread use occurred only in the 1990s as technical advances led to improved image capture, lower costs, and greater accessibility.
There has been a great deal of attention paid to digital imaging over the last few years, and some pressure has been placed on librarians and preservation professionals to relinquish microfilm in favor of a technology that greatly improves access. However, some fervent advocates of the technology have scant understanding of what is involved in scanning images and preserving them. Digital imaging is expensive; labor costs associated with identifying, preparing, inspecting, and indexing digital information far exceed the costs of the scan itself.
Preservation professionals must ensure that the process of scanning does not damage an artifact, and that the scanning apparatus is appropriate to that artifact. For example, a fragile book should not be crushed down onto a flat-bed scanner. Preservation professionals should establish a conservation plan for each artifact or group of materials to be scanned. The plan of work would include the following:
Training scanning technicians in the safe handling of artifacts, for example, wearing cotton gloves when handling photographs. Examining artifacts to identify damage risks such as fragile surfaces, tears, soiling, and mold or insect holes.
Ensuring that artifacts are in appropriate condition for handling by researchers. In some cases, the widespread appearance of digital images has stimulated interest in viewing original artifacts.
Unique properties of digital technology
Digital images are distinct from other types of electronic files. Also known as "raster" or "bitmapped" images, digital images differ from vector images, in which graphic information is represented as mathematically defined lines and curves rather than as dots ordered on a grid. The quality of a digital image is resolution dependent; the quality of a vector image is not.
A digital image is sampled and mapped as a grid of dots or picture elements (pixels). Each pixel is assigned a tonal value (black, white, shades of gray, or color), and digitally represented in binary code (zeros and/or ones). The binary digits (bits) for each pixel are stored by the computer in a sequence, or reduced to a mathematical representation. The bits are then interpreted and read by the computer to produce an analog, bitmapped representation for display or printing. (For more detail, see http://www.library.cornell.edu/preservation/tutorial/index.html).
Unlike alphanumeric text files such as ASCII, digital images are "dumb" files because their information cannot be searched or manipulated. However, digital images are the source used to create text-searchable files through the application of optical character recognition (OCR) programs. The accuracy of such programs varies tremendously, depending on the attributes of the source documents and the digital images produced.
Pros and cons of digital
Digital images are much better than analog counterparts in terms of preservation and access. Digital images remain more faithful to the original, and from one copy to another. They also allow for flexible image capture, image enhancement, and continuing access through network distribution. Digital technology also holds potential for cost and space savings. Although digital imaging technology is promising, there are a number of drawbacks that currently limit its utility in libraries and archives. These include:
A rapidly changing technological base
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