Securing Digital Image Assets in Museums and Libraries: A Risk Management Approach

Library Trends, Fall, 1999 by Teresa Grose Beamsley

Dynamic range defines the ability of the file structure to convey tonal information about each pixel captured. Every digital image is composed of a fixed number of pixels--tiny discrete blocks of tone. Bi-tonal images can only convey information in black and white. A bit (the basic building block of digital information) can only convey two possible values; therefore, bi-tonal information is conveyed using one bit per pixel. This type of information encoding produces the smallest possible files, but the resulting image cannot represent any range of shades between black and white. It is recommended for uses that involve modern printed works and line drawings or graphics and is frequently employed when the desired use is a printed reproduction of such materials. Gray scale uses 8 bits to represent each pixel, providing the capability of representing up to 256 shades ranging from pure white through gray to pure black. This format is usually recommended for representing black and white photographs, half-tone illustrations, and other two-dimensional representations that convey shading or variation in ink density. Color is best represented using 24 bits per pixel, which provides about 16 million different colors but which results in much larger file sizes. Color conveys much more information than gray-scale or bi-tonal files and is required for images in which color must be maintained but is also recommended for use in digitizing images of older documents (http:// www.columbia.edu/acis/dl/imagespec.html; http://lcweb2.1oc.gov/ ammem/pictel/index.html). While software, printing, and display hardware designs determine the nature of the end product, the dynamic range of the image file establishes the bases from which these devices perform in tests of tone and color reproduction. Recording image data in a file structure that uses 8-bit color, for example, will in most cases result in image information that offers only a general approximation of the tonality of the original and severely impact the utility of the image surrogate for many uses.

Many institutions have chosen to protect their digital image assets by providing general access to only low-resolution files. Resolution refers to the number of pixels that are used to describe a single image (the fixed number mentioned earlier); it is usually expressed in terms of horizontal and vertical dimensions. An image recorded at a resolution of 512 x 768, for instance, has 512 rows and 768 columns of pixels. Resolution affects the level of detail that can be depicted by the image file. If a lower resolution is specified, fewer pixels will be used to describe the image and therefore edges may be blurred, areas of the displayed or printed image may appear blocky, tonal transitions may seem more abrupt, and detail may be lost altogether. An illustration will assist in visualizing the loss of information that may result from the use of lower resolutions.

Information conveyed by Figure 1, an extremely detailed photograph, would undoubtedly be lost if its digital surrogates were created using lower resolutions. Edge blurring would prevent a researcher from studying wheels, spokes, and hubs, and clothing detail would become invisible. The wide range of tonal contrast across very limited spaces would also be obscured, and the overall effect would be a smoothing of shadows and features.