Digital Imaging Standards for Research and Teaching

Information Technology and Hispanic Studies DIGITAL IMAGING STANDARDS FOR RESEARCH AND TEACHING Mark Johnston Newberry Library Kenney and Rieger's Moving Theory into Practice: Digital Imaging forLibraries andArchives is the most comprehensive and up-to-date guide available for the creation and reproduction of digital images.1 Drawing on their own extensive work at Cornell University as well as contributions from other experts, the volume addresses chiefly the needs of professional archivists and librarians, but offers much basic practical advice ofvalue to the individual researcher or teacher. Following is a distillation of the standards and procedures applicable to any serious academic project involving digitization of images, from photographing manuscripts, printed texts, or artifacts to preparing illustrations for publication, the classroom, or a website. Basic terms and units First, it's important to understand the standards of measurement commonly used in digitization. The basic unit of display and capture is the "pixel" (dot). All digital video screens use arrays of glowing dots to display images; all digital scanners and cameras use arrays of tiny dot-sized sensors, each of which captures one pixel of an image. 1 Anne R. Kenney and Oya Y. Rieger, Eds. Moving Theory into Practice: Digital Imagingfor Libraries and Archives. Mountain View, CA: Research Libraries Group, 2000. x+ 189 pages. ISBN 0-9700225-0-6. $80.00. U CORoNiCA 30.1 (Fall, 2001): 205-13 206MarkJohnstonLa coránica 30.1, 2001 Regular computer monitors display a rectangular array of pixels where the ratio of width to height is 4:3. The number of pixels in an array ranges from 640 by 480 (VGA) to 1600 by 1200, although 800 by 600 (SVGA) is still the most common. The actual size in inches of the screen determines the number of dots per inch (dpi), which is the measure of display "resolution". Remember that manufacturers still advertise monitors by their diagonal measurement: a "14-inch" screen is actually 1 1 inches wide by 8 inches high. Set to a display of 800 by 600 pixels, this screen offers a rather low resolution of 72-75 dpi (800 dots divided by 1 1 inches or 600 dots divided by 8 inches); set to 1600 by 1200, it still yields a resolution of only 144-150 dpi. Even a giant "27-inch" monitor (actually 21 inches wide by 15 inches high) only attains a resolution of 76-80 dpi when displaying 1600 by 1200 pixels. The low resolution of screen displays is due to limitations in manufacturing technology and in the amount of computer memory (RAM) required to display adequate "color-depth" for each pixel (discussed below). The resolution of digital video displays remains far below that ofprinted images: regular offset printing uses 1200 dpi. Nonetheless, most people will not distinguish the finer detail provided by resolutions beyond 600 dpi. The resolution of capture devices now exceeds that of most displays . The best flatbed scanners can achieve resolutions of 3000 dpi for images up to 1 1 by 17 inches in size. The latest digital cameras use somewhat lower resolutions, usually advertised in terms of"megapixels" (millions of dots), which is the product of the width and height in pixels of their sensor arrays. A camera that takes a picture with an array of sensors measuring 2000 by 2000 pixels captures 4 megapixels. Since a camera's array of sensors is only the size of a fingertip, the resolution of the tiny array itself is impressive (perhaps 2500 dpi for a 2 megapixel camera), but the size of the image as finally displayed or printed, of course, determines the resolution offered to a viewer. By comparison, high-quality, fine-grain 35 millimeter film captures a picture using 12.5 megapixels (over 4000 by 3000 dots). The number of possible colors displayed ("color-depth") is a second essential unit of measurement in digital imaging. Each two colors that a pixel might display requires 1 additional bit ofcomputer memory. In the binary mathematics of bits, displaying 256 possible colors requires 8 bits or 1 byte ofmemoryperpixel (8 bits equal one byte). Since a SVGA display of800 by 600 dots has a total of480,000 pixels, it therefore requires 480 kilobytes of RAM simply...