PART 3. OPTICAL DISKS
Optical storage technology uses lasers to record information by selectively altering the light reflectance characteristics of a given medium. The alteration may be accomplished in various ways: by forming microscopic pits or bubbles in the medium's surface (ablative recording), by fusing two metallic layers to create a reflective alloy (dual alloy recording), by altering an organic dye material (dye-based recording), by changing the direction of light reflected from a magnetizable surface (magneto-optical recording), by inducing crystalline-to-amorphous transitions (phase-change recording), or by other means. Regardless of method, a "playback" laser detects the alterations and decodes (reads) the recorded information. The playback laser typically operates at lower power or a different wavelength than the laser that is used for recording.
As their most attractive and important characteristic, optical storage media support very high areal recording densities. Manufacturers of magnetic storage devices and media have steadily increased their recording densities from 10,000 bits per square inch in the early 1960s to the more than 60 million bits per square inch supported by new products; optical storage media, by contrast, routinely record hundreds of millions of bits per square inch. They consequently offer much higher storage capacities than magnetic media of comparable size. For libraries and other information-intensive work environments, optical disks offer sufficient capacity to store huge quantities of character-coded text, computer data bases, graphic images, and audio signals, thereby permitting the implementation of large-scale electronic information systems. When automated jukebox retrieval units are utilized, optical disk installations can provide unattended access to hundreds of billions or even trillions of bytes of data.
Areal density aside, optical and magnetic media support similar applications; any computer, audio, or video information that can be recorded on magnetic tapes or magnetic disks can be stored on optical media, and vice versa. While all magnetic media support direct recording, however, optical disks are available in read/write and read-only varieties. Read/ write optical disks, as their name suggests, resemble their magnetic counterparts in supporting both recording and playback of machine-readable information. Read-only optical disks, in contrast, have no direct recording capabilities; they are limited to playback of prerecorded information generated by a mastering and replication process described below. Depending on the equipment configuration employed in a specific situation and the application in which they are used, optical disks may contain computer-generated data, video images, and/or audio signals. Depending on the disk format, the information itself may be encoded in either digital or analog form, the former being the most common. The following discussion surveys the most important types of optical disks, emphasizing physical and recording characteristics that can have an impact on media stability, care, and handling.
Read/write optical media, as noted above, support direct recording of machine-readable information. Such media are purchased blank, although they may be pregrooved or contain some prerecorded control signals. Like their magnetic counterparts, they can accept information from various sources. This report emphasizes optical disks, the most widely encountered read/write optical media and the only ones for which library applications have been reported. Optical cards and optical tapes, described briefly below, offer interesting capabilities that may eventually lead to library-related implementations, but installations to date have consisted mainly of field trials, demonstration projects, and highly customized systems.
Optical cards, also known as optical memory cards and optical digital data cards, were introduced in the early 1980s as compact, portable storage media for microcomputer configurations. …