Punch card in which a piece of microfilm is mounted
An aperture card is a type of punched card with a cut-out window into which a chip of microfilm is mounted. Such a card is used for archiving or for making multiple inexpensive copies of a document for ease of distribution. The card is typically punched with machine-readable metadata associated with the microfilm image, and printed across the top of the card for visual identification; it may also be punched by hand in the form of an edge-notched card.[1] The microfilm chip is most commonly 35mm in height, and contains an optically reduced image, usually of some type of reference document, such as an engineering drawing, that is the focus of the archiving process. Machinery exists to automatically store, retrieve, sort, duplicate, create, and digitize cards with a high level of automation.
Aperture cards have several advantages and disadvantages when compared to digital systems. While many aperture cards still play an important role in archiving, their role is gradually being replaced by digital systems.
Usage
Aperture cards are used for engineering drawings from all engineering disciplines. The U.S. Department of Defense once made extensive use of aperture cards, and some are still in use, but most data is now digital.[2]
Details about the drawing, such as its number, could be either punched or printed on the remaining portion of the card. With appropriate machinery, this enables automated handling. However, even without such equipment, the cards remain readable to humans using a lens and a light source.
Advantages
Aperture cards have, for archival purposes, some advantages over digital systems. They have a 500-year lifetime, they are human readable, and there is no expense or risk in converting from one digital format to the next when computer systems become obsolete.[3]
Disadvantages
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Many of the drawbacks are rooted in the inherent differences between analog and digital technologies. For instance, searching for specific strings within content is notably slower in analog formats. Moreover, handling physical cards necessitates specialized machinery, and processing optical film is time-consuming.
Microfilm cameras and the properties of microfilm stock itself impose limitations on the level of detail that can be captured, especially at higher reduction ratios required for larger drawings. Faded or low-contrast drawings may not reproduce well, leading to the loss of significant detail or annotations.
Like other forms of microfilm, misfiling cards in large archives can effectively render them lost unless discovered by chance later on.
Aperture cards made from 35mm roll film affixed to blank cards require careful handling. Bending the card can cause film detachment, and excessive pressure on a stack of cards can result in mounting glue seepage, leading to clumps of cards that may not feed properly through machinery. Attempting to feed a delaminated card through machinery risks damaging both the image and the equipment itself.
Machinery
A set of cards could be rapidly sorted by drawing number or other punched data using a card sorter. Machines are now available that scan aperture cards and produce a digital version.[4] Aperture card plotters are machines that use a laser to create the image on the film.[5]
Conversion
Aperture cards can be converted to digital documents using scanning equipment and software. Scanning can allow for significant image cleanup and enhancement. Often, the digital image produced is better than the visual quality available prescan.[6]
Cady, Susan A. (1999). "Microfilm technology and information systems". In Bowden, Mary Ellen; Hahn, Trudi Bellardo; Williams, Robert Virgil (eds.). Proceedings of the 1998 Conference on the History and Heritage of Science Information Systems. ASIS monograph series. Medford, NJ: Published for the American Society for Information Science and the Chemical Heritage Foundation by Information Today. pp.177–186. CiteSeerX10.1.1.99.3796. ISBN1573870803. OCLC42022214. Early aperture cards were sometimes mounted on McBee Keysort cards that could be notched on the margins to indicate an index term and then sorted manually with tools resembling knitting needles.
Ronald Kay, chief executive of UCCA, is seen holding an aperture card. The cards held a microfilm image of a candidate's applications to UK universities, and were used to print batches of applications sorted by the university and course applied to.
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