An aperture card is a punch card which includes an aperture into which a piece of microfilm is inserted. The microfilm contains data such as a drawing, while the punch card includes details about the data. Aperture cards were designed for archival use in a range of applications, and some are still used in this sense today. For the most part, however, digital archiving systems have replaced the aperture card.
Some examples of things which can be mounted on an aperture card include blueprints, engineering drawings, and microfilms of newspaper pages. The size of the film varies, depending on where and when the film was produced. The card, marked in Hollerith code, usually also has human-readable information so that the card can be readily identified.
A specialized card reader is required to view an aperture card. The card reader can interpret the metadata on the punch card and enlarge the film so that it can be easily seen. Card readers may include the ability to sort through and catalog a large collection of aperture cards, allowing people to manage a large archive.
The distinct advantage to an aperture card from an archival perspective is that it takes up far less room than full-sized blueprints and drawings, along with their accompanying information. The standardized size and shape also make aperture cards easier to store when contrasted with original drawings. For example, a county records office can store deed records and blueprints in a small room when they are on aperture cards, requiring a much larger space for original documents.
The disadvantage, of course, is that aperture cards still take up space and they require some manual activity, unlike digital information. With a digital archiving system, information can be cataloged in a variety of ways and called up instantly whenever it is required. Digital systems, however, can degrade data if they are compromised, and it can be costly to replace systems as newer technology is developed.
Substantial archives of aperture cards have been converted to digital form for convenience in some regions. The original cards may be retained, or discarded once it is clear that the digital system is functional. For people with a particularly burning interest, aperture card blanks are still available, allowing people to make new aperture cards. Such cards are primarily used for personal interest rather than institutional archiving, because digital technology has so many clear advantages, making it a preferred choice.