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Photographic plates preceded film as the primary medium for capturing images in photography. These plates, made of metal or Glass and coated with a light-sensitive emulsion, were integral to early photographic processes such as heliography, , and photogravure. Glass plates, thinner than standard window glass, became widely used in the late 19th century for their clarity and reliability. Although largely replaced by film during the 20th century, plates continued to be used for specialised scientific and medical purposes until the late 20th century.
Glass plate photographic material largely faded from the consumer market in the early years of the 20th century, as more convenient and less fragile films were increasingly adopted. However, photographic plates were reportedly still being used by one photography business in London until the 1970s, and by one in Bradford called the Belle Vue Studio that closed in 1975. They were in wide use for professional astrophotography as late as the 1990s. Workshops on the use of glass plate photography as an alternative medium or for artistic use are still being conducted in the early 21st century.
| + Sizes !Known as ! Imperial !! Metric | ||
| Quarter-plate | 3¼ × 4¼ in. | 83mm × 108mm |
| Half-plate | 4¾ × 6½ in. | 120mm × 165mm |
| Full-plate | 6½ × 8½ in. | 216mm × 165mm |
Many solar system objects were discovered by using photographic plates, superseding earlier visual methods. Discovery of using photographic plates was pioneered by Max Wolf beginning with his discovery of 323 Brucia in 1891. The first natural satellite discovered using photographic plates was Phoebe in 1898. Pluto was discovered using photographic plates in a blink comparator; its moon Charon was discovered 48 years later in 1978 by U.S. Naval Observatory astronomer James W. Christy by carefully examining a bulge in Pluto's image on a photographic plate.
Glass-backed plates, rather than film, were generally used in astronomy because they do not shrink or deform noticeably in the development process or under environmental changes. Several important applications of astrophotography, including astronomical spectroscopy and astrometry, continued using plates until digital imaging improved to the point where it could outmatch photographic results. Eastman Kodak and other manufacturers discontinued production of most kinds of plates as the market for them dwindled between 1980 and 2000, terminating most remaining astronomical use, including for sky surveys.
The persistent use of plates in astronomical and other scientific applications started to decline in the early 1980s as they were gradually replaced by charge-coupled devices (CCDs), which also provide outstanding dimensional stability. CCD cameras have several advantages over glass plates, including high efficiency, linear light response, and simplified image acquisition and processing. However, even the largest CCD formats (e.g., 8192 × 8192 pixels) still do not have the detecting area and Image resolution of most photographic plates, which has forced modern survey cameras to use large CCD arrays to obtain the same coverage.
The manufacture of photographic plates has been discontinued by Kodak, Agfa and other widely known traditional makers. Eastern European sources have subsequently catered to the minimal remaining demand, practically all of it for use in holography, which requires a recording medium with a large surface area and a submicroscopic level of resolution that currently (2014) available electronic image sensors cannot provide. In the realm of traditional photography, a small number of historical process enthusiasts make their own wet or dry plates from raw materials and use them in vintage large-format cameras.
Another astronomical plate archive is the Astronomical Photographic Data Archive (APDA) at the Pisgah Astronomical Research Institute (PARI). APDA was created in response to recommendations of a group of international scientists who gathered in 2007 to discuss how to best preserve astronomical plates (see the Osborn and Robbins reference listed under Further reading). The discussions revealed that some observatories could no longer maintain their plate collections and needed a place to archive them. APDA is dedicated to housing and cataloging unwanted plates, with the goal of eventually cataloging the plates and creating a database of images that can be accessed via the Internet by the global community of scientists, researchers, and students. APDA now has a collection of more than 404,000 photographic images from over 40 observatories that are housed in a secure building with environmental control. The facility possesses several plate scanners, including two high-precision ones, GAMMA I and GAMMA II, built for NASA and the Space Telescope Science Institute (STScI) and used by a team under the leadership of the late Barry Lasker to develop the Guide Star Catalog and Digitized Sky Survey that are used to guide and direct the Hubble Space Telescope. APDA's networked storage system can store and analyze more than 100 terabytes of data.
A historical collection of photographic plates from Mt. Wilson Observatory is available at the Carnegie Observatories. Metadata is available via a searchable database, while a portion of the plates has been digitized.
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