Photographic plates preceded photographic film as a target medium in photography. A light-sensitive emulsion of silver salts was applied to a glass plate. This form of photographic material largely faded from the consumer market in the early years of the 20th century, as more convenient and less fragile films were introduced. However, photographic plates were still in use by some photography businesses until the 1970s, and were in wide use by the professional astronomical community as late as the 1990s. Such plates respond to ~2% of light received. Glass plates were far superior to film for research-quality imaging because they were extremely stable and less likely to bend or distort, especially in large-format frames for wide-field imaging.
Scientific uses 
Many famous astronomical surveys were taken using photographic plates, including the first Palomar Observatory Sky Survey (POSS) of the 1950s, the follow-up POSS-II survey of the 1990s, and the UK Schmidt survey of southern declinations. A number of observatories, including Harvard College and Sonneberg, maintain large archives of photographic plates, which are used primarily for historical research on variable stars.
Many solar system objects were discovered by using photographic plates, superseding earlier visual methods. Discovery of minor planets 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 by carefully examining a bulge in Pluto's image on a plate.
Glass-backed plates were used rather than film, because they do not shrink or deform in going between wet and dry condition, or under other disturbances. Kodak discontinued producing several kinds of plates between 1980 and 2000, terminating the production of sky surveys.
Several important applications of astrography, including astronomical spectroscopy and astrometry require greater dimensional stability than film could provide, and continued using plates until digital imaging improved to the point of supplanting photochemical photography for these purposes.
Photographic plates were also an important tool in early high-energy physics, as they get blackened by ionizing radiation. For example, in the 1910s, Victor Franz Hess discovered [[cosmic radiatiit left traces on stacks of photographic plates, which he left for that purpose on high mountains or sent into the even higher atmosphere using balloons.
Medical imaging 
The sensitivity of certain types of photographic plates to ionizing radiation (usually X-rays) is also useful in medical imaging and material science applications, although they have been largely replaced with reusable and computer readable image plate detectors and other types of X-ray detectors.
Use of photographic plates has declined significantly since the early 1980s, replaced by charge-coupled devices (CCD). CCD cameras have several benefits over glass plates, including high efficiency, linear light response, and simplified image acquisition and processing. However, even the largest CCD formats (e.g., 8192x8192 pixels) still do not have the detecting area and resolution of most photographic plates, which has forced modern survey cameras to use large CCD arrays to obtain the same coverage.
Several institutions have established archives to preserve photographic plates and prevent valuable historical information from being lost. This is a particular need in astronomy, where the plates represent irreplaceable records of the sky and astronomical objects that extend back over 100 years. S
One example of an astronomical plate archive is the Astronomical Photographic Data Archive (APDA) at the Pisgah Astronomical Research Institute (PARI). APDA was created in 2007 in response to recommendations of a group of 31 international scientists who gathered in 2007 to discuss the preservation of astronomical plates. The discussions revealed that some observatories no longer could maintain their plate collections and needed a place to archive them. APDA is dedicated to housing and cataloging unwanted plates, with the goal to eventually catalog the plates and create 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 200,000 photographic images. They 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, that were built for NASA and the Space Telescope Science Institute (STScI) and used by a team of scientists under the leadership of the late Dr. Barry Lasker to develop the Guide Star Catalog and Digitized Sky Survey that are used to guide and direct the Hubble Space Telescope. A networked storage system and software that can store and analyze more than 100 terabytes of research data, donated by EMC Corporation, is available.
See also 
- Harrow Photos - Hills & Saunders Photography Business - Glass plates used until the 1970s
- Girard, Terrence M. (2004). "The Southern Proper Motion Program. III. A Near-Complete Catalog toV = 17.5". The Astronomical Journal 127: 3060. arXiv:astro-ph/0402411. Bibcode:2004AJ....127.3060G. doi:10.1086/383545.
- Peter Kroll, Constanze La Dous, Hans-Jürgen Bräuer: "Treasure Hunting in Astronomical Plate Archives." (Proceedings of the international Workshop held at Sonneberg Observatory, March 4 to 6, 1999.) Verlag Herri Deutsch, Frankfurt am Main (1999), ISBN 3-8171-1599-7
- Wayne Osborn, Lee Robbins: "Preserving Astronomy's Photographic Legacy: Current State and the Future of North American Astronomical Plates." Astronomical Society of the Pacific Conference Series, Vol. 410 (2009), ISBN 978-1-58381-700-1
- Pisgah Astronomical Research Institute (PARI) Astronomical Photographic Data Archive (APDA) http://www.pari.edu/about_pari/facts/astronomical-plate-preservation/