Photographic plates preceded
photographic filmPhotographic film is a sheet of plastic coated with an emulsion containing light-sensitive silver halide salts with variable crystal sizes that determine the sensitivity, contrast and resolution of the film...
as a means of photography. A light-sensitive
emulsionAn emulsion is a mixture of two or more liquids that are normally immiscible . Emulsions are part of a more general class of two-phase systems of matter called colloids. Although the terms colloid and emulsion are sometimes used interchangeably, emulsion is used when both the dispersed and the...
of
silverSilver is a metallic chemical element with the chemical symbol Ag and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal...
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
astronomicalAstronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...
community as late as the 1990s. Such plates respond to ~2% of light received. Glass plates were far superior to
filmPhotographic film is a sheet of plastic coated with an emulsion containing light-sensitive silver halide salts with variable crystal sizes that determine the sensitivity, contrast and resolution of the 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.
Early plates used the very inconvenient wet
collodion processThe collodion process is an early photographic process. It was introduced in the 1850s and by the end of that decade it had almost entirely replaced the first practical photographic process, the daguerreotype. During the 1880s the collodion process, in turn, was largely replaced by gelatin dry...
which was replaced late in the 19th century by gelatin
dry plateDry plate, also known as gelatin process, is an improved type of photographic plate. It was invented by Dr. Richard L. Maddox in 1871, and by 1879 it was so well introduced that the first dry plate factory had been established...
s.
Astronomy
Many famous astronomical surveys were taken using photographic plates, including the first
Palomar ObservatoryPalomar Observatory is a privately owned observatory located in San Diego County, California, southeast of Pasadena's Mount Wilson Observatory, in the Palomar Mountain Range. At approximately elevation, it is owned and operated by the California Institute of Technology...
Sky Survey (
POSSThe National Geographic Society – Palomar Observatory Sky Survey is a major photographic survey of the night sky that was completed at Palomar Observatory in 1958.-Observations:...
) of the 1950s, the follow-up POSS-II survey of the 1990s, and the UK Schmidt
surveyTimeline of astronomical maps, catalogs and surveys* ca. 1800 BC — Babylonian star catalog* ca. 350 BC — Shi Shen's star catalog has almost 800 entries* ca. 300 BC — star catalog of Timocharis of Alexandria...
of southern
declinationIn astronomy, declination is one of the two coordinates of the equatorial coordinate system, the other being either right ascension or hour angle. Declination in astronomy is comparable to geographic latitude, but projected onto the celestial sphere. Declination is measured in degrees north and...
s. A number of
observatoriesAn observatory is a location used for observing terrestrial or celestial events. Astronomy, climatology/meteorology, geology, oceanography and volcanology are examples of disciplines for which observatories have been constructed...
, including
Harvard College ObservatoryThe Harvard College Observatory is an institution managing a complex of buildings and multiple instruments used for astronomical research by the Harvard University Department of Astronomy. It is located in Cambridge, Massachusetts, USA, and was founded in 1839...
and
Sonneberg ObservatorySternwarte Sonneberg is an astronomical observatory and was formerly an institute of the Academy of Science in the German Democratic Republic. It was founded in 1925 by Cuno Hoffmeister and is located in Sonneberg, Thuringia, Germany. Sonnenberg Observatory has one of the world's largest...
, maintain large archives of photographic plates, which are used primarily for historical research on
variable starA star is classified as variable if its apparent magnitude as seen from Earth changes over time, whether the changes are due to variations in the star's actual luminosity, or to variations in the amount of the star's light that is blocked from reaching Earth...
s.
Many solar system objects were discovered by using photographic plates, superseding earlier visual methods. Discovery of
minor planetAn asteroid group or minor-planet group is a population of minor planets that have a share broadly similar orbits. Members are generally unrelated to each other, unlike in an asteroid family, which often results from the break-up of a single asteroid...
s using photographic plates was pioneered by
Max WolfMaximilian Franz Joseph Cornelius Wolf was a German astronomer and a pioneer in the field of astrophotography...
beginning with his discovery of
323 Brucia323 Brucia was the first asteroid to be discovered by the use of astrophotography. It was also the first of over 200 asteroids discovered by Max Wolf, a pioneer in that method of finding astronomical objects...
in 1891. The first
natural satelliteA natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called its primary. The two terms are used synonymously for non-artificial satellites of planets, of dwarf planets, and of minor planets....
discovered using photographic plates was
PhoebePhoebe is an irregular satellite of Saturn. It was discovered by William Henry Pickering on 17 March 1899 from photographic plates that had been taken starting on 16 August 1898 at the Boyden Observatory near Arequipa, Peru, by DeLisle Stewart...
in 1898.
PlutoPluto, formal designation 134340 Pluto, is the second-most-massive known dwarf planet in the Solar System and the tenth-most-massive body observed directly orbiting the Sun...
was discovered using photographic plates in a
blink comparatorA blink comparator was a viewing apparatus used by astronomers to find differences between two photographs of the night sky shot using optical telescopes such as astrographs. It permitted rapidly switching from viewing one photograph to viewing the other, "blinking" back and forth between the two...
; its moon
CharonCharon is the largest satellite of the dwarf planet Pluto. It was discovered in 1978 at the United States Naval Observatory Flagstaff Station. Following the 2005 discovery of two other natural satellites of Pluto , Charon may also be referred to as Pluto I...
was discovered 48 years later by carefully examining a bulge in Pluto's image on a plate.
Several important applications of astrography, including
astronomical spectroscopyAstronomical spectroscopy is the technique of spectroscopy used in astronomy. The object of study is the spectrum of electromagnetic radiation, including visible light, which radiates from stars and other celestial objects...
and
astrometryAstrometry is the branch of astronomy that involves precise measurements of the positions and movements of stars and other celestial bodies. The information obtained by astrometric measurements provides information on the kinematics and physical origin of our Solar System and our Galaxy, the Milky...
require greater dimensional stability than film could provide, and continued using plates until
digital imagingDigital imaging or digital image acquisition is the creation of digital images, typically from a physical scene. The term is often assumed to imply or include the processing, compression, storage, printing, and display of such images...
improved to the point of supplanting photochemical photography for these purposes.
Physics
Photographic plates were also an important tool in early high-energy physics, as they get blackened by
ionizing radiationIonizing radiation is radiation composed of particles that individually have sufficient energy to remove an electron from an atom or molecule. This ionization produces free radicals, which are atoms or molecules containing unpaired electrons...
. For example, Victor Franz Hess discovered, in the 1910s, cosmic radiation as it left traces on stacks of photographic plates, which he left for that purpose on high mountains or sent into the even higher atmosphere using
balloonA balloon is an inflatable flexible bag filled with a gas, such as helium, hydrogen, nitrous oxide, oxygen, or air. Modern balloons can be made from materials such as rubber, latex, polychloroprene, or a nylon fabric, while some early balloons were made of dried animal bladders, such as the pig...
s.
Medical imaging
The sensitivity of certain types of photographic plates to ionizing radiation (usually
X-rayX-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...
s) is also a useful in
medical imagingMedical imaging is the technique and process used to create images of the human body for clinical purposes or medical science...
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.
Decline
Use of photographic plates has declined significantly since the early 1980s, replaced by
charge-coupled deviceA charge-coupled device is a device for the movement of electrical charge, usually from within the device to an area where the charge can be manipulated, for example conversion into a digital value. This is achieved by "shifting" the signals between stages within the device one at a time...
s (CCD). CCD cameras have several benefits over glass plates, including high efficiency, linear response to light, and simplicity of image acquisition and
processingDigital image processing is the use of computer algorithms to perform image processing on digital images. As a subcategory or field of digital signal processing, digital image processing has many advantages over analog image processing...
. However, even the largest format CCDs (e.g., 8192x8192 pixels) still do not have the detecting area and
resolutionImage resolution is an umbrella term that describes the detail an image holds. The term applies to raster digital images, film images, and other types of images. Higher resolution means more image detail....
of most photographic plates, which has forced modern survey cameras to use large arrays of CCD chips.
Preservation
Several institutions are setting up archives to preserve the original plates, preventing valuable historical astronomical data from being lost.
In the fall of 2007, a group of 31 international scientists gathered at the
Pisgah Astronomical Research InstitutePisgah Astronomical Research Institute is an astronomical observatory located in the United States in Balsam Grove, North Carolina. It is operated as a not-for-profit foundation. PARI is in the Pisgah National Forest of Western North Carolina at an elevation of 914 m...
(PARI) to develop a national plan for the preservation of astronomical photographic data. They established the Astronomical Photographic Data Archive (APDA), housed at PARI and dedicated to the task of collecting, restoring, preserving and storing photographic data. APDA is also tasked with scanning each image and establishing a database of images that can be accessed via the Internet by the global community of scientists, researchers and students.
Housed in a highly secure building on the PARI campus, the APDA now has a director and a collection of more than 100,000 photographic images. The APDA also possesses two high precision glass plate scanners, GAMMA I and GAMMA II, that were built for NASA and the Space Telescope Science Institute (STScI). The scanners were 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 projects that guide and direct the Hubble Space Telescope. EMC Corporation donated a networked storage system and software that can store and analyze more than 100 terabytes of research data.
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