what is the chemistry of photography
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Photographic processing or development is the chemical means by which photographic film or paper is treated after photographic exposure to produce a negative or positive image.
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Chemistry Of Photography :
It has long been known that certain substances, when illuminated, undergo permanent visible changes. In the early part of the nineteenth century, these materials were sometimes used to make photogenic drawings.
The first known photograph, made in 1826. It shows the courtyard outside the room of Joseph-Nicéphore Niepce.
by exposing them to sunlight through patterned masks. The most light-sensitive compounds are silver salts, and the photography that prospered in the second half of the nineteenth and throughout the twentieth century was based almost entirely on the use of silver halides.
Early Photography
Practical photographic processes were devised in the 1830s by Louis-Jacques-Mandé Daguerre in France and by William Henry Fox Talbot in England. In Daguerre's method, a silver iodide-coated silver plate was exposed to light in a camera, whereby the exposed silver iodide was decomposed to metallic silver and iodine. A clear image was obtained by treating the plate with mercury vapor (which amalgamated the silver) and by rinsing it in a strong salt solution to remove the remaining silver iodide. A positive image could be viewed by holding this "Daguerreotype" in oblique lighting with a dark background, so that the amalgamated silver zones appeared bright and the silver plate appeared dark.
Talbot's procedure consisted of washing paper successively in baths of saltwater and silver nitrate solution, thus depositing silver chloride in the fibers of the paper. The still wet paper was then exposed in a camera until a dark silver image appeared in the light-struck regions, and the remaining silver chloride was removed by washing with a concentrated salt solution or a sodium thiosulfate solution. By waxing or oiling the negative sheet, Talbot made the paper transparent, and then by making an exposure of diffuse light through the negative onto another sensitized sheet, he produced a positive image. An unlimited number of copies of a photograph could thus be made from any one negative.
Improvements in Talbot's Method
Both Daguerre's and Talbot's methods were inconvenient because they required long exposures in the camera—sometimes as long as 60 minutes. In 1840 Talbot greatly improved his process. He found that a very short camera exposure (about 1/60 of that required to give a visible image) left an invisible "latent" image on the sensitized paper. The latent image was then "developed" into a visible image by treatment with a solution of gallic acid and silver nitrate. This modification, together with the negative/positive feature, made Talbot's process so superior that it has survived, in its general form, to the present day. The main difference between Talbot's process and modern photographic practice is that now the silver halide, in the form of approximately micron-sized crystals or "grains," is suspended in gelatin. The gelatin emulsion is coated as a thin film on glass plates or flexible sheets of plastic or paper.
Mechanism of the Photographic Process
When a photon is absorbed by a silver halide grain, an electron is ejected from a halide ion and temporarily held at some site in the crystal. A silver ion can migrate to the site and combine with the electron to form a silver atom. The atom is not stable; it can decompose back into a silver ion and a free electron. However, during its lifetime, the atom can trap a second electron if one becomes available. If this second electron remains trapped until the arrival of a second silver ion, a two-atom cluster forms. This buildup of a silver cluster can continue as long as photoelectrons are available. The smallest cluster corresponding to a stable latent image speck is believed to consist of three or four silver atoms. Specks of this size or greater on the crystal surface can catalyze the subsequent action of a developer.
It has long been known that certain substances, when illuminated, undergo permanent visible changes. In the early part of the nineteenth century, these materials were sometimes used to make photogenic drawings.
The first known photograph, made in 1826. It shows the courtyard outside the room of Joseph-Nicéphore Niepce.
by exposing them to sunlight through patterned masks. The most light-sensitive compounds are silver salts, and the photography that prospered in the second half of the nineteenth and throughout the twentieth century was based almost entirely on the use of silver halides.
Early Photography
Practical photographic processes were devised in the 1830s by Louis-Jacques-Mandé Daguerre in France and by William Henry Fox Talbot in England. In Daguerre's method, a silver iodide-coated silver plate was exposed to light in a camera, whereby the exposed silver iodide was decomposed to metallic silver and iodine. A clear image was obtained by treating the plate with mercury vapor (which amalgamated the silver) and by rinsing it in a strong salt solution to remove the remaining silver iodide. A positive image could be viewed by holding this "Daguerreotype" in oblique lighting with a dark background, so that the amalgamated silver zones appeared bright and the silver plate appeared dark.
Talbot's procedure consisted of washing paper successively in baths of saltwater and silver nitrate solution, thus depositing silver chloride in the fibers of the paper. The still wet paper was then exposed in a camera until a dark silver image appeared in the light-struck regions, and the remaining silver chloride was removed by washing with a concentrated salt solution or a sodium thiosulfate solution. By waxing or oiling the negative sheet, Talbot made the paper transparent, and then by making an exposure of diffuse light through the negative onto another sensitized sheet, he produced a positive image. An unlimited number of copies of a photograph could thus be made from any one negative.
Improvements in Talbot's Method
Both Daguerre's and Talbot's methods were inconvenient because they required long exposures in the camera—sometimes as long as 60 minutes. In 1840 Talbot greatly improved his process. He found that a very short camera exposure (about 1/60 of that required to give a visible image) left an invisible "latent" image on the sensitized paper. The latent image was then "developed" into a visible image by treatment with a solution of gallic acid and silver nitrate. This modification, together with the negative/positive feature, made Talbot's process so superior that it has survived, in its general form, to the present day. The main difference between Talbot's process and modern photographic practice is that now the silver halide, in the form of approximately micron-sized crystals or "grains," is suspended in gelatin. The gelatin emulsion is coated as a thin film on glass plates or flexible sheets of plastic or paper.
Mechanism of the Photographic Process
When a photon is absorbed by a silver halide grain, an electron is ejected from a halide ion and temporarily held at some site in the crystal. A silver ion can migrate to the site and combine with the electron to form a silver atom. The atom is not stable; it can decompose back into a silver ion and a free electron. However, during its lifetime, the atom can trap a second electron if one becomes available. If this second electron remains trapped until the arrival of a second silver ion, a two-atom cluster forms. This buildup of a silver cluster can continue as long as photoelectrons are available. The smallest cluster corresponding to a stable latent image speck is believed to consist of three or four silver atoms. Specks of this size or greater on the crystal surface can catalyze the subsequent action of a developer.
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