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History[ edit ] The effect of light in darkening a prepared paper was discovered by Thomas Wedgwood in After the early days of photography, papers have been manufactured on a large scale with improved consistency and greater light sensitivity.
Types of photographic papers[ edit ] Photographic papers fall into one of three sub-categories: Papers used for negative-positive processes. This includes all current black-and-white papers and chromogenic colour papers. Papers used for positive-positive processes in which the "film" is the same as the final image e.
Papers used for positive-positive film-to-paper processes where a positive image is enlarged and copied onto a photographic paper, for example the Ilfochrome process.
Structure[ edit ] All photographic papers consist of a light-sensitive emulsion Copy paper buyers in india, consisting of silver halide salts suspended in a colloidal material - usually gelatin - coated onto a paper, resin coated paper or polyester support.
Black-and-white papers[ edit ] Modern black-and-white papers are coated on a small range of bases; baryta -coated paper, resin-coated paper or polyester. In the past, linen has been used as a base material. This is called a supercoating. Papers without a supercoating are suitable for use with the bromoil process.
These papers require careful processing and handling, especially when wet. However, they are easier to tonehand-colour and retouch than resin-coated equivalents. Since no chemicals or water are absorbed into the paper base, the time needed for processing, washing and drying durations are significantly reduced in comparison to fiber-based papers.
Resin paper prints can be finished and dried within twenty to thirty minutes. Resin-coated papers have improved dimensional stability, and do not curl upon drying. However, the substance used to coat photographic papers is usually not pure barium sulfatebut a mixture of barium and strontium sulfates.
The ratio of strontium to barium differs among commercial photographic papers, so chemical analysis can be used to identify the maker of the paper used to make a print and sometimes when the paper was made.
The brightening occurs because barium sulfate is in the form of a fine precipitate that scatters light back through the silver image layer. In the early days of photography, before baryta layers were used, impurities from the paper fibers could gradually diffuse into the silver layer and cause an uneven loss of sensitivity before development or mottle unevenly discoluor the silver image after development.
The photographic emulsion used for colour photographic materials consists of three colour emulsion layers cyanyellowand magenta along with other supporting layers. The colour layers are sensitised to their corresponding colours.
Although it is commonly believed that the layers in negative papers are shielded against the intrusion of light of a different wavelength than the actual layer by colour filters which dissolve during processing, this is not so.
The colour layers in negative papers are actually produced to have speeds which increase from cyan red sensitive to magenta green sensitive to yellow blue sensitiveand thus when filtered during printing, the blue light is "normalized" so that there is no crosstalk.
After adding enough yellow filtration to make a neutral, the blue sensitivity of the slow cyan layer is "lost". This is the reverse of the usual layer order in colour films. Type R prints, which are no longer made, were positive—positive chromogenic prints.
Black-and-white papers typically use relatively insensitive emulsions composed of silver bromidesilver chloride or a combination of both.
Chloride emulsions are also used for printing-out papers, or POP, which require no further development after exposure. They produce warm-black to neutral image tones by development, which can be varied by using different developers.
Low contrast negatives can be corrected by printing on a contrasty paper; conversely a very contrasty negative can be printed on a low contrast paper.
VC papers permit the selection of a wide range of contrast grades, in the case of the brand leader between 00 and 5. These papers are coated with a mixture of two or three emulsions, all of equal contrast and sensitivity to blue light. However, each emulsion is sensitised in different proportions to green light.
Upon exposure to blue light, all emulsions act in an additive manner to produce a high contrast image. When exposed to green light alone, the emulsions produce a low contrast image because each is differently sensitised to green.Bell Pepper Buyers, Bell Pepper Buyers Suppliers Directory - Find variety Bell Pepper Buyers Suppliers, Manufacturers, Companies from around the World at bell pepper,dried bell pepper,fresh bell pepper, Fresh Capsicum.
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