Фотографическая бумага: состав, виды и применение.

Light sensitive paper used to make photographic prints
light sensitive photographic media
Photographic paper is a paper coated with a light sensitive chemical formula, like photographic film, used for making photographic prints. When photographic paper is exposed to light, it captures a latent image that is then developed to form a visible image; with most papers the image density from exposure can be sufficient to not require further development, aside from fixing and clearing, though latent exposure is also usually present. The light sensitive layer of the paper is called the emulsion. The most common chemistry was based on silver halide (the focus of this page) but other alternatives have also been used. The print image is traditionally produced by interposing a photographic negative between the light source and the paper, either by direct contact with a large negative (forming a contact print) or by projecting the shadow of the negative onto the paper (producing an enlargement). The initial light exposure is carefully controlled to produce a gray scale image on the paper with appropriate contrast and gradation. Photographic paper may also be exposed to light using digital printers such as the LightJet, with a camera (to produce a photographic negative), by scanning a modulated light source over the paper, or by placing objects upon it (to produce a photogram). Despite the introduction of digital photography, photographic papers are still sold commercially. Photographic papers are manufactured in numerous standard sizes, paper weights and surface finishes. A range of emulsions are also available that differ in their light sensitivity, colour response and the warmth of the final image. Color papers are also available for making colour images.

The effect of light in darkening a prepared paper was discovered by Thomas Wedgwood in 1802. Photographic papers have been used since the beginning of all negative–positive photographic processes as developed and popularized by William Fox Talbot (Great Britain/1841 calotype). After the early days of photography, papers have been manufactured on a large scale with improved consistency and greater light sensitivity.

All photographic papers consist of a light sensitive emulsion, consisting of silver halide salts suspended in a colloidal material – usually gelatin coated onto a paper, resin coated paper or polyester support. In black and white papers, the emulsion is normally sensitised to blue and green light, but is insensitive to wavelengths longer than 600 nm in order to facilitate handling under red or orange safelighting. In chromogenic colour papers, the emulsion layers are sensitive to red, green and blue light, respectively producing cyan, magenta and yellow dye during processing.

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. Tints are sometimes added to the baryta to add subtle colour to the final print; however most modern papers use optical brighteners to extend the paper's tonal range. Most fiber based papers include a clear hardened gelatin layer above the emulsion which protects it from physical damage, especially during processing. This is called a supercoating. Papers without a super coating are suitable for use with the bromoil process. The baryta layer has two functions 1) to brighten the image and 2) to prevent chemicals adsorbed on the fibers from infiltrating the gelatin layer. 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 discolour) the silver image (after development). The emulsion layers can include the colour dyes, as in Ilfochrome; or they can include colour couplers, which react with colour developers to produce colour dyes, as in type C prints or chromogenic negative–positive prints. Type R prints, which are no longer made, were positive–positive chromogenic prints.

The emulsion contains light sensitive silver halide crystals suspended in gelatin. Black and white papers typically use relatively insensitive emulsions composed of agb silver bromide, silver chloride or a combination of both. The silver halide used affects the paper's sensitivity and the image tone of the resulting print. These insensitive papers are suitable for contact printing, and yield warm toned images by development. Chloride emulsions are also used for printing out papers, or POP, which require no further development after exposure.

Containing a blend of silver chloride and silver bromide salts, these emulsions produce papers sensitive enough to be used for enlarging. They produce warm black to neutral image tones by development, which can be varied by using different developers. Filters in the enlarger's light path are a common method of achieving this control. Magenta filters absorb green and transmit blue and red, while yellow filters absorb blue and transmit green and red. The contrast of photographic papers can also be controlled during processing or by the use of bleaches or toners.

Panchromatic black and white photographic printing papers are sensitive to all wavelengths of visible light. They were designed for the printing of full tone black and white images from colour negatives; this is not possible with conventional orthochromatic papers. Panchromatic papers can also be used to produce paper negatives in large format cameras. These materials must be handled and developed in near complete darkness. Kodak Panalure Select RC is an example of a panchromatic black and white paper; it was discontinued in 2005.

Numerous photo sensitive papers that do not use silver chemistry exist. Most are hand made by enthusiasts but cyanotype prints are made on what was commonly sold as blueprint paper. Certain precious metal including platinum and other chemistries have also been in common use at certain periods.

The longevity of any photographic print media will depend upon the processing, display and storage conditions of the print.

Fixing must convert all non image silver into soluble silver compounds that can be removed by washing with water. Washing must remove these compounds and all residual fixing chemicals from the emulsion and paper base. A hypo clearing solution, also referred to as hypo clearing agent, HCA, or a washing aid, and which can consist of a 2% solution of sodium sulfite, can be used to shorten the effective washing time by displacing the thiosulfate fixer, and the byproducts of the process of fixation, that are bound to paper fibers. Toners are sometimes used to convert the metallic silver into more stable compounds. Commonly used archival toners are: selenium, gold and sulfide. Prints on fiber based papers that have been properly fixed and washed should last at least fifty years without fading. Some alternative non silver processes – such as platinum prints – employ metals that are, if processed correctly, inherently more stable than gelatin silver prints.

For colour images, Ilfochrome is often used because of its clarity and the stability of the colour dyes.