Photographic paper is paper coated with light-sensitive chemicals, used for making photographic prints. Photographic paper differs from photo papers: specially coated papers for use in inkjet or laser printers to make digital prints.
Photographic paper is exposed to light in a controlled manner, either by placing a negative in contact with the paper directly to produce a contact print, by using an enlarger in order to create a latent image, by exposing in some types of camera to produce a photographic negative, or by placing objects upon it to produce photograms. Photographic papers are subsequently developed using the gelatin-silver process to create a visible image.
The effect of light in darkening a prepared paper was discovered by M. Charles in 1800 or by Thomas Wedgwood in 1802.
Traditional photographic papers are still sold commercially today.
Photographic papers fall into one of three sub-catergories:
Papers used for positive-positive processes in which the "film" is the same as the final image (e.g. the Polaroid process).
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.
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.
Fibre-based (FB or Baryta) photographic papers consist of a paper base coated with a baryta. 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 fibre-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 supercoating are suitable for use with the bromoil process.
Fibre-based papers are generally chosen as a medium for high-quality prints for exhibition, display and archiving purposes. These papers require careful processing and handling, especially when wet. However, they are easier to tone, hand-colour and retouch than resin-coated equivalents.
The paper base of resin-coated papers is sealed by two polyethylene layers, making it impenetrable to liquids. 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 fibre-based papers. Resin paper prints can be finished and dried within ten to fifteen minutes. Resin-coated papers have improved dimensional stability, and do not curl upon drying.
All colour photographic materials available today are coated on either RC (resin coated) paper or on solid polyester. The photographic emulsion used for colour photographic materials consists of three colour emulsion layers (cyan, yellow and 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 color layers in negative papers are actually produced to have speeds which increase from cyan (red sensitive) to magenta (green sensitive) to yellow (blue sensitive), and thus when filtered during printing, the blue light is "normalized" so that there is no crosstalk. Therefore the yellow (blue sensitive) layer is nearly ISO 100 while the cyan (red) layer is about ISO 25. After adding enough yellow filtration to make a neutral, the blue sensitivity of the slow cyan layer is "lost".
In negative-positive print systems, the blue sensitive layer is on the bottom, and the cyan layer is on the top. This is the reverse of the usual layer order in color films.
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 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.
Popular in the past, chloride papers are nowadays unusual; a single manufacturer produces this this material. 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.
Papers with pure silver bromide emulsions are sensitive and produce neutral black or 'cold' blue-black image tones.
Fixed grade - or graded - black-and-white papers are available in different contrast grades, numbered 0 to 5, with 0 being the softest, or least contrasty paper grade and 5 being the hardest, or most contrasty paper grade. 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. Because of decreased demand, many extreme paper grades are now discontinued.
Variable-contrast - or VC - papers permit the selection of any contrast grade between 00 and 5. These papers are coated with a mixture of 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. By varying the ratio of blue to green light, the contrast of the print can be continuously varied between these extremes, creating all contrast grades from 00 to 5. 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.
The actual life span of any given photographic paper will vary with the environment the paper is stored within and how well the paper was processed.
Fibre-based black-and-white photographic paper that has been processed archivally (i.e. has been fixed and watered properly) is considered archivally "stable" and should last at least 70 years (the life-span of a typical resin-coated print is usually around 30-40 years.) Some special processes include photographic emulsions that are, if processed correctly, inherently more stable than the paper base they are printed upon, such as platinum prints.
For color images, Ilfochrome is often used because of its clarity and the stability of the colour dyes.