Photographic processing

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Photographic processing or photographic development is the chemical means by which photographic film or paper is treated after photographic exposure to produce a negative or positive image. Photographic processing transforms the latent image into a visible image, makes this permanent and renders it insensitive to light. [1]

Contents

All processes based upon the gelatin silver process are similar, regardless of the film or paper's manufacturer. Exceptional variations include instant films such as those made by Polaroid and thermally developed films. Kodachrome required Kodak's proprietary K-14 process. Kodachrome film production ceased in 2009, and K-14 processing is no longer available as of December 30, 2010. [2] Ilfochrome materials use the dye destruction process. Deliberately using the wrong process for a film is known as cross processing.

Common processes

Key stages in production of Ag-based photographs. Two silver halide particles, one of which is impinged with light (hn) resulting in the formation of a latent image (step 1). The latent image is amplified using photographic developers, converting the silver halide crystal to an opaque particle of silver metal (step 2). Finally, the remaining silver halide is removed by fixing (step 3). PhotoDeveloptKeySteps.png
Key stages in production of Ag-based photographs. Two silver halide particles, one of which is impinged with light (hν) resulting in the formation of a latent image (step 1). The latent image is amplified using photographic developers, converting the silver halide crystal to an opaque particle of silver metal (step 2). Finally, the remaining silver halide is removed by fixing (step 3).

All photographic processing use a series of chemical baths. Processing, especially the development stages, requires very close control of temperature, agitation and time.

Black and white negative processing

Black and white negative processing is the chemical means by which photographic film and paper is treated after photographic exposure to produce a negative or positive image. Photographic processing transforms the latent image into a visible image, makes this permanent and renders it insensitive to light. Photographic processing.jpg
Black and white negative processing is the chemical means by which photographic film and paper is treated after photographic exposure to produce a negative or positive image. Photographic processing transforms the latent image into a visible image, makes this permanent and renders it insensitive to light.
  1. The film may be soaked in water to swell the gelatin layer, facilitating the action of the subsequent chemical treatments.
  2. The developer converts the latent image to macroscopic particles of metallic silver. [3]
  3. A stop bath, [lower-alpha 1] typically a dilute solution of acetic acid or citric acid, halts the action of the developer. A rinse with clean water may be substituted.
  4. The fixer makes the image permanent and light-resistant by dissolving remaining silver halide. A common fixer is hypo, specifically ammonium thiosulfate. [4]
  5. Washing in clean water removes any remaining fixer. Residual fixer can corrode the silver image, leading to discolouration, staining and fading. [5]

The washing time can be reduced and the fixer more completely removed if a hypo clearing agent is used after the fixer.

  1. Film may be rinsed in a dilute solution of a non-ionic wetting agent to assist uniform drying, which eliminates drying marks caused by hard water. (In very hard water areas, a pre-rinse in distilled water may be required – otherwise the final rinse wetting agent can cause residual ionic calcium on the film to drop out of solution, causing spotting on the negative.)
  2. Film is then dried in a dust-free environment, cut and placed into protective sleeves.

Once the film is processed, it is then referred to as a negative.

The negative may now be printed; the negative is placed in an enlarger and projected onto a sheet of photographic paper. Many different techniques can be used during the enlargement process. Two examples of enlargement techniques are dodging and burning.

Alternatively (or as well), the negative may be scanned for digital printing or web viewing after adjustment, retouching, and/or manipulation.

From a chemical standpoint, conventional black and white negative film is processed by a developer that reduces silver halide to silver metal, exposed silver halide is reduced faster than unexposed silver halide, which leaves a silver metal image. It is then fixed by converting all remaining silver halide into a soluble silver complex, which is then washed away with water. [6] An example of a black and white developer is Kodak D-76 which has bis(4-hydroxy-N-methylanilinium) sulfate with hydroquinone and sodium sulfite.

In graphic art film, also called lithographic film which is a special type of black and white film used for converting images into halftone images for offset printing, a developer containing methol-hydroquinone and sulfite stabilizers may be used. Exposed silver halide oxidizes the hydroquinone, which then oxidizes a nucleating agent in the film, which is attacked by a hydroxide ion and converts it via hydrolysis into a nucleating agent for silver metal, which it then forms on unexposed silver halide, creating a silver image. The film is then fixed by converting all remaining silver halide into soluble silver complexes. [6]

Black and white reversal processing

This process has three additional stages:

  1. Following the first developer and rinse, the film is bleached to remove the developed negative image. This negative image is composed of metallic silver formed in the first developer step. The bleach used here only affects the negative, metallic silver grains, it does not affect the unexposed and therefore undeveloped silver halide. The film then contains a latent positive image formed from unexposed and undeveloped silver halide salts.
  2. The film is fogged, either chemically or by exposure to light.
  3. The remaining silver halide salts are developed in the second developer, converting them into a positive image composed of metallic silver.
  4. Finally, the film is fixed, washed, dried and cut. [7]

Colour processing

Chromogenic materials use dye couplers to form colour images. Modern colour negative film is developed with the C-41 process and colour negative print materials with the RA-4 process. These processes are very similar, with differences in the first chemical developer.

The C-41 and RA-4 processes consist of the following steps:

  1. The colour developer develops the silver negative image by reducing the silver halide crystals that have been exposed to light to metallic silver, this consists of the developer donating electrons to the silver halide, turning it into metallic silver; the donation oxidizes the developer which then activates the dye couplers to form the colour dyes in each emulsion layer, but only does so in the dye couplers that are around unexposed silver halide. [8] [9]
  2. A rehalogenising bleach converts the developed metallic silver into silver halide.
  3. A fixer removes all silver halide by converting it into soluble silver complexes that are then washed away, leaving only the dyes. [10]
  4. The film is washed, stabilised, dried and cut. [11]

In the RA-4 process, the bleach and fix are combined. This is optional, and reduces the number of processing steps. [12]

Transparency films, except Kodachrome, are developed using the E-6 process, which has the following stages:

  1. A black and white developer develops the silver in each image layer.
  2. Development is stopped with a rinse or a stop bath.
  3. The film is fogged in the reversal step.
  4. The fogged silver halides are developed and oxidized developing agents couple with the dye couplers in each layer.
  5. The film is bleached, fixed, washed/rinsed, stabilised and dried as described above. [11]

The Kodachrome process is called K-14. It is very involved, requiring 4 separate developers, one for black and white and 3 for color, reexposing the film in between development stages, 8 or more tanks of processing chemicals, each with precise concentration, temperature and agitation, resulting in very complex processing equipment with precise chemical control. [8]

In some old processes, the film emulsion was hardened during the process, typically before the bleach. Such a hardening bath often used aldehydes, such as formaldehyde and glutaraldehyde. In modern processing, these hardening steps are unnecessary because the film emulsion is sufficiently hardened to withstand the processing chemicals.

A typical chromogenic color film development process can be described from a chemical standpoint as follows: Exposed silver halide oxidizes the developer. [6] The oxidized developer then reacts with color couplers, [6] which are molecules near the exposed silver halide crystals, [6] to create color dyes [6] which ultimately create a negative image, after this the film is bleached, fixed, washed, stabilized and dried. The dye is only created where the couplers are. Thus the development chemical must travel a short distance from the exposed silver halide to the coupler and create a dye there. The amount of dye created is small and the reaction only occurs near the exposed silver halide [10] and thus doesn't spread throughout the entire layer. The developer diffuses into the film emulsion to react with its layers. [10] This process happens simultaneously for all three colors of couplers in the film: cyan (in the red-sensitive layer in the film), magenta(for the green-sensitive layer), and yellow (for the blue-sensitive layer). [6] Color film has these three layers, to be able to perform subtractive color mixing and be able to replicate colors in images.

Further processing

Black and white emulsions both negative and positive, may be further processed. The image silver may be reacted with elements such as selenium or sulphur to increase image permanence and for aesthetic reasons. This process is known as toning.

In selenium toning, the image silver is changed to silver selenide; in sepia toning, the image is converted to silver sulphide. These chemicals are more resistant to atmospheric oxidising agents than silver.

If colour negative film is processed in conventional black and white developer, and fixed and then bleached with a bath containing hydrochloric acid and potassium dichromate solution, the resultant film, once exposed to light, can be redeveloped in colour developer to produce an unusual pastel colour effect.[ citation needed ]

Processing apparatus

Before processing, the film must be removed from the camera and from its cassette, spool or holder in a light-proof room or container.

Small scale processing

A cut-away illustration of a typical light-trap tank used in small scale developing Light trap tank.jpg
A cut-away illustration of a typical light-trap tank used in small scale developing

In amateur processing, the film is removed from the camera and wound onto a reel in complete darkness (usually inside a darkroom with the safelight turned off or a lightproof bag with arm holes). The reel holds the film in a spiral shape, with space between each successive loop so the chemicals may flow freely across the film's surfaces. The reel is placed in a specially designed light-proof tank (called a daylight processing tank or a light-trap tank) where it is retained until final washing is complete.

Sheet films can be processed in trays, in hangers (which are used in deep tanks), or rotary processing drums. Each sheet can be developed individually for special requirements. Stand development, long development in dilute developer without agitation, is occasionally used.

Commercial processing

In commercial, central processing, the film is removed automatically or by an operator handling the film in a light proof bag from which it is fed into the processing machine. The processing machinery is generally run on a continuous basis with films spliced together in a continuous line. All the processing steps are carried out within a single processing machine with automatically controlled time, temperature and solution replenishment rate. The film or prints emerge washed and dry and ready to be cut by hand. Some modern machines also cut films and prints automatically, sometimes resulting in negatives cut across the middle of the frame where the space between frames is very thin or the frame edge is indistinct, as in an image taken in low light. Alternatively stores may use minilabs to develop films and make prints on the spot automatically without needing to send film to a remote, central facility for processing and printing.

Some processing chemistries used in minilabs require a minimum amount of processing per given amount of time to remain stable and usable. Once rendered unstable due to low use, the chemistry needs to be completely replaced, or replenishers can be added to restore the chemistry to a usable state. Some chemistries have been designed with this in mind given the declining demand for film processing in minilabs, often requiring specific handling. Often chemistries become damaged by oxidation. Also, development chemicals need to be thoroughly agitated constantly to ensure consistent results. The effectiveness (activity) of the chemistry is determined through pre-exposed film control strips. [13]

Environmental and safety issues

Many photographic solutions have high chemical and biological oxygen demand (COD and BOD). These chemical wastes are often treated with ozone, peroxide or aeration to reduce the COD in commercial laboratories.

Exhausted fixer and to some extent rinse water contain silver thiosulfate complex ions. They are far less toxic than free silver ion, and they become silver sulfide sludge in the sewer pipes or treatment plant. However, the maximum silver concentration in discharge is very often tightly regulated. Silver is also a somewhat precious resource. Therefore, in most large scale processing establishments, exhausted fixer is collected for silver recovery and disposal.

Many photographic chemicals use non-biodegradable compounds, such as EDTA, DTPA, NTA and borate. EDTA, DTPA, and NTA are very often used as chelating agents in all processing solutions, particularly in developers and washing aid solutions. EDTA and other polyamine polycarboxylic acids are used as iron ligands in colour bleach solutions. These are relatively nontoxic, and in particular EDTA is approved as a food additive. However, due to poor biodegradability, these chelating agents are found in alarmingly high concentrations in some water sources from which municipal tap water is taken. [14] [15] Water containing these chelating agents can leach metal from water treatment equipment as well as pipes. This is becoming an issue in Europe and some parts of the world.[ citation needed ]

Another non-biodegradable compound in common use is surfactant. A common wetting agent for even drying of processed film uses Union Carbide/Dow Triton X-100 or octylphenol ethoxylate. This surfactant is also found to have estrogenic effect and possibly other harms to organisms including mammals.[ citation needed ]

Development of more biodegradable alternatives to the EDTA and other bleaching agent constituents were sought by major manufacturers, until the industry became less profitable when the digital era began.

In most amateur darkrooms, a popular bleach is potassium ferricyanide. This compound decomposes in the waste water stream to liberate cyanide gas.[ citation needed ] Other popular bleach solutions use potassium dichromate (a hexavalent chromium) or permanganate. Both ferricyanide and dichromate are tightly regulated for sewer disposal from commercial premises in some areas.

Borates, such as borax (sodium tetraborate), boric acid and sodium metaborate, are toxic to plants, even at a concentration of 100 ppm. Many film developers and fixers contain 1 to 20 g/L of these compounds at working strength. Most non-hardening fixers from major manufacturers are now borate-free, but many film developers still use borate as the buffering agent. Also, some, but not all, alkaline fixer formulae and products contain a large amount of borate. New products should phase out borates, because for most photographic purposes, except in acid hardening fixers, borates can be substituted with a suitable biodegradable compound.

Developing agents are commonly hydroxylated benzene compounds or aminated benzene compounds, and they are harmful to humans and experimental animals. Some are mutagens. They also have a large chemical oxygen demand (COD). Ascorbic acid and its isomers, and other similar sugar derived reductone reducing agents are a viable substitute for many developing agents. Developers using these compounds were actively patented in the US, Europe and Japan, until the 1990s but the number of such patents is very low since the late-1990s, when the digital era began.

Development chemicals may be recycled by up to 70% using an absorber resin, only requiring periodic chemical analysis on pH, density and bromide levels. Other developers need ion-exchange columns and chemical analysis, allowing for up to 80% of the developer to be reused. Some bleaches are claimed to be fully bio-degradable while others can be regenerated by adding bleach concentrate to overflow (waste). Used fixers can have 60 to 90% of their silver content removed through electrolysis, in a closed loop where the fixer is continually recycled (regenerated). Stabilizers may or may not contain formaldehyde. [16]

See also

Notes

  1. ↑ In modern automatic processing machines, the stop bath is replaced by mechanical squeegee or pinching rollers. These treatments remove much of the carried-over alkaline developer, and the acid, when used, neutralizes the alkalinity to reduce the contamination of the fixing bath with the developer.

Related Research Articles

<span class="mw-page-title-main">Film stock</span> Medium used for recording motion pictures

Film stock is an analog medium that is used for recording motion pictures or animation. It is recorded on by a movie camera, developed, edited, and projected onto a screen using a movie projector. It is a strip or sheet of transparent plastic film base coated on one side with a gelatin emulsion containing microscopically small light-sensitive silver halide crystals. The sizes and other characteristics of the crystals determine the sensitivity, contrast and resolution of the film. The emulsion will gradually darken if left exposed to light, but the process is too slow and incomplete to be of any practical use. Instead, a very short exposure to the image formed by a camera lens is used to produce only a very slight chemical change, proportional to the amount of light absorbed by each crystal. This creates an invisible latent image in the emulsion, which can be chemically developed into a visible photograph. In addition to visible light, all films are sensitive to X-rays and high-energy particles. Most are at least slightly sensitive to invisible ultraviolet (UV) light. Some special-purpose films are sensitive into the infrared (IR) region of the spectrum.

<span class="mw-page-title-main">Kodachrome</span> Brand name of an Eastman Kodak film

Kodachrome is the brand name for a color reversal film introduced by Eastman Kodak in 1935. It was one of the first successful color materials and was used for both cinematography and still photography. For many years, Kodachrome was widely used for professional color photography, especially for images intended for publication in print media.

<span class="mw-page-title-main">Photographic paper</span> Light-sensitive paper used to make photographic prints

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 but other alternatives have also been used.

<span class="mw-page-title-main">E-6 process</span> Chromogenic photographic process

The E-6 process is a chromogenic photographic process for developing Ektachrome, Fujichrome and other color reversal photographic film.

<span class="mw-page-title-main">Photographic developer</span> Chemical(s) which convert a latent image on photographic film to a visible image

In the processing of photographic films, plates or papers, the photographic developer is one or more chemicals that convert the latent image to a visible image. Developing agents achieve this conversion by reducing the silver halides, which are pale-colored, into silver metal, which is black when in the form of fine particles. The conversion occurs within the gelatine matrix. The special feature of photography is that the developer acts more quickly on those particles of silver halide that have been exposed to light. When left in developer, all the silver halides will eventually be reduced and turn black. Generally, the longer a developer is allowed to work, the darker the image.

<span class="mw-page-title-main">Gelatin silver print</span> Photographic process

The gelatin silver process is the most commonly used chemical process in black-and-white photography, and is the fundamental chemical process for modern analog color photography. As such, films and printing papers available for analog photography rarely rely on any other chemical process to record an image. A suspension of silver salts in gelatin is coated onto a support such as glass, flexible plastic or film, baryta paper, or resin-coated paper. These light-sensitive materials are stable under normal keeping conditions and are able to be exposed and processed even many years after their manufacture. The "dry plate" gelatin process was an improvement on the collodion wet-plate process dominant from the 1850s–1880s, which had to be exposed and developed immediately after coating.

Photographic printing is the process of producing a final image on paper for viewing, using chemically sensitized paper. The paper is exposed to a photographic negative, a positive transparency , or a digital image file projected using an enlarger or digital exposure unit such as a LightJet or Minilab printer. Alternatively, the negative or transparency may be placed atop the paper and directly exposed, creating a contact print. Digital photographs are commonly printed on plain paper, for example by a color printer, but this is not considered "photographic printing".

C-41 is a chromogenic color print film developing process introduced by Kodak in 1972, superseding the C-22 process. C-41, also known as CN-16 by Fuji, CNK-4 by Konica, and AP-70 by AGFA, is the most popular film process in use, with most, if not all photofinishing labs devoting at least one machine to this development process.

Photographic fixer is a mixture of chemicals used in the final step in the photographic processing of film or paper. The fixer stabilises the image, removing the unexposed silver halide remaining on the photographic film or photographic paper, leaving behind the reduced metallic silver that forms the image. By fixation, the film or paper is insensitive to further action by light. Without fixing, the remaining silver halide would darken and cause fogging of the image. Fixation is commonly achieved by treating the film or paper with a solution of thiosulfate salt. Popular salts are sodium thiosulfate—commonly called hypo—and ammonium thiosulfate—commonly used in modern rapid fixer formulae. Fixation involves these chemical reactions (X = halide, typically Br−):

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<span class="mw-page-title-main">Latent image</span> An invisible image produced by the exposure of a photosensitive material to light.

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<span class="mw-page-title-main">Sun printing</span>

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<span class="mw-page-title-main">Minilab</span> Photographic film developing and printing system

A minilab is a small photographic developing and printing system or machine, as opposed to large centralized photo developing labs. Many retail stores use film or digital minilabs to provide on-site photo finishing services.

Fogging in photography is the deterioration in the quality of the image or the negative caused either by extraneous light, other electromagnetic radiation, radioactivity or the effects of a processing chemical. It is seen either as deposition of silver or dyes across all or part of the image unrelated to the original exposure. It can be confused with chemical staining that can be produced from poorly compounded developer, contamination of processing baths or poor washing after processing.

Chromogenic photography is photography that works by a chromogen forming a conventional silver image and then replacing it with a dye image. Most films and papers used for color photography today are chromogenic, using three layers, each providing their own subtractive color. Some chromogenic films provide black-and-white negatives, and are processed in standard color developers. In this case, the dyes are a neutral color.

A chromogenic print, also known as a C-print or C-type print, a silver halide print, or a dye coupler print, is a photographic print made from a color negative, transparency or digital image, and developed using a chromogenic process. They are composed of three layers of gelatin, each containing an emulsion of silver halide, which is used as a light-sensitive material, and a different dye coupler of subtractive color which together, when developed, form a full-color image.

Photographic emulsion is a light-sensitive colloid used in film-based photography. Most commonly, in silver-gelatin photography, it consists of silver halide crystals dispersed in gelatin. The emulsion is usually coated onto a substrate of glass, films, paper, or fabric. The substrate is often flexible and known as a film base.

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<span class="mw-page-title-main">Photographic film</span> Film used by film (analog) cameras

Photographic film is a strip or sheet of transparent film base coated on one side with a gelatin emulsion containing microscopically small light-sensitive silver halide crystals. The sizes and other characteristics of the crystals determine the sensitivity, contrast, and resolution of the film. Film is typically segmented in frames, that give rise to separate photographs.

<span class="mw-page-title-main">Mordançage</span>

Mordançage is an alternative photographic process that alters silver gelatin prints to give them a degraded effect. The mordançage solution works in two ways: it chemically bleaches the print so that it can be redeveloped, and it lifts the black areas of the emulsion away from the paper giving the appearance of veils. Once the emulsion is lifted, it can then be removed or manipulated depending on the desired outcome. Areas where the emulsion was removed appear to be in relief. These prints can become oxidized during their creation, further altering the tonality of the image.

References

  1. ↑ Karlheinz Keller et al. "Photography" in Ullmann's Encyclopedia of Industrial Chemistry, 2005, Wiley-VCH, Weinheim. doi : 10.1002/14356007.a20_001
  2. ↑ Matt Warman (December 31, 2010). "Kodachrome film retires, aged 75". London: Telegraph. Archived from the original on January 2, 2011. Retrieved January 2, 2011.
  3. ↑ Wall, 1890, p. 30–63
  4. ↑ Wall, 1890, p. 88–89
  5. ↑ http://sites.tech.uh.edu/digitalmedia/materials/3351/PHOTCHEM.pdf Archived 2018-01-24 at the Wayback Machine [ bare URL PDF ]
  6. 1 2 3 4 5 6 7 Organic Chemistry of Photography. Shinsaku Fujita. doi: https://doi.org/10.1007/978-3-662-09130-2. Springer-Verlag Berlin Heidelberg, 2004. ISBN 978-3-540-20988-1.
  7. ↑ Photographic Almanac, 1956, p. 149–155
  8. 1 2 "Archived copy" (PDF). Archived from the original (PDF) on 2020-03-25. Retrieved 2020-08-15.{{cite web}}: CS1 maint: archived copy as title (link)
  9. ↑ "Archived copy" (PDF). Archived from the original (PDF) on 2020-03-25. Retrieved 2020-08-15.{{cite web}}: CS1 maint: archived copy as title (link)
  10. 1 2 3 "Archived copy" (PDF). Archived (PDF) from the original on 2023-03-22. Retrieved 2023-03-01.{{cite web}}: CS1 maint: archived copy as title (link)
  11. 1 2 Langford, Michael (2000). Basic Photography . Oxford: Focal Press. pp.  210, 215–216. ISBN   0-240-51592-7.
  12. ↑ Photographic Almanac, 1956, p. 429–423
  13. ↑ https://imaging.kodakalaris.com/sites/uat/files/wysiwyg/retailers/chemistry/techpub/cis246.pdf Archived 2020-08-14 at the Wayback Machine [ bare URL PDF ]
  14. ↑ Fuerhacker, M.; Lorbeer, G.; Haberl, R. (30 June 2003). "Emission factors and sources of ethylene-diaminetetraacetic acid in waste water––a case study". Chemosphere. 52 (1): 253–257. Bibcode:2003Chmsp..52..253F. doi:10.1016/S0045-6535(03)00037-7. PMID   12729709.
  15. ↑ Blair-Tyler, Martha (1995). Look Before You Build. Washington: United States Government Printing Office.
  16. ↑ https://www.fujifilm.eu/fileadmin/countries/europe/United_Kingdom/Photofinishing_data_files/Technical_bulletins/TB_C41_E13_09-10.pdf Archived 2020-08-14 at the Wayback Machine [ bare URL PDF ]

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