Photographic developer

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Developer powder using genol-hydroquinone. Boite avec 5 doses de revelateur universel en poudre au genol-hydroquinone de la societe Guilleminot 02.jpg
Developer powder using genol-hydroquinone.

In the processing of photographic films, plates or papers, the photographic developer (or just 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 a fine particle). [1] The conversion occurs within the gelatine matrix. The special feature of photography is that the developer acts more quickly on those particles of silver halides that have been exposed to light. Paper left in developer will eventually reduce all the silver halides and turn black. Generally, the longer a developer is allowed to work, the darker the image.

Contents

Chemical composition of developers

The developer typically consists of a mixture of chemical compounds prepared as an aqueous solution. For black-and-white photography, three main components of this mixture are: [2] :115

Notable standard formulas include Eastman Kodak D-76 film developer, D-72 print developer, and D-96 motion picture negative developer. [3]

Hydroquinone is superadditive with metol, meaning that it acts to "recharge" the metol after it has been oxidised in the process of reducing silver in the emulsion. Sulfite in a developer not only acts to prevent aerial oxidation of the developing agents in solution, it also facilitates the regeneration of metol by hydroquinone (reducing compensation and adjacency effects) and in high enough concentrations acts as a silver halide solvent. The original lithographic developer contained formaldehyde (often added as paraformaldehyde powder) in a low sulfite/bisulfite solution.

Most developers also contain small amounts of potassium bromide to modify and restrain the action of the developer [2] :218-219 to suppress chemical fogging. Developers for high contrast work have higher concentrations of hydroquinone and lower concentrations of metol and tend to use strong alkalis such as sodium hydroxide to push the pH up to around pH 11 to 12.

Metol is difficult to dissolve in solutions of high salt content and instructions for mixing developer formulae therefore almost always list metol first. It is important to dissolve chemicals in the order in which they are listed. Some photographers add a pinch of sodium sulfite before dissolving the metol to prevent oxidation, but large amounts of sulfite in solution will make it very slow for metol to dissolve.

Because metol is relatively toxic and can cause skin sensitisation, modern commercial developers often use phenidone or dimezone S (4-hydroxymethyl-4-methyl-1-phenyl-3-pyrazolidone) instead. Hydroquinone can also be toxic to the human operator as well as environment; some modern developers replace it with ascorbic acid, or vitamin C. This, however, suffers from poor stability. Ascorbate developers may have the advantage of being compensating and sharpness-enhancing, as oxidation by-products formed during development are acidic, meaning they retard development in and adjacent to areas of high activity. This also explains why ascorbate developers have poor keeping properties, as oxidised ascorbate is both ineffective as a developing agent and lowers the pH of the solution, making the remaining developing agents less active. Recently, claims for practical methods to improve the stability of ascorbate developers have been made by several experimenters [ citation needed ].

Other developing agents in use are p-aminophenol, glycin (N-(4-hydroxyphenyl)glycine), pyrogallol, and catechol. When used in low sulfite developer composition, the latter two compounds cause gelatin to harden and stain in the vicinity of developing grains. Generally, the optical density of the stain increases in the heavily exposed (and heavily developed) area. This is a property that is highly sought after by some photographers because it increases negative contrast in relation to density, meaning that highlight detail can be captured without "blocking" (reaching high enough density that detail and tonality are severely compromised). Hydroquinone shares this property. However, the staining effect only appears in solutions with very little sulfite, and most hydroquinone developers contain substantial quantities of sulfite.

In the early days of photography, a wide range of developing agents were used, including chlorohydroquinone, ferrous oxalate, [2] :131 hydroxylamine, ferrous lactate, ferrous citrate, Eikonogen, atchecin, antipyrin, acetanilid and Amidol (which unusually required mildly acidic conditions).

Developers also contain water softening agent to prevent calcium scum formation (e.g., EDTA salts, sodium tripolyphosphate, NTA salts, etc.).

The original lithographic developer was based upon a low sulfite/bisulfite developer with formaldehyde (added as the powder paraformaldehyde). The very low sulfite, high hydroquinone and high alkalinity encouraged "infectious development" (exposed developing silver halide crystals collided with unexposed silver halide crystals, causing them to also reduce) which enhanced the edge effect in line images. These high energy developers had a short tray life, but when used within their tray life provided consistent usable results.

Modern lithographic developers contain hydrazine compounds, tetrazolium compounds and other amine contrast boosters to increase contrast without relying on the classic hydroquinone-only lithographic developer formulation. The modern formulae are very similar to rapid access developers (except for those additives) and therefore they enjoy long tray life. However, classic lithographic developers using hydroquinone alone suffers very poor tray life and inconsistent results.

Development

The developer selectively reduces silver halide crystals in the emulsion to metallic silver, but only those having latent image centres created by action of light. [4] The light sensitive layer or emulsion consists of silver halide crystals in a gelatin base. Two photons of light must be absorbed by one silver halide crystal to form a stable two atom silver metal crystal. The developer used generally will only reduce silver halide crystals that have an existing silver crystal. Faster exposure or lower light level films usually have larger grains because those images capture less light. Fine grain films, like Kodachrome, require more light to increase the chance that the halide crystal will absorb at least two quanta of light as they have a smaller cross sectional size. Therefore, silver halide crystal size is proportional to film speed. The metallic silver image has dark (black) appearance. Once the desired level of reduction is achieved the development process is halted by washing in a dilute acid and then the undeveloped silver halide is removed by dissolving it in a thiosulfate solution, a process called fixing . Most commercial film developers use a dual solution or "push" (pushes the films speed) development (compensating developer, like Diafine) procedure where the reducing agent e.g. hydroquinone solution soaks into and swells the gelatin then the film is introduced into the alkaline solution which activates (lowers reduction potential) of the developer. The areas with the most light exposure use up the tiny amount of developer in the gelatin and stop making silver crystal before the film at that point is totally opaque. The areas that received the least light continue to develop because they haven't used up their developer. There is less contrast, but time is not critical and films from several customers and different exposures will develop satisfactorily.

The time over which development takes place, and the type of developer, affect the relationship between the density of silver in the developed image and the quantity of light. This study is called sensitometry and was pioneered by F Hurter & V C Driffield in the late 19th century. [5]

Colour development

In colour and chromogenic black-and-white photography, a similar development process is used except that the reduction of silver simultaneously oxidizes the paraphenylene colour developing agent which then takes part in the production of dye-stuffs in the emulsion by reacting with the appropriate couplers. There are three distinct processes used here. The C-41 process is used for almost all colour negative films and in this process dye couplers in the emulsion react with the oxidized colour developing agent in the developer solution to generate the visible dyes. An almost identical process is then used to produce colour prints from films. The developing agents used are derivates of paraphenylene diamine.

In colour negative films, [6] there are 3 types of dye couplers. There are the normal cyan, magenta and yellow dye forming couplers, but also there is a magenta coloured cyan masking coupler and a yellow coloured magenta masking coupler. These form respectively normal cyan dye, and magenta dye, but form an orange positive mask to correct colour. In addition, there is a third type of coupler called a DIR (Developer Inhibitor Release) coupler. This coupler releases a powerful inhibitor during dye formation, which affects edge effects and causes effects between layers to enhance overall image quality.

Reversal film development

In Ektachrome-type (E-6 process) [7] transparencies, the film is first processed in an unusual developer containing phenidone and Hydroquinone-monosulfonate. This black and white developer is used for 6:00 at 100.4°F (38°C), with more time yielding "push" processing to increase the apparent film speed by reducing the Dmax, or maximum density. The first developer is the most critical step in Process E-6. The solution is essentially a black-and-white film developer, because it forms only a negative silver image in each layer of the film; no dye images are yet formed. Then, the film goes directly into the first wash for 2:00 at 100 °F, which acts as a controlled stop bath. Next, the film goes into the reversal bath. This step prepares the film for the colour developer step. In this reversal bath, a chemical reversal agent is absorbed into the emulsion, with no chemical reaction taking place until the film enters the colour developer. The reversal process can also be carried out using 800 footcandle-seconds of light, which is used by process engineers to troubleshoot reversal bath chemistry problems.

Next, the film is developed to completion in the colour developer bath, which contains CD-3 as the colour developing agent. When film enters the colour developer, the reversal agent absorbed by the emulsion in the reversal bath chemically fogs (or "exposes") the unexposed silver halide (if it has not already been fogged by light in the previous step). The colour developer acts on the chemically exposed silver halide to form a positive silver image. However, the metallic silver image formed in the first developer, which is a negative image, is not a part of the reaction that takes place in this step. What is being reacted in this stage is the "leftover" of the negative image, that is, a positive image. As the colour development progresses, metallic silver image is formed, but more importantly, the colour developing agent is oxidised. Oxidised colour developer molecules react with the couplers to form colour dyes in situ. Thus colour dye is formed at the site of development in each of the three layers of the film. Each layer of the film contains different couplers, which react with the same oxidised developer molecules but form different colour dyes. Next, the film goes into the pre-bleach (formerly conditioner) bath, which has a precursor of formaldehyde (as a dye preservative) and EDTA to "kick off" the bleach. Next, the film goes into a bleach solution. The bleach converts metallic silver into silver bromide, which is converted to soluble silver compounds in the fixer. The C-41 color negative process introduced in 1972 uses ferric EDTA. Reversal processes have used ferric EDTA at least since the introduction of the E-6 process in 1976. For Kodachrome ferric EDTA is used at least in the current K-14 process. During bleaching, ferric EDTA is changed to ferrous EDTA before fixing, and final wash.

Fe3+
EDTA + Ag + Br
→ Fe2+
EDTA + AgBr

Previously potassium ferricyanide was often used as bleach. The most common processing chemistry for such films is E6, derived from a long line of developers produced for the Ektachrome range of films.

Ektachrome papers are also available.

Standard black and white stock can also be reversal processed to give black and white slides. [8] After 'first development,' the initial silver image is then removed (e.g. using a potassium bichromate/sulfuric acid bleach, which requires a subsequent "clearing bath" to remove the chromate stain from the film). The unfixed film is then fogged (physically or chemically) and 'second-developed'. However, the process works best with slow films such as Ilford Pan-F processed to give a high gamma. Kodak's chemistry kit for reversing Panatomic-X ("Direct Positive Film Developing Outfit") used sodium bisulfate in place of sulfuric acid in the bleach, and used a fogging developer that was inherently unstable, and had to be mixed and used within a two-hour period. (If two rolls, the maximum capacity of a single pint of redeveloper, were to be processed in succession, the redeveloper had to be mixed while the first roll was in the first developer.)

Proprietary methods

The K-14 process for Kodachrome films involves adding all the dyes to the emulsion during development. Special equipment was needed to process Kodachrome. Since 2010, there has been no commercial entity that processes Kodachrome anywhere in the world.

In colour print development the Ilfochrome, or Cibachrome, process also uses a print material with the dye-stuffs present and which are bleached out in appropriate places during developing. The chemistry involved here is wholly different from C41 chemistry; (it uses azo-dyes which are much more resistant to fading in sunlight).

Related Research Articles

Film stock Medium used for recording motion pictures

Film stock is an analog medium that is used for recording motion pictures or animation. 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.

Kodachrome 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. Because of its complex processing requirements, the film was sold process-paid in the United States until 1954 when a legal ruling prohibited this. Elsewhere, this arrangement continued.

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.

Photographic paper paper coated with a light-sensitive chemical formula, used for making photographic prints

Photographic paper is a paper coated with a light-sensitive chemical formula, 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 salts but other alternatives have also been used.

Reversal film type of photographic film that produces a positive image on a transparent base

In photography, reversal film is a type of photographic film that produces a positive image on a transparent base. The film is processed to produce transparencies or diapositives instead of negatives and prints. Reversal film is produced in various sizes, from 35 mm to roll film to 8×10 inch sheet film.

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

Gelatin silver process photographic process

The gelatin silver process is the photographic process used with currently available black-and-white films and printing papers. 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. This is in contrast to 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 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 photofinishing labs devoting at least one machine to this development process.

Photographic fixer is a mix 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):

A silver halide is one of the chemical compounds that can form between the element silver and one of the halogens. In particular, bromine, chlorine, iodine and fluorine may each combine with silver to produce silver bromide (AgBr), silver chloride (AgCl), silver iodide (AgI), and three forms of silver fluoride, respectively.

Sodium sulfite chemical compound

Sodium sulfite (sodium sulphite) is a soluble sodium salt of sulfurous acid (sulfite) with the chemical formula Na2SO3. It is used as a preservative to prevent dried fruit from discoloring, and for preserving meats, and is used in the same way as sodium thiosulfate to convert elemental halogens to their respective hydrohalic acids, in photography and for reducing chlorine levels in pools. In boiler systems, sulfite and bisulfite are the most commonly employed oxygen scavengers used to prevent pitting corrosion. Sodium sulfite is also a byproduct of sulfur dioxide scrubbing, a part of the flue-gas desulfurization process.

Latent image An invisible image produced by the exposure of a photosensitive material to light.

A latent image is an invisible image produced by the exposure to light of a photosensitive material such as photographic film. When photographic film is developed, the area that was exposed darkens and forms a visible image. In the early days of photography, the nature of the invisible change in the silver halide crystals of the film's emulsion coating was unknown, so the image was said to be "latent" until the film was treated with photographic developer.

Photographic print toning method of changing the color of black-and-white photographs

In photography, toning is a method of changing the color of black-and-white photographs. In analog photography, it is a chemical process carried out on silver-based photographic prints. This darkroom process cannot be performed with a color photograph. The effects of this process can be emulated with software in digital photography. Sepia is considered a form of Black and White or Monochromatic Photography.

Fogging in photography is the deterioration in the quality of the image caused either by extraneous light or the effects of a processing chemical.

Chromogenic refers to photographic processes that work by forming a conventional silver image and then replacing it with a dye image. Most films and papers used for color photography today are chromogenic.

A chromogenic print, also known as 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.

K-14 process developing process for Kodaks Kodachrome transparency film

K-14 was the most recent version of the developing process for Kodak's Kodachrome transparency film before its discontinuation. It superseded previous versions of the Kodachrome process used with older films.

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.

Photographic film sheet of plastic coated with light-sensitive chemicals

Photographic film 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.

References

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  2. 1 2 3 Wall, E.J. (1890). Dictionary of Photography. London: Hassel, Watson and Viney.
  3. "Full text of "Kodak Data Book Volume 1 & 2"". Accessed 30 September 2017
  4. Woodworth, Chuck. "How Film Works". BYG Publishing. Retrieved 14 March 2013.
  5. Papagiannakis, E. E. Krieziz, D. P. Chrissoulidis & A. G. (1992). Electromagnetics and optics. River Edge, N.J.: World Scientific. p. 397. ISBN   978-9810208493.
  6. Photographic Almanac, 1956, p. 429–423
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  8. Advanced Photography, 1980, p. 345