Photographic processing

Processing, especially the development stages, requires very close control of temperature, agitation and time.

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

[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.

[6] This process has three additional stages: Chromogenic materials use dye couplers to form colour images.

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.

[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.

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.

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

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).

Some chemistries have been designed with this in mind given the declining demand for film processing in minilabs, often requiring specific handling.

[13] 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.

Therefore, in most large scale processing establishments, exhausted fixer is collected for silver recovery and disposal.

EDTA and other polyamine polycarboxylic acids are used as iron ligands in colour bleach solutions.

A common wetting agent for even drying of processed film uses Union Carbide/Dow Triton X-100 or octylphenol ethoxylate.

[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.

[citation needed] Other popular bleach solutions use potassium dichromate (a hexavalent chromium) or permanganate.

Most non-hardening fixers from major manufacturers are now borate-free, but many film developers still use borate as the buffering agent.

Key stages in production of Ag -based photographs. Two silver halide particles, one of which is impinged with light ( h ν) 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).
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.
A cut-away illustration of a typical light-trap tank used in small scale developing