Species: Feline   |   Classification: Miscellaneous

Formation of the latent image

  • When film is exposed to x-rays and light from intensifying screens, the silver halide crystals are affected forming theLatent image.
  • The latent image is not visible since, although the reduction of silver halide to metallic silver has begun there is not enough of this to appear black.
  • The silver halide in the emulsion is in the form of a crystal lattice containing (negative) bromide ions and (positive) silver ions.
  • Some of these ions are free to move within the crystal lattice.
  • Around the crystal lattice is a "barrier" of electrons.
  • During manufacture a defect is introduced into the crystal   →  sensitivity speck  Radiation physics: sensitivity speck  .
  • When the film is exposed to ionizing radiation:
    • Some of the bromide ions in the lattice emit electrons.
    • These electrons travel rapidly to the sensitivity speck which acts as anelectron trap.
    • The electron trap builds up a negative charge.
    • Silver ions are attracted to the negative charge.
    • The positive charge of the silver ions is neutralized by the electrons in the trap (reduction) and silver atoms are produced.
    • The presence of metallic silver in the crystal lattice makes a break in the electron barrier which surrounded it and it is therefore susceptible to the action of developer.

Development

  • The action of the developer on the exposed silver halide grains amplifies the latent image until there is enough metallic silver to be visible.
  • The developer works by reducing the silver ions, ie donating electrons.
  • Over-development results in reduction of unexposed crystals   →   over-blackening of film.

Constituents of developer

Developing agents

  • Alkaline reducing agents.
  • Phenidone and Hydroquinone.
  • Together these exhibit superadditivity, ie 2+2=6.

Accelerator

  • Sodium carbonate or hydroxide.
  • Increases alkalinity of solution and therefore availability of electrons.
  • pH should be maintained around 10.

If it is too high selectivity decreases   →  fogging.

Restrainer

  • Usually sodium or potassium bromide.
  • Improve developer selectivity.
  • Careful balancing of restrainer and accelerator controls contrast of finished film.
  •  Restrainer not present in replenisher solutions.

Preservative

  • Anti-oxidizing agent.
  • Sodium or potassium sulfide.
  • Oxidation of halide by developer   →   production of byproducts of oxidation   →   rapid deterioration of developer.
  • Preservative combines with oxidation products to reduce this effect.

Developer should be kept covered to prevent aerial oxidation.

Buffers

  • To limit pH changes.

Sequestering agents

  • To prevent precipitation of "hard water" products.

Antiswelling and hardening agents

  • Aldehydes.
  • Now being eliminated from some solutions as implicated in hypersensitivity in users.

Fungicide

  • To prevent fungal growth while chemicals stored in tanks.

Solvent

  • Water.
  • Cheap and readily available!

Developer replenishment

  • Action of developer is greatly depressed by build up of bromides in solution during use.
  • Some of solution is carried out of tank on each film that is processed.
  • These losses can be compensated for by addition of replenisher solution.

Developer should be completely replaced every 4-6 weeks.

Rinsing

  • Rinsing is essential to prevent development continuing in the fixing tank.
  • If this happensdichroic fogcan result.
  • Dichroic fog appears yellow in reflected light and pink in transmitted light.
  • To stop development the developer must be rinsed from the film and/or alkalinity reduced.
  • The rinse bath can be:
    • A static water bath.
    • A water spray.
    • An acid stop bath (containing 2-4% acetic acid).

Fixing

  • After development undeveloped silver halides remain in the emulsion giving the image a milky appearance.
  • If these are not removed they will darken with exposure to light and the image will quickly be ruined.
  • Fixing removes the residual silver halides by converting them to water-soluble compounds which can be rinsed away.
  • Fixing also starts to harden the emulsion so that it will less likely to be damaged by abrasion and drying time will be reduced.

If a film emerges from an automatic processor before being fully dry, it may be the result of a fixing problem, rather than a fault with the dryer.

Fixer constituents

Ammonium thiosulfate

  • Ammonium thiosulfate + silver bromide = ammonium bromide + ammonium salt of monoargento dithiosulfuric acid.
  • These compounds are readily soluble in water.

Acid

  • sulfuric acid.
  • Neutralizes carried-over developer and ensures adequate pH for emulsion hardening.
  • The acid is presented in a separate container otherwise it causes decomposition of the fixing agent.

This effect is reduced once the solution is diluted with water.

Preservative

  • A sulfite (usually sodium).
  • This prevents the acidity of the solution from causing too much decomposition of fixing agent.

Hardener

  • Gluteraldehyde.
  • Now being removed from some formulations as it is a serious pollutant and fumes can be harmful to health.

Buffer

  • To prevent the pH from rising too much.
  •  A high pH would allow precipitation of aluminum hydroxide.

Solvent

  • Water.

Clearing time

  • The period of time required to remove the original milky opacity of the radiograph is known asclearing time.
  • Can be determined by a simple test which indicates when fixer is exhausted.
    • An unprocessed film is partly immersed in fixer and held steady for about 1 minute.
    • A boundary forms on the film which shows clear film on one part and unprocessed on the other.
    • The film is then immersed further so that the boundary is below the fixer and can be clearly seen.
    • The time taken for the boundary to disappear is the clearing time.

This test can be performed in white light.

  • Thefixing timeis twice the clearing time - after this time it is safe to view a processed film in white light.

Fixer replenishment

  • Reduced fixer efficacy is caused by
    • Dilution of the fixer (by water carried over from the rinse bath).
    • Accumulation of silver salts and bromides which are byproducts of fixing.
    • Dilution of fixing agent as it is used up in reactions.
  • This results in
    • Increased clearing and fixing times.
    • Dichroic fog .
    • Reduced film hardening which can increase drying times.
  • In manual processing fixer levels tend to remain constant as rinse water is carried in as well as fixer being carried out.
  • Replenishment of fixer therefore involves removal of a certain volume and replacement with fresh solution.

This requires careful calculation and it is more practical to replace the entire volume of fixer once the clearing time becomes too long.

Washing

  • Fixer products must be removed from the film surface and within the emulsion to prevent fading and discoloration of the image with time   Film faults: poor washing  .
  • The washing time required depends on the length of time for which storage of films is intended (usually several years).
  • Arunningorcirculatingwater washing time of30 minutesis required.
  • If a film emerges "tacky" from an automatic processor it may be that the rinsing tank is empty and the film is contaminated with fixer.

There must be a continual flow of fresh water to the film surface or washing will not be effective.