Species: Canine   |   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 ionising 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.


  • 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 crytals ’ over-blackening of film.
Constituents of developerDeveloping agents
  • Alkaline reducing agents.
  • Phenidone and Hydroquinone.
  • Together these exhibit superadditivity, ie 2+2=6.
  • 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.
  • 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.
  • 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.
  • To limit pH changes.
Sequestering agents
  • To prevent precipitation of "hard water" products.
Anti-swelling and hardening agents
  • Aldehydes.
  • Now being eliminated from some solutions as implicated in hypersensitivity in users.
  • To prevent fungal growth while chemicals stored in tanks.
  • 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 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).


  • 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 likley 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 contituentsAmmonium thiosulfate
  • Ammonium thiosulfate + silver bromide = ammonium bromide + ammonium salt of monoargento dithiosulphuric acid.
  • These compounds are readily soluble in water.
  • 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.
  • A sulfite (usually sodium).
  • This prevents the acidity of the solution from causing too much decomposition of fixing agent.
  • Gluteraldehyde.
  • Now being removed from some formulations as it is a serious pollutant and fumes can be harmful to health.
  • To prevent the pH from rising too much.
  •  A high pH would allow precipitation of aluminium hydroxide.
  • 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.


  • 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.