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 .
- 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.
- 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
- 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.
- To prevent precipitation of "hard water" products.
Antiswelling and hardening agents
- Now being eliminated from some solutions as implicated in hypersensitivity in users.
- To prevent fungal growth while chemicals stored in tanks.
- Cheap and readily available!
- 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 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.
- Ammonium thiosulfate + silver bromide = ammonium bromide + ammonium salt of monoargento dithiosulfuric 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.
- 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 aluminum hydroxide.
- 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.
- 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 .
- 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.