All photographic films are made up in basically the same way. The film base is usually plastic such as tri-acetate or polyester which is coated with a light sensitive emulsion. The emulsion consists of gelatin containing light sensitive silver halide crystals such as silver bromide and silver chloride. In practice the film will consist of many other layers. Photographic emulsion is not a true emulsion, it is a dispersion of small solid particles in a liquid medium which is then allowed to cool and set.
The light sensitive crystals are prepared by the combination of silver-Ag- and a halogen.
Due to the very low solubility of silver halides mixing aqueous solutions of silver ions and halide ions will result in the precipitation of silver halide crystals.
e.g. silver nitrate (AgNO3) + potassium bromide -----> silver bromide (AgBr) + potassium nitrate (KNO3)
Ag+ (silver ion in solution) + Br- (bromide ion in solution) --------> Ag+Br- (silver bromide crystal)
Silver bromide is a lattice crystal containing millions of pairs of ions.
The Latent image.
It is not fully understood exactly what happens during exposure but the energy released when a photon of light strikes a silver halide crystal frees an electron from the bromide ion. The former bromide ion is released from the crystal as bromine and is absorbed by the gelatin. The free electron moves through the crystal to a 'sensitivity speck' caused by imperfections in the crystal structure or created during the sensitizing process during manufacture. .This now negatively charged speck attracts positive silver ions which are neutralised to form silver atoms. If enough silver atoms form at a single point then a latent image is created. The latent image is not visible, even under a microscope so the only way to tell if it is present is to chemically develop the film to reveal the image.
During development the developing agent supplies electrons to the latent image thus attracting and neutralising silver ions to produce metallic silver which will eventually form a visible image. The latent image acts as a catalyst encouraging development to take place faster in exposed areas. Development takes place in both exposed and unexposed areas of the film just at different rates.
Developing agents: Metol, Phenidone, Hydroquinone.
When the predetermined development time has been reached the film is moved from the developer to a 'stop bath' which neutralises the developer and prevents any further development of the image from taking place.
Developers work most effectively in an alkaline environment which is why an acid stop bath is used.
Stop bath: 1% solution acetic acid.
After development the emulsion still contains unexposed and undeveloped silver halides. The film will look cloudy or milky and given exposure to light the remaining silver halides will be reduced to silver. The fixer, commonly sodium thiosulphate, converts the unexposed silver halide to soluble salts which can be washed out of the emulsion.
The processed film is washed thoroughly to remove any chemical residue before being dried.