Image Characteristics

• When X rays hit the film,the Ag halide get exposed in that area.When this film is processed(developer solution) the Exposed Ag halide gets converted into Ag granules.These granules block the passage of light from the View box,making this area radiolucent.
• This overall degree of darkening of exposed area of film is called as Radio graphic density.
• Radio graphic density depends on-

1. Number of photons hitting the exposed area of film-more number of photons means more darkness,more radio graphic density.
2. Subject thickness-If a kid is exposed to the same amount of tube current,voltage and exposure time as used to take cardiographs for an adult than the radio graphic density would be more.
3. subject density-Enamel,dentin,cementum,bone,muscle,fat,air

• Radio graphic contrast is a term that describes the range of radio graphic densities on a radio graph.
• Radio graphic Speed refers to the amount of exposure needed to produce a standard amount of radio graphic density.Film speed is controlled largely by the size of the silver halide grains and their silver content.
• Radio graphic speed is resembled by letters C;D;E;F.F is the ultra speed films requiring least amount of radiation for producing same amount of radio graphic density as that produced in C at the same exposure level.
• Although film speed can also be increased by processing films at slightly higher temperature
• When an unexposed film is processed than it also show some amount of radio graphic density( due to the thickness of  base and development of unexposed Ag halide crystals.This is called as Gross fog.Its equal to 0.2 to 0.3.