1. X-RAY FILM

2. COMPOSITION X-ray film has two principal components:
Emulsion: The emulsion, which is sensitive to x rays and visible light, records the radiographic image.
base.: The base is a plastic supporting material onto which the emulsion is coated.

3. Emulsion:
The two principal components of emulsion are silver halide grains, which are sensitive to x radiation and visible light, and a vehicle matrix in which the crystals are suspended. The silver halide grains are composed primarily of crystals of silver bromide.
The silver halide grains are flat, tabular crystals with a mean diameter of about 1.8 µm

4. The tabular grains are oriented parallel with the film surface to offer a large cross-sectional area to the x-ray beam. The silver halide grains are suspended in a surrounding vehicle that is applied to both sides of the supporting base.
During film processing the vehicle absorbs processing solutions, allowing the chemicals to reach and react with the silver halide grains. An additional layer of vehicle is added to the film emulsion as an overcoat. This barrier helps protect the film from damage by scratching, contamination, or pressure from rollers when an automatic processor is used.

6. Film emulsions are sensitive to both x-ray photons and visible light
Film emulsions are sensitive to both x-ray photons and visible light. Film intended to be exposed by x rays is called direct exposure film . All intraoral dental film is direct exposure film. Screen film is used with intensifying screens that emit visible light. Screen film and intensifying screens are used for extraoral projections, such as panoramic and cephalometric radiographs.

7. Base:
The function of the film base is to support the emulsion. The base for dental x-ray film is made of polyester polyethylene terephthalate, which provides the proper degree of flexibility to allow easy handling of the film. The film base must also withstand exposure to processing solutions without becoming distorted. The base is uniformly translucent and casts no pattern on the resultant radiograph.

8. INTRAORAL X-RAY FILM
Intraoral dental x-ray film is made as a double-emulsion film—that is, both sides of the base are coated with an emulsion. With a double layer of emulsion, less radiation is required to produce an image. Direct exposure film is used for intraoral examinations because it provides higher resolution images than screen- film combinations. Some diagnostic tasks, such as detection of incipient caries or early periapical disease, require this higher resolution.

9. A thin lead foil backing with an embossed pattern is between the wrappers in the film packet (behind the film, away from the tube), its purposes:
It shields the film from backscatter (secondary) radiation, which fogs the film and reduces subject contrast (image quality).
It also reduces patient exposure by absorbing some of the residual x-ray beam.

11. SCREEN FILM
The extraoral projections used most frequently in dentistry are panoramic and cephalometric views. For these projections, screen film is used with intensifying screens. Screen film is different from dental intraoral film. It is designed to be sensitive to visible light.

12. INTENSIFYING SCREENS
various inorganic salts or phosphors fluoresce (emit visible light) when exposed to an x-ray beam. The intensity of this fluorescence is proportional to the x-ray energy absorbed. The presence of intensifying screens creates an image receptor system that is 10 to 60 times more sensitive to x rays than the film alone.

13. FORMATION OF THE LATENT IMAGE
When a beam of photons exits an object and exposes an x-ray film (either direct-exposure film or screen film exposed by light photons), it chemically changes the photosensitive silver halide crystals in the film emulsion. These chemically altered silver bromide crystals constitute the latent (invisible) image on the film.

14. Before exposure, film emulsion consists of photosensitive crystals containing primarily silver bromide . These silver halide crystals also contain a few free silver ions (interstitial silver ions) and trace amounts of sulfur compounds bound to the surface of the crystals.

15. . Along with physical irregularities in the crystal produced by iodide ions, sulfur compounds create sensitivity sites, sites in the crystals that are sensitive to radiation. Each crystal has many sensitivity sites.

16. When the silver halide crystals are irradiated, x-ray photons release electrons from the bromide ions. The free electrons move through the crystal until they reach a sensitivity site, where they become trapped and impart a negative charge to the site
The negatively charged sensitivity site attracts positively charged free interstitial silver ions When a silver ion reaches the negatively charged sensitivity site, it is reduced and forms a neutral atom of metallic silver.

17. The sites containing these neutral silver atoms are now called latent image sites. This process occurs numerous times within a crystal. The overall distribution of crystals with latent image sites in a film after exposure constitutes the latent image. Processing the exposed film in developer and fixer converts the latent image into the visible radiographic image.