X-ray film has two principal components: (1) emulsion and (2) base. The emulsion, which is sensitive to x rays and visible light, records the radiographic image. The base is a plastic supporting material onto which the emulsion is coated (Fig. 5-1).

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 composition of a dental film emulsion is shown in Table 5-1. The silver halide grains in INSIGHT film and Ultra-speed film (Carestream Dental, a division of Carestream Health, Inc.) are flat, tabular crystals with a mean diameter of about 1.8 µm (Fig. 5-2). The tabular grains are oriented parallel with the film surface to offer a large cross-sectional area to the x-ray beam (Fig. 5-3). INSIGHT film has about twice the number of silver grains so that it requires only half the exposure of Ultra-speed film.

TABLE 5-1

Typical Coating Weights per Film Side (mg/cm²)

Film Type	Silver	Bromide	Emulsion Vehicle	Overcoat Vehicle
InSight (E/F speed)	0.8–1.1	0.6–0.75	0.6–0.8	0.1–0.2
Ultra-Speed (D speed)	0.4–0.55	0.6–0.75	0.4–0.7	0.1–0.2

Data from Carestream Health, Inc., exclusive manufacturer of Kodak dental systems.

The silver halide grains are suspended in a surrounding vehicle that is applied to both sides of the supporting base. During film processing (described later in this chapter) 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.

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 (described later in this chapter) that emit visible light. Screen film and intensifying screens are used for extraoral projections, such as panoramic and cephalometric radiographs.

One corner of each dental film has a small, raised dot that is used for film orientation (Fig. 5-4). The manufacturer orients the film in the packet so that the convex side of the dot is toward the front of the packet and faces the x-ray tube. The side of the film with the depression is thus oriented toward the patient’s tongue. After the film has been exposed and processed, the dot is used to orient the patient’s right and left side images properly. When mounting radiographs, each film is oriented with the convex side of the dot toward the viewer, and on the basis of the features of the teeth and anatomic landmarks in the adjacent bone, the films are arranged in their normal sequential relationship in the mount.

Intraoral x-ray film packets contain either one or two sheets of film (Fig. 5-5). When double-film packs are used, the second film serves as a duplicate record that can be sent to insurance companies or to a colleague. The film is encased in a protective black paper wrapper and then in an outer white paper or plastic wrapping, which is resistant to moisture. The outer wrapping clearly indicates the location of the raised dot and identifies which side of the film should be directed toward the x-ray tube.

A thin lead foil backing with an embossed pattern is between the wrappers in the film packet. The foil is positioned in the film packet behind the film, away from the tube. This lead foil serves several 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. Most importantly, if the film packet is placed backward in the patient’s mouth so that the tube side of the film is facing away from the x-ray machine, the lead foil will be positioned between the subject and the film. In this circumstance, most of the radiation is absorbed by the lead foil, and the resulting radiograph is light and shows the embossed pattern in the lead foil (Fig. 5-6). This combination of a light film with the characteristic pattern indicates that the film packet was exposed backward in the patient’s mouth and that the patient’s right side–left side designation indicated by the film dot is reversed.

Periapical View

Periapical views are used to record the crowns, roots, and surrounding bone. Film packs come in three sizes: (1) size 0 for small children (22 mm × 35 mm); (2) size 1, which is relatively narrow and used for views of the anterior teeth (24 mm × 40 mm); and (3) size 2, the standard film size used for adults (30.5 mm × 40.5 mm) (Fig. 5-7).

Bitewing View

Bitewing (interproximal) views are used to record the coronal portions of the maxillary and mandibular teeth in one image. They are useful for detecting interproximal caries and evaluating the height of alveolar bone. Size 2 film is normally used in adults; the smaller size 1 is preferred in children. In small children, size 0 may be used. A relatively long size 3 is also available.

Bitewing films often have a paper tab projecting from the middle of the film on which the patient bites to support the film (Fig. 5-8). This tab is rarely visualized and does not interfere with the diagnostic quality of the image. Film-holding instruments for bitewing projections also are available.

The extraoral projections used most frequently in dentistry are panoramic and cephalometric views. For these projections, screen film is used with intensifying screens (described later in this chapter) to reduce patient exposure (Fig. 5-9). Screen film is different from dental intraoral film. It is designed to be sensitive to visible light because it is placed between two intensifying screens when an exposure is made. The intensifying screens absorb x rays and emit visible light, which exposes the film. Silver halide crystals are inherently sensitive to ultraviolet (UV) and blue light (300 to 500 nm) and thus are sensitive to screens that emit UV and blue light. When film is used with screens that emit green light, the silver halide crystals are coated with sensitizing dyes to increase absorption. It is important to use the appropriate screen-film combination recommended by the screen and film manufacturer so that the emission characteristics of the screen match the absorption characteristics of the film.

Contemporary screen films use tabular-shaped (flat) grains of silver halide (Fig. 5-10) to capture the image. The tabular grains are oriented with their relatively large, flat surfaces facing the radiation source, providing a larger cross section (target) and resulting in increased speed without loss of sharpness. To increase the sharpness of images, some manufacturers add an absorbing dye in the film emulsion. This dye reduces light from one screen crossing through the film to reach the emulsion on the opposite side. EVG film from Carestream Dental is an example of this type of film.

Intensifying screens are made of a base supporting material, a phosphor layer, and a protective polymeric coat (Fig. 5-11). In all dental applications, intensifying screens are used in pairs, one on each side of the film, and they are positioned inside a cassette (see Fig. 5-9). The purpose of a cassette is to hold each intensifying screen in contact with the x-ray film to maximize the sharpness of the image.

Phosphor Layer

The phosphor layer is composed of phosphorescent crystals suspended in a polymeric binder. When the crystals absorb x-ray photons, they fluoresce (see Fig. 5-11). The phosphor crystals often contain rare earth elements, most commonly lanthanum and gadolinium. Their fluorescence can be increased by the addition of small amounts of elements, such as thulium, niobium, or terbium. Common phosphor combinations used in intensifying screens are shown in Table 5-2. Rare earth screens convert each absorbed x-ray photon into about 4000 lower energy, visible light (green or blue) photons. These visible photons expose the film.

TABLE 5-2

Rare Earth Elements Used in Intensifying Screens

Emission	Phosphor
Green	Gadolinium oxysulfide, terbium activated
Blue and UV	Yttrium tantalite, niobium activated

Different phosphors fluoresce in different portions of the spectrum. For example, light emission from Lanex rare earth intensifying screens ranges from 375 to 600 nm and peaks sharply at 545 nm (green). Figure 5-12 shows the spectral emission of a rare earth screen and the spectral sensitivity of an appropriate film. Other intensifying screens have a major peak at 350 nm (UV) and at 450 nm (blue). It is important to match green-emitting screens with green-sensitive films and blue-emitting screens with blue-sensitive films.

Fast screens have large phosphor crystals and efficiently convert x-ray photons to visible light but produce images with lower resolution. As the size of the crystals or the thickness of the screen decreases, the speed of the screen also declines, but image sharpness increases. Fast screens also have a thicker phosphor layer and a reflective layer, but these properties also decrease sharpness. In deciding on the combination to use, the practitioner must consider the resolution requirements of the task for which the image will be used. Screen-film combinations are rated for speed, a measure of the amount of radiation required for a proper exposure. For dental extraoral diagnostic tasks, it is recommended to use screen-film combinations that have a speed of 400 or faster.

The developer reduces all silver ions in the exposed crystals of silver halide (crystals with a latent image) to metallic silver grains (see Fig. 5-14). To produce a diagnostic image, this reduction process must be restricted to crystals containing latent image sites; to accomplish this, the reducing agents used as developers are catalyzed by the neutral silver atoms at the latent image sites (see Fig. 5-14, B). Individual crystals are developed completely or not at all during the recommended developing times (see Fig. 5-14, C). Variations in density on the processed radiographs are the result of different ratios of developed (exposed) and undeveloped (unexposed) crystals. Areas with many exposed crystals are darker because of their higher concentration of black metallic silver grains after development.

When an exposed film is developed, the developer initially has no visible effect (Fig. 5-15). After this initial phase, the density increases, rapidly at first and then more slowly. Eventually, all the exposed crystals develop (are converted to black metallic silver), and the developing agent starts to reduce the unexposed crystals. The development of/>