The film inside a screen-film cassette converts incident light photons into varying levels of darkness as a function of light flux. It is composed of a thin plastic base coated on one or both sides with a light-sensitive emulsion layer, which consists of silver halide (AgBr and AgI) crystals held inside water-soluble gelatin (Figure 3.1). Each silver halide crystal has on its surface specks of Ag⁺ ions because of artificially introduced defects to the crystal lattice. When a grain of silver halide crystal is exposed to visible light, a small number of Ag⁺ ions on the crystal surface gain electrons and get reduced to metallic Ag atoms, and, as a result, the crystal grain forms a stable latent center locally. The opacities introduced by these Ag atoms are still too weak to form a visible image; hence a developing process is needed to amplify the signal by converting all Ag⁺ ions at each latent center to Ag atoms (Figure 3.2). After film development, the film then has its residual silver halide crystals in unexposed areas dissolved by a bath of an aqueous oxidizing solution called fixer before it is rinsed and dried for image presentation.

A developed film is viewed in front of a light box. The local darkness of the film is caused by the presence of Ag atoms after they are converted from ions and reflects local X-ray exposure made during image acquisition; the higher the exposure, the darker the area. Film images are therefore considered as negative images.

An intensifying screen is primarily made of scintillator materials, which convert incident X-ray exposure into visible light photons, whose wavelengths fall into the sensitive range of the film.

In selecting the appropriate thickness of an intensifying screen, there is a trade-off between the spatial resolution of the image and the efficiency of X-ray absorption. When the screen thickness increases, more incident X-ray photons are absorbed by the screen, but the visible light photons have a broader spread before reaching the film, leading to a blurred image. When the screen thickness decreases, less blurring of the image is expected at the expense of a more grainy/noisy image.

Speed is a common term used to describe the efficiency of X-ray absorption by a radiographic image receptor. The higher the speed, the less radiation it takes for a certain level of signal output (the optical density of the film is the signal output for screen-film radiography).

Radiographic applications often use screen-film cassettes with a double-emulsion and double-screen configuration, in which the film has two emulsion layers and is placed in between two screens for the maximum efficiency of X-ray absorption (Figure 3.3). The configuration becomes different for applications where high spatial resolution is desired, like mammography; only a single screen is used and placed behind the film along the X-ray path so that X-ray photons hit the film first. In this configuration, there are more X-ray photons absorbed by the screen at places near the film because of X-ray attenuation, and the resulting light spread is limited; if X-ray photons hit the screen first, there would be more photons absorbed by the screen at places far from the film and there would be more light spread.

Indirect conversion FPD uses a scintillator mounted on the front surface of the detector. The scintillator functions in the same way as that in a screen-film cassette by converting incident X-ray photons to light photons. The light photons then reach the pixels, and each individual pixel, with its photodiode, converts the light photons to electrical charges before their readout. Light photons produced inside the scintillator layer diffuse in all directions while exiting the scintillator, eventually causing a blurring of the signal and a loss of resolution in the formed image. To reduce such resolution loss, most indirect conversion detectors use cesium iodide (CsI) as the scintillator material. CsI has a columnar crystal structure that acts like light pipes to reduce lateral spread of light.

Direct conversion FPDs eliminate the production of light photons and directly convert X-ray photons into charges. Amorphous selenium is the semiconductor material most commonly used for fabricating the layer at the entrance of the detector. The semiconductor layer produces electron-hole pairs in proportion to the incident X-ray intensity, and the ion pairs are instantly collected by the applied voltage across the layer and transferred to individual pixels for subsequent readout. This design minimizes the spread of signals and results in high spatial resolution in the acquired images.