The phosphor crystals often contain rare earth elements, the most common being:

The phosphor layer is composed of phosphorescent crystals suspended in a polymeric binder. When the crystals absorb X-ray photons, they fluoresce. 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. Rare earth screens convert each absorbed X-ray photon into approximately 4000 lower-energy, visible light (green or blue) photons. These visible photons expose the film.