Which of the following phosphors is classified as a rare earth phosphor?

The classification of phosphors as rare earth phosphors is primarily based on their composition and the materials used in their formulation. Lanthanum oxybromide is specifically recognized as a rare earth phosphor due to the presence of lanthanide ions, which are part of the rare earth elements category in the periodic table. These rare earth elements are typically found in the f-block and have unique luminescent properties that make them particularly effective in applications such as imaging and display technologies.

Rare earth phosphors generally exhibit high efficiency and stability, which is why lanthanum oxybromide is favored in various imaging systems. In contrast, the other phosphors listed do not contain rare earth elements. Cesium iodide and calcium tungstate, while effective in certain applications, do not belong to the rare earth group. Silver bromide is a compound that is primarily used in traditional photographic films and does not fall under the rare earth classification either. Thus, the choice of lanthanum oxybromide aligns with the specific criteria for rare earth phosphors.