Caesium iodide or cesium iodide is the ionic compound of caesium and iodine. It is often used as the input phosphor of an X-ray image intensifier tube found in fluoroscopy equipment. Caesium iodide photocathodes are highly efficient at extreme ultraviolet wavelengths.
cesium iodide is also used in beam splitters for Fourier Transform Infra-Red (FTIR) devices. And in scintillators where it may or may not be doped with Thallium.