Barium absorbs X-rays. It is used in a liquid swallowed by patients before getting a stomach X-ray. Lanthanum and other lanthanides are used in carbon-arc lamps in motion pictures. Praseodymium and Neodymium are used in welder's goggles. Neodymium is used in astronomical spectrometers.
Promethium is used as a power source in spacecraft.
Samarium is used in carbon-arc lamps. Europium is used in red phosphors in televisions. Gadolinium is the best neutron absorber and is used in nuclear reactors. Terbium is used in green lasers. Dysprosium absorbs neutrons and is used to control nuclear reactors.