A soft, silvery metal.Terbium is used to dope calcium fluoride, calcium tungstate and strontium molybdate, all used in solid-state devices. It is also used in low-energy lightbulbs and mercury lamps. It has been used to improve the safety of medical x-rays by allowing the same quality image to be produced with a much shorter exposure time. Terbium salts are used in laser devices.An alloy of terbium, dysprosium and iron lengthens and shortens in a magnetic field. This effect forms the basis of loudspeakers that sit on a flat surface, such as a window pane, which then acts as the speaker.