Fluoro-chloro apatite forms the basis of the now obsolete Halophosphor fluorescent tube phosphor system. Alternatives include brass, phosphor bronze, and beryllium copper. Conformal coating process is frequently used to address the issue of varying phosphor thickness. Illumination of the phosphor is achieved using UV light. Early Telechrome devices used two electron guns aimed at either side of a phosphor plate. Gadolinium as a phosphor is also used in other imaging. The tube contains a flexible shaft formed of three coils of phosphor bronze. The aluminum layer was provided to reflect more light from the phosphor towards the viewer. Light output drops over time due to falling emission and reduction of phosphor efficiency. Lamps can be made with a lithium metaluminate phosphor activated with iron. A grille or mask absorbs the electrons that would otherwise hit the wrong phosphor. These had the added advantage of not degrading the phosphor over time, unlike radium. It is used as a blue phosphor in color CRTs. Sarod strings are either made of steel or phosphor bronze. Some snare drums are constructed with phosphor bronze.