Because of the very high power levels involved, large lamps are water-cooled. In those used in IMAX projectors, the electrode bodies are made from solid Invar and tipped with thoriated tungsten. An O-ring seals the tube, so that the naked electrodes do not contact the water. In low power applications the electrodes are too cold for efficient electron emission and are not cooled. In high power applications an additional water cooling circuit for each electrode is necessary. To reduce cost, the water circuits are often not separated and the water needs to be deionized to make it electrically non-conductive, which lets the quartz or some laser media dissolve into the water.