The power level of the lamps is rated in watts/area, total electrical input power divided by the lamp's inner wall surface. Cooling of the electrodes and the lamp envelope is of high importance at high power levels. Air cooling is sufficient for lower average power levels. High power lamps are cooled with a liquid, typically by flowing demineralized water through a tube in which the lamp is encased. Water-cooled lamps will generally have the glass shrunk around the electrodes, to provide a direct thermal conductor between them and the cooling water. The cooling medium should flow also across the entire length of the lamp and electrodes. High average power or continuous-wave arc lamps must have the water flow across the ends of the lamp, and across the exposed ends of the electrodes as well, so the deionized water is used to prevent a short circuit. Above 15 W/cm2 forced air cooling is required; liquid cooling if in a confined space. Liquid cooling is generally necessary above 30 W/cm2.