The target chamber is a multi-piece steel sphere weighing . Just before reaching the target chamber, the light is reflected off mirrors in the switchyard and target area in order to hit the target from different directions. Since the path length from the Master Oscillator to the target is different for each beamline, optics are used to delay the light in order to ensure that they all reach the center within a few picoseconds of each other. One of the last steps before reaching the target chamber is to convert the infrared (IR) light at 1053 nm into the ultraviolet (UV) at 351 nm in a device known as a frequency converter. These are made of thin sheets (about 1 cm thick) cut from a single crystal of potassium dihydrogen phosphate. When the 1053 nm (IR) light passes through the first of two of these sheets, frequency addition converts a large fraction of the light into 527 nm light (green). On passing through the second sheet, frequency combination converts much of the 527 nm light and the remaining 1053 nm light into 351 nm (UV) light. Infrared (IR) light is much less effective than UV at heating the targets, because IR couples more strongly with hot electrons that absorb a considerable amount of energy and interfere with compression. The conversion process can reach peak efficiencies of about 80 percent for a laser pulse that has a flat temporal shape, but the temporal shape needed for ignition varies significantly over the duration of the pulse. The actual conversion process is about 50 percent efficient, reducing delivered energy to a nominal 1.8 MJ. As of 2010, one important aspect of any ICF research project was ensuring that experiments could be carried out on a timely basis. Previous devices generally had to cool down for many hours to allow the flashlamps and laser glass to regain their shapes after firing (due to thermal expansion), limiting their use to one or fewer firings per day. One of the goals for NIF has been to reduce this time to less than four hours, in order to allow 700 firings a year. Other concepts NIF is also exploring new types of targets. Previous experiments generally used plastic ablators, typically polystyrene (CH). NIF targets are constructed by coating a plastic form with a layer of sputtered beryllium or beryllium–copper alloy, and then oxidizing the plastic out of