The blocking method has found its way into liquid crystal displays (LCDs), video projectors and rear projection TVs. In this type of screens and projectors, the source light is first polarised by a filter in one direction and then passed on to another filter, filled with liquid crystals. By changing the voltage applied to this crystal filter, it will work as a switching polarising filter, giving different gray scales of the light coming out. The light is changed only once for each image frame. The light valve thus consists of the two polarising filters, where one has a voltage controlled switch function thanks to the properties of the liquid crystals. This type of valve is often referred to as a liquid crystal light valve.