When light enters or leaves a lens, there is a loss of some transmitted light due to reflection. By applying a surface coating of an antireflective material such as magnesium fluoride, the transmission can be greatly increased and internal flare can be reduced. When all lens surfaces have been coated they are said to be fully-coated and when the surfaces are coated with multiple layers to maximize transmission, the optics are said to be multi-coated. Coatings also play a big part in the performance of reflectors because not all of the light is reflected; there is a small loss at each mirror surface. Today's reflectors usually have a thin coat of aluminum as the mirror and then an overcoat of silicon monoxide or silicon dioxide to protect it. Silicon dioxide produces a more durable coat than silicon monoxide but requires specialized equipment to apply it and is therefore more expensive. Protection is needed because, in most reflectors, the mirror is open to the elements and deterioration of the reflective layer reduces the resolution of the telescope. All Sky-Watcher reflectors are multi-coated with silicon dioxide for more durability.