fan, however, some have multiple supply or exhaust fans. Very often, the air-flow rate required by the firing range and space constraints for the fans dictate the number and types of fans. Most firing ranges have systems that supply 100% outside air to the firing range and exhaust all of the air to outside the building; but, some firing range ventilation systems are designed to recirculate some of the exhaust air to the supply air system to conserve energy especially in extreme climates. The exhaust air is always filtered before being exhausted outside the building or recirculated to the supply system. Lighting in the range consists of control booth, uprange area, shooting booth, and downrange lighting systems. Control booth lighting is usually manually controlled and consists of general lighting and low-level lighting used during particular shooting conditions. Lighting uprange of the booths is general ceiling-level lighting and can usually be controlled manually or from the central controls. Lights downrange of the firing line are usually spotlights used to illuminate the targets at various distances downrange of the booths. Safety control systems are installed to protect the shooters during range malfunction or emergency situations. Such systems may include warning lights, alarm bells, and air-flow and filtration monitors. Outdoor range Outdoor shooting ranges are used for longer-distance shooting up to or exceeding . Training might also specifically require exposure to the elements such as wind, dust and rain. Outdoor competition shooting is preferred under benign weather conditions, although conditions may change, competition is only abandoned when safety becomes an issue. Outdoor ranges are designed to contain all fired shots. This necessitates a high retaining wall behind the target line called a backstop or stop-butt, comprising an earth mound, sandbag barrier or specially designed funnel-shaped traps to catch and prevent misaligned shots, errant projectile ricochets, or shots going beyond the bounds of the shooting range. Most outdoor ranges restrict the maximum caliber size and/or projectile energy based on the design specification of the range. Some target-shooting ranges have separate facilities devoted to the use of higher-powered firearms such as .50 caliber. Outdoor ranges may be partially enclosed and so have some features in common with indoor ranges, for example the British Armed Forces barrack range has a roofed firing point and normally has 360° walls. As its name suggests, it is generally found in military bases rather than in the more remote areas