All approach Lighting Systems in the United States utilize a feature called the Decision Bar. The Decision Bar is always located 1000' farther away from the threshold in the direction of the arriving aircraft, and it serves as a visible horizon to ease the transition from instrument flight to visual flight.
Approach Light Systems are designed to allow the pilot to quickly and positively identify visibility distances in Instrument meteorological conditions. For example, if the aircraft is at the Middle Marker, and the Middle Marker is located 3600' from the threshold, the Decision Bar is 2600' ahead. If the procedure calls for at least ½ statute mile flight visibility (roughly 2600'), spotting the Decision Bar at the marker would indicate enough flight visibility to continue the procedure. In addition, the shorter bars before and after the Decision Bar are spaced either 100' or 200' apart depending on the ALS type. The number of short bars the pilot can see can be used to determine flight visibility. Approaches with lower minimums use the more precise 100' spacing systems for more accurate identification of visibility.
Read more about this topic: Approach Lighting System
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