Figure 6-37. - Fresnel^® lens optical landing system (FLOLS).

line. They are spaced 1,000 feet apart. The signs have painted numbers that are lit so they can be seen at night and during periods of restricted visibility.

The power supply for serving the runway distance markers should be from a separate series circuit. Do not have the markers supplied from circuits feeding either the runway, threshold, or the approach lighting circuits, since these circuits are operated at various brightness steps, and the signs are operated at their full brightness at all times. Also, do not connect them to taxiway circuits because of the intermittent operation requirements. The cable used for the runway distance marker circuit is a single conductor, No. 8 AWG, stranded, 5,000 volts.

THRESHOLD LIGHTS. - The threshold lights are a part of the approach light system. Four sets of light systems are used in the threshold light configuration. These lights are as follows: inboard threshold lights, winged-out threshold lights, prethreshold wing bars, and a terminating bar. These four sets of lights are installed on both ends of the runway and are used to mark the beginning of the runway.

Inboard Threshold Lights. - Inboard threshold lights are installed in the area at the end of the runway between the two runway edge light lines. This line of lights will be at a right angle to the runway center line. They arc as close to the full-strength paving as possible but not more than 10 feet from it. The lights