25-09-2012, 10:43 AM
Flashing Light Signals
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Flashing light signals consist of two light units that flash alternately at a rate of 45 to 65 times per minute. Thus, like their predecessor, the wigwag, they simulate a watchman swinging a red lantern. Wigwags consist of a single red light unit that sways back and forth.
The main components of a flashing light unit are the hood, background, roundel, lamp, lampholder, reflector, and housing. The background is 20 or 24 inches in diameter and is painted a nonreflecting black to provide a contrast for the red light. The hood is also painted black.
Current standards call for the use of 12-inch diameter heads.
The roundel is red and comes in a variety of designs that direct the light toward the motorist. The “spreadlight” roundel distributes light through the entire angle, one-half the angle on each side of the beam axis. A deflecting roundel directs a portion of the light from the beam to one side of the axis in the direction indicated on the lens. A roundel having both spreadlight and deflecting features is designed so that the deflection is at a right angle to the spread. An example is the 3-degree horizontal deflection and 15-degree vertical spread. A roundel using a 20-degree spread and 32-degree downward deflection can be used on cantilevers. Back light units may use a 70-degree horizontal spread.
The lamp consists of a low-wattage bulb used to ensure operation on stand-by battery power should commercial power fail. The wattage most commonly used is 18 or 25 watts; however, some railroads use quartz iodide bulbs of 16 or 36 watts. The reflector, or mirror, is mounted behind the lamp and directs the light back through the roundel.