"Searchlight" signals were the most often used signal type in the U.S. until recently, although these have become less popular due to vandalism. In these, a single incandescent light bulb is used in each head, and either an A.C. or D.C. relay mechanism is used to move a coloured spectacle (or "roundel") in front of the lamp. In this manner, gravity (fail safe) returns the red roundel into the lamp's optical path. In effect, this mechanism is very similar to the colour light signal that is included in an electrically operated semaphore signal, except that the omission of the semaphore arm allows the roundels to be miniaturized and enclosed in a weatherproof housing. Widely used in the U.S from World War II onward, searchlight signals have the disadvantage of having moving parts which may be deliberately tampered with. This had led to them becoming less common during the last fifteen to twenty years when vandalism began to render them vulnerable to false indications.