locomotive design changed to allow good visibility on both sides of the track, regulations were changed allowing railroads to shift to bi-directional mast type signals, using signal bridges only in special situations involving multiple tracks or restricted views. Dwarf signals are typically mounted on the ground in areas of low speed movements or restricted clearances. Dwarf signals may be sometimes mounted higher up on a small mast or other structure for improved visibility. These can be known as "high dwarfs" or "stick signals," but a tall mounting does not change the lower speed applications of the dwarf signal. Signal colors and lamps Electric signal lamps are typically low power (35 watt) incandescent lamps running off of low voltage DC current or, more recently, high output LED arrays. Incandescent signals use a doublet lens combination to directionally focus their small power out over a long range (3,500 feet in daylight.) New LED signals may either use an unfocused array or act as a drop-in replacement behind a traditional lens. U.S. signal lenses have a standard diameter of . North American signals use a standard set of colors, defined in October 1905, and which became common to other modes of transportation as shown on page 384 of the Simmons-Boardman 1911 Signal Dictionary. Green - Used to indicate "clear" or proceed. Yellow - Used to warn the engineer of an impending stop or speed reduction for an occupied "block" ahead. Also used for low-speed movements. Red - Used to indicate a full stop or other restrictive condition, or used as a "placeholder" light (when that part of a signal is unused but to confirm to the crew the signal is working, so as not to require guessing the rest of the combination in case of a light failure). Blue - When on a signal doll post, indicates intervening track between the signal and the track to which the signal applies, or to indicate all equipment on the section of track to the rear of the blue signal is absolutely not to be moved as men are working under, on, or in said equipment. Purple - Obsolete. Up to around 1940, purple lenses were used rather than red as the "Stop" indication in some yards and derails. In 1952, the Interstate Commerce Commission ruled that purple should no longer be used in the U.S. for that purpose. Lunar White - Blue filtered light to