page_title: Railway signal
section_title: Colour light signals
hierarchical_section_title: Railway signal / Signal forms / Colour light signals
caption_attribution_description: English: German railway signals (Hp system) Polski: Semafory kolei niemieckich (system Hp)
context_page_description: A railway signal is a visual display device that conveys instructions or provides advance warning of instructions regarding the driver’s authority to proceed. The driver interprets the signal's indication and acts accordingly. Typically, a signal might inform the driver of the speed at which the train may safely proceed or it may instruct the driver to stop.
context_section_description: The introduction of electric light bulbs made it possible to produce colour light signals which were bright enough to be seen during daylight, starting in 1904. These at first were "short-range" signals, used in low-speed applications. Improvements in signal glassware by Dr. Churchill of Cornell University in conjunction with Corning Glass Works (Corning, New York) increased the daylight viewing range to that of "medium-range" signals. These were quickly adopted by many U.S. rapid transit and trolley (street, light) railway systems. Ultimately, by 1913, Dr. Churchill developed the "doublet lens" combination wherein an outer clear Fresnel lens was sealed with inwards facing ribs to an inner glass of the desired colour with ribs facing the outer lens. Both inner and outer exposed surfaces were therefore smooth, avoiding dirt accumulation. The inner lens convexity permitted a 35-watt, 10-volt lamp filament to be almost surrounded by this system, gathering much more of the light produced than with previous systems. This yielded the first true "long-range" daylight signal with a range of over 760 metres (2,500 ft) under bright sun conditions. The first application was by the Chicago, Milwaukee, St. Paul and Pacific Railroad's use of the US&S "Style L" colour light signal on their line through the Pacific Northwest in 1914. These were intentionally fitted from the start with two lamps - one ahead of the other -, with a circuitry that assured that the failure of the main lamp caused the lighting the back-up lamp. The final improvement came in the early 1920s with Corning's "High Transmission" glass colours, increasing this range to 1,100 metres (3,500 ft) under bright sun conditions. The results were that in the U.S., all semaphore manufacturing had ceased by 1944.
  The signal head is the portion of a colour light signal which displays the aspects. To display a larger number of indications, a single signal might have multiple signal heads. Some systems used a single head coupled with auxiliary lights to modify the basic aspect.
  Colour light signals come in two forms; both were developed in the first quarter of the 20th century in the U.S. The most prevalent form is the multi-unit type, with separate lights and lenses for each colour, in the manner of a traffic light. Hoods and shields are generally provided to shade the lights from sunlight which could cause false indications; coloured Fresnel lenses are used to focus the beam, though reflectors are often not used, to prevent false indications from reflected sunlight. The lights may be mounted vertically or in a triangle; usually green is on top and red at the bottom. Signals with more than three aspects to display generally have multiple heads to display combinations of colours.
  Some rapid transit systems in the United States, including PATCO and the Washington Metro, use a lunar "white" aspect in lieu of the green and yellow aspects; in this case, the lights are mounted vertically, with red lamps on the top and the bottom of the white lamp.
  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. An elliptical reflector focuses the lamp through the roundel, a small lens and then a larger single or compound lensing arrangement is employed. The Hall Signal Company first produced this type of signal in 1920, having purchased the patent rights from one Mr. Blake in 1918. US&S bought the Hall Company in 1925 thus naming this signal the Style "H," with the later compound lens ver
caption: German railway signals showing aspect Hp0 (Stop)
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