Railway Color light signals
The introduction of electric light bulbs made it possible to produce color light signals which were bright enough to be seen during daylight. Many railroads thus converted to color light signals.
The signal head is the portion of a color 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.
Color light signals come in two forms. The most prevalent form is the multi-unit type, with separate lights and lenses for each color, in the manner of a traffic light. Hoods and shields are generally provided to shade the lights from sunlight which could cause false indications; colored Fresnel lenses are used to focus the beam, though reflectors are often not used in order 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 colors.
Searchlight signals were also used, although these have become less popular. In these, a single incandescent light bulb is used in each head, and a solenoid is used to position a colored spectacle (or ’roundel’) in front of the lamp. In effect, this mechanism is very similar to the color 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. Typically, an elliptical reflector focuses the lamp through the roundel a small lens and then a larger Fresnel lens.[1]Colchester to Clacton Line. The viewing angle for the searchlight beam is frequently very narrow, so these signals have to be carefully sited and aligned in order for the light to be seen properly. Again, to display more than three aspects, multiple heads are used. Searchlight signals have the disadvantage of having moving parts in what can be a hostile location for mechanical equipment and thus need regular maintenance. Examples of searchlight signals in the UK can still be found on the
A variant of this is the Unilens signal made by Safetran Systems Corporation, which uses a single-lens system, fed by three or four individual halogen lamps with parabolic reflectors behind them. These lamps shine through colored filters into individual fiber-optic elements, which join together at the focal point of the lens assembly. This makes it possible to show four different colors (usually red/yellow/green/lunar (white)) from a single signal head, which is impossible for the traditional searchlight mechanism.
More recently, clusters of LEDs have started to be used in place of the incandescent lamps, reflectors and lenses. They have a more even color output, use less power and have a working life of around 10 years, significantly reducing long term costs. These are often arranged so that the same aperture is used for whichever color light is required and are therefore sometimes referred to as modern searchlights.
Operating rules generally dictate that a dark signal be interpreted as giving the most restrictive indication it can display (generally “stop” or “stop and proceed”). Obviously this greatly impedes traffic until repairs are made. Therefore many color light systems have circuitry to detect failures in lamps or mechanism, allowing the signal to compensate for the failure by displaying an aspect which, while more restrictive than that set by the dispatcher or signalling equipment, still allows traffic to pass; for example, if a green lamp is burned out, but the indication to be displayed is “clear”, the signal can detect this and display a cautionary aspect using a different lamp or lamps, allowing traffic to proceed at reduced speeds without stopping. Approach lighting leaves the signal dark (or dimmed) when a train is not present. This may be applied for sighting reasons, or simply to extend the life of the lamp and save the batteries.
In the UK, most filament-type colour light signals are equipped with lamps having two filaments. When the main filament fails, the auxiliary filament automatically comes into use. Failure of the main filament is indicated to the technician (but not the signalman), who will then arrange for the lamp to be replaced. Failure of both filaments, resulting in a ‘dark’ signal, is indicated to the signalman, inside the signal box, also the previous signal may also be restricted to no more than a yellow warning aspect.