<P> A variant of the above, a "medium - range" single lens signal, is the "Unilens" signal made by Safetran Systems Corporation . This uses a single - lens system, fed by three or four individual halogen lamps with parabolic reflectors behind them . These lamps shine through the same coloured roundels used in searchlight signals into individual fibre - optic elements, which are rolled together as sheets to make a solid cable as it were at the focal point of the lens assembly . This makes it possible to show four different colours (usually red / yellow / green / lunar white) from a single signal head, which is not possible for the traditional searchlight mechanism, nor is it necessary . The marginal medium - range aspect of this signal is often somewhat compensated by the use of the illumination of two red lamps simultaneously lit to improve visual range . Current consumption of this arrangement would have been prohibitive during the 20th century, when most signals were battery powered . </P> <P> More recently, clusters of LEDs have started to be used in place of the incandescent lamps, reflectors and lenses . These use less power and have a purported working life of ten years, but this may not in reality be the case . These are often arranged so that the same aperture is used for whichever colour light is required, but again only for medium - or short - range use as the number per colour of discrete LEDs is much less than found on high signals intended for long - range application . </P> <P> 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 colour 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 . </P> <P> 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 . </P>

The diagram shows the circuit for a signal to display a green or a red light