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Colour Light Lens Configurations

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Introduction

A colour light signal head incorporates the light units (each comprising a source of light behind a lens assembly) that are needed to display the desired aspects. The manner in which the units are arranged is determined by certain rules as well as being influenced by technical considerations. As a result, various different standard types of signal heads, with the lenses in different positions, have been used on the British rail network.

The four main aspects displayed by colour light signals are:

Not every signal is capable of displaying all four aspects. A signal may additionally be required to display flashing aspects but, since these use the same light units as non-flashing aspects, their inclusion or otherwise does not affect the format of the signal head. Red (meaning 'danger') is the most restrictive aspect and green (meaning 'clear') is the least restrictive.

The two yellow lights that make a double yellow aspect are always displayed vertically one above the other and far enough apart that they can be clearly distinguished when viewed from a distance. There is enough space between the two yellow lenses to accommodate another light unit, if desired.

Multi-unit Signals

The most common configuration had the light units arranged vertically in a multi-unit signal head. As a rule, the lens for the most restrictive aspect is positioned at the bottom, closest to drivers' eye level. If a signal is equipped to show both single yellow and double yellow aspects, then it is usually the lower yellow lens that illuminates for the single yellow aspect. These rules are reversed for ground-mounted signals or any signal mounted below drivers' eye level.

The earliest four-aspect signal heads had the lenses in the order yellow-red-yellow-green. Around 1937, both the LNER and the Southern Railway started to install signal heads with the lenses arranged in the order red-yellow-green-yellow. Both forms would continue to be installed, until the Ministry of Transport 'Requirements' of 1950 stated that only the latter arrangement was acceptable.

'Cluster' signals, which have the light units evenly distributed around a circular backboard, saw some limited use in the 1920s. These were intended for use on overhead gantries, where it was thought that a driver stopped close to the signal would have had difficulty viewing the aspect on a vertical type signal. Cluster signals were found to offer no real advantage in practice and were soon discontinued.

Only three and four-aspect signals are illustrated in the tables below. A two-aspect signal is generally configured as per the equivalent type of three-aspect signal, with the unneeded aspect or lens omitted. Single-aspect signals exist too but are relatively few in number.

Type Symbol R Y YY G First Introduced
Three-Aspect Vertical Red aspect Single Yellow aspect N/A Green aspect 1923
Four-Aspect Vertical
(Original Configuration)
Red aspect Single Yellow aspect Double Yellow aspect Green aspect 1926
Four-Aspect Vertical
(Later Configuration)
Red aspect Single Yellow aspect Double Yellow aspect Green aspect c.1937
Four-Aspect Cluster Red aspect Single Yellow aspect Double Yellow aspect Green aspect 1926

Single-aperture Signals

Some designs of signals can display up to three different colours through a common aperture. The earliest form of single-aperture signal is the 'searchlight' signal, in which a small roundel of the desired colour is brought in front of the continually-lit lamp by a relay mechanism inside the head. Modern LED or fibre-optic signal heads achieve the same effect but without any moving parts. Single-aperture signals, being smaller than multi-unit types, can be more easily accommodated in places with restricted clearance. If the double yellow aspect is required, a second light unit capable of displaying only yellow is placed at the appropriate distance vertically above the main unit.

'SDO' (optic diode) signals supplied by Ansaldo are used exclusively in the Manchester South area. This kind of signal head has two apertures, both of which are equipped to display three colours. In normal operation, they present a similar appearance to other twin-aperture signals except that a green aspect is displayed in the upper aperture. The lights are derived from single-filament lamps (one for each colour), directed through the aperture by dichroic mirrors. In the event of a lamp failure, a red, single yellow or green aspect will be displayed in the alternative aperture. For that reason, an SDO signal head always has two light units, even if the signal is only required to display two or three aspects.

Type Symbol R Y YY G First Introduced
Three-Aspect Single Aperture Red aspect Single Yellow aspect N/A Green aspect 1923
Four-Aspect Twin Aperture Red aspect Single Yellow aspect Double Yellow aspect Green aspect c.1933
Three or Four-Aspect SDO
 
Red aspect
*
Single Yellow aspect
*
Double Yellow aspect
 
Green aspect
*
2003
Note: This aspect will be displayed in the other aperture in the event of a lamp failure.

Non-standard Variations

Alternative lens configurations are occasionally found in restricted clearance areas where a standard signal head could not be accommodated. For example, the red lens may be in a separate head placed alongside the head with the other lenses or the lenses may be laid out horizontally.

Tunnel Signals

One type of four-aspect multi-unit signal, which is used within certain tunnels, has smaller lenses. In the original multi-unit form, the yellow lenses occupy the top and bottom positions to achieve adequate spacing, with both the red and green lenses placed in the space between them. The more recent LED equivalent has two apertures, each capable of displaying two colours. This form of signal can also function as a miniature co-acting signal.

Type Symbol R Y YY G
Four-Aspect Multi-Unit Tunnel Red aspect Single Yellow aspect Double Yellow aspect Green aspect
Four-Aspect Twin-Aperture Tunnel Red aspect Single Yellow aspect Double Yellow aspect Green aspect