The present invention concerns railway light signals. Light signals are in widespread use in railways, shipping, aerospace or any other environment where optical information may be communicated over distance to a driver, pilot or other vehicle or machine operator. Filament lamps providing an incandescent light source are most commonly installed in light signals and have been in use for a considerable length of time.

Advances in technology have resulted in light-emitting diodes becoming a viable alternative light source to filament lamps in many applications. The sake of brevity the conventional term "LED" shall be used to describe a light-emitting diode hereon. LEDs have a lower energy consumption, longer lifespan and greater reliability than filament lamps which make LEDs an attractive alternative. Consequently, LEDs providing an electroluminescent light source are often used in new light designs. LEDs have also been proposed as plug-in higher efficiency replacement light sources in general domestic lighting applications. However, replacement of filament lamps with LEDs in the railway environment poses specific challenges such as strict performance and compatibility requirements.

Accordingly, in a first embodiment of the present invention, there is provided a light source for use behind a lens arrangement of a signal aspect of a railway light signal configured to use a filament lamp, wherein the light source is a LED light unit comprising: a LED driver circuit comprising an elongate LED element; and a mechanical support member for supporting the LED driver circuit and the elongate LED element on a component of the signal aspect configured to receive a filament lamp, wherein the elongate LED element substantially replicates the location and orientation and incandescent profile of the filament in a filament lamp for which the signal aspect was configured and wherein the LED driver circuit comprises a ballast load causing the LED light unit to consume substantially the same electric power as the filament lamp for which the signal aspect was configured. The present invention enables direct substitution of a filament lamp with the LED light unit and provides all the reliability and long-life benefits of LEDs without needing to renew or modify the entire railway light signal. Moreover, contrary to the principle of known LED replacements, electric power is deliberately "wasted" as it is found that this avoids a problem that equipment associated with the light signal may register the reduced electric power consumption of the replacement LED light unit as fault.

Preferably, the LED element substantially replicates the visual effect of a fault condition in the filament lamp for which the signal aspect was configured. This optical fault indication is to complement a fault alarm signal conveyed to a remote signal box by alarm circuitry associated with the signal aspect.

Preferably, the LED element comprises a plurality of discrete LEDs arranged in a LED array. By clustering discrete LEDs together in an array, commercially available LEDs may be used to match the incandescent profile of a filament.

Preferably, the LED array is arranged in a pair of parallel rows of LEDs in the LED plane, wherein one of the rows is a front row and the other of the rows is a back row located behind the front row in a direction of light from the LEDs and wherein the LEDs are arranged alternately between the front row and the back row. By staggering the LEDs alternately between two rows, the light output from adjacent LEDs may be blended more evenly across the LED array. Preferably, the LED array comprises five LEDs arranged in a W-shape in the LED plane with three LEDs arranged in the front row and two LEDs arranged in the back row. This arrangement most closely replicates the incandescent profile of a main filament in SL18 and SL35 filament lamps commonly used in railway light signal. Preferably, a distance between the centres of electroluminescence in LEDs at either end of the front row of LEDs is substantially 1 1.5 mm and wherein a gap between a line through the centres of electroluminescence in the LEDs in the front row and a line through the centres of electroluminescence in the LEDs in the back row is substantially 2.1 mm. These dimensions are closely equivalent the incandescent profile of a filament in a SL18 single filament lamp and a main filament in a SL35 double filament lamps commonly used in railway light signals.

Preferably, a geometrical centre of the W-shape of the LED array is halfway a between the centres of electroluminescence in the LEDs at either end of the front row of LEDs and halfway between a line through the centres of electroluminescence in the LEDs in the front row and a line through the centres of electroluminescence in the LEDs in the back row, wherein the geometric centre of the W-shape is a light centre of the LED array and wherein the light centre of the LED array is coincident with a focal point of the lens arrangement of the signal aspect of the railway light signal. The location of LEDs in rows on both sides of the lens arrangement focal point slightly de-focuses light output from the LEDs and helps to blend it together.

Preferably, the LED light unit comprises a plurality of mutually spaced boards connected to the support member, wherein the front row of LEDs is mounted upon a first board of the plurality of boards and the back row of LEDs is mounted upon a second board of the plurality of boards, wherein light from each of the LEDs of the back row passes through a respective elongate slit in the first board and wherein the axis of elongation of each elongate slit is substantially perpendicular to the LED plane. The space between the first and second boards is calibrated to maintain exactly the right gap between the front and back rows of LEDs. The light from the back row LEDs is projected through the elongate slits to the lens arrangement.

Preferably, the length of each elongate slit along its axis of elongation is long enough to permit light from the back row of LEDs to fall upon the lens arrangement unobstructed by the first board. The light from the back row LEDs is not obstructed along the axis of elongation. Preferably, the width of each elongate slit perpendicular to its axis of elongation is at least as wide as the LED in the back row which projects light through said elongate slit. The aperture provided by the elongate slit in the LED plane allows light from back row LEDs to blend with light from the front row LEDs.

Preferably, the ballast load is a power resistor which is mounted upon a third board of the plurality of mutually spaced boards and wherein a space exists between the third board and the support member upon which the third board is mounted. The third board and power resistor are thermally decoupled from the support member and the rest of the LED light unit.

Preferably, the power resistor is removable from the third board. A removable power resistor allows a reduction in the electric power consumption of the LED light unit. The LED light unit of the present invention may be modified to reduce its electric power consumption and, in doing so, adapt to any future railway network conversion to lower power LED light signalling systems.

Preferably, each LED comprises a LED chip. LED chips are compact, readily available, and may be adhered directly to the support structure of the LED driver circuit thereby saving space.

Preferably, the colour of electroluminescence from the LED element substantially matches the colour of the lens arrangement. This reduces light intensity losses caused by the filtering effect of colour in the lens arrangement.

Preferably, the support member is connectable by bayonet fitment to a component of the signal aspect configured to receive a filament lamp. Bayonet fitment ensures quick and reliable mechanical connection of LED light unit in the correct orientation. It also replicates the connection system of filament lamp it replaces.

Preferably, the LED driver circuit comprises a pair of electric cables for electrical connection to electric power supplied by the light signal. The electric cables may be hard-wired to the electric power supply of the light signal rather than electrically connected by spring contacts as is commonly the case a filament lamp and which are prone to electrical or mechanical failure.

Preferably, the LED driver circuit comprises means for detecting a fault condition in the LED element and means for reducing electric current drawn by the LED driver circuit in response to the detected fault condition. The reduction of electric current drawn by LED driver circuit is an easily communicated indication to external light signal equipment, like, for example, a signal box, that a fault condition exists within the LED light unit and that replacement is necessary.

Preferably, the fault condition in the LED element is detected by comparing electric current through the LED element and/or comparing voltage drop across the LED element with a threshold. The LED driver circuit uses its own internal circuit elements to monitor LED element performance.

Preferably, the fault condition is indicated by disconnection of the ballast load from the LED driver circuit. Disconnection of the ballast load makes a step change to the power consumption of the LED driver circuit which gives a positive indication to external light signal equipment that a fault condition exists within the LED light unit.

Preferably, disconnection of the ballast load reduces electric power consumption of the LED driver circuit by substantially 40 percent of normal electric power consumption of the LED driver circuit. The ballast load is chosen so that its disconnection occurs, and the LED light unit ceases to operate, at the same electric power supply threshold as a filament lamp for which the signal aspect was configured and which the LED light unit replaces. In a second embodiment of the present invention, there is provided a railway light signal comprising: at least one signal aspect with a lens arrangement for focussing and filtering light from a light source according to the first embodiment into a beam of light projected from the signal aspect; a signal head for housing the at least one signal aspect; a support structure for supporting the signal head; and a signal control circuit for supplying electric power to the light source of the or each signal aspect. Such a railway light signal benefits from the advantages of the LED light unit of the present invention.

Preferably, the signal control circuit of the or each signal aspect is associated with alarm circuitry for detection of disconnection of the ballast load from the electric power supply to the LED driver circuit. The resulting drop in electric current consumption may be used to raise the alarm to a remote signal box that the selected signal aspect is not illuminated as intended.

In a third embodiment of the present invention, there is provided a method of modifying the railway light signal, comprising the steps of: (a) gaining access to the interior of at least one signal aspect; (b) removal of a filament lamp from said signal aspect; (c) mechanically installing the light source according to the first embodiment; (d) electrical connection of the light source; and (e) closure of said signal aspect to external elements. This provides an existing railway light signal with the benefits of the LED light unit of the present invention.

Preferably, the method of modifying the railway light signal comprises the additional step of substitution of a filament switching relay associated with said signal aspect with a hardwired unit in the signal control circuit between steps (c) and (d) above. This provides an existing railway light signal with the benefits of the LED light unit of the present invention. It also provides the benefit of using the existing railway light signal systems to raise the alarm in a remote signal box that the selected colour signal aspect with LED light unit is not properly illuminated and avoids indication of a main filament failure in a double filament lamp which has in reality been replaced by the LED light unit. These and other features and advantages of the present invention will be better understood from the following detailed description, which is given by way of example and with reference to the accompanying drawings, in which:

Figure 1 shows a perspective view of a light signal;

Figure 2 shows a rear view of a colour signal aspect for the light signal of Figure 1 ;

Figure 3 shows a vertical cross-section Ill-Ill through the colour signal aspect of Figure 2; Figure 4 shows the same view as Figure 3 with detail on lens construction;

Figure 5 shows a plan view of beams and sectors which compose light output from a colour signal aspect;

Figure 6 shows a side elevation view of a double filament lamp;

Figure 7 shows a top view of the lamp of Figure 6 without a bulb;

Figure 8 shows a bottom view of electrical circuit of the lamp of Figure 6;

Figure 9 shows a side elevation view of a single filament lamp;

Figure 10 shows a top view of the lamp of Figure 9 without a bulb;

Figure 1 1 shows a control circuit for the colour signal aspects of the light signal of Figure 1 ;

Figure 12 shows a side elevation view of a LED light unit for use in the light signal of Figure 1 ;

Figure 13 shows a top view of the LED light unit of Figure 12;

Figure 14 shows a front perspective view of the LED light unit of Figure 12;

Figure 15 shows a lateral cross-section XV-XV through the LED light unit of Figure 12; Figure 16 shows detail XVI of Figure 15 of an array of five LEDs;

Figure 17 shows a diagram of a LED driver circuit for the LED light unit of Figure 12; and Figure 18 shows the vertical cross-section Ill-Ill through the signal aspect of Figure 2 converted with the LED light unit of Figure 12.

Referring to Figure 1 , there is shown a conventional light signal 10 as is commonly used in railways, but which may be used in other industries or environments. The light signal comprises a signal head 12 mounted upon a support structure. The support structure may be any structure capable of supporting and aiming the signal head correctly, like for example, a signal gantry or a signal post. The support structure shown in Figure 1 comprises a signal post 14 with a foot plate 16 anchored to the ground. Light signals with three or four aspect are the most commonly used in railways, although signals with two or one aspect may be used. Each aspect is illuminated to indicate specific long range line status information to a train driver. The signal head 12 houses two colour signal aspects 18a, 18b each projected through a black signal face 20 on the front of the signal head. The light signal 10 is a close-range two-aspect signal and the aspects are, from top to bottom, green 18a and red 18b. If the light signal 10 were a long-distance two-aspect signal the aspects would be, from top to bottom, green 18a and yellow 18b and in other applications it may be, fom top to bottom, yellow and red. Each signal aspect is sheltered from sunlight by a respective hood 22.

The light signal 10 comprises a junction route indicator 24 mounted diagonally to one side of the signal head 12. The junction route indicator 24 comprises a line of five white signal aspects 26 projected through a black face plate 28. The line of signal aspects corresponds to the approximate direction of a branch line diverging from a main line. The line of signal aspects is illuminated to indicate branch line divergence to a train driver.

A signal aspect converts electrical energy into optical information. Referring to Figures 2 and 3, there is shown a conventional colour signal aspect 18 corresponding to either one of the colour signal aspects 18a, 18b shown in Figure 1. The colour signal aspect 18 is an assembly comprising a filament lamp 100 mechanically connected by bayonet fitment to a socket 30 in a lamp holder 32. The signal aspect comprises a lens housing 34 to support the lamp holder 32 and a lens arrangement comprising an outer lens 36 and an inner lens 38. The outer lens is clamped to the lens housing by three outer lens clamp assemblies 40 each comprising an outer clamp 40a and fastener 40b. The inner lens is clamped to the lens housing by three inner lens clamp assemblies 42 each comprising an inner clamp 42a and fastener 42b. The inner and outer lenses are aligned to have the same the same focal axis 44. The focal axis 44 is normal to the signal face 20 of the signal head 18. The outer lens is a clear. The inner lens is coloured according to the colour (i.e. red, yellow or red) to be projected by the colour signal aspect. The lenses focus and filter light from the filament lamp to produce the correct colour and concentration of light.

The socket 30 comprises three spring contacts: a main spring contact 46m electrically coupled to a main terminal stem 48m located near the rear of the lamp holder 32; a 5 common spring contact 46c electrically coupled to a common terminal stem 48c located near the rear of the lamp holder 32; and an auxiliary spring contact 46a electrically coupled to auxiliary terminal stem 48a located near the rear of the lamp holder 32. The terminal stems are electrically coupled to a signal control circuit 300, which is described in more detail below. Each terminal stem comprises a screw with nuts for fastening an 10 electric cable with a spanner.

The interior of the signal head 12 is coloured matt black. The signal aspect 18 does not have an internal reflector behind the filament lamp 100. Light output from the filament lamp 100 which does not pass through the lenses 36,38 is wasted. This is to avoid low 15 sunlight causing unintended, or phantom, illuminations of the colour signal aspect.

Light output of the filament lamp 100 is from a filament with a light centre located at a combined focal point C of the outer 38 and inner 40 lenses. The filament lamp 100 has a longitudinal central axis 102 which is inclined by an angle a with respect to a focal plane 20 50 including the focal point C and normal to the focal axis 44. The angle a is 15 degrees.

Referring to Figure 4, both the outer 36 and inner 38 lenses are generally circular dishes. The outer lens 36 has a larger diameter than the inner lens 38. The outer lens 36 has a central biconcave lens 36a surrounded by an array of concentric annular Fresnel 25 sections 36b. The inner lens 38 has a central convex-concave lens 38a surrounded by an array of concentric annular Fresnel sections 38b. The combination of the outer 36 and inner 38 lenses produces a central long range beam LR and peripheral close up sector of light CS.

30 The central biconcave 36a and convex-convex 38a lenses provide the long range beam LR with a width and height of that is comparable to a magnified version of a filament in the filament lamp 100 behind the outer 36 and inner 38 lenses. The annular Fresnel sections 36a, 38b provide forward illumination which makes the long range beam LR appear circular when viewed from a distance. The focal axes 44 of the colour signal aspect 18a, 18b are parallel to each other.

Referring to Figure 5, the long range beam LR of a colour signal aspect 18 is relatively narrow, diverging by an angle β of three degrees from either side the focal axis 44, and approximately circular in cross-section. The long range beam LR is directed at the line of sight of a train driver TD sat in an approaching train and can be seen from up to 800 metres. The long range beam LR appears as a continuous, steadily increasing light to a driver approaching the light signal 10 on straight track. In order to maintain sight of the colour signal aspect 18 as the train driver TD approaches, light output from the signal aspect gradually reduces as the train driver's angle of view, measured from the focal axis 44, increases to a sector outside the angular sweep of the long range beam LR. This sector is referred to as the close up sector of light CS. The signal aspect is readable by a train driver TD within the close up sector of light CS. The close up sector of light CS is defined as the sector of light swept between a point at which the long range beam LR ceases to be effective up to a point two metres from the signal face 20 measured along a line of train driver's sight 52 parallel to the optical axis 44. These are stringent requirements to which the performance of the colour signal aspect 18 must comply.

Referring to Figures 6 to 8, there is shown a SL35 double filament lamp 100 rated at 24 Watts power consumption and is commonly used in colour signal aspects of railways light signals. The filament lamp 100 comprises a clear glass bulb 104 fitted to a generally cylindrical cap 106 concentric with the longitudinal central axis 102. The filament lamp 100 is a double filament lamp meaning that its light output can come from one of two sources: a main filament 108 for illumination in normal usage conditions and an auxiliary filament 1 10 for illumination when there is a fault with the main filament. The cap has three bayonet pins 1 12a, 1 12b, 1 12c protruding radially outwardly. The angle subtended by pin 1 12b and pin 1 12c is 90 degrees. The angle subtended by pin 1 12a and pin 1 12b and by pin 1 12a and pin 1 12c is 135 degrees. The three bayonet pins are for mechanical connection by bayonet fitment to three corresponding bayonet grooves in the socket 30 of the lamp holder 32, only one of which (groove 30a) is shown in Figure 2. The irregular angles subtended by the pins prevent incorrect main filament orientation when the filament lamp 100 is mechanically connected to the socket.

A reference plane 1 14 is defined by the top of the bayonet pins. The reference plane 1 14 is normal to the central axis 102. The main filament 108 is parallel to the reference plane 1 14. The auxiliary filament 1 10 is coaxial with the central axis 102. A filament length of a filament is defined as the linear axial dimension of that filament between its ends which contributes to useful incandescent light output. A train driver TD is able to see whether a long range beam LR is illuminated by a main filament 108 or by an auxiliary filament 1 10 by virtue of the different orientations of their filament length which is magnified by the inner 38 and outer 36 lenses. Upon seeing an illuminated auxiliary filament, a train driver can report a faulty main filament.

The light centre of a filament is defined as the geometrical centre of that filament. A light centre length of a filament is defined as the perpendicular distance from that filament's light centre to the reference plane 114. A light centre length 1 16 of the main filament 108 of the filament lamp 100 used in a conventional railway colour light signal is between 41.5 and 42.5 millimetres.

An axial plane 1 18 which includes the central axis 102 and the engagement pin 1 12a passes through the light centres of the main 108 and auxiliary 1 10 filaments. An axial error of a filament is defined as the perpendicular distance of that filament's light centre from the central axis 102 along the axial plane 1 18. An axial error 120 of the main filament 108 of the filament lamp 100 used in a conventional railway colour light signal is between 2.5 and 3.5 millimetres.

The width of the magnified version of the main filament 108 in the long range beam LR, when illuminated behind the central biconcave 36a and convex-convex 38a lenses, diverges by five degrees. The height of the magnified version of main filament diverges by half a degree. The main filament is orientated perpendicular to the auxiliary filament 1 10. The width of the magnified version of the auxiliary filament in the long range beam LR, when illuminated behind the central biconcave and convex-convex lenses, diverges by half a degree. The height of the magnified version of auxiliary filament diverges by 5 five degrees. The different orientations of the main and auxiliary filaments can be seen by a train driver TD. The train driver sees the main filament as a continuous horizontal line in the long range beam. Whereas the auxiliary filament is seen as a momentary vertical line as the train driver's line of vision traverses the long range beam on approach to the light signaMO.

10

The main filament 108 is electrically connected across a metal jacket 106c around the cap 106 and a main electrical stud 122m located on the bottom of the cap. The auxiliary filament 1 10 is electrically connected across the metal jacket 106c around the cap 106 and an auxiliary electrical stud 122a located on the bottom of the cap. The metal jacket 15 106c around the cap 106 is filament lamp's common electrical contact.

Returning to Figure 3, when the filament lamp 100 is mechanically connected to the socket 30, the main 48m and common 48c spring contacts are electrically connected across the main filament 108 via the main electrical stud 122m and the metal jacket 106c 20 of the cap, respectively. The common 48c and auxiliary 48a spring contacts are electrically connected across the auxiliary filament 1 10 via the metal jacket 106c and the auxiliary electrical stud 122a of the cap, respectively.

Each white signal aspect 26 of the junction route indicator 24 is equipped with a single 25 filament lamp 200 instead of the double filament lamp 100. This is because failure of one filament lamp 200 leaves four remaining operational white signal aspects 26 and the operation of the junction route indicator is not sufficiently impaired to justify the added expense of double filament lamps and their associated control circuits.

30 Referring to Figures 9 and 10, there is shown a SL18 single filament lamp 200 rated at 24 Watts power consumption and commonly used in white signal aspects of railways light signals. The single filament lamp 200 comprises a clear glass bulb 204 fitted to a generally cylindrical cap 206 with a longitudinal central axis 202. Light output from the single filament lamp 200 can only come from a main filament 208. The cap has three bayonet pins 212a,212b,212c protruding radially outwardly. The angle subtended by pin 5 212b and pin 212c is 90 degrees. The angle subtended by pin 212a and pin 212b and by pin 212a and pin 212c is 135 degrees. The three bayonet pins are for mechanical connection by bayonet fitment to three corresponding bayonet grooves in the socket 30 of the lamp holder 32, only one of which (groove 30a) is shown in Figure 2. The irregular angles subtended by the pins prevent incorrect main filament orientation when the 10 filament lamp 200 is mechanically connected to the socket.

A reference plane 214 is defined by the top of the bayonet pins 212a, 212b, 212c. The reference plane 214 is normal to the central axis 202. The main filament 208 is parallel to the reference plane 214. A light centre length 216 of the main filament 208 of the 15 filament lamp 200 used in a conventional railway junction route indicator 24 is between 41.5 and 42.5 millimetres.

An axial plane 218 which includes the central axis 202 and the bayonet pin 212a passes through the light centre of the main filament 208. An axial error 220 of the main filament 20 208 of the filament lamp 200 used in a conventional railway junction route indicator 24 is between 2.5 and 3.5 millimetres.

The width of the magnified version of the main filament 208 in the long range beam LR, when illuminated behind the central biconcave 36a and convex-convex 38a lenses, 25 diverges by five degrees. The height of the magnified version of main filament diverges by half a degree.

The main filament 208 is electrically connected across a main electrical stud 222m and a common electrical stud 222c both of which are located on the bottom of the cap. The 30 white signal aspect 26 is similar in design to the colour signal aspect 18 except that the white aspect signal is proportionately about half the size of the colour signal aspect 18. When the single filament lamp 200 is mechanically connected to a socket of the white signal aspect, main and common spring contacts are electrically connected across the main filament via the main electrical stud 222m and the common electrical stud 222c, respectively. The socket of the white signal aspect does not have an auxiliary spring contact because the single filament lamp 200 does not have an auxiliary filament.

Referring to Figure 1 1 , the signal control circuit 300 comprises a step-down transformer T1 ,T2 and filament switching relay ER1 ,ER2 for each respective colour signal aspect 18a, 18b and a terminal board 310 with eight terminals numbered 31 1 to 318. The terminals 31 1 to 315 and 318 are electrically coupled to a railway signal box remote from the signal 10. Terminals 316 and 317 are redundant. The signal control circuit 300 comprises an alarm circuit 340 for detecting faulty main filaments 108 and reporting the fault to a remote signal box connected to the signal control circuit. The signal control circuit 300 is located within the signal head 12.

Terminals 31 1 and 312 are connected across the primary coil of transformer T1. The common terminal stem 48c is connected to one end of the secondary coil of the transformer T1 via line 323a. The main terminal stem 48m is connectable to the other end of the secondary coil of transformer T1 via relay contacts 2-7 of the filament switching relay ER1 and line 324a. The auxiliary terminal stem 48a is connectable to the secondary coil of transformer T1 partway between the main and common terminal stems via relay contact 5-8 of the filament switching relay ER1 and line 325a. Relay contact 2-7 is open and relay contact 5-8 is closed when the filament switching relay ER1 is de- energised.

Terminals 313 and 314 are connected across the primary coil of transformer T2. The common terminal stem 48c is connected to one end of the secondary coil of the transformer T2 via line 323b. The main terminal stem 48m is connectable to the other end of the secondary coil of transformer T2 via relay contact 2-7 of the filament switching relay ER2 and line 324b. The auxiliary terminal stem 48a is connectable to the secondary coil of transformer T2 partway between the main and common terminal stems via relay contact 5-8 of the filament switching relay ER2 and line 325b. Relay contact 2-7 is open and relay contact 5-8 is closed when the filament switching relay ER2 is de- energised. When the green colour signal aspect 18a is to be illuminated, the signal box normally supplies 1 10 Volts a.c. across terminals 31 1 and 312 and the primary coil of transformer T1 , although the signal box may supply 12 Volts a.c. or d.c, in which case the signal control circuit may by-pass the transformer T1 (and transformer T2 mentioned below). The signal box electric supply can fluctuate according to external factors on and around the railway line, like, for example, track circuit interference caused by passing electric trains. The signal control circuit 300 is designed to withstand these fluctuations.

When 1 10 Volts a.c. is supplied across the primary coil of transformer T1 output from the secondary coil of the transformer T1 is about 12 Volts a.c. Normally, a potential difference across relay contacts 2-7 and 5-8 is detected by filament switching relay ER1. The filament switching relay ER1 is energised causing the relay contact 2-7 to close and the relay contact 5-8 to open. An electric current of about 2 Amperes flows along line 324a, the main filament 108 and line 323a to illuminate the main filament 108. The filament switching relay ER1 remains energised and the relay contacts 2-7 remain closed while at least 1.2 Amperes flows through the circuit of the main filament 108 (i.e. about 60% of normal electric current through the main filament).

Exceptionally, if the main filament 108 is faulty, or if electric current flowing through the main filament circuit drops below 1.2 Amperes, the filament switching relay ER1 is de- energised, the relay contact 2-7 opens and the relay contact 5-8 closes. A diminished electric current (i.e. less than 1.2 Amperes) flows through line 325a, the auxiliary filament 1 10 and line 323a to illuminate the auxiliary filament 1 10. The auxiliary filament has a reduced illumination intensity in comparison to the main filament. Also, the auxiliary filament is oriented at 90 degrees with respect to the main filament.

When the green colour signal aspect 18a is to be extinguished, the signal box ceases to supply 1 10 Volts a.c. across the terminals 31 1 and 312. The primary and secondary coils of the transformer T1 are de-energised. Normally, electric current ceases to flow through the main filament circuit and light from the main filament 108 is extinguished. The filament switching relay ER1 is de-energised, the relay contact 2-7 opens and the relay contact 5-8 closes. No electric current flows through the auxiliary filament 1 10.

If the auxiliary filament 1 10 is to be extinguished, because the main filament 108 is faulty, electric current ceases to flow through the auxiliary filament circuit and light from the auxiliary filament 1 10 is extinguished. Relay contact 2-7 remains open and relay contact 5-8 remains closed.

When the red colour signal aspect 18b is to be illuminated, the signal box normally supplies a 1 10 Volts a.c. across terminals 313 and 314 and the primary coil of transformer T2. The signal box supply may fluctuate across terminals 313,314 just like it may across terminals 31 1 ,312. Output from the secondary coil of the transformer T2 is about 12 Volts a.c. Normally, a potential difference across relay contacts 2-7 and 5-8 is detected by filament switching relay ER2. The filament switching relay ER2 is energised causing the relay contact 2-7 to close and the relay contact 5-8 to open. An electric current of about 2 Amperes flows along line 324b, the main filament 108 and line 323b to illuminate the main filament 108. The filament switching relay ER2 remains energised and the relay contact 2-7 remain closed while at least 1.2 Amperes flows through the circuit of the main filament 108 (i.e. about 60% of normal electric current through the main filament). Exceptionally, if the main filament 108 is faulty, or if electric current flowing through the main filament circuit drops below 1.2 Amperes, the filament switching relay ER2 is de- energised, the relay contact 2-7 opens and the relay contact 5-8 closes. A diminished electric current (i.e. less than 1.2 Amperes) flows through line 325b, the auxiliary filament 1 10 and line 323b to illuminate the auxiliary filament 1 10 with reduced intensity in comparison to the main filament. The auxiliary filament has a reduced illumination intensity in comparison to the main filament. Also, the auxiliary filament is oriented at 90 degrees with respect to the main filament.

When the red colour signal aspect 18b is to be extinguished, the signal box ceases to supply 1 10V a.c. across the terminals 313 and 314. The primary and secondary coils of the transformer T2 are de-energised. Normally, electric current ceases to flow through the main filament circuit and light from the main filament 108 is extinguished. The filament switching relay ER2 is de-energised, the relay contact 2-7 opens and the relay contact 5-8 closes. No electric current flows through the auxiliary filament 1 10. If the auxiliary filament 1 10 is to be extinguished, because the main filament 108 is faulty, electric current ceases to flow through the auxiliary filament circuit and light from the auxiliary filament 1 10 is extinguished. Relay contact 2-7 remains open and relay contact 5-8 remains closed. The alarm circuit 340 comprises auxiliary relay contacts 3-1 of filament switching relays ER1 and ER2 connected in parallel across terminals 315 and 317 which are connected to the signal box. Each auxiliary relay contact 3-1 is closed when its respective filament ER1 , ER2 is de-energised. In normal operating conditions, neither main filament 108 is faulty, one of the filament switching relays ER1 ,ER2 is energised, its respective auxiliary relay contact 3-1 is open and its respective colour signal aspect 18a, 18b is illuminated. If one of the main filaments 108 is faulty, its filament switching relay ER1 ,ER2 is not, as explained above, energised and its auxiliary relay contact 3-1 remains closed. This fault condition is transmitted by the alarm circuit 340 to the signal box to alert signal maintenance personnel that a double filament lamp 100 is illuminated by auxiliary filament 1 10. This is in addition to an indication received by separate alarm circuitry outside the signal 10 that electric current consumption of the double filament lamp 100 in a colour signal aspect 18 has dropped below the threshold of 60% of normal electric current through the main filament 108 and that the lamp is faulty. This is also in addition to the optical indication received by the train driver who should have seen that a double filament lamp 100 is illuminated by auxiliary filament 1 10. A signal control circuit of the white signal aspect 26 does not include the alarm circuit 340 because the single filament lamp 200 does not have an auxiliary filament. If the main filament 208 is faulty, a fault indication is received by separate alarm circuitry outside the signal 10 that electric current consumption of the single filament lamp 200 in a white signal aspect has dropped below the threshold of 60% of normal electric current through the main filament 208 and that the lamp is faulty. This is in addition to the optical indication received by the train driver who should have seen that a single filament lamp 200 is extinguished. Referring to Figures 12 to 16, there is shown a LED light unit 400 for replacement of a double filament lamp 100 in a colour signal aspect 18 or for replacement of a single filament lamp 200 in a white signal aspect 26.

The LED light unit 400 comprises a rigid support member 404 mounted upon a generally cylindrical cap 406 concentric with a longitudinal central axis 402. The cap 406 corresponds in shape and dimensions to the cap 106 of the double filament lamp 100 and the cap 206 of the single filament lamp 200. Light output is from a LED element arranged as a LED array 408 of five LEDs 408a,408b,408c,408d,408e. The LEDs are LED chips, although other types of LEDs may be used in the LED light unit 400. The cap has three bayonet pins 412a,412b,412c protruding radially outwardly. The angle subtended by pin 412b and pin 412c is 90 degrees. The angle subtended by pin 412a and pin 412b and by pin 412a and pin 412c is 135 degrees. The three bayonet pins are for mechanical connection by bayonet fitment to three corresponding bayonet grooves in the socket 30 of the lamp holder 32, only one of which (groove 30a) is shown in Figure 2. The irregular angles subtended by the pins prevent incorrect LED element orientation when the LED light unit 400 is mechanically connected to the socket.

The LED light unit 400 comprises three rigid circuit boards 413,415,417 connected by a fastening assembly to the support member 404. The fastening assembly comprises four screws 419 each fastened to a respective nut 421. The second circuit board 415 is connected directly to an upper surface 423 of the support member 404. The first circuit board 413 mounted upon, and parallel to, the second circuit board 415. The first 413 and second 415 circuit boards are spaced apart by spacer washers 424 to create a gap 426 between the upper faces of the first 413 and second 415 circuit boards. The third circuit board 417 is mounted upon, and parallel to, a lower surface 428 of the support member 404. The third circuit board 417 and the lower surface 428 of the support member 404 are spaced apart by spacer washers 430 to thermally de-couple the third circuit board from the support member 404. The three rigid circuit boards 413,415,417 and the support member 404 are inclined with respect to the central axis 402 by the angle a.

Three LEDs 408a,408c,408e are mounted upon the first circuit board 413. Two LEDs 408b, 408d are mounted on the second circuit board 415. The five LEDs project light in a forward direction indicated by an arrow A. Light from the two LEDs 408b, 408d of the second circuit board 415 passes through two elongate slits 432b, 432d in the first circuit board 413 wherein the axis of elongation of each elongate slit is substantially parallel to the central axis 402, as is best shown in Figure 14. The length of each slit 432b,432d along its axis of elongation is 6.7mm which is sufficiently long that light from LEDs 408b and 408d may fall upon the entire height of the portion of the inner lens 38 located directly in front of LEDs 408b and 408d. The width of each slit 432b, 432d perpendicular to its axis of elongation is 2.7mm which is substantially the same as the width of its respective LED 408b,408d.

A reference plane 414 is defined by the top of the bayonet pins 412a, 412b, 412c. The reference plane 414 is normal to the central axis 402. The LED array 408 is generally W-shaped in a LED plane 434 orthogonal to the three circuit boards 413,415,417 and the support member 404, as is best shown by Figures 15 and 16. The LED array 408 appears straight when viewed in an opposite direction to arrow A, as is best shown in Figure 14. A light centre LC of the LED array 408 is defined herein as its geometrical centre which, in this case, is the geometrical centre of its W-shape in the LED plane 434. The geometrical centre of the LED array 408 is halfway between the extreme ends of the W- shape in the LED plane 434 (i.e. the centres of electroluminescence in LEDs 408a and 408e) and halfway between the two sides of the W-shape in the LED plane 434 (i.e. one front row 436 defined by a line through the centres of electroluminescence in LEDs 408a, 408c and 408e and another back row 438 defined by a line through the centres of electroluminescence in LEDs 408b and 408d). A light centre length of the LED array 408 is defined herein as the perpendicular distance 416 from the light centre LC to the reference plane 414. The light centre length 416 of the LED array 408 is the same as the light centre length of the double filament lamp 100 and the single filament lamp 200 and is between 41.5 and 42.5 millimetres.

An axial plane 418 which includes the central axis 402 and the bayonet pin 412a passes through the light centre LC of the LED array 408. An axial error 420 of the LED array 408 is defined herein as the perpendicular distance of the light centre LC from the central axis 402 along the axial plane 418. The axial error 420 of the LED array 408 is the same as the axial error of the main filament 108 of the double filament lamp 100 and the single filament lamp 200 and is between 2.5 and 3.5 millimetres. The axial error 420 is half the length of the gap 426 between the first 413 and second 415 circuit boards. The LED light unit 400 may be used as a direct substitute for a double filament lamp 100 or the single filament lamp 200 and provide all the benefits of LED electroluminescence over filament incandescence. In use, the LED lamp unit 400 is mechanically connected to the lamp holder 32 of the signal aspect 18,26 by bayonet fitment between the bayonet pins 412a,412b,412c and corresponding bayonet grooves 30a in the socket 30 of the lamp holder 32. An LED light unit 400 with commercially available LEDs 408a,408b,408c,408d,408e coloured green is for use in the green signal aspect 18a to ensure the correct colour and intensity of illumination through the inner 38 and outer 36 lenses. A LED light unit 400 with commercially available LEDs coloured red is for use in the red signal aspect 18b to ensure the correct colour and intensity of illumination through the inner 38 and outer 36 lenses. Likewise, an LED light unit 400 with commercially available LEDs coloured white, or yellow, is needed for use in a white, or yellow, signal aspect 18, for the same reasons. In the case of LEDs coloured yellow, the back row LEDs 408b, 408d may need a colour correction filter so that the LED array 408, as a whole, may comply with railway specifications for yellow aspects. Referring to Figure 17, the LED light unit 400 comprises a LED driver circuit 500 for supplying electric power to the LEDs 408a,408b,408c,408d,408e of the LED array 408. The LED driver circuit 500, or parts thereof, is mounted upon one or more of the first 413, second 415 or third 417 circuit boards. The LED driver circuit 500 comprises a common electrical cable 422c for connection to the common terminal stem 48c of the lamp holder 30 and a main electrical cable 422m for connection to the main terminal stem 48m of the lamp holder. As already described above, the common terminal stem 48c is connected to one end of the secondary coil of the transformer T1 via line 323a. The LED driver circuit may operate with the filament switching relay ER1. Optionally, the filament switching relay ER1 may be replaced by a hard-wired unit LR1. The main terminal stem 48m is connected to the other end of the secondary coil of transformer T1 via pins 2-7 of hard-wired unit LR1. The auxiliary terminal stem 48a is connected to a mid-point of the secondary coil of transformer T1 via pins 5-8 of hard-wired unit LR1 to maintain the usual integrity of this line for circuit proving by signal maintenance personnel. However, the auxiliary terminal stem 48a is open circuit and is not connected to the LED driver circuit. The LED light unit 400 has no electrical contacts on or around the cap 406. The spring contacts 46m, 46a, 46c of the socket 30 are redundant. This eliminates a potential electrical connection failure common with conventional filament lamps. The spring contacts are prone to damage during removal or attachment of filament lamps in the socket 30 of the lamp holder 32.

When the green colour signal aspect 18a is to be illuminated, the signal box supplies a 1 10V a.c. across terminals 31 1 and 312 and the primary coil of transformer T1. Output from the secondary coil of the transformer T1 is about 12V a.c. depending on the input supply to the primary coil. The LED driver circuit 500 comprises a bridge rectifier 502. The 12V a.c. output supply from the transformer T1 is connected, via electrical cables 422m, 422c, across the bridge rectifier 502 which rectifies it into a 12V d.c electric supply.

The LED driver circuit 500 comprises the LEDs 408a,408b,408c,408d,408e and associated constant current circuits 504a, 504b, 504c, voltage monitoring circuits 5 506a, 506c, resistor switching circuits 508a, 508c and zener diode 510, as is explained in more detail below.

The LEDs 408a, 408b are connected in series. A constant current circuit 504a receives the electric current from the bridge rectifier 502, via line 530, and supplies the LEDs

10 408a, 408b with a constant electric current below the maximum current rating of the LEDs. Electric current from the LEDs returns to the bridge rectifier 502 via line 531. A voltage monitoring circuit 506a is connected across lines 530,531 and in parallel with the LEDs. The voltage monitoring circuit 506a checks for open circuit and short circuit fault conditions with the LEDs. A resistor switching circuit 508a is connected between the

15 voltage monitoring circuit 506a and the line 530 to detect operation of the LEDs and their electric power consumption.

The LED 408c is connected in series with a zener diode 510 orientated in reverse-biased direction with respect to the LED 408c. A constant current circuit 504b receives the

20 electric current from the bridge rectifier 502, via line 530, and supplies the LED 408c and zener diode 510 with a constant electric current below the maximum current rating of the LED 408c and zener diode 510. Electric current from the LED 408c and the zener diode 510 returns to the bridge rectifier 502 via line 531. The zener diode 510 stabilises voltage drop across LED 408c. The zener diode 510 is selected to have an impedance

25 equivalent to each one of LEDs 408a,408b,408c,408d,408e.

The LEDs 408d,408e are connected in series. A constant current circuit 504c receives the electric current from the bridge rectifier 502, via line 530, and supplies the LEDs with a constant electric current below the maximum current rating of the LEDs. Electric 30 current from the LEDs returns to the bridge rectifier 502 via line 531. A voltage monitoring circuit 506c is connected across lines 530,531 and in parallel with the LEDs. The voltage monitoring circuit 506c checks for open circuit and short circuit fault conditions with the LEDs. A resistor switching circuit 508c is connected between the voltage monitoring circuit 506c and the line 530 to detect operation of the LEDs and their electric power consumption.

5

The pairs of LEDs 408a,408b, LEDs 408d,408e and LED 408c and zener diode are connected in parallel, via lines 530,531 , across the output supply from the bridge rectifier 502. As a result of the zener diode's characteristics, voltage drop across LED 408c and the zener diode 510 is the same, or similar, to voltage drop across the pairs of LEDs 10 408a,408b and LEDs 408d,408e in normal operating conditions.

The LED driver circuit 500 comprises a power resistor 512 to draw surplus electric supply from the transformer T1 and dissipating it in the form of heat. The bridge rectifier 502 and the power resistor 512 are connected in parallel, via electrical cables 422m, 422c,

15 across the output supply from the secondary coil of the transformer T1. The power resistor 512 is connected on one side to cable 422m, via line 532, and on another side to cable 422c, via line 533. An electronic switch 514 is connected between the power resistor 512 and main electrical cable 422m. The electronic switch 514, such as a bidirectional triode thyristor, is for controlling the alternating current output supply from

20 the secondary coil of the transformer T1 to the power resistor 512.

The resistor switching circuit 508c is connected, via line 534, to a junction in the line 532 between the electronic switch 514 and the sink resistor 512. The resistor switching circuits 508a, 508b are connected in series, via line 535. A control line 536 from the 25 resistor switching circuit 508a is connected to the electronic switch.

The power consumption of the LED driver circuit 500 is rated at about 24 Watts which is intended to be the same as the single filament lamp 100 or the double filament lamp 200 that the LED light unit 400 replaces. In normal operating conditions, the electric current 30 flowing through the main 422m and common 422c electrical cables is approximately 2 Amperes. The power consumption of the LEDs 408a,408b,408c,408d,408e and associated circuits is about 14.4W (i.e. about 60% of the overall power rating of the LED driver circuit as a whole). An electric current of about 1.2 Amperes flows through the bridge rectifier 502 to illuminate the LEDs 408a,408b,408c,408d,408e. Provided that the total current flowing through the LEDs, as detected by the resistor switching circuits 5 508a, 508c, is at least 1.2 Amperes, which indicates that the LEDs are illuminated and functioning normally, the resistor switching circuits 508a, 508c switch ON the electronic switch 514 to supply electric current to the power resistor 512. The power consumption of the power resistor 512 is about 9.6W. An electric current of 0.8 Amperes flows through the power resistor 512 which dissipates surplus energy in the form of heat. The 10 power resistor 512 is equipped with a heat sink 512a to augment heat dissipation. Both the power resistor 512 and heat sink 512a are mounted to an exposed part of the third circuit board 417.

Exceptionally, one or more of the LEDs 408a,408b,408c,408d,408e may be faulty and/or

15 combined light output from the LEDs may be degraded. If, as a result of such conditions, the total current flowing through the LEDs, as detected by the resistor switching circuits 508a, 508c, is below about 1.2 Amperes the resistor switching circuits 508a, 508c switch OFF the electronic switch 514 to cut supply of electric current to the power resistor 512. The electric current through the LED driver circuit 500, as a whole, drops by 0.8 Amperes

20 to pass positively below a 1.2 Amperes threshold (i.e. 60% of normal electric current flow through the LED driver circuit 500 as a whole). In this event, the separate alarm circuitry outside the signal 10 detects that electric current consumption of the LED light unit 400 has dropped below the threshold of 60% of normal electric current through a main filament 108 of a filament lamp 100 and that a fault condition exists with the LED light unit

25 400. This is in addition to the optical indication received by a train driver who may have seen a gap caused by an extinguished LED, or seen an orphaned illuminated LED, as a momentary vertical line in the long range beam. This replicates the visual effect on the train driver caused by a faulty double filament lamp with an illuminated auxiliary filament. Replacement of the filament changeover relay ER1 connected to the green signal aspect

30 18a with the hard-wired unit LR1 prevents the alarm circuit 340 from indicating a main filament failure in a double filament lamp 100 which has in reality been replaced by the LED light unit.

The skilled person will understand that a LED driver circuit 500 of a LED light unit 400 used in the red signal aspect 18b is the same as the LED driver circuit 500 described above. A hard-wired unit LR2 which replaces the filament switching relay ER2 operates in the same way as the hard-wired unit LR1 which replaces the filament switching relay ER2. The step-down transformer T2 is the same as step-down transformer T1. The only difference is that a different supply of 1 10V a.c. from the signal box is connected across terminals 313 and 314 and the primary coil of transformer T2 when the red colour signal aspect 18b is to be illuminated.

Referring to Figure 18, the LED light unit 400 is mechanically and electrically connected to the lamp holder 32. The central axis 402 is inclined by the angle a (i.e. 15 degrees) with respect to the focal plane 50 so that the circuit boards 413,415,417 and the support member 404 are parallel to the focal plane 50 of the signal aspect 18. The light centre LC is coincident with the focal point C of the outer 36 and inner 38 lenses. The LED plane 434 includes the focal axis 44 and the LED array 408 projects light in a forward direction indicated by the arrow A. The width of the LED array 408 in the LED plane 434 (i.e. distance between the centres of electroluminescence in LEDs 408a and 408e) is 1 1.5mm. The gap 426 between the two rows of the LED array 408 in the LED plane 434 (i.e. distance between a front row 436 defined by a line through the centres of electroluminescence in LEDs 408a, 408c and 408e and a back row 438 defined by a line through the centres of electroluminescence in LEDs 408b and 408d) is 2.1 mm. The W-shape of the LED array 408, as viewed from above the LED plane 434, staggers and compresses light output from the LEDs 408a, 408b, 408c,408d,408e, as viewed along the focal axis 44 in the opposite direction to arrow A, with the light centre LC located at the geometric centre of the LEDs. This arrangement of the LED array blends the areas of high and low electroluminescence. The LED light unit 400 illuminates a signal aspect 18 in a way that is indistinguishable from the orientation, width, height, continuity and intensity of the filament length of a main filament 108 of a filament lamp 100,200 even when magnified and projected over long distance as is the case when viewed by a train driver in the long range beam LR. The power consumption of the LED light unit 400 is the substantially the same as a filament lamp 100,200. The electric current threshold (i.e. about 1.2 Amperes or 60% of normal electric current consumption) at which the LED light unit 400 functions normally is the same as a filament lamp 100,200. The light signal 10 and its alarm circuitry, which are configured to detect an illumination fault with the filament lamp, can detect an illumination fault with the LED light unit using the existing systems. Also, existing filament lamp proving circuits can be used to test electrical integrity of the LED light unit because its fault condition manifests itself in the same way as the filament lamp it replaces.

Mechanical and electrical detachment of a filament lamp 100,200 from a signal aspect 18 and subsequent mechanical connection of the LED light unit 400 to the socket 30 of the lamp holder 32 benefit from the speed of bayonet fitment. The cap 106,206 of the filament lamp and the cap 406 of the LED light unit are interchangeable. No modification is required to the socket 30 of the lamp holder 32. If the filament changeover relay ER1 ,ER2 is to be replaced by the hard-wired unit LR1 ,LR2, the filament changeover relay is unplugged and the hard-wired unit is plugged in its place. Electrical connection of the main 422m and common 422c electrical cables to the main 48m and common 48c terminal studs is by means of a spanner. The signal aspects in one light signal can be fitted with LED light units, aspect by aspect, and between trains to minimise disruption.