GB2395074A

GB2395074A - Control circuit for rail signal

Info

Publication number: GB2395074A
Application number: GB0225667A
Authority: GB
Inventors: Barry Wright
Original assignee: Dorman Traffic Products Ltd
Current assignee: Dorman Traffic Products Ltd
Priority date: 2002-11-04
Filing date: 2002-11-04
Publication date: 2004-05-12
Anticipated expiration: 2022-11-04
Also published as: GB0225667D0; GB2395074B

Abstract

A control circuit (10) for a light source in a rail signal is disclosed, the light source including an array of LEDs (22) connected in series, where the circuit maintains the colour output of the light source within required limits. The circuit may adjust the current flowing through the LEDs to control the colour output. The circuit may include proving resistors and a temperature sensor whereby the circuit operates to maintain the sensed temperature at or above a certain level.

Description

CONTROL CIRCUIT

The present invention relates to rail signals. It 5 is particularly, but not exclusively, concerned with circuits for controlling rail signals, and in particular rail signals using light emitting diodes (LEDs) as a light source.

Rail signals are used to convey information to train 10 drivers, such as whether it is safe to proceed along a certain track, or whether a particular route has been set. Almost all rail signals currently in use involve the use of one or more white or coloured light elements.

Each particular arrangement of illuminated light elements 15 is called an "aspect", and conveys a different message to a driver. One signal is normally capable of displaying a number of different aspects, typically through illumination of one or more different light elements at any one time.

20 Two particular types of signal can be identified: position light signals (also called ground position lights) are normally located on the ground and are used in areas such as sidings and maintenance areas; colour light signals (CLS - also called colour line signals) are

( normally located directly above, or above and to one side of, a main line. CLS are generally used to control traffic flow along one or more tracks of a main line.

In the UK, all signals which are to be used as CLS 5 must conform to Railtrack line specification RT/E/S/10062

(current issue 1, August 1999). This specification lays

out minimum conditions for, among other things format, reliability, colour and visibility. Similar standards exist for ground position lights and in other countries.

10 Most current light signals used on railways use tungsten filament bulbs as light sources. However, the potential advantages of using LEDs instead of such filament bulbs have recently been realized. The present invention relates to signals in which LEDs are used as 15 the light source(s).

The LED-based signals currently in use generally have some form of feedback circuit which controls the brightness of the light output to maintain it within the required limits. One method of providing this feedback 20 is to monitor the light intensity of a separate feedback LED, which is connected in series with the LEDs which form the light source, with a sensor such as a phototransistor.

( The photosensor produces a feedback current depending on the sensed brightness of the feedback LED.

This feedback current is passed to the LED control circuit which compares it to a reference, and adjusts the 5 input current to the LEDs accordingly. Generally a photosensor will produce a greater current at higher light intensities, and so the control circuit applies an inverse relationship between the feedback current and the LED current.

10 However, over the temperature range over which signals are required to operate (generally -30 C to +40 C, although this varies from country to country), the performance of both the LEDs and the sensor(s) are affected by changes in temperature.

15 Generally the brightness of LEDs increases as the operating temperature decreases. Figure lo shows a typical intensity vs. wavelength graph at 3 different temperatures for an AlInGaP LED.

Light sensors also exhibit performance variations 20 over the temperature ranges. Figure ll shows an output current vs. temperature graph for a typical phototransistor (OP 802 WSL) maintained under constant illumination.

( In current situations, the effect of these changes is to cause the light output from the LEDs to become much brighter at lower operating temperatures. This is disadvantageous as it may decrease the operating life of 5 the LEDs, and it may also cause the LEDs to operate outside the brightness parameters imposed on the signal.

Signals which use LEDs as light sources still have to comply with the same specifications as those using

filament bulbs. In particular the light output from the 10 light sources must fall within the same specifications

regarding colour and visibility.

The colour requirements are generally defined with regard to a CIE(1931) chromaticity chart. A brief discussion of chromaticity is given below, and further Is detail can be obtained from standard texts on the subject such as Light and Sound for Engineers - R. C. Stanley [Nelson 1968], the relevant parts of which are incorporated by reference.

The discussion of chromaticity, both in general and 20 with regard to the invention is made with the specific provisions of Railtrack line specification RT/E/S/10062

(current issue 1, August 1999), Railtrack Group Standard GK/RT0031 (Issue 3 - Section B26), and BS 1376: 1974, which together define the colour requirements for CLS

( which are to be used on railways in the UK. Similar, but not identical, standards exist in many other countries, and the invention is equally applicable to those standards. 5 A CIE(1931) chromaticity chart is shown at Figure 1, with the reference axes being the x and y standard co ordinates for such a chart as discussed below. The relevant areas for signal lights as defined by the above referenced Group Standard are outlined. In the case of 10 each colour, three bands are identified, called classes A, B and C. Different types of signal are constrained to have operating colours within a particular class. For example, in accordance with the above mentioned Group Standard, the yellow light source for CLS must be within 15 the region marked "signal yellow class B" at all times.

Whilst light of a single wavelength ("monochromatic light") can be defined by reference to its wavelength, the visual impression made by a complex light source is rarely so simple. A trichromatic calorimeter allows any 20 colour illumination to be compared to a comparison field

to which a mixture of specified primary sources is applied. The proportions of those primary sources can be varied to produce an identical colour impression. The quantity of each source used is then measured. The

( actual colour can then be plotted on a colour triangle in which the vertices represent the three primary stimuli used (normally blue, green and red) and described by its "tristimulus values" (i.e. the amount of each primary 5 source needed to create the colour). Unfortunately different observers tend to arrive at slightly different tristimulus values for the same colour. The CIE standard brings together mean tristimulus values for observed colours to define a standard observer to which all to calorimetric results can be related.

By convention since white is a neutral colour, it is matched by equal amounts of each of the primaries and lies at the centre of the colour triangle. The amounts of each primary required are not measured in lumens (or 15 other units of luminous flux), but in units of each stimuli. The units are defined such that one unit of red, plus one unit of green, plus one unit of blue produces an observed colour equivalent to that of mean noon sunlight. A given colour can then be defined in 20 terms of the number of units needed to produce one unit of that colour. Together this means that the total number of units of all primaries must equal 1. This obviously means that it is only ever necessary to specify two of the coefficients (the third can be calculated),

( and so these can be easily plotted on a 2 axis graph, which is called a chromaticity chart.

However, one of the weaknesses of the CIE system is that there are no three monochromatic lights whose 5 mixture can replicate all the spectrum colours. If the three primaries chosen were monochromatic, then all spectral colours would lie just outside the colour triangle, requiring some of the coefficients to be negative. To avoid this, three imaginary primaries are 10 chosen (by convention labelled X (substantially red), Y (substantially green) and Z (substantially blue)) and mathematically related to the real monochromatic primaries. Therefore a standard CIE chromaticity chart such as that shown in Figure l plots the values of X and 15 Y units for each given colour. The "spectrum locus" is the plot of all monochromatic colours in the spectrum in terms of their chromaticity coordinates.

A chromatic colour can also be described in terms of the "dominant wavelength" of the light and its purity.

20 To do this a straight line is drawn on the chromaticity chart from the point representing white (l/3, l/3) through the point representing the colour. The point at which this meets the spectrum locus is the dominant wavelength, and the purity is the ratio of the distance

( from white to the colour to the distance from white to the spectrum locus along that line.

A particular problem arises concerning yellow rail signal lights when LEDs are used as the light source.

5 Figure 2 shows a how the chromatic colour output from a typical yellow LED varies with temperature. It can be seen that over the operating temperature range required (-20 C to +40 C) the colour output is not always within the specified ranges. The quadrilateral boundary shown 10 on the diagram is the allowable values for the light to fall within "Signal Yellow Class B" in accordance with BS 1376: 1994. The line running from top-left to bottom-

right of the diagram is the spectrum locus.

From Figure 2 it can be seen that a signal using 15 standard yellow LEDs would not conform to the required specifications for light output over the operating

temperature range.

At it broadest the present invention provides a rail signal including a light source including a plurality of 20 LEDs and control means arranged so as to maintain the colour of the light output from the light source such that it has a dominant wavelength of between 585nm and 605nm and a purity of at least 80% over the temperature range -20 C and +40 C.

( Optionally the signal also includes a temperature sensor, such as a thermistor, which is connected to the control means.

The operation of the control means may be such that 5 the light output of the light source has a minimum purity of 85% or even 90% in the conditions specified.

Similarly the operation of the control means may be such that the light output of the light source has a lower limit to the dominant wavelength of 587nm and/or an upper 10 limit to the dominant wavelength of 600nm or in a further alternative an upper limit of 595nm, again in the conditions specified.

In particular embodiments the operation of the control means may be such that the light output of the 15 light source is maintained within the range of CIE (1931) chromaticity values enclosed by a quadrilateral having the following vertices: (0.546, 0.426); (0.560, 0.440); (0.585, 0. 415); (0.576, 0.406), again in the conditions specified. This quadrilateral corresponds to the 20 allowable chromaticity to conform to "Signal Yellow Class B" in accordance with BS 1376: 1994.

The control means which operates so as to maintain the colour output of the light source within the required limits can work in a number of ways.

( In a first aspect, the present invention provides a rail signal including a light source, the light source including a plurality of LEDs; and control means arranged 5 so as to control the output of the light source, wherein the light source includes at least two separate sets of LEDs, each set of LEDs emitting a different colour of light at a given temperature; the control means is configured to adjust the current flowing through each set 10 of LEDs; the type of LED in each set is chosen such that for any given temperature in a predetermined range, the light output from at least one of the sets of LEDs is within specified conditions; and the control means adjusts the current flowing through each set of LEDs such 15 that the light output from the light source is within specified conditions over the predetermined temperature range. The specified conditions may relate to the brightness and or chromaticity values of the light 20 output. These chromaticity values may relate to any colour of light, including, but not limited to white, red, green and yellow.

The predetermined temperature range may be between -30 C and +400C or between -20 C and +90 C.

( In embodiments of the first aspect, the LEDs are chosen such that at least one set has a dominant wavelength of between 585nm and 605nm and a purity of at least 80%, and the control means operates such that the 5 light output of the light source has a dominant wavelength of between 585nm and 605nm and a purity of at least 80%.

The selection of the LEDs in this first aspect may be such that at least one of the sets has a light output 10 with a minimum purity of 85% or even 90% at any given temperature between -20 C and +40 C. Similarly the selection of the LEDs may be such that at least one of the sets has light output with a lower limit to the dominant wavelength of 587nm and/or an upper limit to the 15 dominant wavelength of 600nm or 595nm, at any given temperature between -20 C and +40 C.

In particular embodiments the selection of the LEDs may be such that at least one of the sets has a light output within the range of CIE (1931) chromaticity values 20 enclosed by a quadrilateral having the following vertices: (0.546, 0.426); (0.560, 0.440); (0.585, 0.415); (0.576, 0.406), at any given temperature between -20 C and +40 C. This quadrilateral corresponds to the allowable

( 12 1 chromaticity to conform to "Signal Yellow Class B" in accordance with BS 1376: 1994.

In a second aspect, there is provided a rail signal 5 including a light source the light source including a plurality of LEDs; control means arranged so as to control the light output of the light source; a proving circuit having at least two proving resistors capable of | drawing a predetermined proving current, wherein the 10 control means controls the proving current flowing through each of the resistors of the proving circuit. A temperature sensor is connected to the control means and located thermally proximate to the LEDs, at least a first of the proving resistors is located in a position 15 thermally proximate to the LEDs, and at least a second of the proving resistors is located in a position thermally distant from the LEDs; the control means operates so as to maintain the sensed temperature at or above a predetermined level; and the LEDs are chosen such that 20 the chromaticity values of the light output from the light source falls within specified conditions for temperatures between said predetermined level and +40 C.

In an embodiment of the second aspect, the LEDs are chosen such that the light output has a dominant

( wavelength of between 585nm and 605nm and a purity of at least 80% for temperatures between said predetermined level and +40 C.

Preferably the control means operates to cause the 5 proving current to pass through said first proving resistor when the temperature is sensed to be below a predetermined value, to maintain the temperature of the circuit board on which the LEDs are mounted at or above a predetermined value. The control means may continually 10 adjust the amount of current flowing through each proving resistor depending on the sensed temperature, or may switch the current between the different proving resistors depending on the sensed temperature.

Preferably the predetermined value of temperature is 15 between +15 and + 25 C. In one embodiment the predetermined value is around +20 C.

The proving system on which the second aspect of the invention is based is a general safety requirement in many rail signals. Each signal is part of a feedback 20 loop connected to a control centre, which monitors whether a signal is considered to be operational or not.

In the UK (and other countries) this monitoring comprises monitoring the current drawn by the signal. For example a signal which is nominally drawing over 250mA is

( considered to be operational. Whilst this system could provide direct feedback for filament lamps, which draw currents greater than this when operational, typical LED based signals may only draw around 75mA or less when 5 operational. Therefore, in order to be compatible with the existing monitoring systems, signals using LEDs are normally required to have a separate "proving circuit" capable of drawing a sufficient current to "prove" the operational status of the signal. The proving circuit 10 typically includes a system that monitors the light output of the signal, for example based on the current drawn by the LEDs, or direct monitoring. A resistor (termed a "proving resistor") ensures that sufficient current is drawn when the signal is deemed to be 15 operational by the proving circuit.

Both the LEDs and the proving resistor generate heat when current flows through them. In normal operation of existing signals, this heat is generally undesirable as, for example, operating the LEDs at a higher temperature 20 than necessary will reduce their active life.

The second aspect above recognises that under some conditions the heat created by the proving resistor can be used beneficially. For example, under normal operating conditions, i.e. when the temperature measured

( is above the predetermined value, the control means passes all or most of the proving current through the second proving resistor, which is thermally remote from the LEDs. As the measured temperature decreases, the 5 control means adjusts the proportion of the proving current flowing through the first proving resistor (which is thermally proximate to the LEDs) so as to maintain the measured temperature above the predetermined value.

In an embodiment of the second aspect, the signal 10 has a metal casing which encloses both the light source and the proving circuit. This casing not only prevents access to the interior workings of the signal by the elements or by animals or humans, but also acts as a heat sink for the various electrical components. The LEDs of 15 the light source may be mounted on a support board, which may also be a circuit board, and either through that support board, or otherwise, the LEDs may be in thermal contact with a part of the casing. The second proving resistor is mounted in a different part of the casing, 20 and is placed in thermal contact with a different part of the casing, preferably located as far as possible from any part of the casing which is in thermal contact with the LEDs. The first proving resistor is mounted in a position where it is in thermal contact with the LEDs.

( The casing may be formed as at least two separate parts, which are joined by an insulating portion, such as a gasket. This allows the two proving resistors to be in thermal contact with a part of the casing for heat 5 dissipation, but thermally isolated from each other.

The casing may include various heat dissipation means, such as fins, at various points to allow it to dissipate heat from the various heat sources more quickly. 10 It is preferable that the LEDs are in thermal contact with the casing as this allows the heat generated by the LEDs to be efficiently dispersed when the signal is operating in temperatures above the predetermined value, for example up to 40 C as required by the UK 15 specification.

The interior of the casing may partially or wholly divided by a heat shield into two regions, the LEDs and the first proving resistor being located in one of said regions and the second proving resistor being located in 20 the other of said regions. This arrangement reduces the amount of heat that can be transferred from the second proving resistor to the LEDs.

The temperature sensor may be positioned on the support board so that the temperature measured by the

( sensor corresponds to the temperature of the LEDs as closely as possible.

The present invention also provides a method of controlling the light output of a rail signal having a 5 light source including a plurality of LEDs including the steps of: monitoring the temperature at or close to the LEDs using a temperature sensor; and controlling the operating conditions of the LEDs to maintain the colour of the light output from the light source within 10 specified conditions over a particular temperature range.

In particular embodiments, the step of controlling maintains the colour of the light output from the light source such that it has a dominant wavelength of between 585nm and 605nm and a purity of at least 80,% over the 15 temperature range -20 C and +40 C.

The method of controlling may be such that the light output of the light source has a minimum purity of 85% or even 90% in the conditions specified. Similarly the method of controlling may be such that the light output 20 of the light source has a lower limit to the dominant wavelength of 587nm and/or an upper limit to the dominant wavelength of 600nm or 595nm, again in the conditions specified.

( In particular embodiments the light output of the light source may be maintained within the range of CIE (1931) chromaticity values enclosed by a quadrilateral having the following vertices: (0.546, 0.426); (0.560, 5 0.440); (0.585, 0.415); (0.576, 0.406), again in the conditions specified. This quadrilateral corresponds to the allowable chromaticity to conform to "Signal Yellow Class B" in accordance with BS 1376: 1994.

10 In a third aspect (which may be used with any or all of the other aspects) the present invention provides a rail signal having a light source including a plurality of LEDs, and a control circuit, wherein the control circuit includes at least one feedback LED and a 15 corresponding photosensor for receiving light from the feedback LED, the control circuit controlling the current passing through all the LEDs in response to an output of the photosensor, further wherein the photosensor and the at least one feedback LED are chosen such that at all 20 temperatures between -30 C and +40 C the luminous intensity of the light source varies by less than 50% above or below a nominal value.

Preferably the photosensor and the at least one feedback LED are chosen such that the luminous intensity

( of the light source varies by less than 20% above or below a nominal value.

More preferably the photosensor and the at least one feedback LED are chosen such that the luminous intensity 5 of the light source varies by less than 15% above or below a nominal value.

The nominal value may be set by the specification of

the rail operator. Alternatively the nominal value may be internally defined the mean of the highest luminous 10 intensity of the light source over the specified temperature range and the lowest luminous intensity of the light source over the specified temperature range.

The feedback LED need not be the same as the LEDs making up the light source, in which case the brightness 15 of the feedback LED need not be maintained within the specified limits over the temperature range.

In one embodiment, the signal LEDs increase in brightness with decreasing temperature (for constant signal current), the effect of the feedback loop (which 20 includes the feedback LED, the photosensor and the LED control unit) will generally be to reduce the signal current in order to maintain the brightness of the signal LEDs within the specified limits.

( In the above embodiment, the relationship between the feedback current and the signal current may generally be fixed in the inverse relationship described above.

Therefore, in order to reduce the brightness of the 5 signal LEDs at the lower temperature, the feedback current from the photosensor must increase.

If the feedback LED in the above embodiment is chosen to be the same as the signal LEDs, then the photosensor must have no difference in sensitivity 10 (feedback current output for a given sensed brightness) with the decreasing temperature. Alternatively, if the feedback LED in the above embodiment is chosen so that at lower temperatures it increases in brightness by more than the signal LEDs (for a given signal current), then 15 the photosensor may decrease in sensitivity with decreasing temperature.

By selecting the feedback LED and the photosensor appropriately, the brightness of the signal LEDs can be maintained within the limits.

A further aspect of the present invention provides a method for controlling a rail signal having a light source including a plurality of LEDs, and a control circuit, the control circuit including a feedback LED and

( a corresponding photosensor, comprising the steps of: sensing the brightness of the feedback LED with the corresponding photosensor; controlling the current flowing through all the LEDs, depending on an output of 5 the photosensor, wherein the feedback LED and the corresponding photosensor are chosen such that at all temperatures between -30 C and +40 C the luminous intensity of the light source varies by less than 50% above or below a nominal value.

JO The invention is equally applicable to all colours of LED rail signals, including, but not limited to red, yellow, green and white aspects of rail signals. The particular selection of the feedback LED and photosensor may need to be specifically selected according to the 15 type of LED used in the light source.

Embodiments of the present invention will now be described in relation to the accompanying figures, in which: 20 Figure 1 is a general CIE(1931) chromaticity diagram, as has already been referred to; Figures 2 is a magnification of the yellow portion of the CIE(1931) chromaticity diagram showing the

( variation in colour observed from a yellow LED, and has already been described; Figure 3 is a cut-away schematic view of a CLS according to one embodiment of the first aspect of the 5 present invention; Figure 4 is a block circuit diagram of an embodiment of the first aspect of the present invention; Figure 5 is a cut-away schematic view of a CLS according to an embodiment of the second aspect of the 10 present invention; Figure 6 is a magnification of the yellow portion of the CIE(1931) chromaticity diagram showing the variation in colour observed from a signal according to an embodiment of the second aspect of the present invention; 15 Figure 7 is a graph showing how the temperature of various components of a CLS according to an embodiment of the second aspect of the present invention vary after an aspect of the signal has been activated; Figure 8 is a circuit diagram of a part of the 20 control circuit of an embodiment of the second aspect of the present invention) and Figure 9 is a block circuit diagram of an embodiment of the second aspect of the present invention.

( Figure 10 is a graph of intensity against wavelength for an AlInGaP LED at three different temperatures and has already been referred to; Figure 11 is a graph of output current against S temperature for a phototransistor maintained at constant illumination and has already been referred to; Figure 12 is a control circuit for a rail signal according to an embodiment of the present invention; and Figure 13 is a graph showing how the light output of 10 a yellow rail signal according to an embodiment of the present invention varies with temperature.

Figure 14 is a graph showing how the light output of a green rail signal according to an embodiment of the present invention varies with temperature.

15 Figure 15 is a graph showing how the light output of a red rail signal according to an embodiment of the present invention varies with temperature.

Figure 3 shows the main components of a CLS 1 20 according to an embodiment of the first aspect of the present invention. The CLS 1 has a casing 11 housing the various components. Light sources 14 and 15 consist of a plurality of LEDs mounted on a support board 12, which may also act as a circuit board for part of the circuit

driving the LEDs 14 and 15. Light from the light sources 14 and 15 passes through a lens unit 17 which creates the required beam pattern. The front of the CLS 1 has a protective transparent cover 13.

5 Control unit 20 takes readings from temperature sensor 19, which may be for example a thermistor. The control unit 20 is set up to adjust thecurrent supplied to each set of LEDs 14 and 15 based on the temperature value provided by temperature sensor 19.

10 The two sets of LEDs 14 and 15 each comprise a number of LEDs. Whilst the LEDs in each set are the same as others in that set, different LEDs are used in each set. The arrangement of the two sets of LEDs 14 and 15 shown is purely schematic, and the sets of LEDs are 15 normally arranged to give consistent geometric light output from either set when independently lit.

The LEDs in the sets are chosen such that at all temperatures within the required operating range the light output of at least one of the types of LEDs falls 20 within the specified conditions. A typical combination would be one set 14 consisting of "orange" LEDs, such as Agilent Technologies LED type HLMP-DL31, colour bin 4, and a second set 15 consisting of "yellow" LEDs, such as Agilent Technologies LED type HLMP- DL31, colour bin 2.

In the examples given the "orange" LEDs 14 have a light output with CIE(1931) chromaticity co-ordinates of x=0.5825 and y=0.4175 and the "yellow" LEDs 15 have a light output with CIE(1931) chromaticity coordinates of 5 x=0.565 and y=0.435, both measured at +20 C.

Comparison of these colours on a CIE(1931) chromaticity chart showing the "Railtrack Signal Yellow boundary" (e.g. Figure 2) will show that at + 20 C both sets have a light output that is compliant. However, as 10 Figure 2 shows for a typical "yellow" LED, these outputs will shift towards red as the temperature increases (increasing dominant wavelength), and towards green as the temperature decreases (decreasing dominant wavelength). 15 Therefore control unit 20 is arranged so as to vary the proportion of the overall lighting current supplied to each set of LEDs 14 and 15 such that the overall output from the light source that falls within the specified limits.

20 For example at -20 C, the majority of the current (in some embodiments 90% of the current) would be supplied to the "orange" set of LEDs 15, since the light output from that set of LEDs would still fall within the required limits whilst the output from the "yellow" set of LEDs 14

! would be significantly outside the limits. Conversely at +40 C, all the current would be supplied to the "yellow" set of LEDs 14, since the light output from that set of LEDs would still fall within the required limits whilst 5 the output from the "orange" set of LEDs 15 would be significantly outside the limits.

In addition, the current flowing through the two sets of LEDs 14 and 15 are balanced to maintain the overall light output of the signal at a constant lO brightness. This may be achieved by conventional means, or as described in relation to the specific embodiment relating to brightness control.

The variation in the proportion of the current supplied to each set of LEDs may be linear over the IS entire temperature range, or may be linear over a smaller portion of the range, such as the middle of the range, or may switch completely from one state to another at a predetermined value.

Control unit 20 may have adjustment facility so that 20 an engineer can adjust characteristics of the system such as the point at which the control unit switches between the two sets of LEDs, or the range over which the transfer of current occurs.

Control unit 20 may also include other control circuitry associated with the signal and in particular with the LEDs, such as brightness control and a proving circuit. S Figure 4 shows a block circuit diagram of the control circuit 20 of an embodiment of the first aspect of the present invention. A power supply 47 is connected to two LED drive circuits 44 and 45, which respectively drive two sets of LEDs 14 and 15 (only a sample of the 10 LEDs being shown for clarity).

The LEDs 14 and 15 are mounted on a support board 12 (which, as before may also be a circuit board). Also mounted on support board 12 is a temperature sensor 19.

Feedback unit 43 provides brightness control IS feedback to the LED control units 44 and 45, and also an input to proving circuit 42 dependent on the overall light output. The proving circuit 42 determines whether the signal light source is operational, and if so directs current through a proving resistor 22 which is mounted on 20 the case 11 of the signal unit.

It will be appreciated that in the first aspect of this invention, the proving resistor 22 may be mounted anywhere in the signal unit, and may even be mounted on the support board 12.

( LED temperature control unit 41 is connected to temperature sensor 19, and also to both LED drive units 44 and 45. LED temperature control unit 41 determines on the basis of temperature information received from the 5 temperature sensor the proportion of current flowing to each set of LEDs 14 and 15.

Figure 5 shows the main components of a CLS 1 according to an embodiment of the second aspect of the present invention.

10 The CLS 1 has a casing 11, 31 and 33 which houses the various components of the signal. The light source consists of a plurality of LEDs 14 mounted on a support board 12 which may also act as a circuit board for part of the circuit driving the LEDs 14. Light from the LEDs 15 14 passes through a lens unit 17 which creates the required beam pattern. The front of the CLS 1 has a protective transparent cover 13.

The casing is divided into two sections: main casing 11 and back plate 31. A thermally insulating gasket 33 20 separates the two sections and prevents conductive heat transfer between them.

The support board 12 is mounted on the back plate 31 on a number of thermally conducting pillars 26. Back plate 31 therefore acts as a heat sink for the LEDs 14

( mounted on the support board 12. Back plate 31 may have one or more fins (not shown) to increase the rate of heat dissipation. A temperature sensor 19, which may be for example a 5 thermistor, is mounted on the support board 12, and is thereby in thermal contact with and proximate to the LEDs 14. Control unit 20 monitors the status of the LEDs 14 and operates a proving circuit which provides feedback to 10 the signal controller. The proving circuit includes two proving resistors, the first 24 is mounted on a thermally conductive plate 16 which is mounted on the pillars 26 which support the support board 12, and the second 22 is affixed to, and in thermal contact with, the main part of 15 the casing 11.

Control unit 20 takes readings from temperature sensor 19. The control unit 20 controls the proportion of the proving current which is passed through each of the proving resistors in order to keep the temperature 20 measured by temperature sensor 19 above a predetermined value. Control unit 20 may have adjustment means by which an engineer can adjust that predetermined value.

At high ambient temperatures, the sensed temperature is generally well above the predetermined value, since

the LEDs 14 generate heat when operational. The control means 20 causes all or most of the proving current to flow through second proving resistor 22. The heat generated by this proving resistor 22 is conducted to the 5 main body 11 of the casing and dissipated. The heat generated by the operation of the LEDs 14 is conducted to the back plate 31 where it is dissipated. No or little heat transfer occurs between the first proving resistor 22 and the LEDs 14 due to the insulation (e.g. gasket 33) 10 between the two different thermal circuits.

As the sensed temperature drops, control means 20 increases the proportion of current flowing through the first proving resistor 24. This may be a gradual, possibly linear, increase with dropping temperature, or 15 it may be achieved by switching all or most of the current from one resistor 22 to the other 24 at or around a given temperature. That given temperature may be equal to the predetermined temperature, or may be slightly higher so that the sensed temperature never drops below 20 the predetermined temperature.

Therefore at low ambient temperatures, all or most of the proving current will be passing through first proving resistor. In the arrangement shown in Figure 5, the location of this first proving resistor 24 prevents

( heat loss from the support board 12 and therefore from the LEDs 14 to the back plate 31 which acts as a heat sink. At very low ambient temperatures, the first proving resistor 24 may be at a higher temperature than 5 either the support board 12 and the LEDs 14, in which case the latter two components are actually heated by the proving resistor 24. However, LEDs 14 generate some heat themselves whilst lit, and so it is possible that the heat from the proving resistor 24 simply acts to heat the JO conductive plate 16, maintaining it at the same temperature as the support plate 12 and the LEDs 14, and thereby preventing heat from the support plate 12 and the LEDs 14 from being conducted to the back plate 31 through the thermally conductive pillars 26.

15 Other arrangements of the first proving resistor are contemplated, but the arrangement shown in Figure 5 is thought to be particularly beneficial since it allows the temperature of the LEDs 14 to be maintained stably over the temperature range required by allowing heat 20 conduction at higher ambient temperatures and preventing it at lower ambient temperatures.

At all times when the CLS 1 is operational (i.e. displaying an aspect), the control means maintains the total proving current above the minimum specified in the

( appropriate safety requirements, e.g. a 250mA minimum in the UK.

A heat shield 28 splits the interior of the CLS 1 into two sections: a first section (lowest in the Figure) 5 containing the LEDs and the first proving resistor 24, and a second section (highest in the Figure) containing the second proving resistor 22. In this way the heat produced by each of the two proving resistors is further isolated from the other. Heat shield 28 may completely 10 divide the interior of the casing, in which case it is preferable that at least part of the heat shield is made from thermally insulating material so that no conduction of heat from the main body 11 of the casing to the back plate 31.

15 A heat shield which completely divides the interior of the casing prevents conductive heat transfer between the two proving resistors, and between the second proving resistor and the LEDs.

Control unit 20 is shown located in the second 20 section, but can be located anywhere within the casing.

Figure 6 is a magnification of the yellow portion of the CIE(1931) chromaticity diagram showing the variation in colour observed from a CLS such as that shown in the above embodiment of the second aspect of the present

( 33! invention. The quadrilateral boundary shown on the diagram is the allowable values for the light to fall within "Signal Yellow Class B" in accordance with BS 1376: 1994. The line running from top-left to bottom-

5 right of the diagram is the spectrum locus.

The CLS tested in Figure 6 used a set of Agilent Technologies HLMP-EL 31 LEDs as its light source. For each temperature measured the signal was allowed to stabilise at that temperature before being activated from 10 a "cold-start". The light output was then allowed to stabilise before being measured. The testing apparatus could only operate to ambient temperatures down to -8OC and so the results were extrapolated using a non-linear calculation on three prior data points to give results 15 down to -20 C which are plotted in Figure 6.

It can be seen that the CLS used in this test produces a light output which is within the boundaries of "Signal Yellow Class B" in accordance with BS 1376: 1994 for temperatures from -20 C to +40 C. It is thought that 20 performance below -8OC is actually better than that predicted as the operating temperature of the LEDs is maintained approximately stable at temperatures below -8Oc.

( Figure 7 is a graph showing how the temperature of various components of a CLS such as that in the above embodiment vary after an aspect of the signal has been activated in an ambient temperature of -20 C. The 5 triangular points represent the temperature of the LED board (12 in Figure 5); the diamond points the ambient temperature; the open square points the temperature of the signal case at a point which is thermally distant from the LEDs (all on the left-hand scale); and the to filled square points represent the current flowing through the first proving resistor (24 in Figure 5) (right-hand scale). The horizontal axis is the time measured in minutes from the aspect being activated.

It can be seen from Figure 7 that in approximately l5 20 minutes after activation of the aspect, the temperature of the LED board has been raised to its normal operating temperature (which corresponds to the predetermined value of the sensed temperature) of around +20 C.

20 Figure 8 is a circuit diagram of part of the control means which may be used in the above embodiment. This is essentially a "flip-flop" type circuit, which is connected in place of the standard proving circuit normally found in LED signals. The circuit has two

proving resistors 24 and 22 controlled by transistors 40 and 41. Thermistor 19 is connected in a potential divider with adjustment means 21 such that at high sensed temperature, the proving current flows through second 5 proving resistor 22 and when the temperature drops the current switches over to flow through first proving resistor 24. The proving resistors 22 and 24 and the thermistor 19 are not formed as part of the circuit, but are separately mounted in the appropriate positions (for 10 example as shown in Figure 5) and connected back to the circuit. Figure 9 shows a block circuit diagram of the control circuit 20 of an embodiment of the second aspect of the present invention. A power supply 47 is connected 15 to an LED drive circuit 44 which drives LEDs 14 (only a sample of the LEDs being shown for clarity).

The LEDs 14 are mounted on a support board 12 (which, as before, may also be a circuit board). Also mounted on support board 12 is a temperature sensor 19 20 and a first proving resistor 24.

A second proving resistor 22 is mounted on a part of the case 11 thermally distant from the LEDs. LED temperature control unit 41 takes temperature data from the temperature sensor 19, and determines the proportion

( of the proving current flowing through each of the two proving resistors 22 and 24, provided that the signal is determined to be operational by the proving circuit 42 (see below).

5 Feedback unit 43 provides brightness control feedback to the LED control units 44 and 45, and also an input to proving circuit 42 dependent on the light output. The proving circuit 42 thereby determines whether the signal light source is operational, and if so 10 directs a proving current through one or both proving resistors 22 and 24, depending on the control of LED temperature control unit 41.

Figures 12 to 15 describe an embodiment of the IS invention which relates to matching a photosensor to a feedback LED to provide suitable light output control over a range of temperatures. This aspect of the invention may also be used in conjunction with the preceding aspects and embodiments of the invention.

Figure 12 shows a control circuit 10 for a rail signal, having two principal circuits: a red circuit 12 and a white circuit 14. Each circuit drives an LED matrix 22 and 24 respectively, the LED matrices being

! 37 I composed of a plurality of LEDs 32, 34 of the respective colour. A proving circuit 15 ensures that the control circuit 10 draws sufficient current from power supply 17 5 to provide the requisite feedback to the signal controller that the signal is operational.

A switch 16 connected in one input of the power supply 17 switches the light output of the signal between white and red. The proving circuit is powered from lo either input.

The following features will be described in relation to the white circuit 12 and the corresponding white LED matrix 22, but equivalent features can be seen in the red circuit 14.

15 In the embodiment shown the power supply 17 is an alternating current (AC) supply and so the input to each part of the control circuit is rectified by rectifier 18 and smoothed by smoothing capacitor 19.

One LED 26 from the matrix 22 is a feedback LED, 20 which is connected in series with the other LEDs but is housed separately in conjunction with a photosensor, in this case phototransistor 28. Therefore the current which passes through the feedback LED 26 is the same as that passing through all the other LEDs 32 in matrix 22

/ which together make up the light source for that particular part of the rail signal.

The photosensor 28 forms a potential divider which adjusts the potential across LED matrix 22, and therefore 5 the current flowing through the LEDs 32.

Therefore, if the brightness of feedback LED 26 drops, the photosensor 28 increases in resistance, and so the potential across the LED matrix 22 increases, and the current through the LEDs 32 increases thereby returning lo the brightness of the feedback LED to a predetermined level. In this manner, the brightness of the LEDs 32 is maintained at a predetermined level.

However, variations in temperature will cause both the feedback LED 26 and the photosensor 28 to vary in 15 performance. In an embodiment of the present invention, the photosensor 28 is selected to be OP802 WSL, and the feedback LED 26 is selected to be HLMP-EL31 (yellow).

A typical LED which is used as the red light source 20 LEDs 32 is HLMP ED25. In this embodiment, this LED is different from the feedback LED 26. Therefore the characteristics of that LED and its corresponding photosensor 28 can be chosen so that the variation in the

brightness of the light source LEDs 32 over the operating temperature range is reduced.

An embodiment using green LED's may use NSPE5905.

Figure 13 shows that for a yellow rail signal having S a control circuit as described in the above embodiment, the luminous intensity of the output of the light source over the temperature range of -5OC to 40 C only varies by +5%. Figure 14 shows that for a green rail signal having lo a control circuit as described in the above embodiment, the luminous intensity of the output of the light source over the temperature range of -5OC to 40 C only varies by +15%. Figure 15 shows that for a red rail signal having a 15 control circuit as described in the above embodiment, the luminous intensity of the output of the light source over the temperature range of -5OC to 40 C only varies by +2%.

The above embodiments are intended to be examples of 20 the present invention and variants and modifications of those embodiments, such as would be readily apparent to the skilled person, are envisaged and may be made without departing from the scope of the present invention.

Claims

! CLAIMS

1. A rail signal including a light source, the light source including a plurality of LEDs, and control means 5 arranged so as to maintain the light output from the light source such that it has a dominant wavelength of between 585nm and 605nm and a purity of at least 80% over the temperature range -20 C and +40 C. i 10

2. A signal according to claim 1 wherein the control means is arranged so as to maintain the light output from the light source such that it has a purity of at least 85% over the temperature range specified.

IS

3. A signal according to claim 2 wherein the control means is arranged so as to maintain light output from the light source such that it has a purity of at least 90% over the temperature range specified.

20

4. A signal according to any one of the preceding claims wherein the control means is arranged so as to maintain the light output from the light source such that it has a dominant wavelength of at least 587nm over the temperature range specified.

/

5. A signal according to any one of the preceding claims wherein the control means is arranged so as to maintain the light output from the light source such that S it has a dominant wavelength of no more than 600nm over the temperature range specified.

6. A signal according to any one of the preceding claims wherein the control means is arranged so as to 10 maintain the light output from the light source within the range of CIE (1931) chromaticity values enclosed by a quadrilateral having the following vertices: (0.546, 0.426); (0.560, 0.440); (0.585, 0.415); (0.576, 0.406),

over the temperature range specified.

7. A signal according to any one of the preceding claims further including a temperature sensor connected to the control means.

20

8. A signal according to any one of the preceding claims wherein said light source includes at least two separate sets of LEDs, each set of LEDs emitting light having a different chromaticity value at a given temperature; and the control means is configured to

adjust the current flowing through each set of LEDs, wherein the control means adjusts the current flowing through each set of LEDs such that the overall light output from the light source is within the specified 5 conditions.

9. A signal according to claim 8 wherein the type of LED in each set is chosen such that for any given temperature between -20 C and +40 C the light output from 10 at least one of the sets of LEDs has a dominant wavelength of between 585nm and 605nm and a purity of at least 80%.

10. A signal according to claim 9 wherein the type of 15 LED in each set is chosen such that for any given temperature between -20 C and +40 C the light output from at least one of the sets of LEDs has a purity of at least 85%. 20

ll. A signal according to claim 9 wherein the type of LED in each set is chosen such that for any given temperature between -20 C and +40 C the light output from at least one of the sets of LEDs has a purity of at least 90%.

12. A signal according to any one of claims 9 to 11 wherein the type of LED in each set is chosen such that for any given temperature between 20 C and +40 C the 5 light output from at least one of the sets of LEDs has a dominant wavelength of at least 587nm.

13. A signal according to any one of claims 9 to 12 wherein the type of LED in each set is chosen such that 10 for any given temperature between 20 C and +40 C the light output from at least one of the sets of LEDs has a dominant wavelength of no more than 600nm.

14. A signal according to claim 13 wherein the type of 15 LED in each set is chosen such that for any given temperature between 20 C and +40 C the light output from at least one of the sets of LEDs is within the range of CIE ( 1931) chromaticity values enclosed by a quadrilateral having the following vertices: (0.546, 20 0.426); (0.560, 0.440); (0.585, 0.415); (0. 576, 0.406),

over the temperature range specified.

15. A signal according to claim 7 further including a proving circuit having at least two proving resistors

capable of drawing a predetermined proving current, wherein: the temperature sensor is located thermally proximate to the LEDs; 5 at least a first of the proving resistors is located in a position thermally proximate to the LEDs; at least a second of the proving resistors is located in a position thermally distant from the LEDs; the control means controls the proving current 10 flowing through each of the resistors of the proving circuit and so as to maintain the sensed temperature at or above a predetermined level; and the LEDs are chosen such that the light output from the light source is within the specified conditions for 15 temperatures between said predetermined level and +40 C.

16. A signal according to claim 15 wherein the LEDs are chosen such that for temperatures between said predetermined level and +40 C the light output from the 20 light source has a purity of at least 85%.

17. A signal according to claim 16 wherein the LEDs are chosen such that for temperatures between said

predetermined level and +40 C the light output from the light source has a purity of at least 90%.

18. A signal according to any one of claims 15 to 17 S wherein the LEDs are chosen such that for temperatures between said predetermined level and +40 C the light output from the light source has a dominant wavelength of at least 587nm.

10

19. A signal according to any one of claims 15 to 18 wherein the LEDs are chosen such that for temperatures between said predetermined level and +40 C the light output from the light source has a dominant wavelength of no more than 600nm.

20. A signal according to claim 19 wherein the LEDs are chosen such that for temperatures between said predetermined level and +40 C the light output from the light source is within the range of CIE (1931) 20 chromaticity values enclosed by a quadrilateral having the following vertices: (0.546, 0.426); (0.560, 0.440); (0.585, 0.415); (0.576, 0.406).

21. A signal according to any one of claims 15 to 20 further including a metal casing which encloses both the light source and the proving circuit, wherein the casing is in thermal contact with at least one part of the light 5 source or the proving circuit.

22. A signal according to claim 21 wherein the LEDs are in thermal contact with a part of the casing.

10

23. A signal according to claim 22 wherein the LEDs are mounted on a support board and the LEDs are in thermal contact with said part of the casing through the support board. 15

24. A signal according to either claim 22 or claim 23 wherein the second proving resistor is in thermal contact with a second part of the casing such that it is thermally distant from the LEDs.

20

25. A signal according to any one of claims 21 to 24 wherein the casing includes heat dissipation means to allow it to dissipate heat to the surrounding air more quickly.

26. A signal according to any one of claims 21 to 25 wherein the interior of the casing is divided into two regions by a partition, the LEDs and the first proving resistor being located in one of said regions and the 5 second proving resistor being located in the other of said regions.

27. A signal according to claim 23 or any claim dependent on claim 23 wherein the temperature sensor is 10 located on the support board.

28. A signal according to claim 22 or any claim dependent on claim 22 wherein the first proving resistor is located between the LEDs and said part of the casing.

29. A method of controlling the light output of a rail signal having a light source including a plurality of LEDs including the steps of: monitoring the temperature at or close to the LEDs using a temperature sensor; and 20 controlling the operating conditions of the LEDs to maintain the light output from the light source such that it has a dominant wavelength of between 585nm and 605nm and a purity of at least 80% over the temperature range 20 C and +40 C.

30. A method according to claim 29 wherein said step of controlling maintains the light output from the light source such that it has a purity of at least 85% over the 5 temperature range specified.

31. A method according to claim 30 wherein said step of controlling maintains the light output from the light source such that it has a purity of at least 90% over the 10 temperature range specified.

32. A method according to any one of claims 29 to 31 wherein said step of controlling maintains the light output from the light source such that it has a dominant 15 wavelength of at least 587nm over the temperature range specified.

33. A method according to any one of claims 29 to 32 wherein said step of controlling maintains the light 20 output from the light source such that it has a dominant wavelength of no more than 600nm over the temperature range specified.

34. A method according to any one of claims 29 to 33 wherein said step of controlling maintains the light output from the light source within the range of CIE (1931) chromaticity values enclosed by a quadrilateral 5 having the following vertices: (0.546, 0.426); (0.560, 0.440); (0.585, 0.415); (0.576, 0.406), over the temperature range specified.

35. A method according to any one of claims 29 to 34 10 wherein said light source includes at least two separate sets of LEDs, each set of LEDs emitting a different colour of light at a given temperature, and wherein said step of controlling includes adjusting the current flowing through each set of LEDs such that the overall 15 colour output from the light source is within the specified conditions.

36. A method according to any one of claims 29 to 34 wherein the signal further includes a proving circuit 20 having at least two proving resistors capable of drawing a predetermined proving current, wherein at least a first of the proving resistors is located in a position thermally proximate to the LEDs;

/ so at least a second of the proving resistors is located in a position thermally distant from the LEDs; and the LEDs are chosen such that the light output from S the light source has a dominant wavelength of between 585nm and 605nm and a purity of at least 80% for temperatures between a predetermined level and +40 C, and further wherein said step of controlling includes controlling the 10 proving current flowing through each of the resistors of the proving circuit so as to maintain the sensed temperature at or above said predetermined level.

37. A rail signal according to claim 1 including a IS control circuit, wherein the control circuit includes a feedback LED and a corresponding photosensor for receiving light from the feedback LED, the control circuit controlling the current passing through all the LEDs in response to an output of the photosensor, further 20 wherein the photosensor and the feedback LED are chosen such that at all temperatures between -30 C and +40 C the luminous intensity of the light source varies by less than 50% above or below a nominal value.

38. A rail signal according to claim 37 wherein the photosensor and the feedback LED are chosen such that the luminous intensity of the light source varies by less than 20% above or below a nominal value.

39. A rail signal according to claim 38 wherein the photosensor and the feedback LED are chosen such that the luminous intensity of the light source varies by less than 15% above or below a nominal value.

40. A rail signal according any one of claims 37 to 39 wherein the nominal value is the mean of the highest luminous intensity of the light source over the specified temperature range and the lowest luminous intensity of 1S the light source over the specified temperature range.

41. A rail signal according to any one of claims 37-40 wherein the feedback LED is not of the same type as the signal LEDs.

42. A rail signal according to claim 41 wherein the feedback LED increases in brightness at a constant current by more than the signal LEDs increase in

( brightness at a constant current for a given temperature decrease.

43. A rail signal according to any one of claims 37 to S 42 wherein the signal is a colour light signal.

44. A method of controlling a rail signal according to claim 29, the signal having a control circuit, the control circuit including a feedback LED and a 10 corresponding photosensor, comprising the steps of: sensing the brightness of the feedback LED with the corresponding photosensor; controlling the current flowing through all the LEDs, depending on an output of the photosensor, wherein IS the feedback LED and the corresponding photosensor are chosen such that at all temperatures between -30 C and +40 C the luminous intensity of the light source varies by less than 50% above or below a nominal value.

20

45. A method according to claim 44 wherein the feedback LED and the corresponding photosensor are chosen such that at all temperatures between -30 C and +40 C the luminous intensity of the light source varies by less than 20% above or below a nominal value.

(

46. A method according to claim 44 wherein the feedback LED and the corresponding photosensor are chosen such that at all temperatures between -30 C and +40 C the S luminous intensity of the light source varies by less than 15% from above or below a nominal value.

47. A method according to any one of claims 44 to 46 wherein the nominal value is the mean of the highest lO luminous intensity of the light source over the specified temperature range and the lowest luminous intensity of the light source over the specified temperature range.

48. A rail colour light signal substantially as herein IS described with reference to, or as illustrated in, the accompanying drawings.

49. A method for controlling a rail colour light signal substantially as herein described with reference to, or 20 as illustrated in, the accompanying drawings.