GB2495135A

GB2495135A - Elongate LED lighting system to replace fluorescent lighting in mass transit vehicles

Info

Publication number: GB2495135A
Application number: GB1116878.8A
Authority: GB
Inventors: Daniel Alan Rust
Original assignee: Saf T Glo Ltd
Current assignee: Saf T Glo Ltd
Priority date: 2011-09-30
Filing date: 2011-09-30
Publication date: 2013-04-03
Anticipated expiration: 2031-09-30
Also published as: GB201116878D0; GB2495135B

Abstract

A vehicle lighting system has an LED light fitting 100 with an elongate housing 102 containing one or more LED strings 116 for illumination. The fitting 100 includes a ballast having an optimal load and load line for non-optimal load to ballast impedance matches. The length of each LED string 116 is calculated based on the optimal load or load line of the ballast. Each LED string 116 is connected either directly across the main output of the ballast or supplied from the main output of the ballast via one or more rectification circuits. The number of LEDs in the string 116 may be adjusted and zener diodes may be added. The LED light fitting 100 may have a dimmed mode of operation which is achieved by adding a series resistor to the circuit. The housing may act as a heat sink and may contain a variable thickness wall having ripples. A method of converting a fluorescent light fitting into a LED light fitting 100 is also disclosed.

Description

LIGHTING SYST EMS

This invention relates to lighting systems This invention has particular but not exclusive application to passenger transport vehicles (PTVs) such as aircraft and coaches.

Fluorescent lighting has commonly been used to illuminate passenger compartments in PTVs for normal activities. The lighting can be adjusted to alter the brightness to suit particular activities such as reading, sleeping or eating within the passenger compartment of a PTV.

The use of LED lighting which can be adjusted to alter both brightness and colour is disclosed in our co pending applicaLion number (}B115994.4. The LEI) light fitting can be used to replace a fluorescent light tube of an existing fluorescent light fitting.

According to a first aspect of the invention there is provided an LED light fitting comprising an elongate housing containing one or more LED strings for illumination wherein the housing is provided with terminals at each end for connecting the fitting to a power source and wherein the terminals are inclined at an angle to a longitudinal axis of the housing.

The light fitting may be mounted in a base fitting and the terminals may engage contacts of the base fitting for connecting the LIU) light fitting to a power source The light fitting may be detachable from the base fitting for replacing the light fitting.

The base fitting may be the base fitting of a fluorescent light unit and the LED light fitting may be employed to replace the fluorescent tube without modification of the base fitting.

The angled terminals may comprise pins. Angling the pins allows a more reliable contact to be provided with contacts of the base fitting. l'his provides a significant advantage in improving the reliability of lighting systems in aircraft where the monetary and time cost of modifying a vehicle side base connector is significant particularly when the vehicle is an aircraft.

It has been found that connections between the pins of fluorescent tubes and contacts of the base fitting in which they are mounted are poor in existing fluorescent lighting systems and these poor connections can result in at least one fluorescent tube not The pins of the LED light fitting preferably project from end fittings or caps at each end of the housing and are arranged to be inclined at an angle 0 relative to the longitudinal axis a of the housing.

The LED light fitting is preferably arranged so that in use, the LED light fitting is arranged to illuminate outwardly and the pins are angled such that the pins press harder on a lower contact of the base fitting.

According to a second aspect of the invention there is provided an LED light fitting comprising an elongate housing provided with one or more LED strings for illumination wherein the housing is provided with terminals at each end for connecting the light fitting to a power source, and wherein the housing is arranged to act as a heat sink.

It is desirable that the fitting is light weight. Preferably the housing has a casing formed of a high thermal conductivity material such as metal which acts as a heat sink and in a particularly preferred form the casing is made of aluminium.

In one embodiment the housing comprises an extruded aluminium casing having a sclcctcd wall thickness and in which regularly repeated features are extruded which have-a reduced wall thickness.

Preferably the profile of the casing has repeating peaks and troughs forming a profile having a rippled surface. Preferably the rippled surface is at least partially an external surface of the profile. More preferably a substantial proportion of an external surface of the profile is rippled.

Using a rippled or undulating sLlrfacc allows the mean wall thickness of the profile of the casing to be reduced below that winch can be uniformly extruded. it has been found, for example, that if a minimum wall thickness of 0.4mm is required for extrusion of a preferred profile, the profile can be extruded with a number of peaks and troughs where the wall thickness at the base of each trough may he reduced to 0.25mm resulting in Lhe mean wall thickness being reduced to 0.3mm.

It will be appreciated that the reduced mean wall thickness lessens the overall mass of the casing and allows the extruded aluminium profile to be sufficiently light weight to be used. A particular advantage of the variable thickness wall with a rippled surface is that a surface area of the profile is increased approximately by the ratio u/2 and therefore the profile exhibits better heat dissipation to surrounding ambient air than a corresponding profile having a flat surface.

The form of the troughs may vary. The troughs may be curved or may be stepped. It is appreciated that at minimal extrusion limits for both thickness of material and external radii in aluminium that the surface may resemble a series of convex and concave semi-circles arranged end to end, each of radius 0.2-0.3mm.

In one form, the LED strings are mounted on a printed circuit board controlling the LEDs. In another form the LED strings are mounted on a separate LED board and connected to the printed circuit board controlling the LEDs. The casing may be provided with means of supporting the printed circuit board and, where provided, the separate LED board. More than one printed circuit board may be provided. More than one LED board may be provided.

Preferably the easing is in thermal interface to the LEDs. In some embodiments the easing may be in direct thermal interface with the LI-iDs. The casing may remove excess heat from the light fitting by heat exchange with the surrounding ambient air.

The casing may be provided with means such as slots or apertures to allow air to circulate within the casing to assist removal of excess heat and/or air pressure from the light fitting.

The casing assists thermal optimisation of the LED light fitting by facilitating removal of heat from the light fitting and reduction of the temperature at which the LEDs are operative. It is known that a lifetime of LEDs may be maximised by reducing the temperature at which the LEDs operate.

Preferably the LEDs are high efficacy LEDs so as to reduce the percentage of heat generated per unit of light generated. Use of high efficacy LEDs enables the LEDs 10 be run at significantly lower currents than their rated currents. In a preferred embodiment the LEDS can be operated at approximately 50% of their rated current.

According to a third aspect of the invention there is provided an LF3D light fitting adapted to be used in a lighting system of a vehicle, the system comprising a ballast having an optimal load and load line (voltage versus current supplied) for non-optimal load to ballast impedance matches, the LED light fitting comprising an elongate housing containing one or more Liii) strings for illumination, wherein a length of the or each LED string is calculated based on the optimal load or load line of the ballast and the or each LED string is connected either directly across the main outpuL of the ballast or supplied from the main output of the ballast via one or more rectification circuit(s).

The ballast serves to regulate the current through the LIII) string as a function of the string operating voltage (and therefore string length, where each LED of the string has a known operating forward voltage/current relationship).

It is desirable that an overall light output be selected to be a predeteniiined value depending on the intended use of the light fitting. The existing lighting system ballast can be used to directly control the current in a series connected LED string across its output in place of a fluorescent tube. The output of the ballast may be rectified before supplying the LED string. The current supplied to the string is varied by adjusting the operating voltage of the string to worlc at a specific point or range of the ballast load line. The operating voltage of the string is controlled by varying the quantity of LEDs in the string. Preferably a number of zener diodes are used in the string in place of some LEDs, each zcncr diode being used in place of a respective LEI) to provide further control of current where the quantity of LEDs desired may not result in an appropriate string voltage.

It has been found that the use of zener diodes in the LED string in place of [LOs allows the overall light output of the LED string to be controlled further. A ratio of Llil)s to zener diodes and total string length and therefore voltage can be selected to control the overall light output of the LED string while the power requirements of each LED string is controlled at a selected level.

In some embodiments the LEE) Eight fitting may have a dimmed mode of operatiorn Preferably the ballast comprises a series resistor which is added to the circuit in the dimmed mode. In this embodiment, a non-optimal load to ballast match is found such that thc LED string of the light fitting operates nominally at a voltage the same as or higher than that of a standard fluorescent tube operating from the ballast in dimmed mode. The efficacy of the LEDs, quantity of LEDs and the LED to Zener diode ratio is selected to provide the required string voltage and light output.

According to a fourth aspect of the invention there is provided an LED light fitting adapted to be used in a base unit of a lighting system of a vehicle, the base unit having an optimal load and a load line, the LED Eight fitting comprising an elongate housing containing one or more LED strings for illumination, each LED string comprising a number of LEDs, wherein each LED string comprises redundant circuits. such that the LED light fitting is arranged to continue providing illumination from any one of the number of bEDs.

Preferably each LEE) is connected in series with a reverse biased zener diode. In such a circuit even if any of the oumber of LEDs fail the LED string will continue to conduct a current and the remaining LEDs are able to output illumination.

Features of any of the preceding aspects of the invention may be employed separately or in combination with any feature of any of the other aspects of the invention. All combinations of features are envisaged and within the scope of the invention.

According to a fifth aspect of the invention their is provided a method of improving the reliability of a lighting system in a passenger transport vehicle including one or more base units for a lighting fitting the mctLod comprising mounting at least one light fitting comprising an elongate housing containing one or more LED strings for illumination, wherein one the LED light fitting comprises one or more of time following features: the elongate housing being provided with terminals at each end for connecting to a power source wherein the terminals are inclined at an angle to an elongate axis of the housing; * the housing comprises a easing arranged to act as a heat sink; * a length of the or each LED string is calculated based on the voltage versus current load line of a ballast to which the light fitting is to be connected, or * each LED string comprising a number of LEDs, and wherein each LED string comprises redundant circuits such that the LED light fitting is arranged to provide illumination from any one of the number of LhDs.

Preferably the terminals are provided by end fittings at each end of the housing.

Preferably the casing has a rippled surface that increases a rate of heat transfer from the light fitting to the surrounding ambient air. It is desirable that the casing is also in thermal contact with the LED strings. It is desirable that the LEDs are high efficacy LEDs and can operate at lower currents than would otherwise be required.

An advantage of the high efficacy LEDs requiring lower currents is that the power required by the LED light fitting is less than that of a fluorescent light tube.

Typically the base fitting for a fluorescent light unit comprises a ballast which supplies power to the fluorescent tube and contains additional circuitry controlling operation of the light fitting. In replacing the fluorescent tube with an [hI) light fitting according to the invention the ballast of the base fitting is able to operate at below half toad.

The ballast of the base fitting acts to supply a voltage to the LED light fitting hut is not required to supply power to a heater coil as required in a conventional fluorescent light fitting. In a conventional fluorescent light fitting the heater coil operates during start-up and in some cases continuously, drawing power from the ballast in the base fitting. When modified by the use of an LED light fitting according to the present invention the ballast is able to operate at a cooler temperature and under less stress.

The ballast thus becomes an over engineered part of the system and is unlikely to fail.

The invention will now be further described by way of example only with reference to the accompanying figures in which: Figure Ia is a view of a prior art fluorescent tube; Figure lb is a view of two fluorescent tubes linked in a prior art system; Figure 2 is a diagrammatic layout of a vehicle lighting system embodying the i iiv e nt i oil; Figure 3 is a partially exploded view of an li-I) light fitting in accordance with a first embodiment of the invention; Figure 4 is a detail of the LED light fitting of Figure 3; Figure 5 is a schematic diagram of a circuit used in the TED light fitting; Figure 6 is a detail of a circuit used in light strings in the LEE) light fitting; Figure 7 is a partially exploded view of one end of an LED fitting in accordance with a second embodiment of the invention; Figure 8 shows the LED light fitting of Figure 7 assembled; Figure 9 is a cross section of the casing of the LED light fitting of Figures 7 and 8; and Figure 10 is a side view of the end of tile LED light fitting of Figures 7 and 8 In order to understand the invention and the advantages of the invention a brief discussion of the prior art is considered to be helpful.

Figure 1 shows a conventional fluorescent tube 1 having terminals 2 in the form of pins 4 at each end. The pins 4 make connections with a power supply in a base fitting of a lighting system (not shown). The terminals comprise pins labelled Al, A2 at a first end 6 and BI B2 at a second end 8.

In a lighting system power is supplied to the tube 1 via the pins 4 from a ballast in a base fitting. The ballast provides a small voltage across the pins Al and A2 in order to power a thermionic emitter inside the tube. The thermionic emitter may also be referred to as a heater coil. A voltage is also applied across the pins Bl and B2 to power a further thermionic emitter. The thermionic emitters produce ions.

The ballast also applies a large drive voltage across a length of the tube 1. The voltage is applied between the pins Al, A2 and the pins Bl, B2. Once the thermionic devices have initiated a flow of ions the large voltage across the length of the tube starts a current flow between the first end 6 and second end 8 of the tube 1. Once the current flow is established the heaters are not longer essential.

In conventional prior art fluorescent tubes the ballast is designed to supply power to the heaters whenever the lighting is turned on. It has been found that this can lead to early heater failure. It is also to he noted that if any one of the connections Al, A2, Bl, B2 fails then the tube is incapable of starting. If one of the connection fails after starting the tube the tube will generally continue to emit light but will not be able to start the next time that the light fitting is operated.

Figure lb shows a typical arrangement of fluorescent tubes in a passenger vehicle in which a first tube 1 is connected in series with a neighbouring second tube 10. The first tube 1 is connected to the second tube 10 and the terminal 2 at the second end 8 of the first tube 1 is connected to a terminal 12 comprising pins 14 at a first end 16 of the second tube 10. A further terminal I 2 comprising pins 14 is provided at a second end 18 of the second tube 10. Pins 111, B2 of the first tube I are connected to the pins Al, A2 of the second tube 10. A ballast, not shown, in the base fitting applies a heater current across each end 6, 8, 16, 18 of each tube 1, 10 and a main drive voltage across the series pair of tubes such that a current flows in the two tubes.

It will be appreciated that a current can only flow in one of the tubes if it is also able to flow in the other of the tubes. A failure of one connection will cause a tube to fail to start, such that a current cannot flow through the tube, and this will prevent a current flowing through the other tube. A failure in anyone one of the eight connections at either first or second ends of the first and second tubes will lead to an apparent failure of the first and second tubes. In addition, if the thermionic heater devices inside either tube fails then tile tube in which the heater has failed cannot start and a current cannot flow through that tube or through the other of the tubes in the series.

S Figure 2 shows a hghting system for a passenger cabin of an aircraft and includes a plurality of lighting units spaced apart along the length of the cabin. One or more control units for operating and adjusting an illumination from the light units may be provided. Each lighting unit comprises a base fitting arranged Lo receive and connect to a light fitting and a ballast comprising circuitry arranged to connect to and control a power supply to a respective light fitting connected to the base fitting.

Figures 3 and 4 illustrate an LED light fitting 100 in accordance with a first embodiment of the invention. The fitting 100 has an elongate housing 102 provided with two printed circuit boards 104,106 and two end fittings 108,110. Each end fitting 108,1 10 has a pair of pins 112 and each printed circuit board 104,106 has a pair of terminals 114 at one end.

The printed circuit boards 104,106 are positioned in the housing 102 so that the terminals 114 of the board 104 are received in the pins 112 of the end fitting 108 and the terminals 114 of the board 106 are received in the pins 112 of the end fitting 110.

The pins 112 are arranged to connect to a power source, in this case the ballast of the base fitting of the fluorescent light unit described above such that the LED light fitting 100 can directly replace the fluorescent tube 1.

Each printed circuit board 104,106 is provided with one or more strings of light emitting diodes (LEDs) 116 mounted on one side. Other arrangements having one or more printed circuit boards may be employed.

The housing 102 includes a metal easing 118, for example an extruded aluminium profile, and a transparent or translucent plastic cover 120, for example an extruded polyearbonate profile. The casing 118 and cover 120 are preferably of semi-circular section and are cut to length from extruded profiles.

The casing 118 has longitudinal edge formations 122 that provide internal grooves 124 to receive and locate longitudinal edges of the printed circuit boards 104,106 and external channels 126.

The cover 120 has longitudinal edge formations 128 that provide internal flanges 130 that engage the external channels 126 of the easing 118 to connect releasably the covcr 120 to Lhe casing 118.

The flanges 130 of the cover 120 may engage the channels 126 of the casing 118 with a snap action that allows the cover 120 to be clipped onto the casing 118.

The printed circuit boards 104,106 arc connected together by links (not shown) soldered into holes in the adjacent inner ends of the boards 104,106.

The end fittings 108,110 are releasably secured to the casing 118 at opposite ends of the housing 102 by screws (not shown) inserted through holes 134 in the end fittings 108,110 to engage the channels 126, The housing 102 formed by the assembly of the cover 120 and casing 18 generally has the form of a tube with a circular section that resembles a fluorescent tube and the light fitting 100 can be provided in a range of lengths for direct replacement of a fluorescent tube of the same length.

The printed circuit boards 104,106 extend lengthwise of the housing 102 and are enclosed on the side provided with the LED strings by the cover 120 and on the other side by the casing 118.

The casing 116 provides mechanical rigidity to the elongate housing 102 and, being formed of metal, the casing 122 is opaque. The cover 120 protects the LEDs 116 and, being formed of transparent or translucent plastics, transmits light emitted by the LED strings. It is desirable that the printed circuit boards 104,106 cannot be mounted in a reversed position with the LEDs 116 orientated towards the opaque metal casing. Il

The casing 118 is mechanically and thermally attached to the printed circuit boards 104,106 and acts as heat sink to remove heat from the printed circuit boards 104,106 and LEDs 116.

The casing 118 may be provided with apertures or slots to allow air to circulate through the casing. Alternatively or additionally, the end fittings 108,110 may be provided with holes to allow air to circulate through the cover 120.

Circulation of air through the casing 118 and/or cover 120 may remove excess heat from the LED light fitting.

The LED strings preferably include one or more lighting circuits having one or more light emitting diodes 116 that provide visible light for illumination. Where an LED string has more than one light emitting diode, the light emitting diodes of that string are preferably of the sanie type, for example, white, red, green or blue.

This invention is primarily concerned with the use of white light emitting diodes but it is envisaged that the invention could be used with LED strings comprising strings of thc same or different types of LEDs that provide the illumination. Other combinations of the LED strings are envisaged and within the scope of the invention.

The housing may accommodate coloured films in addition to a cover, in place of a cover or the cover may be coloured, allowing the filtering of light to provide different colour light outputs.

The cover may provide lens effects to control the wash of light provided to a surface by the fitting. Diffuse coatings, divergent or convergent lens effects or projection effects (such as for adverting or branding with projections of logos onto a surface) are all envisaged within the scope of the cover or films placed under or over the cover.

We may substitute zcncr diodes for one or more LEDs of an LED string to obtain the correct length of string and ratio of LEDs to zener diodes to extract the correct power from the ballast, give enough light and have a tube voltage that the ballast can operate normally with in both bright and dim modes of operation.

In the dim mode the ballast adds a series resistor to the circuit and as such, the tube voltage should be sufficiently high to prevent excessive power dissipation in the resistor. In another embodiment we may provide one or more strings of zcner diodes.

The individual components are conventional hut an improved and simpler means of powering the LED strings has been devised. In the past a driver circuit has been provided as part of Llie printed circuit board 104,106 and the driver circuit operates to maintain a constant input voltage to the light fitting and to drive the l,FDs accordingly.

It has however been realised that in an LED circuit the ballast can provide direct regulation to the LED circuit provided that the ballast is always in the circuit. Lach ballast has an optimal load point and a load linc of voltage versus current supplied on which the optimal point sits and the forward voltage of the light fitting 100 can he set to a point on the load line of the ballast. Once the forward voltage has been determined, LEDs and/or zcner diode types and quantities can he selected.

It is preferred to use high efficacy LEDs so that the heat output is relatively low for the light output of each LED. the forward voltage of each LED can be used to calculate an optimal number of LEDs in the light string for the optimal load point of the ballast.

If a number of LEDs would provide an overall light output that is greater than a desired predetermined overall light output zener diodes are substituted for LEDs. The use of zener diodes allows the optimal load point of the ballast to be utilised without increasing the overall light output beyond the predetermined level. A non optimal match to the ballast may be selected to reduce current in the LED string.

In the LED light fitting 100 it is not necessary to provide a heater to start current flow in the light fitting. In the LED light fitting 100 it is possible to create redundancy in the supply of power to the light fitting. Power may be taken from either of the two pins 112 at each end of the light fitting.

In order to create a full redundancy at each end of the light fitting a set of eight diodes are arranged as two bridges. A suitable circuit arrangement is shown in Figure 5. It will he appreciatcd that either pin 112 at each of the first 110 and second 112 ends can fail to connect and a power supply will stilt remain internal to the light fitting 100.

Additionally redundant circuits in the t,l-tD strings are uscd to allow a current to flow even if one or more LEDs fail. In a preferred embodiment illustrated in Figure 6 a reverse biased zener diode 132is arranged around each single LU) 116 to allow for redundancy in the LED string. In this way any number or indccd all of the LEDs 116 in a string may fail but the string will continue to conduct a current and to illuminate other LEDs and any series connected LED light fitting can continue to operate.

The circuits in the or each light fitting 100 are arranged to allow the light fitting to provide a light output down to the last working LED 116 and to continue to provide a palh for a current to flow through the light fitting to a neighbouring series connected LED light fitting even if all the LEDs in the light fitting have failed.

Referring now to Figures 7 to 10, a light fitting 200 is shown according to a second embodiment of the invention. in this embodiment the cover of the previous embodiment is omitted and the housing 202 comprises a tubular casing 204 and end fittings 206 (one only shown) at each end.

The casing has opposed internal grooves 208 to locate one or more printed circuit boards 2i0 that extend lengthwise of the housing 202 within the casing 204 as described previously. The casing 204 is made of metal, for example an extruded aluminium metal profile having a semi circular rear portion 212, a pair of angled side portions 214,216 that extend inwardly from the semicircular rear portion 212 to a channel-section front portion 218.

In this embodiment, the LEDs 220 are mounted on one or more LED boards 222 that are located in the channel-section front portion 218 and secured by any suitable means, for example adhesive such as with double sided adhesive tape. The or each LED board 222 is connected to a printed circuit board 210 within the easing 204 which in turn is connected to pins 224 of the end fittings 206 as described previously.

l'he end fittings 206 may be secured to the easing 204 by any suitable means and may be detachable.

The LEDs 220 may be arranged in strings and the LED strings may bc similar to the strings of the first embodiment. The channel-section front portion 218 may provide sufficient protection for the LEDs 220, Alternatively, a cover may he provided to extend over and protect the LEDs 220. The cover may he made of transparent or translucent plastics material, for example polyearhonate, and may be secured by any suitable means and may he detachable.

The tubular casing 204 has a rippled profile with peaks 226 providing a mean wall thickness as generally indicated at X and troughs 228 having a reduced wall thickness indicated at Y in one surface of the profile.

In this embodiment the rippled surface is provided on an external surface of the semi circular portion 212 and the angled side portions 214,216 and on the internal surface of the channel-section front portion 218.

Ihe troughs 228 are provided at regular intervals and have a relatively gentle curvature. It will be appreciated that the rippled effect increases the surface area of the profile and increases a rate of transfer of heat from the interior of the casing 204 to ambient air.

The reduction in wall thickness provided by rippled effect also reduces the weight of the light fitting. For example if the mean wall thickness X is 0.4mm and the wall thickness at the base of a trough Y is 0.25mm the average wall thickness is 0.3mm and this represents a significant reduction in the weight of the extruded profile. The easing of the first embodiment may he provided with a rippled surface in similar in anner.

The end fittings 206 are shaped to be complementary to the profile of the easing 204 and apertures 230 allow the passage of air into the easing 204 to allow a flow of air around the or each printed circuit board 210. Alternatively or additionally an air flow within the casing 204 may be provided by one or more slots or holes in the nietal casing 204.

As can be best seen in Figure 10 the pins 224 of each end fitting 206 project from the end fitting 206 at an inclined angle 0 to a longitudinal axis x of the housing 202.

Preferably, when the LEDs 220 direct light downwards as shown in Figure 10, the pins 224 extend upwards. In this way when the light fitting 200 is mounted in the base fitting of a light unit, for example when replacing a fluorescent tube of an existing light unit, the pins 224 will press harder on a lower contact of the base fitting.

It will be understood that the terms downwards and upwards refer to the orientation of the [liD light fitting shown in Figure 10 and that the light fitting may he mounted so that the light From the LEDs 20 may be directed upwards, downwards, sideways or in any other direction as dictated by the position of the base fitting in which the LED light fitting is to be mounted. Whatever the orientation of the LED light fitting, the pins 224 extend away from the direction of illumination so as to press harder on the lower contact of the base fitting.

It has been found that tIns inclination of the pins 224 provides an improved electrical connection with the base fitting and does not require any modification of an existing base fitting when replacing a fluorescent tube of an existing light unit with the LED light fitting. The pins 224 are conventional other than being mounted at an angle 0 relative to the longitudinal axis a of the elongate housing. The pins 112 of the light fitting according to the first embodiment may be inclined in similar manner, The light fitting 200 is asymmetrical and it is therefore apparent to a user that the light fitting has a single orienLation in which the light fitting 200 should be located in the base fitting so that the pins 224 will be connected in the correct orientation to improve the connection between the pins and the base fitting.

Use of the LED light fittings has been found to provide significant advantages over the prior art fluorescent light fittings and these are further discussed below.

As discussed above the LED light fitting do not require any heaters and can operate from either or both of the pins at each end. In addition the LED light fitting is asymmetrical and will always be mounted in a known configuration such that an asymmetric pin configuration can be utilised to provide a more reliable contact with a connector in the base fitting. Use of the LED light fitting will reduce the number of failures or apparent failures as a result of poor end connections or failed heaters.

Table 1, below, sets out ii1ures as they relate to existing fluorescent light fittings and indicates whether the LED light fitting will overcome the problem.

Table 1

Mitigated by LED light fitting and

Failure Description

Ballast Failed Connection Al Yes Failed Connection A2 Yes Failcd Connection 131 Yes Failed Connection 132 Ycs -Failed Connection Al +B1 Yes Failed Connection Al + B2 Yes Failed Connection A2+Bl Yes Failed Connection A2+B2 Yes Failed Connection Al+A2 No --Failed Connection BlF-B2 Ballast Heater Supply I fail Yes Ballast Ileater supply 2 fail Yes Ballast heater supply I and 2 fail Yes Tube I Heater I fail Yes Tube 1 heater 2 fail Tube 2 heater 1 fail Yes Tube 2 heater 2 fail Yes Ballast main supply fail No All of the failure modes listed in Table I will cause two conventional fluorescent tubes to stop operating. Of these all but three failure modcs arc overcome by use the LED light fitting. Of those remaining two occur only if there is a double connection failure in an end fitting or the pins thereof The likelihood of a double failure iu an end fitting is small and the asymmetric design of the LED light fitting with the inclined pins improves the contact between the end fitting and the base fitting such that a double failure of the contacts is unlikely.

Failure of the main ballast supply is unusual and has a very low occurrence in prior art systems. As discussed above a LED light fitting as described is designed Lo operate at about half the power of a fluorescent tube. Removal of the additional load of heater power also reduces the power that the ballast is required to supply. The ballast S typically operates at around half its normal load and is able to operate at a cooler temperature and under less stress so that a failure of the ballast main supply is less likely to occur.

Claims

<claim-text>CLAIMS1. A vehicle lighting system including a ballast having an optimal load and load line for non-optimal load to ballast impedance matches, an LED light fitting comprising an elongate housing containing one or morc LED strings for illumination, wherein a length of the or each LED string is calculated based on the optimal load or load line of the ballast and the or each LED string is connected either directly across the main output of the ballast or supplied from the main output of the ballast via one or more rectification circuits.</claim-text> <claim-text>2. The system according to claim 1 wherein the current supplied to the or each LED string is varied by adjusting the operating voltage of the string to work at a specific point or range of the ballast load line.</claim-text> <claim-text>3. The systeni according to claim 2 wherein the operating voltage of the or each string is controlled by varying the quantity of LEDs in the string.r 4. The system according to claim 2 or claim 3 wherein the string includes LEDs and zener diodes.5. The system according to claim 4 wherein the overall light output of the LED string is controlled by the ratio of LEDs to zener diodes.6. The system according to any preceding claim wherein the LED light fitting has a dimmed mode of operation.7. The system according to claim 6 wherein the ballast includes a series resistor which is added to the circuit in the dimmed mode of operation.8. The system according to any preceding claim wherein the or each LED string includes redundant circuits for each LED in the string such that if any LED fails the remaining LEDs in the string can continue to provide illumination.9. The system according to claim 8 wherein the LEDs in the string are connected in series and a reverse biased zener diode connected across each LED.10. The systeni according to any preceding claim wherein the housing is provided with a pair of terminals at each end for connecting the fitting to a power source and wherein the terminals are inclined at an angle to a longitudinal axis of the housing.11. The system according to claim 10 wherein the light fitting is detachably mounted in a base fitting and the terminals are inclined to enhance contact with the base fitting.12. The system according to claini 10 or claim II wherein one terminal of each pair is connected across a diode bridge at one end of the housing and the other terminal of each pair is connected across a further diode bridge at the other end of the housing.13. The system according to any preceding claim wherein the housing acts as a heat sink and has a variable wall thickness providing a rippled outer surface with repeating peaks and troughs. r14. A method of converting a fluorescent light fitting having a ballast and a C fluorescent tube detachably mounted in a base unit to an LED light fitting, the method comprising detaching the fluorescent tube from the base unit, providing an elongate housing containing one or more LED strings for illumination, mounting the housing in the base unit such that the or each LED string is connected either directly across a main output of the ballast or supplied from the main output of the ballast via one or more rectification circuit(s), wherein a length of the or each LED string is calculated based on the optimal load or load line of the ballast.15. The niethod according to claim 14 wherein current supplied to the or each LED string is varied by adjusting the operating voltage of the string to work at a specific point or range of the ballast load line.16. The niethod according to claim 15 wherein the operating voltage of the or each string is controlled by varying the quantity of LEDs in the string.17. The method according to claim 15 or claim 16 wherein the string includes LEDs and zener diodes such that an overall light output of the LED string is controlled by the ratio of LEDs to zener diodes.18. The method according to any of claims 14 to 17 wherein the ballast includes a series resistor which is added to the circuit to provide a dimmed mode of operation.19. The method according to any of claims 14 to 18 whcrein the or each LED string includes redundant circuits for each LED in the string such that if any LED fails the remaining LEDs in the string can continue to provide illumination.20. The method according to claim 19 wherein the LEDs in the string are connected in series and a reverse biased zener diodc is connected across each LED.21. The method according to any of claim 14 to 20 wherein the housing has a pair c'J of terminals at each end for connection with contacts of the base unit and wherein one r terminal of each pair is connected across a diode bridge at one end of the housing and the other terminal of each pair is connected across a further diode bridge at the other C end of the housing. Ci,</claim-text>