GB2408315A

GB2408315A - Illumination control system for light emitters

Info

Publication number: GB2408315A
Application number: GB0321875A
Authority: GB
Inventors: Geoffrey Howard Gil Archenhold; Keith Anderson
Original assignee: Radiant Research Ltd
Current assignee: Radiant Research Ltd
Priority date: 2003-09-18
Filing date: 2003-09-18
Publication date: 2005-05-25
Anticipated expiration: 2023-09-18
Also published as: GB2408315B; GB0321875D0; GB2408315A9

Abstract

An illumination control system 100 for light emitter such as LEDs 5 provides synchronised driving waveforms to the light emitters 5, preferably using pulse amplitude modulation. The system 100 may include an optical sensor to provide feedback to the control system 100. The optical sensor (1116, figure 22) may be arranged to detect light that has been totally internally reflected in a cover (1114) over the light emitters (1102-1110). The optical sensor (1116) may be arranged to detect light from the light emitters whilst they are actuated and to detect ambient light whilst the light emitters are not actuated. The control system 100 may drive the lighting emitters at a power greater than their rated power if the output of the light emitters has deteriorated. A temperature sensor 2 may be mounted in proximity to a light emitter to provide thermal feedback to the control system 100. A colour sensor 3 may similarly provide feedback regarding the colour of a non-LED source, whose colour output may vary as it deteriorates.

Description

- , 240831 5 Illumination control system for Light Emitters The present

invention relates to an illumination control system for Light Emitters such as Light Emitting Diodes (LEDs), to lighting fixtures embodying such control and to methods of controlling Light Emitters.

The present applicant's earlier application WO 03/022009 (the contents of which are incorporated herein by reference) addresses, inter alla, an issue of colour coordination when multiple light sources at different wavelengths are combined to provide a composite output.

Driving multiple light sources, such as LEDs, using Pulse Amplitude Modulation (PAM) provides appreciable advantages over alternative techniques such as Pulse Width Modulation (PWM) and Pulse Frequency Modulation (PFM).

The present invention provides a number of enhancements to the disclosures therein.

A first aspect of the present invention addresses a problem of limited operating range. It is well known that LEDs are non-linear devices and if they are driven at too low a voltage they will, at best, be somewhat inefficient in terms of light output per electrical power input and at worst will fail to generate any light at all.

A first aspect of the present invention addresses this by providing synchronised driving waveforms in which at least one additional operating regime is added to the operating range.

In this regime the period and/or the mark-space ratio of the pulse driving signal is altered from that used in a pure PAM regime.

Note that the remaining aspects of the invention are applicable, albeit with possible reduction in performance, to PWM and other non-PAM driving techniques.

A second aspect of the invention addresses applications in which a minimum light output is required. Once such application is automotive lighting in which the minimum light output will typically be specified by the relevant regulatory body. A difficulty arises where the light output deteriorates over time such as is the case with LEDs. (Compare this with the catastrophic failure that occurs when filament lamps reach the end of their lives). Because the deterioration of light output will be gradual it will probably not be discernible to the user, r)

N

or in the case of the car, a driver. While the diminished light output will be detectable at a roadworthiness test station using special equipment, such tests are typically not conducted very frequently and so a considerable period of time could elapse without a driver realising that his or her lamps are dimmer than they ought to be.

According to this aspect of the present invention there is provided a drive circuit for at least one LED which also has means for receiving an intensity measurement of the light emitted by at least one LED. By driving the LED or LEDs in a fixture to higher and higher currents (beyond the recommended values specified by the manufacturer) the light output will be maintained at an acceptable intensity beyond the period of useful life of the LED or fixture.

In addition, the increased power supply to the LED will eventually cause thermally-induced failure and it will then be clear that the LED or fixture needs replacing. This aspect of the invention, therefore, not only extends the useful life of an LED or LED fixture but also ensure that the LED or fixture are replaced in a timely fashion.

A third aspect of the invention provides enhanced thermal feedback. In the applicant's previous patent application, WO 03/022009, temperature sensors are disclosed for the purpose of measuring ambient temperature. However, the inventors have now come to appreciate that it is also important to measure the junction temperature of the LED. In high- power and high-efficiency LED lighting systems the junction temperature is an important control parameter. However, measurement of the junction temperature directly has not hitherto been possible. According to the third aspect, a temperature sensor is provided in sufficient proximity to the LED billet to track the junction temperature thereof. The present inventors have discovered that by mounting a temperature sensor sufficiently close to the LED billet (possibly even within the same housing) the detected temperature is found to track the junction temperature with a negative offset of around 5 C over the temperature range of interest.

A fourth aspect of the present invention addresses the problem of how to obtain a feedback signal for an optical sensor from a plurality of light emitters.

In accordance with the fourth aspect a light fitting is provided comprising a plurality of light emitters whose light, in operation, passes through a cover which may be a lens, diffuser, \ scatterer or simply a transparent protective layer. The light that is totally internally reflected within this layer is a representative sample in terms of colour and intensity of the light passing through the cover. By arranging an optical sensor to capture some of this light, an effective optical feedback loop may be formed.

A fifth aspect of the present invention permits sophisticated control based on ambient light conditions to be effected with little or no increase in component count compared with comparable approaches.

According to a fifth aspect of the present invention, an optical sensor is arranged so that it detects both light issueing from the light emitters but also ambient light. The fact that, when using pulsed driving, there are OFF periods in which no light is produced by the light emitters means that the same sensor can be read during this period to provide an ambient reading. Control circuitry may then exploit this extra information to effect the control of the light emitters. This may be of particular relevance in the automotive field where collisions are particularly common at dawn and dusk. By factoring in the ambient light conditions, the control circuitry may be arranged to provide a safer environment for the driver.

A sixth aspect of the present invention is concerned with improving the quality of the feedback signals in the closed loop control circuits for light emitters. Particularly in noisy environments and as drive pulses become shorter, there is a danger that feedback signals, be they current feedback, temperature feedback or optical feedback, will be corrupted by transient effects.

This is addressed, in accordance with the sixth aspect, by sampling at a given delay and/or integrating (low pass filtering) the feedback signal prior to feeding the signal to the control circuitry.

According to a seventh aspect of the present invention a wavelength or colour detector is provided to sample the combined output of at least two different LEDs having different wavelengths, the drive to the at least two LEDs being responsive to the output of the detector.

One possible arrangement is that at least two LEDs comprise a white phosphor-based LED and a monochromatic LED. \F 4

According to an eighth aspect of the present invention an LED compensation system is provided for another type of light emitter such as a fluorescent tube. By sensing the colour of the output of the tube (and preferably the colour of the light from the tube and LEDs combined), control of, inter alla, colour temperature may be effected. This may be used to compensate for aging and/or to customise the characteristics of a fluorescent (or other) fitting.

The present invention will now be described, by way of example, with reference to the accompanying drawings in which: Figure 1 illustrates by way of a timing diagram the colour imbalance that can result from

(prior art) PWM drive techniques;

Figure 2 illustrates by way of a timing diagram the colour imbalance that can result from

(prior art) PFM drive technology;

Figure 3 illustrates by way of a timing diagram, the colour balance that results from PAM driving technology; Figure 4 shows a block schematic diagram of a first embodiment of the present invention comprising microprocessor control to effect synchronized LED drive signals and including an optional thermoelectric feedback circuit; Figure 5 shows a block schematic diagram of a second embodiment of the present invention utilising a control module embodied in an Application Specific Integrated Circuit (ASIC) to generate synchronized LED drive signals; Figure 6 is a circuit schematic of an embodiment of a pulse signal generator to generate a pulse modulator clock signal with a defined frequency and duty cycle and a corresponding sample and hold signal for the load current feedback module; Figure 7 is a circuit schematic of one embodiement of a pulse modulated colour synchronization load drive section and a load current feedback module; Figure 8 is a timing diagram illustrating the Idac, load, clock and serial data input control signals required to program a quadruple packaged digital to analogue converter; Figure 9 is a circuit schematic of one alternative embodiment of a pulse-modulated colour synchronization load drive section and a load current feedback module; Figure 10 is a circuit schematic of a further alternative embodiment of a pulse-modulated colour synchronization load drive section and a load current feedback module for four colour channels; Figure 11 is a circuit schematic of an embodiment of a communications module for use with embodiments of the present invention; Figure 12a is a graph illustrating the forward DC current versus the ambient temperature for a typical high-brightness blue or green InGan LED at various values of thermal resistance; Figure 12b is a graph illustrating the normalised relative luminous flux output versus the average forward DC current for a typical high brightness blue or green InGan LED; Figure 1 3a is a graph illustrating the relative light output versus the junction temperature for a typical range of high brightness LEDs Figure 13b is a graph illustrating the dominant wavelength versus the temperature for a typical high brightness InGan LED; Figure 14 is an electrical circuit schematic of one embodiment of the thermoelectric load drive section, thermoelectric cooling load and cooling load feedback module; Figure 1 Sa is a cross section of the human eye; Figure 15b is a block diagram of the functions of the human eye illustrating the ability of the human eye to detect colours from a range of light receptors; Figure 16a graphically represents the relative photopic and scotopic responsivity of the human eye as a function of wavelength; Figure 16b graphically represents the typical spectral sensitivity of a three-colour colour sensor; Figure 17 is an electrical curcuit schematic of one embodiment of the colour sensor feedback module; Figure 1 8a is a graphical representation of the XYZ colour matching functions defined by the Commision Lnternationale de L'Eclairage in 1931; Figure 1 8b is a graphcal representation of the relative spectral power distributions for a range of different high brightness LEDs; Figure 19 represents a typical chromaticity diagram illustrating the standardised white illuminants A, B. C, D and their respective colour temperatures; Figure 20a is a graphical representation of the relative spectral power distributions of a metal halide, fluorescent and standard daylight white light sources; Figure 20b is a graphical representation of the relative spectral power distributions of a typical phosphor-based white LED light source; Figure 21 shows an embodiment of the invention utilising a fluorescent light-emitting unit together with LED light emitters; Figure 22 shows a schematic view of an embodiment of the invention utilising optical feedback derived from total internal reflection within a cover of a light unit; Figure 23 is a graph showing intensity and drive current for a further embodiment of the invention, and Figure 24 shows a number of timing diagrams that illustrate the performance of the first embodiment of the present invention.

Figure 1 shows driving waveforms of four colour channels covering red, green, blue and amber parts of the visible spectrum that are driven by a PWM technique. It is clear that for particular colour settings, the LED cluster will exhibit several different colours and intensities that makes it impossible, especially for use with solid state TV cameras, to adequately control colour, intensity, correlated colour temperature (CCT) and colour rendering index (CRI).

Figure 1 assumes that all colour channels are synchronised at TO but in practice this may not occur and further colour and intensity discrepancies would result.

Figure 2 shows an alternative method for driving LEDs, that of Pulse Frequency Modulation (PFM). PFM utilises pulses of equal amplitude and duration that are generated at a rate determined by the signals' frequency but again this technique does not enable true colour mixing or precise CCT control when two or more wavelengths or colours are used within an LED cluster. The figure illustrates how using PFM leads to very poor colour mixing, CCT and CRI properties as well as different signal frequencies for each colour channel.

An additional method of driving LEDs is that of Pulse Amplitude Modulation (PAM) which colntrols the current through each LED colour channel by varying the amplitude of the current. This method has many advantages for driving LEDs such that all of the colour channels can be pulsed at exactly the same time enabling true control of colour mixing, CCT and CRI. Figure 3 illustrates such PAM driving waveforms. While square waves are shown it should be noted that other waveforms such as trapezoidal or even analogue waveforms such as sinusoidal my be used.

Figure 24 illustrates the principles underlying the first embodiment of the invention by way of a timing diagram. Figure 24(a) shows the PAM driving waveforms for red (R), green (G) and blue (B) emitters at a low intensity. Unfortunately, since LEDs are non-linear devices, driving them at very low current levels results in inefficient driving or in extreme cases, no driving at all.

To address this, the present embodiment may vary the pulse width of the driving waveforms.

Figure 24(b) illustrates the same amount of power being applied to each of the three colour g channels but the power is located in pulses of around half the previous duration. The intensity of the drive can thus be increased to a level at which each LED or LEDs are driven in a more efficient, or in a preferred arrangement linear, operating region. Because the duration of the pulses has been reduced, the overall light output is still at the (low) desired level.

Figures 24 (c) and 24(d) illustrate the opposite scenario in which a very high intensity is required and thismay even be sufficiently high to place the LEDs themselves at risk. Instead of using the 50% mark-space ratio shown in Figure 24(c), the pulse ON time is lengthened within a cycle to permit a lower absolute drive amplitude to be applied to the LEDs. This is shown in Figure 24 (d).

Alternatively, the width of the individual pulse may be maintained while altering the pulse repetition period (lengthening for low light values and shortening for high light values). As in the first alternative, this permits an appropriate level of drive to be applied to LED or LEDs for a given power output. Note that this differs from PFM in that the colour outputs remain synchronized.

Both the PWM and PFM techniques are widely utilised within LED based lighting and display applications as they often assume that the light will be received by the human eye. As the human eye (Figure 15a) has an integrating function, it is assumed that colour discrepancies will not be noticed provided the pulse repetition frequency is above a few hundred Hertz. However, this is not applicable to solid state camera systems in which illumination of the subject using PWM will result in flicker and poor colour balance. It should also be noted that the current generation of high brightness inorganic semiconductor LEDs available from LumiLEDs Lighting of San Jose, CA, USA advise using somewhat lower modulation frequencies than this.

Figure 4 shows a block diagram of an illumination control system 100 according to the first embodiment. The system comprises a microprocessor 1 used to control the system and to generate amplitude modulation signals in response to inputs from a temperature sensor module 2 and a colour sensor module 3. A load drive section 4 receives inputs from both the microprocessor I and the load current feedback module 6. It also has an output connected to the load 5 which in turn provides an output to the load current feedback module. An optional thermoelectric drive section 7 has inputs from the microprocessor and a cooling load q feedback module 9. The section 7 has an output connected to a thermoelectric load module 8 which in turn provides an output to the cooling load feedback module 9. An optional communications module 10 provides bi-directional data communication between the microprocessor and one or more external devices or controllers. An external lighting synchronization module 11 is provided to form an interface to an external synchronization signal such as from a solid state camera when such synchronization is required.

A temperature sensor module 2 includes a plurality of temperature sensors ( 13), the colour sensor feedback module 3 includes at least one and preferably a plurality of colour sensors 12 and the load drive section includes at least one and preferably a plurality of load drivers (not shown) for driving a plurlity of loads. The load current feedback module comprises a plurality of load current feedback sensors while the thermoelectric drive section 7 includes a plurality of thermoelectric drivers (not shown) used to drive a plurality of cooling loads 16 and the thermoelectric current feedback module contains a plurality of cooling load current feedback sensors.

The embodiment operates as follows. A required light intensity and colour are received from an external controller via communications module 10. The microprocessor then calculates the appropriate drive intensities for each of the different colour channels within the system. In doing so, any feedback from the optical colour feedback module 3 will be taken into account.

The microprocessor will then determine the drive current, pulse width and pulse repetition frequency that best matches the required intensities in each channel. For instance, that channel with the lowest required intensity must be driven at a sufficiently high drive level to ensure illumination, while that channel at the highest required intensity must not be driven to such a level (considering the thermal feedback from the temperature sensing module 2) that the Load LEDs are jeopardised. Once appropriate pulse widths and intensities have been calculated, the load drive section 4 is instructed to apply the appropriate pulses to the loads - always within the requirement that the duration and timing of the pulses on each channel are the same. This results in synchronized control.

Where one or more colour channels require a much lower drive intensity than other channels is may be appropriate not to drive some of the LEDs in that or those colour channels.

The Load Drive Section 4 and the load current feedback module 6 cooperate to control the current through the load LEDs to ensure both adequate drive and overload protection.

Referring to the block diagram of Figure 5 a second embodiment of the illumination control system 100 is shown. In this embodiment the communications module 10 communicates not with a microprocessor but with a logic control circuit embodied in an ASIC 91. In this embodiment the temperature feedback is applied to the load drive section 4, as is the optical colour feedback from module 3. The optional thermoelectric drive section has been omitted.

The ASIC 91 is designed to perform the functions (with the exception of the temperature and colour feedback) previously performed by the microprocessor in the first embodiment. The ASIC may comprise a small, cheap Field Programmable Gate Array (FPGA) such as the Xilinx XC9572XL from Xilinx Inc. San Jose CA, USA to a mixed signal ASIC carrying additional circuitry for the illumination control system and even further circuitry for any system integrated therewith.

Figure 6 shows an embodiment of a pulse signal generator 21 which provides a pulse modulator clock signal output 19 to the load drive section 4 and a corresponding sample-and- hold output signal 20 for the load current feedback module 6. The pulse modulator clock input may be derived from either the microprocessor 1 or an ASIC 91 with a defined frequency and duty cycle. The frequency and duty cycle may be programmed by changing data values held within registers of the microprocessor 1 or by timing synchronization pulses within the ASIC 91 to enable the illumination system 100 to vary its frequency or duty cycle according to the intended application. An application brief from Agilent Technologies entitled PULSED OPERATING RANGES FOR ALINGAP LEDS VS PROJECTED LONG TERM LIGHT OUTPUT PERFORMANCE recommends typical duty cycle ratios for common LED-based applications. By utilising one or more output pins located on the microprocessor 1 a highly stable duty cycle and frequency can be achieved which in turn enables precise control of colour synchronization and therefore CCT and CRI. Additional advantages are a reduction in external component requirements and the ability to change the frequency and duty cycle from software operating on the microprocessor 1.

In general the illumination control system will be operated in a noncontinuous pulse mode of operation that will provide current to the LED loads 15 through the load drive section 4. In ll practice the load current feedback circuits will benefit from amelioration of transient effects in the sensed current. In this embodiment this is achieved using a sample and hold circuit to ensure that the pulse modulated colour synchronised current feedback control signal is maintained during a time period when the Load LEDs are not energised. Without such a circuit, the control system is likely to see transient switching voltages which could result in unsettled drive currents through the loads 15 producing variations in the intensity of the LED light output. A delay is advantageously added to reduce the effect of the initial switching transient.

An inverted and time delayed clock signal relative to the pulse modulator clock signal 19 is thus derived. The sample and hold output signal 20 and clock output signal 19 can be created using NPN transistor Ql and resistor R2 to invert the pulse modulator clock signal input from a microprocessor 1 or ASIC 91. The inverted signal is buffered and inverted by the dual input NAND gate (U1 a) with its output connected directly to both inputs of NANDgate (Ulc) and also to one of the inputs of NAND gate (Ulb) and to the other input of the NAND gate (Ulb) via resistor (R3) capactor (C1) network. The resistor capacitor(RC) network provides a time delay according to a well known formula and typical values of R3 is 68kOhms and C1 is lnF which provides a time constant of around 68 microseconds. The signal is then inverted and delayed again by Ulc and the RC network R4 and C2 with a final inversion by NANDgate (Uld) to create the pulse modulator clock signal output 19 that represents the ON portion of the duty cycle in Figure 3. To those of skill in the art, selection of appropriate component values can be performed to accommodate values of maximum and minimum desired values of modulation signal frequency. Equally, alternative circuits to perform the requisite function could readily be designed the skilled person.

This technique is also applicable to the other feedback circuits within the system such as the colour feedback circuit.

Figure 7 shows the electrical schematic of an embodiment of the load drive section 4 with a load current feedback module 6 whereby a pulse modulated colour synchronised control signal 30 is formed to drive a voltage controlled current source or load driver (31), such as an e- channel MOSFET through a differential amplifier (U3a) and analogue switching arrangement (U4a, U4b). The pulse modulated colour synchronised control signal is created by combining the amplitude modulated control signal 27 using the output of a Digital to \9 Analogue Converter (DAC) U2 and the current feedback control signal is generated using a sense resistor, which is consequently pulse modulated by employing an analogue switching network using the pulse modulator clock signal outputs representing the time periods Ton (19) and Toff (20).

An exemplary DAC is the 14 pin TLC5620 with serial programming interface available fron Texas Instruments Incorprated, Dallas TX, USA. In the current embodiment the DAC is used to generate an amplitude modulated control signal 27 in the form of a voltage output, which is created as a proportion of the DAC's input reference voltage 26 using an 8-bit data register representing one of 256 different voltage levels between ground and the DAC's reference voltage input. Therefore,by selecting the appropriate value to be stored in the DAC's data register an amplitude (or voltage) of the pulse modulated colour synchronized control signal can be altered accordingly. The DAC's input reference voltage 26 may be generated using a standard voltage divider arrangement containing two resistors R5 and R6 in series between the logic supply voltage and Ground. A typical value for R5 and R6 is 2k20hms, which provides a DAC input reference voltage 26 of approximately 50% of the logic supply voltage, or 2.5 volts. From the foregoing description it will be appreciated that the DAC's input reference voltage 26 may be provided using alternative techniques.

The amplitude modulated control signal 27 output ismodified by programming the DAC (U2) using a serialinterface which is connected to amicroprocessor 1 or ASIC 91. To program the DAC with a new output voltage the microprocessor or ASIC sets the DAC Load control line 22 high and then clocks in a command byte followed by a data byte on the DAC Data line 24. Once all the bits have been clocked in, the DAC load control line is pulsed low to transfer the data from the serial input register to the selected DAC data register. When the DAC LDAC control line 25 is set high during serial programming and then pulsed low the DAC data register value is transferred to the DAC output directly. The DAC LDAC control line 25 enables each of the four DACs outputs to be updated simultaneously enabling precise synchronization of all DAC output channels. The command and data bytes are clocked into the DAC (U2) on the falling edge of the DAC clock line 23 provided by either a microprocessor or ASIC. Figure 8 is a timing diagram illustrating the sequence for programming the DAC, where the command bits are set to instruct to write the data contained within the data byte to the DAC determined by the channel select bits A0 and A 1. The range bit RG controls the DAC output range. When RNG is set low, the output range is between the applied reference voltage and GND, and when RNG is set high, the range is between twice the reference voltage and GND.

Referring to Figure 7, the amplitude modulated control signal voltage is fed to a non- inverting amplifier (U3a) configured to provide a continuous differential output voltage generated by the voltage difference between the amplitude- modulated control signal voltage 27 and the current feedback control signal voltage 28. The current feedback control signal voltage is generated by a load feedback current sensor 14 in the form of a sense resistor (Rsense) used to measure the current flowing from a power source (+Vdd) through the load and the load driver MOSFET 31 to GND. The sense resistor may be connected between the source terminal of the load driver MOSFET and ground to generate a small voltage(Vs) which is proportional to the load current that flows thorough the load 15. A low value is preferred for Rsense to minimise dissipation and the resistor commonly comprises a discrete metallic resistor with zero temperature coefficient such as manganin or constantin. The sense resistor maycomprise a portion of track on the Printed Circuit Board (PCB).

A pulse modulated colour synchronized control signal30 is created by connecting the output voltage from the differential operational amplifier (U3a) stage to the input (B1) of a two- channel, single-pole double-throw analogue switch (U4a) through a resistor (R13). The second input (B0) of the analogue switch (U4a) is connected directly to circuit ground while the output (A)of the analogue switch (U4a) is connected to the gate electrode 30 of the load driver MOSFET 31 through a coupling resistor R9. The control input (S) of the analogue switch (U4a) is connected to the pulse modulator clock signal output 19 which connects the appropriate input Be or B 1 of the analogue switch (U4a) to the output (A) according to the logic level 0 or 1 of the pulse modulator clock output signal 19 respectively. During the ON period Ton, of the pulse modulator clock output signal 19 the analogue switch (U4a) output (A) is connected to the coupling resistor (R9) and charges the capacitor (C3) which produces a voltage potential proportional to the output voltage of the differential operational amplifier (U3a). However, during the off period, Toff, the analogue switch (U4a) output (A) is connected to circuit ground and ensures that the load driver MOSFET 31 does not allow current to pass through the load 15. The voltage potential across the capacitor (C3) remains stable and does not discharge during the time period, Toff, as the inputs B1 and BOof the analogue switches U4a and U4b become high impedance ensuring little or no current discharge occurs.

During the off time period, Toff, of the sample-and-hold output signal 20 the input B0 of the analogue switch (U4b) is connected to output (A) and hence to the coupling resistor (R10) enabling the voltage potential (Vs) across the sense resistor (Rsense) or current feedback control signal voltage 28 to appear at the negative input of the operational amplifier (U3a). A protection diode (Dl) is used to protect the operational amplifier (U3a) from negative voltage spikes that may occur during a failure of the microprocessor 1 or ASIC 91. Accordingly, during the on time, Ton, of the sample-and-hold output signal 20 the input (B 1) of the analogue switch (U4b) is connected to output (A) leaving a high impedance state at the input (B0). The capacitor (C3) ensures that a voltage is maintained during the timing period, Toff, of the pulsed modulated colour synchronized control signal and therefore does not require a large transition current through the load driver MOSFET when entering the timing period, Ton, thus ensuring the current through the load 15 is stable.

A Zobel filter is present across the load drive signal 33 and the circuit ground in the form of resistor (R11) and capacitor (C4) in series. The presence of a Zobel filter ensures that the load appears resistive at high frequencies and helps to ensure oscillation is attenuated at high frequencies. Typical values for the resistor (R11) in 10 Ohms and capacitor (C4) is 10 nanoFarad.

In one alternative embodiment of the load drive section 4 and the load current feedback module 6 shown in Figure 9, the continuous amplitude modulated control signal 27 is generated by combining two timing signals (one timer represents frequency and the other duty cycle) to form a timing control signal 90 from a microprocessor 1 or ASIC 91. The timing control signal is then integrated through a low pass filter in the form of a resistor/capacitor network. The foregoing embodiment has considerable advantages in practice as the DAC uses low cost passive components and the DAC resolution is software- configurable. Alternatively, the timing control signal 90 may be generated by a dedicated PWM timer within the microprocessor 1 or ASIC 91 and produces continuous output voltage with an amplitude resolution proportional to the resolution of the input timing control signal 90. Similar DAC conversion techniques are well known to those skilled in the art.

In yet another separate embodiment of the load drive section 4 shown in Figure 10 up to 4 separate load driver 4 and four separate load current feedback sensor 14 chapels can be independently controlled by the microprocessor lor ASIC 91. All four of the DAC outputs (DACA, DACB, DACC, DACD) are used to create corresponding continuous amplitude modulated control signals (27, 40, 41, 42). The DAC (U2a) provides increased amplitude resolution due to its 10-bit data control enabling 1024 different output voltage levels to be achieved per channel. The pulse modulated colour synchronized load drive signals (33, 47, 48, 49) are then used to drive the respective loads 15 in exact synchronization. In the foregoing embodiment the colour, CCT, CRI and intensity of the illumination control system are synchronized to produce a pure additive mixed light output.

Figure 11 illustrates a stylised electrical circuit schematic of one embodiment of a communications module (10) that provides a data connection interface between the microprocessor 1 or ASIC 91 and an internal or external controlling device. The data connection interface can be understood to encompass any circuit or system that passes control instructions between the microprocessor 1 or ASIC 91 and a controlling device to control the functionality of the illumination control system 100 such as to precisely change colour, CCT or CRI amongst others. In other embodiments of the invention, the communications module 8 may be equipped with a suitable transmitter, receiver or transceiver to facilitate data communications using different techniques such as radio frequency, infrared, electrical or other suitable methods. The embodiment of Figure 11 incorporates an external control receiver circuit 45 whereby a 4 pin XLR type socket connector 55 is used as the serial line data input to the serial port of the microprocessor 1 or ASIC 91 where the serial data is decoded according to the protocol used. Power for the illumination control system 100 may also be supplied with a voltage of + Vdd (56) being available on pin 2 and circuit ground being available on pin 4 of the XLR type socket connector 55.

In order to increase the distance between the power supply units used to provide the power signals +Vdd (56) and Ground (58) the serial line protocol should use a relatively low baud rate to reduce errors along the transmission cable, typical values should be from 2400 to 19200 baud. The serial line data input 57 voltage level is reduced to voltage levels compatible with general logic circuits using a potential divider of resistors R12 and R13 with typical values of 22k and 6k Ohms respectively. A protection diode (D2) is used to protect the communications circuit from negative voltage spikes that may occur within lengthy transmission cables. An NPN transistor (Q3) and a capcitor C5 acting as a Millar integrator

IG

are used to invert the serial line data input 57 and slow down the transistor operation. Typical values for capacitor C5 and resistor R14 are lOOpF and 22Kohms respectively.

Figure 21 shows a lighting arrangement 1010 in accordance with an embodiment of the present invention. A fluorescent tube 1012 is shown arranged in traditional fashion between a reflector 1036 (shown in section) and a diffuser 1014. Light output from the tube passes through the diffuser either directly or via the reflector which is typically shaped and arranged to minimise the amount of"wasted" light.

However, such fluorescent tubes deteriorate with age and, to counter this, a feedback circuit 1020 and six LEDs 1022 - 1032 are provided to compensate. The diffuser 1014 is provided with a colour sensor 1016 which detects light that has been totally internally reflected within the diffuser. This light will be representative of the light passing through the diffuser and so the output ofthe sensor 1016 provides a colour feedback signal to the feedback control circuit 1020 via signal line 1018.

The feedback control circuit is arranged to respond to the output ofthe colour sensor 1016 to control the light issueing from the fixture. To this end the circuit 1020 is connected via line 1034 to six amber LEDs 1022, 1024, 1026, 1028, 1030, 1032 which are mounted in the reflector 1036. Because of the nature of the deterioration of fluorescent tubes, adding some amber light provides compensation for the aging process. As the tube 1012 deteriorates still further, a greater amount of light will be provided by the LEDs to compensate. The closed loop feedback provided by the colour sensor 1016, feedback control circuit 1020 and the LEDs allows at least one colour parameter (for example colour temperature) to be consitently maintained.

The fluorescent tube 1012 may be arranged to be separately removable since the lifetime of such tubes is typically 1000 hours while the lifetime of LEDs is typically 10000 hours.

Alternatively, since LEDs arc comparatively cheap, they could be incorporated with the tube and discarded when the tube is renewed. lo

While the colour sensor 1016 is shown as sensing the output from the whole fixture, i.e. the fluorescent tube and the LEDs, it is possible for the sensor to be arranged to detect only the output from the fluorescent tube.

While this embodiment has been described as providing compensation for the aging of the fluorescent tube, it should be noted that it could equally be used to set a desired characteristic, such as colour temperature, for a tube of any age. Since fluorescent tubes are currently provided in various colour temperatures for various purposes, this would allow manufacturers and dealers in fluorescent tubes to reduce their inventory with attendant reduction in cost.

Moreover, this embodiment of the invention may equally be applied to other non-LED sources of light that deteriorate with age or which would benefit from the ability to provide custom colour features such as incandescent lamps.

Figure 22 shows another embodiment 1100 of the present invention that exploits the total internal reflection of light within a diffuser, lens or cover. The control circuitry has been omitted for clarity. The embodiment may preferably be a headlamp for a car but this aspect of the invention is applicable to a wide range of lighting fixtures.

A circuit board 1 102 carries a number of LEDs of which five 1 104, 1 106, 1 108, 1 1 10 and 1112 are shown. Light from these LEDs passes through a cover 1114 which may be a lens, diffuser etc. constructed from glass or plastics. By the nature of such an arrangement, some of the light incident upon the cover 1114 will be totally internally reflected within the cover as depicted by the zigzag lines. A portion of this totally internally reflected light will impinge upon an optical sensor I 1 16 mounted somewhere on the periphery of the cover.

The sensor 1116 could be a colour sensor as used in the embodiment shown in the previous figure or it might simply be an intensity sensor an required by the following embodiment.

By utilising such as sensor, the collective output of a large number of LEDs may be sensed using a single sensor. This has the benefit over mounting one or more sensors on the circuit board of sensing the entire light output and will not be distorted by the performance (or otherwise) of individual LEDs. 1%

Figure 23 shows a diagrammatic graph of LED light intensity 1202 and drive current 1204 over time.

Imagine that light intensity 1202 is generated in response to current 1204 towards the left hand side of the graph. In the case of a car headlamp, for example, there is a minimum acceptable intensity value determined by the regulatory authorities. In other applications there wild be a minimum specification of light output and this value is illustrated by the horizontal broken line 1206.

After a long usage time the output of the LED will deteriorate as illustrated by the downward turn of curve 1202 at time A. At time B the intensity of the LED output falls below specification. One possibility here would be to detect this and light a warning light on the dashboard of the car (or somewhere else on another, non-automotive, light fitting) to inform the driver that his or her lighting unit needs to be replaced. However, this would quite likely be ignored as the deterioration in intensity would probably not be discernable to the driver.

In the present embodiment, this is addressed by increasing the current drive provided to the LED at time B to cause the light intensity to follow the dot-dash line 1208 (i.e. continue at or above the minimum output level). As a consequence of this the LED will fail catastrophically at time C as shown by the line 1210. The advantages are that the lifetime of the LED is extended beyond that at which it fails to perform to specification and the catastrophic failure ensures that the driver or other user has to replace the LED, lighting unit or fixture and cannot continue to operate a failing unit with the attendant safety implications.

The present disclosure extends to any novel feature or combination of features disclosed herein whether express or implied and to any generalization thereof.

A

CLAIMS: 1. A lighting control circuit for driving a plurality of LEDs of at least two different types, each type having a different characteristic wavelength, the control circuit comprising means for driving the two different types of LEDs to desired light intensities during coincident drive periods wherein means are provided to alter at least one of duration and repetition frequency of the drive period.

2. A lighting circuit as claimed in claim 1, wherein the drive periods start in response to an external signal.

3. A lighting circuit as claimed in claim 1 or claim 2, further comprising a plurality of non-organic LEDs 4. A drive circuit for LEDs comprising power control means for controlling the amount of power supplied to at least one LED, means for measuring the intensity of the light emitted by the at least one LED, the power control means being responsive to the intensity of the light emitted wherein the power control means is arranged to drive the at least one LED at a power greater than specified.

5. A lighting fixture with a temperature sensor mounted in proximity to an LED to track the junction temperature thereof.

6. A lighting fixture as claimed in Claim 5 further comprising a feedback circuit for ensuring that the power supplied to the LED is, at least in part, responsive to the junction temperature thereof.

7. A lighting fixture as claimed in claim 5 or claim 6, wherein the sensor is mounted in he same package as the LED.

8. A lighting fixture comprising a plurality of emitters and a cover through which light from the emitters travel when they are activated, an optical sensor arranged to detect light that has been totally internally reflected in the cover.

9. A lighting fixture as claimed in claim 8 wherein the plurality of emitters comprise LEDs.

10. A lighting fixture as claimed in Claim 8 or claim 9 wherein the optical sensor comprises a colour sensor.

11. A lighting fixture as claimed in Claim 8, claim 9 or claim 10 further comprising a feedback circuit responsive to the output of the optical sensor to control the plurality of emitters.

12. A lighting control circuit for pulsed driving of a plurality of light emitters and receiving optical output information from at least one optical sensor, the control circuit comprising means for receiving information concerning the light output of the light emitters while at least one of the light emitters is actuated and for receiving information concerning ambient light output when none of the light emitters are activated.

13. A lighting control circuit as claimed in Claim 12 further comprising a feedback circuit for controlling the drive to the plurality of light emitters in response to the actuated light output and ambient light output.

14. A lighting control circuit for intermittently driving a plurality of light emitters and receiving feedback information from at least one feedback sensor, the control circuit comprising means for ameliorating transient effects on the output of the feedback sensor.

15. A lighting control circuit as claimed in Claim 14, wherein the means for eliminating transient effects comprises an averaging circuit.

16. A lighting control circuit as claimed in Claim 14 or 15 wherein the means for eliminating transient effects include a delay between the actuation of at least one of the l l all plurality of light emitters and the receiving of feedback information from the at least one feedback sensor.

17. A lighting control circuit for driving at least two optical emitters having different wavelength characteristics and for receiving a signal from a sensor which sensor can detect wavelength information, the control circuit comprising means for altering the amount of drive applied to the at least two optical emitters in response to colour information derived from the sensor.

18. A lighting control circuit for driving at least one LED and for receiving colour information from a sensor, the sensor located to receive light from at least one other light source which does not comprise an LED, the control unit comprising means for driving the at least one LED in response to the colour information received from the sensor regarding the light from the at least another light source.

19. A lighting control circuit as claimed in Claim 18 wherein the means for driving the at least one LED are further responsive to colour information received from the sensor which colour information is derived from light generated by both the at least one LED and the at least one other light source.

20. A lighting control circuit as claimed in Claim 18 or Claim 19 wherein the another light source comprises a fluorescent light fitting.

Claims

1. A lighting fixture comprising a plurality of emitters and a cover through which light from the emitters travel when they are activated, an optical sensor arranged to detect light that has been totally internally reflected in the cover.

2. A lighting fixture as claimed in claim 1 wherein the plurality of emitters comprise LEDs.

3. A lighting fixture as claimed in claim 1 or claim 2 wherein the optical sensor comprises a colour sensor.

4. A lighting fixture as claimed in claim 1, claim 2 or claim 3 further comprising a feedback circuit responsive to the output of the optical sensor to control the plurality of emitters.

5. A lighting control circuit for pulsed driving of a plurality of light emitters and receiving optical output information from at least one optical sensor, the control circuit comprising means for receiving information concerning the light output of the light emitters while at least one of the light emitters is actuated and for receiving information concerning ambient light output when none of the light emitters are activated.

6. A lighting circuit as claimed in claim 5 further comprising a feedback circuit for controlling the drive to the plurality of light emitters in response to the actuated light output and ambient light output.

7. A lighting control circuit for driving at least two optical emitters having different wavelength characteristics and for receiving a signal from a sensor which sensor can detect wavelength information, the control circuit comprising means for altering the amount of drive applied to the at least two optical emitters in response to colour information derived from the sensor.

8. A lighting control circuit for driving a plurality of LEDs of at least two different types, each type having a different characteristic wavelength, the control circuit comprising means for driving the two different types of LEDs to desired light intensities during coincident drive periods wherein means are provided to alter at least one of duration and repetition frequency of the drive period.

9. A lighting circuit as claimed in claim 8, wherein the drive periods start in response to an external signal.

10. A lighting circuit as claimed in claim 8 or claim 9, further comprising a plurality of non-organic LEDs.

11. A drive circuit for LEDs comprising power control means for controlling the amount of power supplied to at least one LED, means for measuring the intensity of the light emitted by the at least one LED, the power control means being responsive to the intensity of the light emitted wherein the power control means is arranged to drive the at least one LED at a power greater than specified.

12. A lighting fixture with a temperature sensor mounted in proximity to an LED to track the junction temperature thereof.

13. A lighting fixture as claimed in claim 12 further comprising a feedback circuit for ensuring that the power supplied to the LED is, at least in part, responsive to the junction temperature thereof.

14. A lighting fixture as claimed in claim 12 or claim 13, wherein the sensor is mounted in the same package as the LED.

15. A lighting control circuit for intermittently driving a plurality of light emitters and receiving feedback information from at least one feedback sensor, the control circuit comprising means for ameliorating transient effects on the output of the feedback sensor.

16. A lighting control circuit as claimed in claim 15, wherein the means for eliminating transient effects comprises an averaging circuit.

17. A lighting control circuit as claimed in claim 15 or 16 wherein the means for eliminating transient effects include a delay between the actuation of at least one of the plurality of light emitters and the receiving of feedback information from the at least one feedback sensor.