Abstract:
A method for measuring a color temperature (TC) of a light source ( 2 ) comprises the steps of: measuring the partial intensity of one predefined blue spectral conpponent (B); measuring the luminance (V); and calculating the quotient BN as representing the color temperature (TC). The color temperature (TC) is calculated on the basis of a predetermined relationship between the color temperature (TC) and the quotient BN. Sensor assembly capable of generating a measuring signal containing information regarding the color temperature of a light source. Switch stage for cooperation with a sensor assembly. Driver for during a lamp with variable color temperature properties. Lamp system comprising a lamp, a sensor assembly and a lamp driver.

Description:
FIELD OF THE INVENTION  
       [0001]     In one aspect, the present invention relates in general to a method and device for measuring the color temperature of a light source.  
         [0000]     More specifically, the present invention relates to a driver device for driving a light source having variable color temperature.  
       BACKGROUND OF THE INVENTION  
       [0002]     In general, there is a need for providing a method and device for measuring the color temperature of a light source. The color temperature of a light source can be defined as the temperature which a black body must have so that, in the chromaticity diagram, its color point is closest to the color point of the light source. Therefore, a conventional method of measuring color temperature comprises the step of first measuring the color point, and then calculating the closest point on the black body line. A first disadvantage of such conventional method is the relative complexity of such calculation.  
         [0000]     Color points of a light source are usually given in a space having three coordinates x, y, z, wherein 
 
 x=X /( X+Y+Z ),  y=Y /( X+Y+Z ),  z=Z //( X+Y+Z ) 
 
 wherein X, Y and Z indicate the absolute intensities of certain pre-defined spectral components. A direct way of measuring the three coordinates x, y and z involves actually measuring the three corresponding intensities, which involves the use of three color sensors, each including a corresponding color filter and a light intensity detector. Such color sensors are relatively expensive. 
 
         [0003]     A more economic approach of measuring a color point is based on the fact that, per definition, x+y+z=1. Therefore, it suffices to measure only two coordinates x and y, and to calculate the third coordinate z according to z=1−x−y. This still involves the use of two color sensors. An example of a method and device according to this principle is disclosed in DE-4421919.  
         [0004]     A main objective of the present invention is to provide a more economic way of measuring the color temperature of a light source.  
         [0005]     In a specific aspect, the present invention relates to a driver device for a gas discharge lamp, specifically a HID lamp, more specifically a metal halide lamp. Typical lamp drivers comprise a stage generating a substantially constant current, followed by a commutator for commutating the lamp current, i.e. regularly changing the direction of the current in the lamp. Conventionally, such commutator operates at a duty cycle of 50%, i.e. in each current period, the duration of the current flow from one electrode to the other is equal to the duration of the current flow in the opposite direction. In an earlier patent application PCT/IB03/01547, the present applicant has described a gas discharge lamp with variable color properties. By changing the average lamp current, specifically the duty cycle of the lamp current, the color temperature is varied over a wide temperature range; depending on the composition of the lamp filling, the temperature range may extend from about 2500 K to about 6000 K.  
         [0006]     In principle, there is a one-to-one relationship between duty cycle and color temperature. A problem is, that this relationship appears to be not constant in time. Therefore, if it is intended to keep the color temperature constant, it does not suffice to keep the duty cycle constant.  
         [0007]     It is a specific objective of the present invention to solve this problem.  
         [0008]     In a specific aspect, the present invention relates to the aspect of transferring two measuring signals to a processing circuit. Normally, this requires three wires: one wire for each measuring signal, and a common ground wire. Each wire involves costs of wiring and associated connectors. Further, with each wire, assembly complexity and assembly time increase.  
         [0009]     It is a further objective of the present invention to reduce this problem.  
       SUMMARY OF THE INVENTION  
       [0010]     According to an important aspect of the present invention, a method is provided for measuring a color temperature, wherein the absolute intensity of one predefined blue spectral component B as well as the overall light intensity or luminance V are measured, and the quotient B/V is calculated. This method, which is based on the insight that said quotient B/V appears to have an almost linear relationship to the color temperature, involves only one relatively expensive color sensor and one relatively inexpensive luminance sensor (i.e. a light intensity sensor). A further advantage of this method is the fact that the overall light intensity, which is typically an important parameter of interest, is also directly made available; in the conventional method, the overall light intensity must be determined indirectly, or, if it is to be determined directly, a further detector is required.  
         [0011]     According to another important aspect of the present invention, a driver for a light source is provided, specifically a gas discharge lamp, comprising a sensor assembly for generating a measuring signal indicating the color temperature, which measuring signal is fed back to a controller of the driver, which is designed to adapt its settings such as to keep the color temperature substantially constant. Advantageously, this sensor assembly comprises a blue sensor and a luminance sensor, allowing the controller to determine the ratio B/V.  
         [0012]     According to another important aspect of the present invention, a sensor assembly comprising two sensor diodes is provided, each sensor diode being connected in series with a corresponding auxiliary diode in an opposite direction, these two series arrangements being connected anti-parallel to each other. When a supply voltage having a first polarity is applied to this assembly, a current is generated indicating the measuring signal of a first sensor diode. When the supply voltage has opposite polarity, the current indicates the measuring signal of the other sensor diode. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]     These and other aspects, features and advantages of the present invention will be further explained by the following description with reference to the drawings, in which same reference numerals indicate same or similar parts, and in which:  
         [0014]      FIG. 1  is a block diagram, schematically showing a driver device according to the present invention;  
         [0015]      FIG. 2  is a graph schematically illustrating lamp current as a function of time;  
         [0016]      FIG. 3  is a graph schematically illustrating color temperature as a function of duty cycle;  
         [0017]      FIG. 4  is a block diagram schematically illustrating a preferred embodiment of some components of a lamp driver;  
         [0018]      FIG. 5  is a graph schematically illustrating a relationship between B/V and color temperature;  
         [0019]      FIG. 6  is a block diagram schematically illustrating a preferred embodiment of some components of a lamp driver. 
     
    
     DESCRIPTION OF THE INVENTION  
       [0020]      FIG. 1  is a block diagram schematically illustrating a preferred embodiment of a driver device or electronic ballast  10  according to the invention for driving a gas discharge lamp  2  in a lamp system  1  with variable color properties. The present invention will be explained for an embodiment where the ballast  10  typically comprises:  
         [0021]     an input  11  for receiving AC mains;  
         [0022]     a rectifier  12  for rectifying the AC mains voltage to a rectified DC voltage;  
         [0023]     a DC/DC up-converter  13  for converting the rectified mains DC voltage to a higher DC voltage and for performing power factor correction;  
         [0024]     a down-converter  14  for converting said higher DC voltage to a lower DC voltage (lamp voltage) and a corresponding DC current (lamp current);  
         [0025]     and a commutator  15  for regularly changing the direction of this DC current within a very brief time (commutating periods).  
         [0026]     It is noted, however, that the ballast may have a different design.  
         [0027]     The down converter behaves as a current source. Typically, the commutator operates at a frequency in the order of about 50-400 Hz. Therefore, in principle, the lamp is operated at constant current magnitude, the lamp current regularly changing its direction within a very brief time (commutating periods), i.e. an electrode is operated as a cathode in a first part of each current period and is operated as anode during the remainder of each current period. This is illustrated by  FIG. 2 , which is a graph schematically illustrating the current I L  through the lamp  2  as a function of time. In a current period P, the current I L  flows from one lamp electrode to the other during a first time interval t 1 , and flows in the opposite direction during a second time interval t 2 , wherein P=t 1 +t 2 . A duty cycle D is defined as D=(t 1 /P)−100%. During the current period P, the lamp current I L  has constant magnitude but changing direction. On a time scale larger than the current period, an average current I AV  may be defined as (t 1 −t 2 )I L /P. Conventionally, a driver is designed such that its output may be considered as constituting a current source with alternating current direction but constant current magnitude, having a duty cycle of 50%; in that case, the average current I AV  is zero.  
         [0028]     Some types of HID lamps have a property that the color temperature T C  is variable as a function of the average current I AV , which can be varied by varying the duty cycle D, as explained more elaborately in PCT/IB03/01547, incorporated herein by reference. When the lamp current is given an average current I AV  differing from zero, a shift is induced of the distribution of the particles in the lamp, resulting, in some types of lamps, in a change in color temperature. Therefore, the driver  10  is capable of driving the lamp  2  with variable average lamp current I AV .  
         [0029]     In one possibility of implementing the present invention, the average current I AV  differs from zero because the current intensity during the positive current period differs from the current intensity during the negative current period, in which case the current may have a duty cycle of 50%. However, this type of implementation is not preferred, one reason being that the lamp current magnitude during one half of a current period differs from the current magnitude during the other half of the current period, i.e. the current intensity is not constant in time. Since the light intensity is proportional to the current intensity, this might lead to undesirable flicker of the lamp. Another reason is that it is relatively difficult to implement this method in existing driver designs.  
         [0030]     In the following, the present invention will be explained in more detail for the case of a preferred implementation of the present invention, in which this disadvantage is avoided, and which furthermore is easier to implement by an appropriate software or hardware adaptation in existing lamp drivers. However, it is noted that the same or similar results can be obtained by having the positive current magnitude and the negative current amplitude differing from each other.  
         [0031]     In this preferred implementation, the duty cycle differs from 50% and the current intensity remains constant at all times, i.e. the lamp current magnitude during the “positive” half of a current period (t 1 ) is equal to the current magnitude during the “negative” half of the current period (t 2 ) (see  FIG. 2 )  
         [0032]     Thus, according to this preferred aspect of the present invention, the driver  10  is designed to have an adaptable duty cycle.  
         [0033]     In general, the relationship between the color temperature T C  and the duty cycle D is as depicted in  FIG. 3 , where the horizontal axis represents the duty cycle and the vertical axis represents the color temperature.  
         [0034]     The exact values of the color temperature depend on the precise composition of the lamp filling.  
         [0035]     It has been found that the relationship between D and T C  is not constant over the life time of the lamp. To solve this problem, the driver  10  comprises a light sensor assembly  20 , arranged in the proximity of the lamp  2 , for receiving light from the lamp  2  and generating a sensor signal S(T C ) which contains information regarding the color temperature of the lamp light. The driver  10  further comprises a controller  50 , which has a measuring input  51  and a first control output  52 . The sensor assembly  20  is coupled to the measuring input  51  of the controller  50 . The controller  50  is adapted for generating, at its first control output  52 , a commutator control signal S D  for controlling the commutator  15 , more particularly for controlling its duty cycle D, on the basis of the sensor signal S(T C ), such as to keep the sensor signal S(T C ) and hence the lamp color temperature constant.  
         [0036]     The lamp driver may be designed for one specific color temperature setting in association with one specific lamp type, but typically the lamp driver will allow a user to set a specific color temperature. To this end, the controller  50  has a first user input  54  for receiving a first user control signal S U1  as a user-generated color setting signal. The driver  10  further comprises a control setting device  57 , such as for instance a potentiometer, generating the first user control signal S U1  which can be varied continuously within a predetermined range. The control setting device  57  can be user-controllable, but it can also be a suitably programmed controller.  
         [0037]     Preferably, and as shown in  FIG. 1 , the controller  50  is also provided with a dimming facility, i.e. a facility for setting the intensity of the light generated by the lamp  2 . To this end, the controller  50  has a second user input  55  and a second control output  53 . At its second user input  55 , the controller  50  receives a second user control signal S U2  as a user-generated intensity setting signal. The driver  10  further comprises an intensity setting device  58 , such as for instance a potentiometer, generating the second user control signal S U2  which can be varied continuously within a predetermined range. The intensity setting device  58  can be user-controllable, but it can also be a suitably programmed controller. At its second control output  53 , the controller  50  generates an intensity control signal S I  for the down-converter  14  to control the magnitude of the lamp current I L .  
         [0038]     The controller  50  may be designed to generate its intensity control signal S I  on the basis of the actual second user input signal S U2  only. Preferably, however, in a control mode, the controller keeps the light intensity constant on the basis of the measuring signal from the sensor assembly  20 .  
         [0039]     In principle, the sensor assembly  20  may be any suitable sensor assembly capable of generating an adequate measuring signal containing information regarding color temperature and light intensity. A preferred embodiment of such sensor assembly  20 , which is preferred in view of its relative simplicity and relative low costs, is illustrated in the schematic block diagram of  FIG. 4 . This preferred sensor assembly  20  comprises two light sensors  21  and  22 . The first sensor  21  is sensitive to all visible light and generates a first sensor signal S V  indicating the luminance of the light, i.e. the total intensity in the visible range of the spectrum; this first sensor  21  will hereinafter also be indicated as luminance sensor, and its sensor signal will hereinafter be indicated as luminance signal. The second sensor  22  is sensitive to blue light only, and generates a second sensor signal S B  indicating the amount of blue light, i.e. the partial intensity in the blue range of the spectrum; this second sensor  22  will hereinafter also be indicated as blue sensor, and its sensor signal will hereinafter be indicated as blue signal. In this respect, “blue light” will be understood as light having a wavelength in the range of approximately 380 nm to approximately 480 nm. Preferably, the blue sensor  22  is sensitive to substantially the entire blue range. It is noted that it is not necessary that the blue sensor  22  has equal sensitivity for all wavelengths within its sensitivity range; usually, a sensor has a peak sensitivity at one wavelength, and a decreasing sensitivity with increasing distance from this one wavelength, as will be clear to a person skilled in the art. The blue sensor  22  may have a narrow sensitivity range around any wavelength within the blue range. Preferably, the blue sensor  22  has a peak sensitivity in the order of about 440 nm.  
         [0040]     The measuring input  51  of the controller  50  actually comprises two input terminals  51   a  and  51   b , the first one for receiving the luminance signal S V  and the second for receiving the blue signal S B . The luminance signal S V  can be used in a simple straight-forward way for controlling the light intensity. The controller  50  comprises a first comparator  60 , having one input receiving the luminance signal S V  and having another input receiving a reference light intensity signal REF L . This reference light intensity signal may be the user input signal received at the user input  55 , or a reference value stored in a memory  56 . The comparator output signal is coupled to the second control output  53  of the controller  50 .  
         [0041]     The controller  50  further comprises a divider  70 , having two inputs coupled to the controller measuring input terminals  51   a  and  51   b  for receiving the luminance signal S V  as well as the blue signal S B . The divider  70  is arranged to divide the blue signal S B  by the luminance signal S V , and to generate an output signal B/V corresponding to S B /S V . The controller  50  comprises a second comparator  71 , having one input receiving the divider output signal B/V and having another input receiving a reference color signal REF C . This reference color signal may be the first user input signal S U1  received at the first user input  54 , or a reference value stored in said memory  56 . The comparator output signal is coupled to the first control output  52  of the controller  50 , either directly or, in the example illustrated, via a pulse generator  72  which generates timing pulses for determining the duration of the first duty cycle time interval t 1  and the duration of the second duty cycle time interval t 2 , respectively.  
         [0042]     By keeping the ratio B/V substantially constant, the controller  50  assures that the color temperature remains substantially constant, based on the finding that B/V is a parameter which is a good representative for the color temperature, as illustrated by  FIG. 5 .  FIG. 5  is a graph showing experimental results of measurements regarding the relationship between B/V (vertical axis) and the color temperature TC (horizontal axis).  
         [0043]     For transferring the sensor signals from two sensor devices to a processing circuit, in a preferred implementation, as illustrated in  FIG. 6 , only two wires are required.  
         [0044]     In this preferred embodiment, the two sensors  21  and  22  are each implemented as a photo diode. The first photo diode  21  is connected in opposite direction in series with a first auxiliary diode  23 , while the second diode  22  is connected in opposite direction in series with a second auxiliary diode  24 . The free electrode of the first photo diode  21  is connected to the free electrode of the second auxiliary diode  24 , and this node is connected to a first output terminal  25  of the sensor assembly  20 , while the free electrode of the second photo diode  22  is connected to the free electrode of the first auxiliary diode  23 , and this node is connected to a second output terminal  26  of the sensor assembly  20 . In this case, the diodes  21 ,  23  and  22 ,  24  in each series connection have their anode connected together, so each diode has its cathode connected to an output terminal, but the diodes may have their orientations inverted. Also, the order of the diodes in each series connection may be reversed.  
         [0045]     The controller  50  is provided with a commutating switch stage  90  having input terminals  91   a  and  91   b  and an output terminal  99 . This stage  90  is shown as an external stage, having its output terminal  99  connected to an input terminal  51  of the controller  50 , but the stage  90  and the controller  50  may be one integrated unit, as should be clear to a person skilled in the art.  
         [0046]     The switch stage  90  comprises three switches  82 ,  83 ,  84 . Each switch ( 82 ) [ 83 ] { 84 } has a central switch terminal ( 82   c ) [ 83   c ] { 84   c }, a first switch terminal ( 82   a ) [ 83   a ] { 84   a }, and a second switch terminal ( 82   b ) [ 83   b ] { 84   b }. The controller  50  has a switch control output  98 , generating a switch control signal S CS  for controlling the operative states of the switches  82 ,  83 ,  84 . In a first operative state, each switch ( 82 ) [ 83 ] { 84 } has its central switch terminal ( 82   c ) [ 83   c ] { 84   c } connected to its first switch terminal ( 82   a ) [ 83   a ] { 84   a }. In a second operative state, each switch ( 82 ) [ 83 ] { 84 } has its central switch terminal ( 82   c ) [ 83   c ] { 84   c } connected to its second switch terminal ( 82   b ) [ 83   b ] { 84   b}.    
         [0047]     The first switch  82  has its central terminal  82   c  connected to the first input terminal  91   a  of the switch stage  90 , which is connected to the first output terminal  25  of the sensor assembly  20 . The second switch  83  has its central terminal  83   c  connected to the second input terminal  91   b  of the switch stage  90 , which is connected to the second output terminal  26  of the sensor assembly  20 . The third switch  84  has its central terminal  84   c  connected to the output terminal  99  of the switch stage  90 .  
         [0048]     The first switch terminal  82   a  of the first switch  82  and the second switch terminal  83   b  of the second switch  83  are connected to a positive reference voltage V CC . The second switch terminal  82   b  of the first switch  82  and the first switch terminal  83   a  of the second switch  83  are connected to ground through corresponding resistors R 1  and R 2 , respectively. The first switch terminal  84   a  of the third switch  84  is connected to the first input terminal  91   a  of the switch stage  90 , and the second switch terminal  84   b  of the third switch  84  is connected to the second input terminal  91   b  of the switch stage  90 .  
         [0049]     The operation is as follows. In the first operative state, the cathodes of the first sensor diode  21  and the second auxiliary diode  24  are connected to the positive reference voltage, while the cathodes of the second sensor diode  22  and the first auxiliary diode  23  are connected to the second measuring resistor R 2 . The second auxiliary diode  24  blocks any current through the second sensor diode  22 . The first sensor diode  21  generates a sensor current on the basis of the amount of light received by the first sensor diode  21 , which current flows into the second measuring resistor R 2 , developing a voltage over this second resistor R 2 . This voltage is provided at output terminal  99  as output signal, reflecting the measuring signal from the first sensor diode  21 .  
         [0050]     In the second operative state, the situation is opposite, and the voltage developed over the first measuring resistor R 1 , reflecting the measuring signal from the second sensor diode  22 , is provided as output signal at output terminal  99 .  
         [0051]     The controller  50  controls the switch stage  90  to regularly switch from the first operative state to the second and vice versa. In the case of measuring a color temperature, The commutation frequency of the switching stage  90  does not need to be a high frequency: since the color temperature changes only slowly, the commutation cycle may have a duration in the order of seconds. At its input  51 , the controller  50  receives the measuring signals S V  and S B  from the first and second sensors  21  and  22  in an alternating way. The controller is adapted to calculate B/V=S B /S V , representing color temperature.  
         [0052]     It is noted that the measuring signals B and V are influenced by the resistance values of R 1  and R 2 . Since the controller  50  only keeps the ratio B/V constant, the exact values of B and V, and therefore R 1  and R 2 , are not important. It is even not necessary that the controller  50  knows which signal indicates S B  and which signal indicates S V . After all, it is immaterial whether the controller  50  is designed to keep constant the ratio B/V or the ratio V/B. In fact, if the ratio V/B is kept constant, the ratio B/V is also kept constant, per definition, and one may consider measuring B/V to be equivalent to measuring V/B. With reference to the implementation of  FIG. 4 , it will be clear to a person skilled in the art which modifications are required.  
         [0053]     On the other hand, if it is desired that the controller knows which signal is which, for instance because the controller  50  is adapted to control the lamp current intensity to control the overall light intensity, as illustrated in  FIG. 4 , the values of the measuring resistors R 1  and R 2  may be chosen such that S V  is always larger than S B , or vice versa, in which case the relative magnitudes of the first and second measuring signals give the controller  50  the required information regarding which signal is which. Suitably selecting the resistance values of the measuring resistors R 1  and R 2  requires, however, knowledge on the characteristics of the sensors.  
         [0054]     It is also possible that the controller  50  is designed for performing a sensor identification test. Such a test involves the step of deliberately changing the driver settings (briefly) such that the relative amount of blue light is increased (or decreased); for instance, the driver settings may be set to values of which it is known that the relative amount of blue light is maximal (or minimal). By monitoring the response of the sensor signals, the controller  50  can determine which sensor is the blue sensor.  
         [0055]     It should be clear to a person skilled in the art that the present invention is not limited to the exemplary embodiments discussed above, but that several variations and modifications are possible within the protective scope of the invention as defined in the appending claims.  
         [0056]     For instance, the present invention is not applicable only to gas discharge lamps, or HID lamps. In other types of light sources, it may also be possible to achieve a variation of the color temperature by varying a control parameter (e.g. TL lamps). In that case, a driver for controlling the light source on the basis of a measuring signal indicating B/V is also useful. Further, the sensor assembly and two-wire connection as proposed by the present invention are also useful.  
         [0057]     Further, although in the embodiment described, it suffices to measure B/V in order to keep a color temperature constant, it is also possible to actually find the value of the color temperature itself. For instance, the controller  50  may be provided with a look-up table or a formula, based on the results of a measurement like shown in  FIG. 5 , so that the controller  50  is capable of retrieving or calculating T C  once the ratio B/V is determined.  
         [0058]     Further, in stead of using blue light, it is possible to use light from a different wavelength range within the visible range. As a very suitable alternative range, a red range is mentioned, i.e. the range from approximately 610 nm to approximately 760 nm.  
         [0059]     Further, with reference to  FIG. 6 , an advantageous sensor assembly is described which has two sensors generating two measuring signals, only requiring two signal paths (wires) for connection to a signal processor. In the embodiment discussed, the sensors are photo diodes sensitive to light. However, the measuring principle involved in the sensor assembly is not limited to diodes: other types of light-sensitive devices may be used also, such as for instance light-dependent resistors (LDRs). The measuring principle involved in the sensor assembly is even not limited to measuring light: the design of the sensor assembly can be applied using any type of sensor, sensitive to a certain parameter such that at least one electrical characteristic, e.g. the electrical resistance between two sensor terminals (LDR) or a current generated (photodiode), depends on this parameter. The sensor assembly comprises a series connection of a diode with such sensor: as a result, a measuring signal (current) is only generated when a voltage having the correct polarity is applied across this series connection; in the case of opposite polarity, the series diode will block any measuring signal from its associated sensor. The sensor assembly further comprises a second series connection of a second diode with a second sensor (which need not necessarily be of the same type as the first sensor: the parameters to be measured may be quite different). The second series connection is connected anti-parallel to the first series connection, as far as the directions of the diodes is concerned.  
         [0060]     Further, with reference to  FIG. 6 , the switch stage  90  is explained in relation to positive supply voltage V CC  and ground. However, it is also possible to use a negative reference voltage. Also, the measuring resistance may be connected in series with the reference voltage instead of the ground terminal.  
         [0061]     In the above, the present invention has been explained with reference to block diagrams, which illustrate functional blocks of the device according to the present invention. It is to be understood that one or more of these functional blocks may be implemented in hardware, where the function of such functional block is performed by individual hardware components, but it is also possible that one or more of these functional blocks are implemented in software, so that the function of such functional block is performed by one or more program lines of a computer program or a programmable device such as a microprocessor, microcontroller, digital signal processor, etc.