Patent Application
20060202641
In one aspect, the present invention relates in general to a method and device for measuring the color temperature of a light source.
More specifically, the present invention relates to a driver device for driving a light source having variable color temperature.
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.
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.
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.
A main objective of the present invention is to provide a more economic way of measuring the color temperature of a light source.
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.
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.
It is a specific objective of the present invention to solve this problem.
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.
It is a further objective of the present invention to reduce this problem.
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.
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.
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.
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:
an input 11 for receiving AC mains;
a rectifier 12 for rectifying the AC mains voltage to a rectified DC voltage;
a DC/DC up-converter 13 for converting the rectified mains DC voltage to a higher DC voltage and for performing power factor correction;
a down-converter 14 for converting said higher DC voltage to a lower DC voltage (lamp voltage) and a corresponding DC current (lamp current);
and a commutator 15 for regularly changing the direction of this DC current within a very brief time (commutating periods).
It is noted, however, that the ballast may have a different design.
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
Some types of HID lamps have a property that the color temperature TC is variable as a function of the average current IAV, 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 IAV 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 IAV.
In one possibility of implementing the present invention, the average current IAV 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.
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.
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 (t1) is equal to the current magnitude during the “negative” half of the current period (t2) (see
Thus, according to this preferred aspect of the present invention, the driver 10 is designed to have an adaptable duty cycle.
In general, the relationship between the color temperature TC and the duty cycle D is as depicted in
The exact values of the color temperature depend on the precise composition of the lamp filling.
It has been found that the relationship between D and TC 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(TC) 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 SD for controlling the commutator 15, more particularly for controlling its duty cycle D, on the basis of the sensor signal S(TC), such as to keep the sensor signal S(TC) and hence the lamp color temperature constant.
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 SU1 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 SU1 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.
Preferably, and as shown in
The controller 50 may be designed to generate its intensity control signal SI on the basis of the actual second user input signal SU2 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.
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
The measuring input 51 of the controller 50 actually comprises two input terminals 51a and 51b, the first one for receiving the luminance signal SV and the second for receiving the blue signal SB. The luminance signal SV 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 SV and having another input receiving a reference light intensity signal REFL. 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.
The controller 50 further comprises a divider 70, having two inputs coupled to the controller measuring input terminals 51a and 51b for receiving the luminance signal SV as well as the blue signal SB. The divider 70 is arranged to divide the blue signal SB by the luminance signal SV, and to generate an output signal B/V corresponding to SB/SV. 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 REFC. This reference color signal may be the first user input signal SU1 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 t1 and the duration of the second duty cycle time interval t2, respectively.
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
For transferring the sensor signals from two sensor devices to a processing circuit, in a preferred implementation, as illustrated in
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.
The controller 50 is provided with a commutating switch stage 90 having input terminals 91a and 91b 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.
The switch stage 90 comprises three switches 82, 83, 84. Each switch (82) [83] {84} has a central switch terminal (82c) [83c] {84c}, a first switch terminal (82a) [83a] {84a}, and a second switch terminal (82b) [83b] {84b}. The controller 50 has a switch control output 98, generating a switch control signal SCS 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 (82c) [83c] {84c} connected to its first switch terminal (82a) [83a] {84a}. In a second operative state, each switch (82) [83] {84} has its central switch terminal (82c) [83c] {84c} connected to its second switch terminal (82b) [83b] {84b}.
The first switch 82 has its central terminal 82c connected to the first input terminal 91a 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 83c connected to the second input terminal 91b 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 84c connected to the output terminal 99 of the switch stage 90.
The first switch terminal 82a of the first switch 82 and the second switch terminal 83b of the second switch 83 are connected to a positive reference voltage VCC. The second switch terminal 82b of the first switch 82 and the first switch terminal 83a of the second switch 83 are connected to ground through corresponding resistors R1 and R2, respectively. The first switch terminal 84a of the third switch 84 is connected to the first input terminal 91a of the switch stage 90, and the second switch terminal 84b of the third switch 84 is connected to the second input terminal 91b of the switch stage 90.
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 R2. 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 R2, developing a voltage over this second resistor R2. This voltage is provided at output terminal 99 as output signal, reflecting the measuring signal from the first sensor diode 21.
In the second operative state, the situation is opposite, and the voltage developed over the first measuring resistor R1, reflecting the measuring signal from the second sensor diode 22, is provided as output signal at output terminal 99.
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 SV and SB from the first and second sensors 21 and 22 in an alternating way. The controller is adapted to calculate B/V=SB/SV, representing color temperature.
It is noted that the measuring signals B and V are influenced by the resistance values of R1 and R2. Since the controller 50 only keeps the ratio B/V constant, the exact values of B and V, and therefore R1 and R2, are not important. It is even not necessary that the controller 50 knows which signal indicates SB and which signal indicates SV. 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
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
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.
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.
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.
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
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.
Further, with reference to
Further, with reference to
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.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/IB04/51196 | 7/12/2004 | WO | 1/19/2006 |
