Patent Grant
7352137
The invention relates to controlling the color temperature of a lighting fixture utilizing light emitting diodes as the light source.
Solutions are previously known, in which white light emitting diodes are used as the light source of a lighting fixture in order to generate white light. Since the light level of a single light emitting diode, i.e. the light intensity, is relatively low, a large number of light emitting diodes are typically used in a single lighting fixture so as to acquire an adequately high level of light.
A drawback is associated with the known solution mentioned above that the white light emitting diodes are provided with a high color temperature (approximately 6000K), in which case the shade of the white color generated thereby becomes unpleasant in many situations.
Whether a particular shade of white light is unpleasant or not depends in addition to the color temperature of the lighting fixture for instance on the target, from which light is reflected, and on the light level, i.e. the light intensity. In practice, it is therefore impossible to provide a light emitting diode with such a color temperature, the shade of white light generated thereby would suit all required lighting targets and conditions. As regards fluorescent lamps, a solution has been reached, in which several alternative fluorescent lamps are manufactured provided with a mutually different color temperature so that each application could be provided with a fluorescent lamp that produces a shade of white light appropriate for the purpose. However, such a solution is clumsy as far as it results in a situation, in which several alternative fluorescent lamps have to be manufactured in order to find a fluorescent lamp providing an appropriate shade of white for each purpose.
It is an object of the present invention to solve the problem explained above and to provide a solution that allows controlling the color temperature of a lighting fixture in order to achieve an appropriate shade of white light when light emitting diodes are used as light sources. This object is achieved with the method according to the accompanying independent claim 1 and the lighting fixture according to the accompanying independent claim 5.
The invention is based on the idea that mixing light generated by differently colored light emitting diodes can be utilized in lighting fixtures using light emitting diodes as light sources in order to achieve a desired shade of white light. This becomes possible since the light emitting diodes are placed sufficiently close to one another so that the light generated by differently colored light emitting diodes can be adequately mixed among each other. Hence, the end result is white light, the shade of which depends on the mutual intensity of the light generated by the differently colored light emitting diodes. Selecting the currents flowing through the differently colored light emitting diodes appropriately allows achieving a desired shade of white light, in other words the color temperature of the lighting fixture can be controlled in accordance with the desires of the user.
The most significant advantage of the solution according to the invention is that using the same light sources, i.e. light emitting diodes, an appropriate shade of white light can case-specifically be achieved. Thus, the need to produce several white light emitting diodes provided with different color temperatures is avoided. In addition, in a particular target the shade of the white color can be changed in accordance with the situation to suit the particular conditions concerned without having to change the light sources, i.e. the light emitting diodes. The invention is applicable to be used for instance in vehicles such as buses, in which the invention can be applied to illuminate interior spaces for instance with spot lights or light lines with an appropriate shade of white light.
The preferred embodiments of the method and the lighting fixture according to the invention are disclosed in the accompanying dependent claims 2 to 4 and 6 to 10.
In the following the invention will be described in greater detail with reference to the accompanying drawings, in which:
In accordance with the invention light is generated using differently colored light emitting diodes. Generating light using light emitting diodes has the same meaning as controlling diodes to a conducting state or conducting current through them. Further according to the invention, current flowing through the differently colored light emitting diodes is controlled in order to achieve a desired color temperature for a lighting fixture. The color temperature is set as desired by controlling currents flowing through the light emitting diodes that allow achieving the desired color temperature. How to control the color temperature may vary. An alternative for controlling the color temperature can be implemented so that transistors, whose base control can be used to set the current flowing through the light emitting diodes, are connected in series with the light emitting diodes. Such a solution and the advantages and properties thereof are explained below with reference to
It is assumed by way of example in
In order to achieve different shades of color, in other words controlling the color temperature of the lighting fixture according to the desires of the user, the arrangement shown in
The voltage division formed by the resistors R1, R1′; R2, R2′; R3, R3′ connected in series allows controlling the color shade according to the invention. The voltage division is dimensioned so that the control voltage Vc affects the base voltages of the transistors in such a manner that a change in the control voltage provides each transistor with base voltages of different magnitudes, whereby the ratio of currents flowing through the transistors changes and thus also the color of the light generated by the combination of the light emitting diodes changes. The ratios of the resistor divisions can therefore be used to implement the control characteristic of the lighting temperature, along which the lighting changes when the control voltage is changed.
The function of the coupling and the invention shown in
The following calculation has been made assuming that:
the values of all other resistors except R1, R2 and R3 are 1 kΩ;
the common supply voltage Vs of the light emitting diode chains is a standard 13 V;
the excess voltage VD of the light emitting diodes in each light emitting diode chain (R, G, B) is 3.0 V;
the base emitter voltage Vbe of the transistors is 0.6 V;
R1=5 kΩ;
R2=3 kΩ;
R3=1 kΩ;
The base voltages Vb of the transistors are obtained with these initial values as a function of the control voltage Vc:
VbR=Vc×1k/(1k+R1)=Vc×0.17
VbG=Vc×1k/(1k+R2)=Vc×0.25
VbB=Vc×1k/(1k+R3)=Vc×0.5
The base voltages then obtain the following values using the different values of the control voltage Vc:
A current Id of each light emitting diode chain can thus be stated as the function of the base voltage Vb of the transistor in the light emitting diode chain in the following form:
where Re is the resistor between the emitter of the transistor and the light emitting diode chain and URe is the voltage drop above this resistor.
Thus the different control voltage values Vc are used to obtain the currents of the light emitting diode chain
The above presented exemplary dimensionings allow reducing the current of the blue LED chain B the most when the control voltage Vs is increased, whereby the shade of the light to be generated by the lighting fixture of the invention changes more towards the red. Thus combined with the embodiment of
An embodiment is shown in
The voltage dividers implemented using the resistors R1, R1′; R2, R2′; r3, R3′ of the controller 1 shown in the embodiments of
For clarity,
It is to be understood that the above description and the drawings associated therewith are merely intended to illustrate the present invention. Different variations and modifications of the invention will be apparent to those skilled in the art without having to deviate from the scope of the invention shown in the accompanying claims.
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| 20030853 | Jun 2003 | FI | national |
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| 20040264186 A1 | Dec 2004 | US |
