Patent Application
20080203945
The present invention relates to a light emitting diode (LED) lighting system for producing white light, and a method for controlling three or more sets of LEDs for providing white light.
A current issue for a color adjustable light emitting diode (LED) light source is color rendering properties. To obtain a sufficiently large color gamut, the light source normally comprises three color LEDs: red, green, and blue. This is described in U.S. Pat. No. 6,411,046, where light output and the color of the LEDs are controlled by measuring color coordinates for each LED light source for different temperatures, storing the expressions of the color coordinates as a function of the temperatures, deriving equations for the color coordinates as a function of temperature, calculating the color coordinates and lumen output fractions on-line, and controlling the light output and color of the LEDs based upon the calculated color coordinates and lumen based upon the calculated color coordinates and lumen output fractions. However, the demand on calculation power will increase cost of the lighting system. Further, the color rendering properties of a three-color system may not be satisfactory. Note that the color rendering index can only be optimized by choosing the wavelengths of the LEDs when designing the lighting system. This can be overcome by using more colors. However, the demand on calculation power would then raise even more, and thus the cost. Therefore, there is a need for an improved LED lighting system, and an improved method of controlling such a LED lighting system.
In view of the above, an objective of the invention is to solve or at least reduce the problems discussed above. In particular, an objective is to improve optimization of color rendering in sense of complexity.
The present invention is based on the understanding that complexity in controlling color rendering can be reduced by driving LEDs of different colors jointly, and how this can be implemented to obtain satisfactory color rendering and control properties although changes in temperature of the LEDs.
According to a first aspect of the present invention, there is provided a light emitting diode (LED) lighting system for producing white light, the system comprising a first set of LEDs arranged to emit light with a first wavelength range and a first set of characteristics; a second set of LEDs arranged to emit light with a second wavelength range and a second set of characteristics; a third set of LEDs arranged to emit light with a third wavelength range and a third set of characteristics; and a driving circuit arranged to drive said sets of LEDs. The driving circuit comprises an input for parameters determining desired light intensity and color; an input for signals for LED temperatures of the sets of LEDs; a model for determining driving currents for said sets of LEDs from said parameters, signals, and sets of characteristics for each of said sets of LEDs; and a current driver for providing said determined currents to said sets of LEDs. The system is characterized in that said third set of LEDs comprises a first subset of LEDs with a first wavelength sub-range and a first set of characteristics, and a second subset of LEDs with a second wavelength sub-range and a second set of characteristics, wherein said third wavelength range is a lumped wavelength range of said first and second wavelength sub-ranges, and said third set of characteristics is a function of said first and second sets of characteristics.
An advantage of this is improved color rendering without increased complexity of controlling. With the use of more than three colors, the color rendering index can be optimized after choosing the color to be generated.
Said sets of characteristics may comprise temperature dependency of light output, temperature dependency of wavelength, or current dependency of light output, or any combination thereof.
The first and second sub-set of light emitting diodes are electrically connected in series.
An advantage of this is that equal current is provided to the two sets of LEDs.
The lighting system may further comprise a temperature sensor for providing said signals for LED temperatures of the sets of LEDs, wherein said temperature sensor is arranged in a heat sink arranged at said sets of LEDs.
Said model for each set of LEDs may comprise a flux function of LED temperature being an exponential function of quotient of a difference between LED temperature and a reference temperature, and a flux dependency on temperature parameter according to the characteristics of each set of LEDs. Said model for each set of LEDs may comprise a wavelength function of LED temperature being dependent on a difference between LED temperature and a reference temperature, and a wavelength dependency on temperature parameter according to the characteristics of each set of LEDs.
According to a second aspect of the present invention, there is provided a method for controlling three sets of LEDs, each arranged to emit light with a wavelength range and with a set of characteristics, to provide white light, comprising the steps of: determining a desired light intensity and color; determining LED temperatures of the sets of LEDs; determining for each set of LEDs a driving current for each of said sets of LEDs from said desired light intensity and color, and said LED temperatures; and providing said driving currents to said sets of LEDs. The method is characterized in that at least one of said sets of LEDs comprises a first subset of LEDs with a first wavelength sub-range and a first set of characteristics, and a second subset of LEDs with a second wavelength sub-range and a second set of characteristics, wherein a wavelength range of said set of LEDs is a lumped wavelength range of said first and second wavelength sub-ranges, and a set of characteristics of said set of LEDs is a function of said first and second sets of characteristics.
Said step of determining for each set of LEDs a driving current for each of said sets of LEDs may use a model for each set of LEDs comprising a wavelength function of LED temperature being dependent on a difference between LED temperature and a reference temperature, and a wavelength dependency on temperature parameter according to the characteristics of each set of LEDs.
The sets of LEDs may comprise one or more LEDs.
By LED temperature, it is meant a temperature under which an LED works. Physically, this is the junction temperature; practically and measurably, this is a temperature of a medium close to the junction, e.g. the capsule of the LED or a heat sink at the LED.
By reference temperature, it is meant a nominal temperature, at which properties of e.g. an LED is specified.
Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to “a/an/the [element, device, component, means, step, etc]” are to be interpreted openly as referring to at least one instance of said element, device, component, means, step, etc., unless explicitly stated otherwise. The steps of any method disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated.
Other objectives, features and advantages of the present invention will appear from the following detailed disclosure, from the attached dependent claims as well as from the drawings.
The above, as well as additional objects, features and advantages of the present invention, will be better understood through the following illustrative and non-limiting detailed description of preferred embodiments of the present invention, with reference to the appended drawings, where the same reference numerals will be used for similar elements, wherein:
The sets of LEDs can comprise one or more LEDs. The number of LEDs in each set can be chosen to optimize the balance between the various wavelength to enable feasible control of provision of white light with a desired color and intensity.
The light of the first wavelength range can be green, i.e. the center wavelength is somewhere in the range of 520 nm to 550 nm. The light of the third wavelength range can be blue, i.e. the center wavelength is somewhere in the range of 450 nm to 490 nm. The first and second wavelength sub-ranges can be red and amber, respectively, i.e. center wavelengths somewhere in the range of 610 nm to 645 nm and 580 nm to 600 nm, respectively. Due to the nature of LEDs, wavelengths around the center wavelengths are also provided. Further, the center wavelength is dependent on the junction temperature of the LED.
The above wavelengths are examples, and other wavelengths and ranges of wavelengths are possible within the scope of the present invention.
The characteristics of the LEDs can be, apart from reference wavelength range, reference light output, reference temperature, etc from e.g. a data sheet of the LED, temperature dependency of wavelength and light output. Empirically, it is found that light output (flux) can be derived from for example
where T0 is a characteristic variable. Further, peak wavelength shift can be described for example by the empirically found relation
λp˜λp0+β(Tj−Tref), (Eq. 2)
where β is a characteristic property. The values of the characteristics are different for LEDs of different color, as can be seen in exemplary Table 1.
The differences in characteristics means that it is not clear that a combination of red and amber LEDs can be seen as a single lumped LED. Earlier tests with a color feedback system utilizing these four colors showed that the differences in temperature behaviour are too significant to just lump the red and amber LEDs in a single degree of freedom, while only taking the optical properties at a single temperature into account. The combined LED can be modeled as a lumped LED with a similar radiation pattern and behaviour as a normal LED. By simulation, radiation pattern in both x- and y-coordinates and flux output of the combined red and amber LED is determined. Table 2 shows an example of a simulation.
Based on the simulation results of Table 2, the combination of 6 amber and 2 red LEDs yields both very good color rendering properties and easy driving, color feedback, and color adjustability. Easy, because there are only three degrees of freedom, which are explicitly determined by choosing a desired color-point. Note that a similar lumped LED can also be defined for a different combination of red and amber LEDs, or for a different combination of colors, e.g. blue and cyan, blue and green, or green and cyan LEDs.
In an alternative embodiment, it can be desirable to provide equal voltage for the sets being jointly driven.
The sets and subsets of LEDs can comprise one or more LEDs. The number of LEDs and wavelength in each set and subset can be chosen to optimize the balance between the various wavelength to enable control of provision of white light with a desired color temperature range and color rendering index, color and intensity. The number of LEDs per color and their wavelength should be optimized for a certain color rendering in a desired color temperature range, color range and light intensity range.
The light of the first wavelength can be red, i.e. the center wavelength is somewhere in the range of 610 nm to 645 nm. The colors of the first and second subsets of LEDs can be blue and green, respectively, i.e. center wavelengths somewhere in the range of 450 nm to 490 nm and 520 nm to 550 nm, respectively.
The above embodiments of the present invention suggest driving more than one subset of LEDs jointly to facilitate control of color temperature of generated white light. Suggestions have been made to reduce a four-color system to three degrees of freedom and to reduce a three-color system to two degrees of freedom. However, the present invention can be used to implement a system with any number of colors with a reduced number of freedoms of control. Thus, a lighting system can comprise two or more lumped sets of LEDs similar to what is described above.
The methods according to the described embodiments of the present invention comprises a number of steps. The steps can be performed in any order, consecutively or parallelly, due to the real-time constraints of the art.
The invention has mainly been described above with reference to a few embodiments. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the invention, as defined by the appended patent claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 05104441.0 | May 2005 | EP | regional |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/IB2006/051483 | 5/11/2006 | WO | 00 | 11/20/2007 |
