WO2012095763A1

WO2012095763A1 - A tunable white light source

Info

Publication number: WO2012095763A1
Application number: PCT/IB2012/050054
Authority: WO
Inventors: Huub BOREL; Aldegonda Lucia Weijers
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2011-01-14
Filing date: 2012-01-05
Publication date: 2012-07-19

Abstract

A tunable white light source (1), comprising: at least one first light emitting diode (2) adapted to emit light of a first integrated color point; at least one second light emitting diode (3) adapted to emit light of a second integrated color point different from said first integrated color point, wherein said first and second integrated color points are selected such that the combined light output of the first and second light emitting diodes appears white in color; and a control unit (5) for tuning a color temperature of white light output by said tunable white light source by adjusting the relative light output between said at least one first light emitting diode (2) and said at least one second light emitting diode (3), wherein the control unit (5) is configured to restrict the color temperature of the white light output by the tunable white light source (1) to a tunable color temperature range (8) where both the at least one first light emitting diode (2) and the at least one second light emitting diode (3) emit light for all color temperatures in the tunable color temperature range.

Description

A tunable white light source

FIELD OF THE INVENTION

The present invention relates to a tunable white light source comprising at least one first light emitting diode adapted to emit light of a first integrated color point; and at least one second light emitting diode adapted to emit light of a second integrated color point different from said first integrated color point, wherein the first and second integrated color points are selected such that the combined light output of the first and second light emitting diodes appears white in color.

BACKGROUND OF THE INVENTION

In some lighting applications, there is a desire to adjust the color of the illumination. In particular, there may be a desire to adjust the color temperature of white illumination e.g. between cool (bluish) white and warm (yellowish) white. There are several known ways to change the color temperature of the white light. For instance, one may change the lamp, or use filters in front of an exit window of the lamp to change the color

temperature. However, this is often considered to be too much trouble for the end user. An alternative is a dynamic color light emitting diode (LED) system that includes red, green and blue LEDs. However, due to e.g. different temperature and aging behavior of different LED materials, such as InGaN for blue LEDs and green LEDs, and AlInGaP for red LEDs, this system requires a sophisticated control system with feedback loops to compensate for the performance difference.

Another alternative is a tunable white light source that combines warm white and neutral/cold white LEDs together in a single light source to make it tunable. For example, US 2008/0252197 discloses a color temperature tunable white light source comprising first and second LED arrangements operable to emit light of first and second wavelength range respectively that are configured such that their combined light output appears white in color. The color temperature of output white light is tunable by controlling the relative light outputs of the LED arrangements.

However, since only part of the LEDs will emit light for certain color temperatures the LED utilization is relatively low, and a relatively large number of LEDs is required to ensure sufficient luminance throughout the tunable range. This leads to a high price and a relatively large size of the light source.

Thus, there is a need for a more compact and cost efficient tunable white light source.

SUMMARY OF THE INVENTION

It is an object of the present invention to alleviate the above problem, and to provide a more compact and cost efficient tunable white light source.

According to an aspect of the invention, this and other objects are achieved by a tunable white light source, comprising: at least one first light emitting diode adapted to emit light of a first integrated color point; at least one second light emitting diode adapted to emit light of a second integrated color point different from said first integrated color point, wherein said first and second integrated color points are selected such that the combined light output of the first and second light emitting diodes appears white in color; and a control unit for tuning a color temperature of white light output by the tunable white light source by adjusting the relative light output (or intensity) between said at least one first light emitting diode and said at least one second light emitting diode, wherein the control unit is configured to restrict the color temperature of the white light output by the tunable white light source to a tunable color temperature range where both the at least one first light emitting diode and the at least one second light emitting diode emit light for all color temperatures in the tunable color temperature range.

The term "white light" is intended to indicate light with an integrated color point at the black body curve, or at least sufficiently near the black body curve for the light to be perceived as white light. By black body curve should be understood the black body curve in CIE 1931 x,y color space, extending between different color temperatures of white light as is well known to the skilled person.

The term "tuning a color temperature" is intended to indicate that the integrated color point of the white light can be adjusted along the black body curve, such that the color temperature is changed between cooler color temperature (e.g. blueish white) and warmer color temperature (e.g. yellowish white).

The present invention is based on the realization that a more efficient utilization of the light emitting diodes and an enhanced luminous efficiency can be achieved by determining a range of desired color temperatures for the light source, restricting the tunable color temperature range to the desired range and extending the integrated color point range between the at least one first light emitting diode and the at least one second light emitting diode such that they are outside the tunable color temperature range of the tunable white light source. Thereby all light emitting diodes can emit light for all color temperatures in the tunable color temperature range. This can be achieved by keeping a distance in CIE 1931 x,y color space between the end-points of the tunable color temperature range and an integrated color point of the respective first and second light emitting diode. The inventors have further realized that the further away from the end-points of tunable color temperature range the respective integrated color points of the first and second light emitting diodes are, the higher is the luminous efficiency. Since both the first light emitting diode and second light emitting diode can emit a significant amount of light for all color temperatures in the tunable color temperature range, also near the end-points of the tunable color temperature range, the required number of light emitting diodes can be reduced enabling a more compact illumination device at a reduced cost. In some applications, such as in spot lights, a compact arrangement may be essential. Another advantage is that the heat generated by the light emitting diodes is spread in a better way as all light emitting diodes contribute to the illumination.

The tunable color temperature range may be restricted such that a distance

Dl = - (x_s— .¾"i)² 4 {>¾— i)² in CIE 1931 x,y color space between an integrated color point (xo,yo) of the at least one first light emitting diode and an integrated color point (xi,yi) of the tunable color temperature range situated closest to the integrated color point (xo,yo) of the at least one first light emitting diode exceeds 0.05, more preferably exceeds 0.1, even more preferably exceeds 0.15, and most preferably exceeds 0.2. By increasing the distance Dl, a more efficient utilization of the light emitting diodes can be achieved.

The tunable color temperature range may be restricted such that a distance D2 = « (x_2.— x_.¾}² 4 (_v₂ - 3¾}² in CIE 1931 x,y color space between an integrated color point (x₃,y₃) of the at least one second light emitting diode and an integrated color point (x₂,y₂) of the tunable color temperature range situated closest to the integrated color point (x₃,y₃) of the at least one second light emitting diode exceeds 0.05, more preferably exceeds 0.1, even more preferably exceeds 0.15, and most preferably exceeds 0.2. By increasing the distance D2, a more efficient utilization of the light emitting diodes can be achieved.

All light emitting diodes (i.e. both the first and second light emitting diodes) may use the same die material, wherein at least one of the first and second light emitting diodes may comprise a phosphor material adapted to absorb at least a part of the light from the die, such that the first integrated color point emitted by the at least one first light emitting diode is different from the second integrated color point emitted by the at least one second light emitting diode. By using a single die type for both first and second light emitting diodes, the temperature and aging behavior is equal for all light emitting diodes creating a more robust system and eliminating the need for feedback and feed forward systems, thereby enabling a more cost-efficient color tunable white light source. The phosphor can e.g. be a powder phosphor applied on the light emitting diode, a ceramic phosphor plate (Lumiramic) attached to the light emitting diode, or be provided remotely, e.g. on a diffusive exit window of the color tunable white light source. Using a single phosphor attached to each light emitting diode may be advantageous as it reduces the phosphor interaction such that a higher color rendering index (CRI) is obtained.

The die material in the first and second light emitting diodes may be InGaN. An InGaN die may typically emit blueish light.

The first light emitting diode may be provided with a LuAg phosphor, or a YAG:Ce phosphor. A LuAg phosphor or a YAG:Ce phosphor can be combined with an InGaN die, to generate e.g. a greenish light.

The second light emitting diode may be provided with a BSSN phosphor. A BSSN phosphor can be combined with an InGaN die, to generate e.g. a reddish light.

The relative light output between the first and second light emitting diodes may be controlled by controlling a relative magnitude of drive currents of the light emitting diodes.

The relative light output between the first and second light emitting diodes may be controlled by controlling a duty cycle of a pulse width modulated drive current.

It is noted that the invention relates to all possible combinations of features recited in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

This and other aspects of the present invention will now be described in more detail, with reference to the appended drawings showing embodiment(s) of the invention.

Fig. 1 schematically illustrates a tunable white light source according to an embodiment of the invention;

Fig. 2 shows an example of a tunable range and integrated color points of the first and second light emitting diodes in CIE 1931 x,y color space; Fig. 3 shows additional examples of integrated color points of the first and second light emitting diodes and associated tunable ranges in CIE 1931 x,y color space;

Fig. 4 shows how the luminous efficiency can be enhanced by extending the integrated color point range between the first light emitting diodes and the second light emitting diodes such that they are well outside the desired tunable color temperature range.

DETAILED DESCRIPTION

A tunable white light source 1 according to an embodiment of the invention will now be described with reference to Fig. 1 and Fig. 2.

In the exemplary embodiment of Fig. 1, the tunable white light source is a spotlight comprising a plurality of first light emitting diodes 2 and a plurality of second light emitting diodes 3. The light emitting diodes 2,3 can be arranged in a collimator 4 to provide a collimated beam. Preferably, both the first 2 and second 3 light emitting diodes utilizes the same die material 2a,3a. For example, all light emitting diodes 2,3 can be InGaN LEDs, such as royal blue LEDs from Luxeon® that emit bluish light of a peak wavelength of about 450 nm.

The first light emitting diodes 2 can be provided with a phosphor material 2b adapted to absorb at least a part of the light spectrum of the light emitted by the die 2a to change the wavelength spectrum such that the first light emitting diodes emit light of a first integrated color point. For example, a LuAg phosphor (or other greenish/yellowish phosphor such as YAG) can be applied to the InGaN LED to generate a greenish light. The phosphor can e.g. be a powder phosphor applied on the light emitting diode, or a ceramic phosphor plate (Lumiramic) attached to the light emitting diode. The thickness/density of the phosphor can be chosen in such a way that a desired integrated color point is achieved for the first light emitting diodes. In Fig. 2, line 6 depicts various integrated color points that can be achieved by combining an InGaN LED and LuAg phosphor. Here, the thickness/density of the LuAg phosphor is chosen such that the light emitted by the first light emitting diode has an integrated color point that corresponds to an integrated color point x₀,yo in CIE 1931 x,y color space, somewhere along line 6.

Similarly, the second light emitting diodes 3 can be provided with a different phosphor material 3b such that the second light emitting diodes emit light of a second integrated color point, different from the first integrated color point. For example, a BSSN phosphor can be applied to the InGaN LED to generate a reddish light. The phosphor can e.g. be a powder phosphor applied on the light emitting diode, or a ceramic phosphor plate (Lumiramic) attached to the light emitting diode. In Fig. 2, line 9 depicts various integrated color points that can be achieved by combining an InGaN LED and BSSN phosphor. Here the thickness/density of the BSSN phosphor is chosen such that the light emitted by the second light emitting diode has an integrated color point that corresponds to an integrated color point x₃,y₃ in CIE 1931 x,y color space, somewhere along line 9.

The light emitted by the first 2 and second 3 light emitting diodes can be combined to generate light with integrated color points in-between the integrated color point xo,yo of the first light emitting diodes, and the integrated color point x₃,y₃ of the second light emitting diodes as indicated by line 7. In particular, the light emitted by the first 2 and second 3 light emitting diodes can be combined to generate white light of different color temperatures, i.e. light at or near the black body line BBL.

The color tunable white light source further comprises a control unit 5 arranged to control the operation of the first 2 and second 3 light emitting diodes. The control unit may control the light emitting diodes via drive circuits. In particular, the control unit can tune a color temperature of white light output by the tunable white light source by adjusting the relative light output between the first light emitting diodes and the second light emitting diodes. This can be achieved by controlling a relative magnitude of drive currents of the light emitting diodes, or by controlling a duty cycle of a pulse width modulated drive current.

The control unit is configured to restrict the color temperature of the white light output by the tunable white light source to a tunable color temperature range 8 having end-points (xi,yi) and (x₂,y₂). The tunable color temperature range 8 may vary depending on the application, but may typically be 2700K to 4000K.

For instance, in the embodiment illustrated by Fig. 2, the first light emitting diodes 2 emits light with a first integrated color point xo=0.276,yo=0.421 ; the second light emitting diodes 3 emits light of a second integrated color point x₃=0.576,y₃=0.410; and the tunable color temperature range is 2828 K to 3541 K, such that the tunable color temperature range has end-points with color points xi=0.41 l ,yi=0.412, and x₂=0.453,y₂=0.414. Thus, a distance Dl = _*J(x_$— x_t )² { ¾ - ¾)² in CIE 1931 x,y color space between the integrated color point (xo,yo) of the first light emitting diode 2 and the integrated color point (xi,yi) of the tunable color temperature range situated closest to the integrated color point

(x₀,yo) is Dl = 0,276 - 0.4.11) ² - (0.421 - 0.412} ² = Ο.Ϊ 35. Similarly, a distance

D2 = {¾-, - _jj ) ² - U½ — 'a P in CIE 1931 x,y color space between an integrated color point (x₃,y₃) of the second light emitting diode 3 and the integrated color point (x₂,y₂) of the tunable color tem erature range situated closest to the integrated color point (x3,y3) is

In operation, the control unit 5 can adjust the relative light output between the first light emitting diodes 2 and the second light emitting diodes 3 e.g. by controlling the relative magnitude of the drive currents of the light emitting diodes or a duty cycle of a pulse width modulated drive current. As the integrated color point of the white light output by the tunable white light source is adjusted along the black body curve BBL, the color temperature is changed between cooler color temperature (e.g. bluish white) and warmer color

temperature (e.g. yellowish white). Moreover, since the respective integrated color points xo,yo and X3,y3 of the first and second light emitting diodes are well outside the tunable color temperature range 8, both the first light emitting diodes 2 and second light emitting diodes 3 emit a significant amount of light for all color temperatures in the tunable range, also near the end-points of the tunable range, hereby both adding to the optical flux. This leads to a more efficient utilization of the light emitting diodes and enhanced luminous efficiency. Further the heat generated by the light emitting diodes are being spread in a better way as all light emitting diodes contribute to the illumination throughout the tunable color temperature range.

Fig. 3 illustrates additional examples of various integrated color points of the first light emitting diode that can be achieved by modifying the thickness/density of the LuAg phosphor when combined with an InGaN LED, allowing light sources with different tunable color temperature ranges while maintaining a high luminous efficiency.

Line 31 illustrates how a tunable range of 2750-4000K can be achieved with the following integrated color points:

0.261 0.376 0.449 0.397 0.384 0.389 0.576 0.410

This results in Dl=0.189, and D2=0.193.

Line 32 illustrates how a tunable range of 3000-4000K can be achieved with the following integrated color points:

0.260 0373 0432 0393 0383 0387 0576 0.410

This results in Dl=0.173, and D2=0.194.

Line 33 illustrates how a tunable range of 2750-6000K can be achieved with the following integrated color points:

0.250 034Ϊ 044Ϊ 0382 O320 0356 0576 0.410

This results in Dl=0.195, and D2=0.26. Fig. 4 shows measurement data on demonstrator lamps and illustrates how the luminous efficiency is enhanced by extending the integrated color point range between the first light emitting diodes and the second light emitting diodes such that they are well outside the desired tunable color temperature range. The desired tunable range is here assumed to be 275 OK to 3900K, and the gain in flux is calculated for two light sources having the same number of light emitting diodes. The lower line 42 illustrates the flux for a prior art light source where the first light emitting diode is a neutral white LED (NW) with a color temperature about 3900K, and the second light emitting diode is a warm white LED (WW) with a color temperature about 2750K. The upper line 41 illustrates the flux for a light source where the tunable range is restricted to the desired tunable range of 2750 to 3900K by the control unit, and where the light emitting diodes have been selected far outside the desired tunable range.

The person skilled in the art realizes that the present invention by no means is limited to the preferred embodiments described above. On the contrary, many modifications and variations are possible within the scope of the appended claims. For instance, the tunable white light source is not limited to a spotlight, but may also be used for other types of light sources such as luminaires, e.g. a downlight. Further, to provide a more uniform illumination, the light emitting diodes may be arranged in a white mixing chamber and/or the tunable white light source may include a diffuse exit window. Further, although in the above described embodiment, both first and second light emitting diodes are provided with phosphors it may suffice to alter the color point of either the first light emitting diode or the second light emitting diode. It is also possible to arrange a phosphor on the exit window of the tunable white light source, to obtain desired integrated color points of the first and second light emitting diodes.

Claims

CLAIMS:

1. A tunable white light source (1), comprising:

at least one first light emitting diode (2) adapted to emit light of a first integrated color point;

at least one second light emitting diode (3) adapted to emit light of a second integrated color point different from said first integrated color point, wherein said first and second integrated color points are selected such that the combined light output of the first and second light emitting diodes appears white in color; and

a control unit (5) for tuning a color temperature of white light output by said tunable white light source by adjusting the relative light output between said at least one first light emitting diode (2) and said at least one second light emitting diode (3),

characterized in that

the control unit (5) is configured to restrict the color temperature of the white light output by the tunable white light source (1) to a tunable color temperature range (8) where both the at least one first light emitting diode (2) and the at least one second light emitting diode (3) emit light for all color temperatures in the tunable color temperature range.

2. A tunable white light source according to claim 1 , wherein the tunable color temperature range is restricted such that a distance Dl = < (x_p— x^}² ÷ i ^'■ ¾— ¾ ² in CIE 1931 x,y color space between an integrated color point (xo,y₀) of the at least one first light emitting diode (2) and an integrated color point (xi,yi) of the tunable color temperature range situated closest to the integrated color point (xo,y₀) of the at least one first light emitting diode exceeds 0.05, more preferably exceeds 0.1 , even more preferably exceeds 0.15, and most preferably exceeds 0.2

3. A tunable white light source according to claim 1 or 2, wherein the tunable color temperature range is restricted such that a distance D2 = ; c~— x_¾)² -f- (y₇— y₃ }² in CIE 1931 x,y color space between an integrated color point (x₃,y₃) of the at least one second light emitting diode (2) and an integrated color point (x₂,y₂) of the tunable color temperature range situated closest to the integrated color point (x₃,y₃) of the at least one second light emitting diode exceeds 0.05, more preferably exceeds 0.1, even more preferably exceeds 0.15, and most preferably exceeds 0.2.

4. A tunable white light source according to any one of the preceding claims, wherein all light emitting diodes (2, 3) use the same die material, wherein at least one of said first (2) and second (3) light emitting diodes comprises a phosphor material adapted to absorb at least a part of the light from the die, such that the first integrated color point emitted by the at least one first light emitting diode is different from the second integrated color point emitted by the at least one second light emitting diode.

5. A tunable white light source according to claim 4, wherein said die material is InGaN.

6. A tunable white light source according to claim 4 or 5, wherein said first light emitting diode (2) is provided with a LuAg phosphor.

7. A tunable white light source according to claim 4 or 5, wherein said first light emitting diode (2) is provided with a YAG:Ce phosphor.

8. A tunable white light source according to any of claims 4 to 7, wherein said second light emitting diode (3) is provided with a BSSN phosphor.

9. A tunable white light source according to any one of the preceding claims, wherein the relative light output between said first (2) and second (3) light emitting diodes is controlled by controlling a relative magnitude of drive currents of the light emitting diodes.

10. A tunable white light source according to any one of the preceding claims, wherein the relative light output between said first (2) and second (3) light emitting diodes is controlled by controlling a duty cycle of a pulse width modulated drive current.