WO2013011409A1

WO2013011409A1 - Incandescent lamp resembling lighting device

Info

Publication number: WO2013011409A1
Application number: PCT/IB2012/053495
Authority: WO
Inventors: Constant Paul Marie Jozef Baggen; Paulus Henricus Antonius Damink; Hugo Johan Cornelissen; Rifat Ata Mustafa Hikmet; Hans-Helmut Bechtel; Arnoldus Theodorus Martinus Hendricus Van Keersop
Original assignee: Koninklijke Philips Electronics N.V.; Philips Intellectual Property & Standards Gmbh
Priority date: 2011-07-19
Filing date: 2012-07-09
Publication date: 2013-01-24

Abstract

The present invention relates to a lighting device (7) comprising a first and a second light generating means (701, 704), and a method (1300) of manufacturing such a lighting device. A linear combination (103) of the first and second light generating means provides an output optical spectrum having an output CCT between a first CCT of the first light generating means and the second CCT of the second light generating means. Further, color points (101, 102) of the first and second light generating means are more green and/or yellow than the black body locus (10) of a chromaticity space (1), and the optical spectrum of at least one of the achievable combinations provides an output color point in the chromaticity space being more red than the black body locus. The present invention is advantageous in that the output CCT of the lighting device may be tuned and a high illumination quality is provided.

Description

INCANDESCENT LAMP RESEMBLING LIGHTING DEVICE

FIELD OF THE INVENTION

The present invention generally relates to the field of lighting devices comprising light generating means of two or more colors. BACKGROUND OF THE INVENTION

Traditional incandescent light bulbs provide a change in color temperature as they are dimmed (i.e. when their light intensity decreases). The color temperature for high light intensities is high, which is perceived as a relatively cool light, and the color

temperature for low light intensities is low, which is perceived as a warm light. Further, traditional incandescent light bulbs have a very high color rendering index (CRI) which is a measure of illumination quality indicating the ability of a light source to reproduce the colors of an illuminated object in comparison with a black body radiator (which is a near ideal light source).

The traditional incandescent light bulbs are currently being replaced with more energy efficient alternatives, such as light emitting diode (LED) based light sources.

Normally, LED-based light sources cannot change their color temperature while being dimmed as incandescent light bulbs do. Moreover, their illumination quality is often not sufficient (i.e. the CRI is too low) to replace the traditional incandescent light bulbs.

WO2009/104136 shows an LED light source comprising an LED of UV-type, a blue LED and a luminescent layer having a response optimized to the emission spectrum of the UV-type LED. The two LEDs are selectively and independently controlled, whereby the overall light output can be tuned. The resulting LED lamp shows a red-shift of the color point upon dimming. Such an LED light source, although being able to change color temperature while being dimmed, still suffers from having an illumination quality too far from a traditional incandescent light bulb, especially for low color temperatures.

SUMMARY OF THE INVENTION

Thus, there is a need for providing alternatives and/or new devices that would overcome, or at least alleviate or mitigate, at least some of the above mentioned drawbacks. It is with respect to the above considerations that the present invention has been made. An object of the present invention is to provide an improved alternative to the above mentioned technique and prior art. More specifically, it is an object of the present invention to provide a lighting device with an improved illumination quality. It is also an object of the present invention to provide a lighting device being able to provide light of different color temperatures. Further, it is an object of the present invention to provide an LED-based light source (or energy efficient light source) whose illumination behavior resembles that of an incandescent light bulb (i.e. providing both cold and warm light).

These and other objects of the present invention are achieved by means of a lighting device and a method of manufacturing such a lighting device having the features defined in the independent claims. Preferable embodiments of the invention are characterized by the dependent claims.

Hence, according to a first aspect of the present invention, a lighting device is provided. The lighting device comprises a first light generating means arranged to provide light with a first optical spectrum having a first correlated color temperature (CCT) and a first color point in a chromaticity space (or chromaticity diagram), wherein the spectral power distribution of the first optical spectrum is close to the spectral power distribution of a black body radiator of a temperature corresponding to the first CCT. The lighting device further comprises a second light generating means arranged to provide light with a second optical spectrum having a second CCT and a second color point in the chromaticity space, wherein the spectral power distribution of the second optical spectrum is close to the spectral power distribution of a black body radiator of a temperature corresponding to the second CCT. A linear combination of the first light generating means and the second light generating means provides an output optical spectrum having an output CCT between the first CCT and the second CCT. Further, the first light generating means and the second light generating means are configured such that the first color point and the second color point are more green and/or yellow than the black body locus (also referred to as the Planckian locus) of the chromaticity space, and such that the optical spectrum of at least one of the achievable combinations provides an output color point in the chromaticity space being more red than the black body locus.

According to a second aspect of the present invention, a method of manufacturing a lighting device is provided. The method comprises the step of providing a first light generating means arranged to provide light with a first optical spectrum having a first CCT and a first color point in a chromaticity space, wherein the spectral power distribution of the first optical spectrum is close to the spectral power distribution of a black body radiator of a temperature corresponding to the first CCT. The method further comprises the step of providing a second light generating means arranged to provide light with a second optical spectrum having a second CCT and a second color point in the chromaticity space, wherein the spectral power distribution of the second optical spectrum is close to the spectral power distribution of a black body radiator of a temperature corresponding to the second CCT. A linear combination of the first light generating means and the second light generating means provides an output optical spectrum having an output CCT between the first CCT and the second CCT. Further, the first light generating means and the second light generating means are selected such that the first color point and the second color point are more green and/or yellow than the black body locus of the chromaticity space, and such that the optical spectrum of at least one of the achievable combinations provides an output color point in the chromaticity space being more red than the black body locus.

The present invention is based on the insight that the illumination quality of non- incandescent lighting devices according to prior art techniques is unsatisfactory because of the absence or insufficient presence of certain colors in the generated light. Hence, the present invention is based on the idea of resembling a traditional incandescent light bulb by providing two light generating means emitting two optical spectrums being close to (or essentially matching) the optical spectrums of a black body radiator and having color points close to the black body locus, which means that each one of the two light generating means radiates light almost like a black body radiator of a temperature corresponding to the CCTs of the light generating means. The linear combination of the two optical spectrums provides a range of output optical spectrums having color points represented by a straight line between the color points of the light generating means in the chromaticity space. Further, the spectral power distribution of the output optical spectrum will be close to a spectral power

distribution of a black body radiator of a temperature corresponding to the output CCT. The inventors have recognized that, since the black body locus is slightly convex, the illumination quality of the achievable combinations (i.e. their proximity to the black body locus) can be increased by selecting light generating means emitting slightly more yellow and/or green light than an ideal black body radiator. The amount of extra yellow and/or green color provided by the light generating means is adjusted such that a portion of the line representing the achievable linear combinations is slightly more red than the black body locus. In other words, the lighting device is designed such that the line representing the achievable linear combinations, almost corresponds to (or matches) the black body locus. With the spectral power distributions of the first and second optical spectrums being close to the spectral power distributions of a black body radiator of temperatures corresponding to the first and second CCTs respectively, it is meant that the wavelength distributions of the first and second light generating means imitates the wavelength distributions of a black body radiator. It will be appreciated that the spectral power distributions of the light generating means may not exactly match that of a black body radiator since this is very difficult (or even impossible) with current non- incandescent lighting techniques. For example, currently available LEDs and phosphor provides an optical spectrum typically lacking some light intensity for longer wavelengths. Instead, the spectral power distributions of the first and second light generating means may essentially resemble the spectral power distributions of the optical spectrums of a black body radiator.

A black body radiator is an idealized physical body which incandescently radiates light of decreasing wavelength as its temperature increases. The black body locus of a chromaticity space represents the color of the black body radiator at a range of

temperatures. An incandescent light bulb radiates light almost like a black body radiator, and hence, it is desirable to provide a lighting device which radiates visible light as similar as a black body radiator as possible to obtain as high illumination quality as possible.

With the present invention, a linear combination of the two light generating means being particularly designed to have a high illumination quality (e.g. a high CRI) obtains a range of output optical spectrums being close to (or approximating) that of a black body radiator.

The present invention is advantageous in that the CRI (or another quality measure) is enhanced over the range of required CCTs if a mixture of light from the two light generating means is used. Hence, the output CCT of the lighting device may be tuned while still providing a high illumination quality for all CCTs. Accordingly, the lighting device according to the present invention emits light with an enhanced rendering of the color of illuminated objects.

Further, the present invention is advantageous in that an enhanced resemblance to a traditional light bulb is obtained since the output optical spectrum of the lighting device approximates a black body radiator over a range of CCTs. For example, the lighting device may be configured to decrease the output CCT while being dimmed (i.e. when the light intensity of the lighting device is decreased), thereby gradually providing a warmer light of lower intensity and resembling the behavior of a traditional incandescent light bulb. The present invention is also advantageous in that it teaches how to select light generating means to provide a lighting device offering improved illumination quality over a range of CCTs.

According to an embodiment of the present invention, the first light generating means and the second light generating means may further be configured such that the maximum distance between the output color points of the linear combination being more red than the black body locus and the black body locus, the distance between the first color point and the black body locus, and the distance between the second color point and the black body locus are of the same order of magnitude (or approximately the same). The present embodiment is advantageous in that the maximum distance from the line representing the achievable linear combinations to the black body locus is reduced, which means that the color deviation from a black body radiator (or an ideal light source) is reduced. Hence, the colors of the lighting device better resembles the colors of an incandescent lamp and the illumination quality is improved or (at least almost) optimized over the range of output CCTs of the lighting device.

According to an embodiment of the invention, the first light generating means and the second light generating means may further be configured such that the lowest CRI of the linear combination is at least 80, preferably at least 90, and even more preferably at least 95, which is advantageous in that the light emitted by the lighting device will render illuminated objects near as good as an ideal light source for the achievable output CCTs. Accordingly, a human will perceive the light from the lighting device as natural for all the CCTs ranging from the first CCT to the second CCT, almost like light emitted by a traditional incandescent light bulb.

In an embodiment of the present invention, the chromaticity space may be one of the CIE color spaces, such as the CIE 1931 color space, the CIE 1960 color space or the

CIE 1976 color space, which is advantageous in that those chromaticity spaces are commonly used to define and evaluate colors of light and provides a representation of the black body locus.

According to an embodiment of the present invention, the first CCT may be between 1500 K and 1900 K, and the second CCT may be between 2500 K and 2900 K, which is advantageous in that such upper and lower CCT-boundaries correspond to the upper and lower CCT-boundaries of the visible spectrum of an incandescent light bulb, thereby further contributing to an enhanced resemblance to a traditional incandescent light bulb. In addition, these two ranges are complementary in that they provide both cold and warm light, thereby enabling a wide range of other color temperature possibilities via linear combination. However, it will be appreciated that the upper and lower CCT-boundaries of the lighting device may be adapted to any corresponding natural light source, such as the sun, which is desirable to resemble. The lighting device may e.g. be configured to resemble a range of daylight spectra.

In an embodiment of the present invention, the first light generating means may comprise a first light source and a first wavelength converting material arranged to receive light emitted by the first light source, and the second light generating means may comprise a second light source and a second wavelength converting material arranged to receive light emitted by the second light source. Accordingly, when the first wavelength converting material is illuminated by the first light source, the first optical spectrum is provided. Likewise when the second wavelength converting material is illuminated by the second light source, the second optical spectrum is provided. The present embodiment is advantageous in that it facilitates the technical implementation of the invention, thereby reducing costs. The output CCT of the lighting device can be controlled by adjusting the light intensities of the first and second light sources relative to each other.

Further, according to an embodiment of the present invention, the first light source may be arranged at a central portion of a base of the lighting device and the second light generating means may comprise a plurality of second light sources arranged around (or circumferentially around) the first light source at the base. Further, the first wavelength converting material may be arranged at a central portion of a surface and the second wavelength converting material may be arranged around (or circumferentially around) the central portion at the surface. The surface may be arranged to receive light from the first light source and the second light sources such that the light from the first light source is received by the central portion of the surface provided with the first wavelength converting material, and the light from the second light sources is received by the outer (or circumferential) portion of the surface provided with the second wavelength converting material. Preferably, the first and second light generating means may be concentrically arranged. In the present application, with the term "surface", it is meant for instance a sheet, film or screen.

The present embodiment is advantageous in that the arrangement of the light generating means facilitates mixing the light from the two light generating means (i.e. the illumination generated by the central and circumferential portions of wavelength concerting material), thereby reducing the need of a separate mixing device or at least facilitating the mixing in such separate mixing device. Further, the present embodiment is advantageous in that the light sources are arranged at the base (or bottom plane) of the lighting device, thus facilitating the electrical and thermal construction of the lighting device.

According to another embodiment of the present invention, the lighting device may comprise a light source, a surface (or sheet) comprising a first portion of a first wavelength converting material and a second portion of a second wavelength converting material, and a means for changing, relative to the surface, the direction and/or shape of a beam of rays emitted from the light source. The present embodiment is advantageous in that merely one light source is needed for providing the two light generating means, which e.g. enables design of a lighting device with a reduced size. By directing the beam of rays emitted by the light source towards different portions of the surface, or by changing the shape of the beam of rays such that differently sized or shaped areas or portions of the surface are illuminated, the output optical spectrum may comprise different amounts of the first and second optical spectrum and hence, the output CCT may be adjusted.

In an embodiment of the present invention, the lighting device may further comprise a beam control means, such as a polymer dispersed liquid crystal (PDLC) beam shaper, a switchable micro lens, a switchable beam deflector or a switchable beam splitter, configured to direct (or aim) the light from the light source towards (or at) different portions (or areas) of the surface. For example, the beam control means may be configured to selectively concentrate the light from the light source at a relatively small area of the surface comprising the first wavelength converting material, thereby providing the first optical spectrum, or spreading the light over a larger area of the surface comprising both the first wavelength converting material and the second wavelength converting material, thereby providing the second optical spectrum. Alternatively, the beam control means may be configured to selectively aim the light from the light source at different separate portions of the surface comprising the different wavelength converting materials. The beam control means may further be configured to provide gradual transition between its utter modes, thereby providing a linear combination of the two optical spectrums. The present

embodiment is an alternative design reducing the need of electromechanical means for changing the direction and/or shape of a beam of rays emitted from the light source relative to the surface. Further, it facilitates reducing the size of the lighting device.

In an embodiment of the invention, the lighting device may further comprise a mixing means for mixing at least a part of the light emitted by the first light generating means with at least a part of the light emitted by the second light generating means, which is advantageous in that the light from the two light generating means is more efficiently mixed, thereby providing an output light having a more spatially uniform CCT and illumination quality, with reduced fringe effect.

According to an embodiment of the present invention, the amount (or intensity) of the light emitted by the first light generating means may be adjustable relative to the amount (or intensity) of the light emitted by the second light generating means, which is advantageous in that a tuning of the output CCT of the lighting device is provided. Hence, the light from the lighting device may be tuned between cool and warm color tones. For example, the tuning of color (i.e. the relative amounts of light from the first and second light generating means) may be a function of an input signal provided by a dimmer of the lighting device such that the color is tuned while dimming the overall light output of the lighting device.

Referring now in particular to the second aspect of the present invention, embodiments of the present invention will be described.

According to embodiments of the present invention, the method of

manufacturing a lighting device may further comprise the step of reducing the maximum distance between the output color points of the linear combination and the black body locus, which is advantageous in that the maximum distance from the line representing the achievable linear combinations is reduced, which means that the color deviation from a blackbody radiator (or an ideal light source) is reduced. Hence, the illumination quality is improved or (at least almost) optimized over the range of CCTs of the lighting device.

Further, the method may comprise the step of increasing the lowest value of an illumination quality measure, such as the CRI, of the linear combinations, which is advantageous in that the light emitted by the lighting device will render illuminated objects near as good as an ideal light source for the range of achievable output CCTs.

Further objectives of, features of, and advantages with, the present invention will become apparent when studying the following detailed disclosure, the drawings and the appended claims. Those skilled in the art realize that different features of the present invention can be combined to create embodiments other than those described in the following. In particular, it will be appreciated that the various embodiments described for the lighting device are all combinable with the method as defined in accordance with the second aspect of the present invention. BRIEF DESCRIPTION OF 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, in which:

Fig. 1 shows a chromaticity space in which color points of a lighting device according to an embodiment of the invention are indicated;

Fig. 2 shows an enlarged view of a portion of the chromaticity space illustrated in Fig. 1;

Fig. 3 shows a diagram indicating the wavelength spectrums of a lighting device according to an embodiment of the present invention;

Fig. 4 shows a diagram indicating CRI measures according to embodiments of the present invention;

Fig. 5 shows a diagram indicating the wavelength spectrums of a lighting device according to an embodiment of the present invention;

Fig. 6 shows a diagram indicating the CRI of a lighting device according to an embodiment of the present invention;

Fig. 7 shows a lighting device according to an embodiment of the present invention;

Fig. 8 shows a lighting device according to another embodiment of the present invention;

Fig 9 shows a lighting device according to another embodiment of the present invention;

Fig. 10 shows a lighting device according to another embodiment of the present invention;

Fig. 11 shows a lighting device with different alternatives of beam control means according to embodiments of the present invention; and

Fig. 12 illustrates the outlines of a method of manufacturing a lighting device according to an embodiment of the present invention.

All the figures are schematic, not necessarily to scale, and generally only show parts which are necessary in order to elucidate the invention, wherein other parts may be omitted or merely suggested. DETAILED DESCRIPTION OF EMBODIMENTS

With reference to Fig. 1, there is shown a chromaticity space 1, more specifically, the CIE 1931 color space. In the chromaticity space 1, the black body locus 10 is indicated, as well as a CCT scale 11 of the colors of and in proximity to the black body locus 10. Further, the colors of the chromaticity space are indicated as follows:

121 purplish blue, 131 orange,

122 blue, 132 yellowish pink,

123 greenish blue, 133 reddish orange,

124 bluish green, 134 pink,

125 green, 135 red,

126 yellowish green, 136 purplish pink,

127 yellow green, 137 purplish red,

128 greenish yellow, 138 reddish purple, and

129 yellow, 139 purple/violet.

130 orange yellow,

A first light generating means of a lighting device according to an embodiment of the present invention is arranged to provide light with a first optical spectrum having a first CCT and a first color point 101 in the chromaticity space 1. A second light generating means of the lighting device is arranged to provide light with a second optical spectrum having a second CCT and a second color point 102 in the chromaticity space 1. For example, the first CCT may be about 2700° K and the second CCT may be about 1700° K, as shown in Fig. 1. Preferably, the second CCT may be about 1800° K since matching of the optical spectrum of an LED to a black body radiator is better for higher CCTs and a CCT of 1800° K is still low enough to resemble an incandescent light bulb of a lower light intensity (and thus a lower CCT). The color points 101, 102 may also be identified using the (x, y) coordinates of the chromaticity space 1, as shown in Fig. 1.

Further, the spectral power distributions of the first and second optical spectrums are close to the spectral power distributions of the optical spectrums of a black body radiator of temperatures corresponding to the first and second CCTs, respectively. Hence, in addition to the color points being close to the black body locus (providing a color resembling an incandescent lamp), the whole spectrum of each light generating means imitates an incandescent lamp, which improves the CRI of the lighting device. A linear combination 103 of the first light generating means and the second light generating means provides an output optical spectrum having an output CCT between the first CCT and the second CCT, hence in the present example between 2700° K and 1700° K. Further, the spectral power distribution of the output optical spectrum is close to the spectral power distribution of a black body radiator of a temperature corresponding to the output CCT. As indicated in the chromaticity space 1 in Fig. 1, and in the enlarged portion of the chromaticity space 1 depicted in Fig. 2, the first color point 101 and the second color point 102 are selected to be slightly more green and/or yellow (and less red) than the black body locus 10. In other words, the first color point 101 and the second color point 102 are positioned slightly above the black body locus 10 (on the outside of the black body locus curve), and slightly closer to the yellowish greenl26, yellow green 127, greenish yellow 128, yellow 129 and orange yellow 130 portions in the chromaticity space 1. In addition, the color points 101, 102 are selected such that some of the achievable combinations provide an output color point being more red (and less green and/or yellow) than the black body locus 10, i.e. positioned slightly below the black body locus 10 (on the inside of the black body locus curve), and slightly closer to the yellowish pink 132, reddish orange 133, pink 134 and purplish pink 136 portions in the chromaticity space 1.

It will be appreciated that an achievable linear combination is represented in the chromaticity space 1 by a color point (such as the color point 103) located on a line joining the first color point 101 and the second color point 102.

Fig. 3 shows a diagram in which it is indicated with dashed lines a wavelength spectrum (or spectral power distribution) 301 with the CCT 2700° K for the first light generating means, a wavelength spectrum 303 with the CCT 1700° K for the second light generating means, and a wavelength spectrum 302 (i.e. a specific output optical spectrum) having the output CCT 2100° K provided by mixing the light from the first and second light generating means. Further, it is indicated, with solid lines, the black body wavelength spectrums 311, 313, 312 with the CCTs 2700° K, 1700° K and 2100° K respectively. As can be seen in the diagram, each one of the wavelength spectrums 301, 303, 302 of the lighting device only slightly deviates from its corresponding black body wavelength spectrum 311, 313, 312. It will be appreciated that such deviation is the result of a slight mismatch between the linear combination and the black body locus. The good match of the wavelength spectrums 301, 303, 302 of the lighting device with the corresponding black body

wavelength spectrums 311, 313, 312 is the main reason why the color rendering of the lighting device is very close to that of an incandescent lamp. To evaluate the illumination quality of the linear combinations of output light from the lighting device according to embodiments of the present invention, the CRI can be calculated. The CRI varies as a function of the generated CCTs. Fig. 4 shows a diagram in which the CRI is indicated with line 401 for the output light from the lighting device in the present example, wherein the color points 101, 102 of the first and second light generating means are shifted slightly towards green/yellow such that the maximum distance from the achievable linear combinations to the black body locus is reduced (or even minimized). As depicted in the diagram, the CRI for such a lighting device do not drop below 93 (out of 100) for any CCT. In contrast, by choosing color points of the first and second light generating means right on the black body locus, higher CRI values (close to 100) can be obtained at the endpoints of the achievable linear combination as indicated with line 402 in the diagram, but the minimum CRI (and the illumination quality for intermediate CCTs) will be lower as compared to the case in which the first and second color points are shifted slightly towards green/yellow. If the first CCT is 2700° K and the second CCT is 1700° K and the first and second color points are positioned right on the black body locus, the minimum CRI is approximately 90 as indicated with line 402.

According to an embodiment, LEDs may be used as light sources together with wavelength converting material for providing the first and second light generating means. For example, Fig. 5 shows a diagram with the wavelength spectrums generated by the first and second light generating means when using a currently available LED having a peak wavelength of 450 nm and organic phosphors as wavelength converting materials. The wavelength spectrum of the first light generating means having a CCT of 2700° K generated accordingly is indicated by the solid line 501, and the wavelength spectrum of the second light generating means having a CCT of 1700° K generated accordingly is indicated by the solid line 502. As a comparison, also the wavelength spectrums of ideal black body radiators of the temperatures 2700° K and 1700° K are indicated by the dashed lines 503 and 504, respectively. As shown in Fig. 5, the wavelength spectrums (or spectral power distributions) 501, 502 of the LEDs are relatively close to the wavelength spectrums 503, 504 of the black body radiators. Using linear combinations of the LED-based first and second light generating means, light of all CCTs between 2700 °K and 1700° K can be obtained.

Fig. 6 shows a diagram depicting the CRI as a function of the CCT for the example with an LED-based light generating means described with reference to Fig. 5. As can be seen, for CCTs above 1900° K the CRI already rises above 90. However, the comparably low CRI for longer wavelengths (and lower CCTs) is a result of a well known problem associated with currently available LED-based light sources, namely the difficulties to obtain a sufficiently red spectrum.

In the following, examples of various possible designs of lighting devices according to embodiments of the present invention will be described.

Fig. 7 shows a lighting device 7 according to an embodiment of the present invention. The lighting device 7 comprises a first light generating means 701 including a first light source 702 and a wavelength converting material 703, and a second light generating means 704 including a second light source 705 and a wavelength converting material 706.

The light sources 702, 705 may for instance be LEDs or any other, preferably low energy light sources. Each one of the light sources 702, 705 preferably emits a fixed optical spectrum which does not remarkably change (except in terms of light intensity) depending on the drive current. For example, the light sources 702, 705 may emit light having a wavelength peak at about 450 nm (or possibly a shorter wavelength). The wavelength converting materials 703, 706 are then selected to be excited by the wavelengths of the light sources 702, 705 for providing stable spectrums having desired CCTs. The wavelength converting materials 703, 706 may for instance comprise phosphorous material (e.g. phosphor, organic phosphor or quantum dots) provided on a remote surface (such as a sheet or screen) or directly on the light sources 702, 705. By appropriate selection of the drive currents for each light source, a linear combination of the light emitted from the light generating means 701, 704 can be generated.

To more efficiently mix the light from the two spatially separated light generating means 701, 704, the lighting device 7 may further comprise a mixing means 707, such as a mixing chamber or a mixing rod. Optionally, a partition wall 711 may be provided between the light generating means 701, 704 for limiting light from the first light source 702 to strike or impinge the second wavelength converting material 706 and light from the second light sources 705 to strike or impinge at the first wavelength converting material 703, which is advantageous in that a more accurate individual control of the first and second light generating means 701, 704 can be provided. Further, the lighting device 7 may be equipped with a bulb or envelope 708 into which the light from the mixing means 707 is fed for scattering and providing a proper angular distribution of the light from the lighting device 7. Optionally, the bulb 708 may be shaped to resemble the appearance of a traditional incandescent light bulb.

In the present embodiment, the light output from the lighting device 7 may be linearly shifted between the CCT of the first light generating means 701 and the CCT of the second light generating means 704 by adjusting the light intensities of the first and second light sources 702, 705 relative to each other. Hence, both cold light and warm light may be provided by the lighting device 7.

Fig. 8 shows a lighting device 8 according to another embodiment of the present invention. The lighting device 8 may comprise a box having a base (or bottom plate) 810 at which at least one first light source 802 (or three as depicted in Fig. 8) is arranged at a central portion of the base 810 and a plurality of second light sources 805 are arranged circumferentially around the first light source 802.

The lighting device 8 further comprises a surface or sheet 809 provided with a first wavelength converting material at its central portion 803 and a second wavelength converting material at an outer circumferential portion 806 surrounding the central portion 803. The surface 809 may be remotely arranged to receive light from the light sources 802, 805. The size of the central portion of the base 810 may be (at least almost) the same as or correspond to the size of the central portion 803 of the surface 809. Likewise, the size of the outer area of the base 810 with the second light sources 805 and size of the outer portion 806 of the surface 811 may be (at least almost) the same or correspond to each other.

Alternatively, some optical elements may be provided such that light from the first light source 802 is directed towards the central portion 803 of the surface 809 and such that light from the second light sources 805 is directed towards the outer portion 806 of the surface 809.

A partition wall 811, preferably highly reflective, may be arranged to optically separate the first and second light sources 802, 805, i.e. to avoid or at least limit that light from the first light source 802 strikes the second wavelength converting material on the outer portion 806 of the surface 811 and that light from the second light sources 805 strikes the first wavelength converting material on the central portion 803 of the surface 809.

Preferably, the first and second light sources 802, 805, as well as the central and outer portions 803, 806 of the base 810 are arranged concentrically. It will be appreciated that the shape of the light generating means of the lighting device 8 may be circular (as shown in Fig. 8), square shaped or of any other suitable shape. It is advantageous to let the outer light generating means (the larger annulus) correspond to the CCT that requires the highest operating luminance. Hence, the outer light generating means may preferably have a higher CCT providing a cooler light than the central light generating means. In this case, the central light generating means may be significantly smaller than the outer light generating means since the produced luminosity for low CCTs is to be much smaller than for high CCT for resembling an incandescent- like behaviour.

As in the previously described embodiment (shown in Fig. 7), the light output from the lighting device 8 may be linearly shifted between the CCT of the first light generating means and the CCT of the second light generating means by adjusting the light intensities of the first and second light sources 802, 805 relative to each other.

Fig. 9 shows a lighting device 9 according to another embodiment of the present invention. The lighting device 9 comprises a light source 902 and a surface 909 including a first portion 903 of a first wavelength converting material and a second portion 906 of a second wavelength converting material. Hence, the wavelength converting material composition varies as a function of the spatial coordinates in two dimensions of the surface 909. The first and second portions 903, 906 may for instance be concentrically arranged as depicted in Fig. 9. Further, the lighting device 9 comprises a beam control means 912, such as a PDLC beam shaper for changing the angular light distribution from the light source 902 relative to the surface 912. Hence, the light emitted by the light source 902 falls on to the surface 909 having a spatially varying composition of wavelength converting material such that the CCT of the output light is controllable by varying the angular distribution of the light from the light source 902.

As an example, the light emitted by the light source 902 after shaping by the beam control means 912 provides a light intensity distribution on the surface 909 having a circular symmetry. Further, the composition of the wavelength converting material on the surface 909 also has a circular symmetry. In other words, both the light intensity distribution on the surface 909 and the wavelength converting composition on the surface 909 can be described by functions depending only on the radius R of the circular symmetry. The light intensity distribution profile may for instance follow a raised cosine function (which angular distribution would correspond to a Lambertian radiation pattern) of which the width (or radius) is controlled by the beam control means 912. The light intensity distribution may alternatively correspond to a (chopped) Gaussian distribution, a modified Bessel distribution or any other pattern having a rotational symmetry.

For example, the radius dependent wavelength converting composition may be arranged such that the lighting device 9 produces incandescent- like light of 1700° K if the beam control means 912 has a setting of R_max = 1 cm, and incandescent-like light of 2700° K if the beam control means 912 has a setting of R_max= 2.5 cm. As mentioned above, it is advantageous to let the inner annulus correspond to the lower color temperatures, because for an incandescent light bulb, the lower color temperatures also correspond to lower total intensities.

Hence, the first portion 903 of the first wavelength converting material may cover the annulus 0 cm < R < 1 cm. This first composition of wavelength converting material produces 1700° K if the beam control means 912 has a setting of R_max = 1 cm. The second portion 906 of the second wavelength converting material may cover the annulus 1 cm < R < 2.5 cm. This second composition of wavelength converting material together with the first composition in the inner annulus produces together 2700° K if the beam control means 912 has a setting of R_max = 2.5 cm. By adjusting the settings of R_max of the beam control means 912 between 1 cm and 2.5 cm, different linear combinations are generated, thus allowing for all CCTs between 1700° K and 2700° K.

It will be appreciated that the surface 909 may comprise several (preferably annulus) portions with different wavelength converting material, thereby allowing to better approximate the visible black body spectrum for the intermediate settings of the beam width.

Another embodiment of the invention, still in connection with the lastly described embodiment (shown in Fig. 9), is shown in Fig. 10. A lighting device 1000 comprises a light source 1002 injecting light into a light guide 1012 comprising polymer dispersed liquid crystal (PDLC) parts that can be switched from an optically transparent state to a light scattering state by means of locally applied electric fields. Due to the scattering, light is coupled out of the light guide through a surface 1003 provided with spatially varying wavelength converting material. The light may hence be coupled out through different portions of the surface 1003 to provide different CCTs.

Different alternatives for changing the direction and/or shape of the beam of rays emitted from the light source relative to the surface of wavelength converting material are possible. In addition to the alternatives already described above, further alternatives for changing the direction and/or shape of the beam of rays are shown in Fig. 11.

Fig. 11 shows a lighting device 1200 comprising a light source 1202 and a collimator 1216 arranged to collimate the light from the light source 1202. The lighting device 1200 further comprises a micro lens array 1204 at which beam control means are provided. Such a micro lens array efficiently creates a multitude of micro beams. The lighting device 1200 further comprises a surface 1209 patterned with two different wavelength converting materials 1203 and 1206. The beam control means are adapted to selectively aim the micro beams at the two wavelength converting materials 1203 and 1206. The patterned surface 1209 is advantageous in that it enhances a color mixing since the areas of alternating wavelength converting material are spatially close.

The beam control means may e.g. comprise a switchable micro lens 1213 which defocuses (spreads) the micro beams on the neighboring wavelength converting portions in the pattern, a switchable beam deflector 1214 which deflects the micro beams to a neighboring wavelength converting portion, and/or a switchable beam splitter 1215 which splits the micro beams onto the neighboring wavelength converting portions. The patterned surface 1209 may alternatively be electromechanically moved for changing the mapping of the micro beam pattern onto the patterned surface 1209.

A method 1300 of manufacturing a lighting device according to an embodiment of the present invention is schematically shown in Fig. 12. The method 1300 comprises the steps of providing 1301 a first light generating means arranged to provide light with a first optical spectrum having a first CCT and a first color point in a chromaticity space, wherein the spectral power distribution of the first optical spectrum is close to the spectral power distribution of a black body radiator of a temperature corresponding to the first CCT, and providing 1302 a second light generating means arranged to provide light with a second optical spectrum having a second CCT and a second color point in the chromaticity space, wherein the spectral power distribution of the second optical spectrum is close to the spectral power distribution of a black body radiator of a temperature corresponding to the second CCT. A linear combination of the first light generating means and the second light generating means provides an output optical spectrum having an output CCT between the first CCT and the second CCT. Further, the first light generating means and the second light generating means are selected such that the first color point and the second color point are more green and/or yellow than the black body locus of the chromaticity space, and such that the optical spectrum of at least one of the achievable combinations provides an output color point in the chromaticity space being more red than the black body locus.

The method 1300 may further comprise the step of reducing 1303 the maximum distance between the output color points of the linear combination and the black body locus, and the step of increasing 1304 the lowest value of an illumination quality measure, such as the CRI, of the linear combinations.

While specific embodiments have been described, the skilled person will understand that various modifications and alterations are conceivable within the scope as defined in the appended claims. For example, the examples of light sources, wavelength converting materials and orientation of the surface given with reference to the embodiment of the lighting device shown in Fig. 7 are applicable also for the other described embodiments (with reference to Figs. 8-11). Further, features like bulbs, mixing chambers and light intensity adjusting means may be applicable to any one of the described embodiments.

Further, the lighting device may comprise several light generating means having color points close to the black body locus, thereby providing a more accurate approximation to the black body locus and an improved resemblance to the behavior of a traditional incandescent lamp.

Further, it will be appreciated that the lighting device according to the invention may be used in any lighting application requiring improved illumination quality and tunable CCT, such as if an incandescent illumination behavior is to be realized using modern (e.g. LED-based) light sources. Examples of applications of the present invention are general lighting, task lighting and decorative lighting in households and commercial environments.

Claims

CLAIMS:

1. A lighting device 7 comprising:

a first light generating means (701) arranged to provide light with a first optical spectrum having a first correlated color temperature, CCT, and a first color point (101) in a chromaticity space (1), wherein the spectral power distribution of the first optical spectrum is close to the spectral power distribution of a black body radiator of a temperature corresponding to the first CCT; and

a second light generating means (704) arranged to provide light with a second optical spectrum having a second CCT and a second color point (102) in the chromaticity space, wherein the spectral power distribution of the second optical spectrum is close to the spectral power distribution of a black body radiator of a temperature corresponding to the second CCT,

wherein a linear combination (103) of the first light generating means and the second light generating means provides an output optical spectrum having an output CCT between the first CCT and the second CCT, and

wherein the first light generating means and the second light generating means are configured such that the first color point and the second color point are more green and/or yellow than the black body locus (10) of the chromaticity space, and such that the optical spectrum of at least one of the achievable combinations provides an output color point in the chromaticity space being more red than said black body locus.

2. A lighting device as defined in claim 1, wherein the first light generating means and the second light generating means are further configured such that the maximum distance between the output color points of the linear combination being more red than the black body locus and the black body locus, the distance between the first color point and the black body locus, and the distance between the second color point and the black body locus are of the same order of magnitude.

3. A lighting device as defined in claim 1 or 2, wherein the first light generating means and the second light generating means are further configured such that the lowest color rendering index of the linear combination is at least 80, preferably at least 90, and even more preferably at least 95.

4. A lighting device as defined in any one of the preceding claims, wherein the chromaticity space is one of the CIE color spaces, such as the CIE 1931 color space, the CIE 1960 color space or the CIE 1976 color space.

5. A lighting device as defined in any one of the preceding claims, wherein the first CCT is between 1500 Kand 1900 K, and the second CCT is between 2500 K and ₂900 K.

6. A lighting device as defined in any one of the preceding claims, wherein:

the first light generating means comprises a first light source (702) and a first wavelength converting material (703) arranged to receive light emitted by the first light source; and

the second light generating means comprises a second light source (705) and a second wavelength converting material (706) arranged to receive light emitted by the second light source.

7. A lighting device as defined in claim 6, wherein:

the first light source is arranged at a central portion of a base (810) of the lighting device and the second light generating means comprises a plurality of second light sources (805) arranged around the first light source at the base; and

- the first wavelength converting material is arranged at a central portion (803) of a surface (809) and the second wavelength converting material is arranged around the central portion at the surface, the surface being arranged to receive light from the first light source and the second light sources.

8. A lighting device as defined in any one of the claims 1-5, wherein the lighting device comprises a light source (902), a surface (909) comprising a first portion (903) of a first wavelength converting material and a second portion (906) of a second wavelength converting material, and a means for changing, relative to the surface, the direction and/or shape of a beam of rays emitted from the light source.

9. A lighting device as defined in claim 8, further comprising a beam control means (912), such as a polymer dispersed liquid crystal, PDLC, beam shaper, a switchable micro lens (1213), a switchable beam deflector (1214) or a switchable beam splitter (1215), configured to direct the light from the light source towards different portions of the surface.

10. A lighting device as defined in any one of the preceding claims, further comprising a mixing means (707) for mixing at least a part of the light emitted by the first light generating means with at least a part of the light emitted by the second light generating means.

11. A lighting device as defined in any one of the preceding claims, wherein the amount of the light emitted by the first light generating means is adjustable relative to the amount of the light emitted by the second light generating means.

12. A method of manufacturing a lighting device, the method comprising the steps of:

providing (1301) a first light generating means arranged to provide light with a first optical spectrum having a first correlated color temperature, CCT, and a first color point in a chromaticity space, wherein the spectral power distribution of the first optical spectrum is close to the spectral power distribution of a black body radiator of a temperature corresponding to the first CCT; and

providing (1302) a second light generating means arranged to provide light with a second optical spectrum having a second CCT and a second color point in the chromaticity space, wherein the spectral power distribution of the second optical spectrum is close to the spectral power distribution of a black body radiator of a temperature

corresponding to the second CCT,

wherein a linear combination of the first light generating means and the second light generating means provides an output optical spectrum having an output CCT between the first CCT and the second CCT, and

wherein the first light generating means and the second light generating means are selected such that the first color point and the second color point are more green and/or yellow than the black body locus of the chromaticity space, and such that the optical spectrum of at least one of the achievable combinations provides an output color point in the chromaticity space being more red than said black body locus.

13. A method as defined in claim 12, further comprising the step of reducing (1303) the maximum distance between the output color points of the linear combination and the black body locus.

14. A method as defined in claim 12 or 13, further comprising the step of increasing (1303) the lowest value of an illumination quality measure, such as the CRI, of the linear combinations.