COLOR CONTROL FOR LUMINESCENT LIGHT GUIDE

Abstract

The invention provides a lighting device (100) configured to provide lighting device light (101). The lighting device (100) comprises a plurality of light sources (200) configured to provide light source light (201), the plurality of light sources (200) comprising at least a first light source (210) configured to generate first light source light (211) and a second light source (220) configured to generate second light source light (221). The lighting device further comprises a light guide (300) comprising: a luminescent material (310) excitable by the light source light (201), and configured to provide luminescent material light (311), wherein the luminescent material (310) is configured to reabsorb at least part of its luminescent material light (311), a light exit window (330) for escape of the luminescent material light (311) from the light guide (300), and a plurality of light incoupling areas (320) arranged perpendicular to the light exit window (330) and comprising at least a first light incoupling area (321) configured at a first distance (d1) from the light exit window (330), and a second light incoupling area (322) configured at a second distance (d2) from the light exit window (330). The first light source (210) is configured to provide said first light source light (211) to the first light incoupling area (321), wherein the second light source (220) is configured to provide said second light source light (221) to the second light incoupling area (322), wherein the first distance (d1) is unequal to the second distance (d2). The lighting device further comprises a control unit (500) arranged for controlling the color temperature of the lighting device light by independently controlling the plurality (m) of light sources (200) dependent on the distance of each light source from the light exit window (330).

Claims

1. A lighting device configured to provide lighting device light, the lighting device comprising: a plurality of light sources configured to provide light source light, the plurality of light sources comprising at least a first light source configured to generate first light source light and a second light source configured to generate second light source light; a light guide comprising: a luminescent material excitable by the light source light, and configured to provide luminescent material light, wherein the luminescent material is configured to reabsorb at least part of its luminescent material light, a light exit window for escape of the luminescent material light from the light guide, and a plurality of light incoupling areas arranged perpendicular to the light exit window and comprising at least a first light incoupling area configured at a first distance from the light exit window, and a second light incoupling area configured at a second distance from the light exit window; wherein the first light source is configured to provide said first light source light to the first light incoupling area, wherein the second light source is configured to provide said second light source light to the second light incoupling area, wherein the first distance is unequal to the second distance; and the lighting device further comprising: a control unit arranged for controlling the color temperature of the lighting device light by independently controlling the plurality of light sources dependent on the distance of each light source from the light exit window, wherein as distance the shortest distance from the light incoupling area through light guide material to the light exit window is chosen.

2. The lighting device according to claim 1, wherein the light guide comprises a plurality of faces, wherein at least part of a first face is configured as first light incoupling area and second light incoupling area, and wherein at least part of a second face is configured as light exit window.

3. The lighting device according to claim 1, wherein the control unit is further arranged to select which light source to switch on and to adjust the relative intensity of the plurality of light sources dependent on the distance of each light source from the light exit window.

4. The lighting device according to claim 1, comprising a plurality of light exit windows arranged perpendicular to the incoupling areas.

5. The lighting device according to claim 1, comprising said plurality of light sources wherein m is at least 3, configured to provide light source light to irradiate incoupling areas of the light guide, with the light incoupling areas configured at distances from the light exit window, wherein each distance is different.

6. The lighting device according to claim 1, wherein the plurality of light sources comprises two or more subsets each comprising one or more light sources, wherein each subset of light sources is configured to provide light source light to another face of the light guide.

7. The lighting device according to claim 1, wherein the light guide comprises a light guide material wherein the luminescent material is embedded.

8. The lighting device according to claim 7, wherein the light guide material comprises a polymeric material.

9. The lighting device according to claim 7, wherein the light guide material comprises a material selected from the group consisting of an inorganic material and a hybrid material having both inorganic and organic character.

10. The lighting device according to claim 7, wherein the light guide material comprises a ceramic material.

11. The lighting device according to claim 1, wherein the luminescent material comprises one or more of an organic dye, quantum dots, and a luminescent ion based luminescent material.

12. The lighting device according to claim 1, wherein the luminescent material has an emission spectrum and an excitation spectrum having spectral overlap, wherein the normalized spectral overlap is 0.02<SO≦0.5.

13. The lighting device according to claim 1, wherein the light source light of the plurality of light sources have spectral overlap.

14. The lighting device according to claim 1, wherein the first light source is arranged at a largest distance from the light exit window of the light guide and is arranged to produce a red-shifted spectral distribution with respect to light generated by the second light source which is arranged at a smallest distance from the light exit window of the light guide.

15. A method of controlling a lighting device configured to provide lighting device light, the lighting device comprising: a plurality of light sources providing light source light, the plurality of light sources comprising at least a first light source configured to generate first light source light and a second light source configured to generate second light source light; a light guide comprising: a luminescent material excitable by the light source light, and configured to provide luminescent material light, wherein the luminescent material is configured to reabsorb at least part of its luminescent material light, a light exit window for escape of the luminescent material light from the light guide, and a plurality of light incoupling areas arranged perpendicular to the light exit window and comprising at least a first light incoupling area configured at a first distance from the light exit window, and a second light incoupling area configured at a second distance from the light exit window; wherein the first light source is configured to provide said first light source light to the first light incoupling area, wherein the second light source is configured to provide said second light source light to the second light incoupling area, wherein the first distance is unequal to the second distance; the method comprising the steps of: controlling the color temperature of the lighting device light by independently controlling the plurality of light sources dependent on the distance of each light source from the light exit window, wherein as distance the shortest distance from the light incoupling area through light guide material to the light exit window is chosen.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0071] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which:

[0072] FIGS. 1a-1d schematically depict some aspects of the invention; and

[0073] FIGS. 2a-2h schematically depict some aspects of the invention.

[0074] The drawings are not necessarily on scale.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0075] FIG. 1a schematically depicts an embodiment of a lighting device 100 configured to provide lighting device light 101. The lighting device 100 comprises a plurality m (here by way of example m=4) of light sources 200 configured to provide light source light 201. The plurality m of light sources 200 comprises at least a first light source 210 configured to generate first light source light 211 and a second light source 220 configured to generate second light source light 221, (and in this example also a third light source 230 configured to generate third light source light 231 and a fourth light source 240 configured to generate fourth light source light 241.

[0076] Phrases like light “light source 210, 220, . . . ” and similar phrases, such as “light source light 211, 221, . . . ” or “distance d1, d2, . . . ” are used to indicate plurality of items or references. For instance, five light sources would provide light sources 210, 220, 230, 240, and 250, and in these cases also light source light 211, 221, 321, 421, 521, and, dependent upon the arrangement of the light sources also five distances d1, d2, d3, d4, and d5. Note that the term “plurality” always indicates at least two. Hence, herein the basic definitions of pluralities of references in general indicate at least two numbers (such as “a plurality of light sources 210, 220, . . . ”, etc.).

[0077] The light source may in principle be any type of point light source, but is in an embodiment a solid state light source such as a Light Emitting Diode (LED), a Laser Diode or Organic Light Emitting Diode (OLED), a plurality of LEDs or Laser Diodes or OLEDs or an array of LEDs or Laser Diodes or OLEDs, or a combination of any of these. The LED may in principle be an LED of any color, or a combination of these, but is in an embodiment a blue light source producing light source light in the blue color-range which is defined as a wavelength range of between 380 nm and 495 nm. In another embodiment, the light source is an UV or violet light source, i.e. emitting in a wavelength range of below 420 nm.

[0078] Further, the lighting device 100 comprises a light guide 300 comprising a luminescent material 310 excitable by the light source light 201, and configured to provide luminescent material light 311. As indicated above, the luminescent material 310 in the light guide 300 is configured to reabsorb at least part of its luminescent material light 311. The extent of reabsorption may depend upon the type of luminescent material (Stokes shift) and also the concentration of the luminescent material or luminescent species in the light guide (material). The light guide material is indicated with reference 301.

[0079] The light guide 300 comprises a light exit window 330 (here second face 342, see also below) for escape (or outcoupling) of the luminescent material light 311 from the light guide 300 and a plurality of light incoupling areas 320 comprising at least a first light incoupling area 321 for receipt of first light source light 211 and configured at a first distance d1 from the light exit window 330, and a second light incoupling area 322 for receipt of the second light source light 21 and configured at a second distance d2 from the light exit window 330. Note that the first distance d1 is unequal to the second distance d2. Here, all four distances d1, d2, d3 and d4 are different. The light sources are thus arranged upstream of the light guide 300 and the light exit window 330 is arranged downstream from the light incoupling areas 320. Further optics may be available downstream of the light exit window 330, but these are not further discussed in detail. For instance, in an embodiment a total internal reflection (TIR) collimator or reflector can be applied. The light exit window (here second face 342) is configured (substantially) perpendicular to the first face 341, which comprises in this example a plurality of four light incoupling areas 320 (i.e. first-fourth light incoupling areas 321, 322, 323, 324).

[0080] The first light source 210 is configured to provide said first light source light 211 to the first light incoupling area 321, and the second light source 220 is configured to provide said second light source light 221 to the second light incoupling area 322. Likewise, this will in general be the case for any further light sources (such as 230, 240).

[0081] Further, the lighting device may include a control unit 500, for independently controlling each of the plurality of light sources 200. By this control, the color point and/or color temperature of the lighting device light 101 may be controlled (see also below). The lighting device light 101 at least comprises luminescent material light 311, but may optionally also comprise light source light 201 (from one or more of the light sources 200), e.g. blue light source light 201 and yellow luminescent material light 311 (making e.g. white light).

[0082] Hence, with the present invention, e.g. it may be possible to tune between cool white and warm white.

[0083] The light guide 300 in this embodiment comprises a plurality of faces (or surfaces) 340, such as the first face 341, which includes in this embodiment a plurality of light incoupling areas 320, a second face 342, which comprises the light exit window 311, and further faces 343 (third face) and 344 (fourth face), etc. Optionally, these faces may be used to couple light source light of other light sources into the light guide 300. Further, parts that are not used to couple light source light in or out may be provided with a reflector (see below). Hence, the light guide 300 comprises one or more faces 340, in general at least two faces 340 (e.g. a cone (not depicted) may be considered to have two faces, and a rod (see FIG. 2g) may be considered to have three faces).

[0084] Reference A indicates a body axis, such as in this case a length axis. In general, the light guide 300 will include one or more body axes (such as a length axis, a width axis or a height axis), with the exit window 330 being configured (substantially) perpendicular to such axis. Hence, in general, the light incoupling areas 340 will be configured (substantially) perpendicular to the light exit window 330.

[0085] In the schematic drawings, a power supply or other items are not shown in the drawings.

[0086] FIG. 1b schematically depicts an emission curve and associated excitation curve, with on the x-axis wavelength (λ) in nanometer (nm) and on the y-axis normalized intensity (N.I.) (in arbitrary units). For the sake of argument, the absorption curve is defined as being identical to the excitation curve, especially within the relevant wavelength range of 400-600, especially even 300-600 nm. The excitation or absorption curve is identified with reference EX. The emission curve is identified with reference EM, i.e. the radiation converter element radiation 311. Further, the (diagonally) hashed curve is the solid state light source radiation 21. The curves are normalized. It is apparent that the radiation converter element can be excited over nearly the whole wavelength range of 300-500 in this example. However, most efficient excitation will be in the range around the maximum λ.sub.XM, i.e. the excitation maximum. The solid state light source radiation has a maximum at radiation maximum λ.sub.RM. Especially, the solid state light source is chosen such that its radiation maximum λ.sub.RM is within the range of 70-100% of λ.sub.XM. In the drawing, the 80-100% range is indicated. Here, in this embodiment the solid state light source radiation overlaps substantially completely with the excitation band, and the maxima λ.sub.XM and λ.sub.RM are substantially on top of each other. Further, the emission band and excitation band overlap. Here, in this schematic drawing about 10-15% of the emission band EM (radiation converter element radiation 311) overlaps with the excitation band. The spectral overlap is determined by normalizing the excitation spectrum and emission spectrum in the visible range to 100 (or 1, etc.), and defining the area under the emission curve overlapping with the area under the excitation curve. For a good reabsorption such overlap is beneficial. The overlapping area is (horizontally) hashed and is indicated with reference SO (spectral overlap). The wavelength at maximum emission is indicated with reference λ.sub.MM. The wavelength difference between λ.sub.XM and λ.sub.MM is the Stokes shift. Note that in general each light source 200 (i.e. first light source 210, second light source 220, etc.) will generate light source light 201 (i.e. first light source light 211, second light source light 221, etc.) being substantially identical, i.e. all substantially having the same spectral distribution. Hence, referring to FIG. 1a, all four light sources 200 may generate the same light source emission 201.

[0087] For measuring an emission and excitation spectrum for estimating the spectral overlap, especially a very thin piece, such as less than 1 mm thick, like less than 0.5 mm thick, may be used. The thin part is irradiated with the excitation light and emission coming from the light incoupling area is measured. Also thicker pieces may be used. In such case, especially excitation is provided under a grazing angle, and emission light emanating from the light incoupling area is measured. In these ways, substantially only emission may be measured that has hardly (or not) been affected by reabsorption. Optionally, an identical light guide may be measured, but with a 100 times lower concentration of luminescent material (i.e. a much lower absorbance). Also this reduces self-absorption and may provide excitation and emission spectra for determining the spectral overlap. Yet another option is to convert (part of) the light guide into particulate material. This will substantially reduce self-absorption and lead to an emission spectrum that is not (substantially) affected by self-absorption. Alternatively, the emission spectra may be measured at different thicknesses (especially of a flat light guide). In this way, the emission spectrum at substantially no self-absorption may be extrapolated from these emissions at different thicknesses. A combination of these methods may also be used.

[0088] High brightness light sources are interesting for various applications including spots and digital light projection. For this purpose, but also for other purposes, it is possible to make use of so-called light concentrators where shorter wavelength light is converted to longer wavelengths in a highly transparent luminescent material. A rod of such a transparent luminescent material can be illuminated by LEDs to produce longer wavelengths within the rod. For various applications such as light engines, lamps, luminaires and projectors, one would like to control the color (temperature) of the light source e.g. for black body dimming (for mimicking the behavior of a light source such as an incandescent light source) or adapting the color gamut of a light system (for adjusting the color quality). This can be done by combining light emitted from multiple sources. However, use of multiple LEDs leads to the increase in the area of the emitting surface. It is therefore desired to control the color (temperature) of a light emitting surface without increasing the area so that the intensity remains high.

[0089] As indicated above we suggest the use of a luminescent concentrator based light source which is pumped by at least two light sources. The light concentrator is chosen that there is (preferably) a large overlap between the emission and absorption spectra. The large overlap leads to a strong re-absorption of light emitted by the luminescent concentrator and hence to a strong dependence of the end spectrum to the length of the luminescent rod. When such a rod is pumped from the sides with a number of LEDs along the length then the spectrum of the light emitted from the exit surface will depend on the position of the LED w.r.t the exit surface. Therefore, by choosing which LED or LEDs to switch on and adjusting their relative intensity, the color (temperature) of the light coming out of the exit surface can be adjusted (FIGS. 1c-1d). Thus, each LED will be controlled differently depending on the distance from, or position of the LED w.r.t., the exit surface and depending on the required color temperature of the light exiting the rod. The control unit takes into account the distance from each LED to the exit surface of the rod and controls each LED accordingly in order to provide for the required color temperature of the light exiting the rod. In the same way, by controlling the number of LEDs the peak width and/or shape can be adapted (FIG. 1d). The top curves in FIGS. 1c and 1d schematically depict an emission without substantial reabsorption. The second curves in FIGS. 1c and 1d, respectively, correspond to the irradiation by the third light source 230 only. By way of example, this curve is already a bit red shifted, indicative of some reabsorption. The third curve in FIG. 1c corresponds with the second light source 220 irradiating the light guide 300; the emission is more red shifted than the former, as the emission light 311 has to travel a longer path through the light guide 300. The lowest curve in FIG. 1c corresponds to the emission generated with the lighting device 100 when the first light source 210 irradiates the light guide 200. The emission is most red-shifted. FIG. 1d shows that by choosing the light sources, not only the peak position can be tuned, but also the shape of the emission. When light sources at different distances are used (see the third and fourth curves wherein respectively the third light source 230 and the second light source 220, or all three light sources 230, 220, 210 irradiate the light guide 200), a superposition of the respective emissions may be obtained. Note that these graphs are schematical.

[0090] If desired, the light sources output may be adapted such that the total intensity escaping the light exit surface stays the same.

[0091] It goes without saying, that a plurality of LEDs can be used, such as 40 LED, or 2 rows of 20 LEDs, or 20 LEDs illuminating the rod from the top and 20 LEDs illuminating the rod from the bottom.

[0092] Hence, in an embodiment a light emitting device is provided, the light emitting device comprising at least two light sources adapted for, in operation, emitting light with a first spectral distribution, a first light guide comprising a first light input surface and a first light exit surface, the first light guide being adapted for receiving the light with the first spectral distribution at the first light input surface, converting at least a part of the light with the first spectral distribution to light with a second spectral distribution, guiding the light with the second spectral distribution to the first light exit surface and coupling the light with the second spectral distribution out of the first light exit surface, the at least two light sources being arranged at mutually different distances from the first light exit surface of the first light guide, and a control device adapted for changing the spectral characteristics of the second spectral distribution by individually controlling the light sources, wherein the light source of the at least two light sources which is arranged at a largest distance from the first light exit surface of the first light guide produces a red-shifted second spectral distribution with respect to light generated by the light source of the at least two light sources which is arranged at a smallest distance from the first light exit surface of the first light guide.

[0093] As indicated above, we suggest the use of a luminescent material chosen such that there is a large overlap between the emission and absorption spectra (FIG. 1b). The (large) overlap leads to a strong re-absorption of light emitted by the luminescent concentrator and hence to a strong dependence of the end spectrum to the length of the luminescent rod.

[0094] The lighting device may comprise in an embodiment a single light exit surface (FIG. 2a). By way of example, the difference in color of the lighting device light as function of the light source that is switched on is indicated by difference in shading of the lighting device light 101. This difference is especially due to the difference in the luminescent material light 311.

[0095] The light guide 300, such as a luminescent concentrator, might also comprise a back reflector (FIG. 2b). However, in this configuration light might be reflected back by the back reflector and therefore this might slightly change the spectral distributions. Due to light losses in the rod (e.g. by slight surface scattering, small scattering in the rod, reflection losses) the contribution of the back reflected light is relatively small. The (back) reflector might be without optical contact with the light guide 300, such as the luminescent rod. Reflectors are herein indicated with reference 400.

[0096] The light guide 300, such as a luminescent concentrator, might also comprise addition reflectors 400 which are positioned preferably without any optical contact with the luminescent concentrator. Further, the lighting device may include a heat sink 117. In FIG. 2c schematically an embodiment is depicted wherein additional reflectors 400 comprise a heat sink element 117.

[0097] In yet another embodiment, the lighting device comprises more than one light exit surface 330 such as two light exit surfaces. It goes without saying that the light characteristics of the light exiting the second light exit surface also depends on the position of the LED with respect to the light exit surface. The light emitting device may also comprise a matrix of LEDs (FIG. 2d). In this way, also more complex light spectral distribution can be obtained (FIGS. 2e and 20. In the latter figure, the light emitting device might also comprise three light exit surfaces (FIG. 20. Of course, the light emitting device might also comprise four light exit surfaces. In FIG. 2f, by way of example a plurality of body axes A are indicated. The light incoupling areas illuminated by the light sources 200 are substantially perpendicular to the light exit windows 330.

[0098] FIG. 2g schematically depict some further embodiments of the light guide 300, with a tubular type of light guide, a wedge shaped type of light guide, and a planar (rod or beam) shaped type of light guide 300. Other shapes may also be possible. By way of example, a plurality of light sources is depicted, to show that different subsets 2000 of light sources may be to irradiate the light guide.

[0099] FIG. 2h schematically depicts an embodiment of the lighting device 100 comprising a further light source 1200. This light source may for instance have no interaction with the light guide 300. Further light source light 1201 and the luminescent material light 311, and optionally light source light may be comprised by the lighting device light 101. A diffusor 118 may be used to mixed the different types of light.

[0100] The light emitting device might also comprise a sensor/detector for detecting a specific signal (e.g. temperature, humidity, position, location, time, intensity, color, etc.) and the light sources may be controlled accordingly to the signal sensed by the controller.

[0101] In another embodiment, the lighting device may comprise a sensor which integrated in the system such that it measures the light generation of the light sources or lighting device.