LED LAMP WITH SLOW DECAY RED PHOSPHOR RESULTING IN CCT VARIATION WITH LIGHT OUTPUT

Abstract

The invention provides a lighting device (100) comprising a) a light source (10) configured to provide blue light source light (11), b) a first luminescent material (210) configured to convert at least part of the light source light (11) into first luminescent material light (211) with light intensity in one or more of the green spectral region and yellow spectral region, c) a second luminescent material (220) configured to convert (i) at least part of the light source light (11), or (ii) at least part of the light light (11) and at least part of the first luminescent material light (221) with light intensity in the red spectral region, and d) a light exit face (110), wherein the lighting device (100) is configured to provide lighting device light (101) downstream from said light exit face (110), wherein the lighting device light (101) comprises one or more of said light source light (11), said first luminescent material light (211), and said second luminescent material light (221), and wherein the second luminescent material (220) is configured to be at least partly saturated with (i) light source light (11), or (ii) light source light (11) and first luminescent material light (211), at or above at least 50% of nominal operation power of the lighting device (100).

Claims

1. A lighting device comprising: a light source configured to provide blue light source light; a layer of a first luminescent material configured to convert at least part of the light source light into first luminescent material light with light intensity in one or more of the green spectral region and yellow spectral region; a layer of a second luminescent material configured to convert at least part of the light source light into second luminescent material light with light intensity in the red spectral region; wherein the light source is covered by the layer of the second luminescent material, followed by the layer of the first luminescent material, wherein the integrated spectral overlap between the absorption curve of the second luminescent material with the emission spectrum of the light source light is at least four times larger than the integrated spectral overlap between the absorption curve of the second luminescent material with the emission spectrum of the first luminescent material, a light exit face; wherein: the lighting device is configured to provide lighting device light downstream from said light exit face, wherein the lighting device light comprises one or more of said light source light, said first luminescent material light, and said second luminescent material light; wherein the second luminescent material is configured to be at least partly saturated with light source light at or above at least 50% of nominal operation power of the lighting device.

2. The lighting device according to claim 1, wherein the second luminescent material is configured to be at least partly saturated with light source light at or above at least 30% of nominal operation power of the lighting device.

3. The lighting device according to claim 1, wherein the second luminescent material has a decay time .sub.r of at least 1 ms, and the ratio between the decay time of the first luminescent material .sub.y and the decay time of the second luminescent material .sub.r is in the range of 0.1<.sub.y/.sub.r<0.8

4. The lighting device according claim 1, wherein the second luminescent material comprises M.sub.2AX.sub.6 doped with tetravalent manganese, wherein M comprises an alkaline cation, wherein A comprises a tetravalent cation, and wherein X comprises a monovalent anion, at least comprising fluorine.

5. The lighting device according to claim 4, wherein M comprises at least one or more of K and Rb, wherein A comprises one or more of Si and Ti, and wherein X=F.

6. The lighting device according to claim 1, wherein the first luminescent material comprises M.sub.3A.sub.6O.sub.12:Ce.sup.3+, wherein M is selected from the group consisting of Sc, Y, Tb, Gd, and Lu, and wherein A is selected from the group consisting of Al, Ga, Sc and In.

7. The lighting device according to claim 1, wherein the integrated spectral overlap between the absorption curve of the second luminescent material with the emission spectrum of the light source light is at least five times larger than the integrated spectral overlap between the absorption curve of the second luminescent material with the emission spectrum of the first luminescent.

8. The lighting device according to claim 7, wherein M at least comprises Gd and wherein A at least comprises Al and Ga.

9. The lighting device according to claim 1, wherein the light source comprises a solid state light source comprising a light exit surface, wherein the lighting device further comprises a converter element configured downstream from the light exit surface, wherein the converter element comprises the layer of the first luminescent material and the layer of the second luminescent material, and wherein the converter element further comprises said light exit face.

10. The lighting device according to claim 1, further comprising a control system configured to control the power provided to the light source.

11. The lighting device according to claim 10, wherein the control system is configured to control the power provided to the light source as function of an input signal of a user interface.

12. The lighting device according to claim 10, wherein the control system is configured to control the power provided to the light source as function of one or more of a sensor signal and a timer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0041] 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:

[0042] FIG. 1 shows color points of (dimmed) halogen lamp (black open circles) and color points of LEDs with slow red phosphor (with indicated decay time) a same relative intensity; the squares indicate a decay time of 8 ms and the triangles indicate a decay time of 12 ms for the second luminescent material (red);

[0043] FIG. 2 shows color points as a function of the LED drive condition for a device with the Blue-Red-Yellow structure, wherein both the first luminescent material and second luminescent material can be brought into saturation;

[0044] FIGS. 3a-3c schematically depicts some aspects of the invention.

[0045] The schematic drawings are not necessarily on scale.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0046] In an embodiment, a blue LED is covered with a mixture of phosphors. A normal yellow phosphor is used in combination with a slow red phosphor. The red phosphor has a broad excitation spectrum (absorbing both blue and yellow/green) and a long decay time. The decay time of the red phosphor should be chosen such that at nominal drive condition of the LED a considerable part of the red-phosphor is saturated, e.g. 30-90%.

[0047] Dimming a halogen bulb will lead to color point on the BBL, varying between 3000K (100% intensity) and 2200K (3% intensity). At intermediate CCT's the light output of the halogen bulb varies between these two levels.

[0048] The color point of a LED device with a slow red phosphor was calculated at these light levels (9 steps). A perfect device would yield a color point on the BBL, with 100K spacing. The results of these simulations are given in FIG. 1. The decay time determines the CCT range that can be made; in case the decay time is <<1 ms, the CCT-range is 0, for a decay time of 8 ms the CCT range is approximately 700K, for a decay time of 12 ms the CCT range becomes 1100K. The 8 ms decay time data are squares, indicated with A; the 12 ms decay data are triagles and are indicated with B. Halogen lamp data follow very well the BBL and are indicated with H. The values 2500-3500 indicate color temperatures in Kelvin.

[0049] In an embodiment, a blue LED is covered with 2 layers of phosphor. Both phosphors, the first luminescent material (yellow/green) and the second luminescent material (red) have a long decay time. Both the yellow and the red phosphor only absorb blue light. The Blue LED is covered by a red phosphor layer, followed by a yellow phosphor layer (BRY structure). The decay time of the yellow and the red phosphor should be chosen such that at nominal drive condition of the LED a considerable part of the phosphors is saturated, e.g. 40%. With the proper decay times for the yellow and red phosphor, the color point variation upon dimming can be following the BBL as shown in FIG. 2. Here, the references indicate the following:

TABLE-US-00001 Decay time red Decay time yellow luminescent luminescent Symbol material (ms) material (ms) A Triangle 6 6 B Solid square 8 8 C Circle (grey) 10 10 D + 8 7 H Open circle

[0050] Hence, the invention shows that when using a slow red phosphor (decay time several ms), the amount of red in the emission spectrum of the LED is determined by the light intensity of the source. If the red phosphor in addition has a broad absorption spectrum, the color point follows the BBL. If the LED is used at nominal current, the amount of red light in the spectrum is decreased due to saturation of the red phosphor; dimming the LED leads to decreased saturation of the red phosphor (apparent thickness of the red phosphor layer increases), resulting in light with a lower CCT. Due to the broad excitation spectrum the light generated will be close to the BBL.

[0051] FIG. 3a schematically depicts an embodiment of a lighting device 100 as described herein. The lighting device 100 comprises a light source 10 configured to provide blue light source light 11, a first luminescent material 210 configured to convert at least part of the light source light 11 into first luminescent material light 211 with light intensity in one or more of the green spectral region and yellow spectral region and a second luminescent material 220 configured to convert at least part of the light source light 11 into second luminescent material light 221 with light intensity in the red spectral region.

[0052] Further, the lighting device comprises a light exit face 110. Herein in the embodiment of FIG. 3a, this may be the downstream face of a window 105. In FIG. 3b this is the downstream face of a converter 200. Here, in FIGS. 3a-3c the converter 200 comprises the first luminescent material 210 and the second luminescent material 220, e.g. a layers (FIG. 3a), or as mixture (FIGS. 3b-3c). Note that the converter 200 may also include materials and/or layers other than the first luminescent material 210 and the second luminescent material 220. In FIG. 3a, the converter is configured upstream of the light exit face, here upstream of window 105. Especially, when using separate layers of the first luminescent material 210 and the second luminescent material 220, the latter is configured downstream of the former, in order to further facilitate absorption of the first luminescent material light 211. Would the second luminescent material 220 substantially not absorb first luminescent material light 211, then the order of the layers may also be revered. Further, also mixtures may be applied (see FIGS. 3b-3c).

[0053] Further, the lighting device 100 is configured to provide lighting device light 101 downstream from said light exit face 110. Here, as shown in FIG. 3a, the lighting device light 101 comprises one or more of said light source light 11, said first luminescent material light 211, and said second luminescent material light 221. As indicated above, the second luminescent material 220 is configured to be at least partly saturated with light source light 11 at or above at least 50% of nominal operation power of the lighting device 100.

[0054] The distance between the first and/or the second luminescent materials is indicated with reference d1, which is (substantially) zero in the case of FIG. 3c (d1 not depicted in FIG. 3c) and which may be in the range of 0.1-50 mm, especially 1-20 mm in e.g. the embodiment of FIGS. 3a-3b. In the schematically depicted embodiment, the distance d1 is the distance between a light exit surface 122 of a solid state light source 120.

[0055] FIG. 3b schematically further depicts a control system 130, which may include a user interface 140.

[0056] The lighting device 100 may especially be applied for providing white lighting device light (101) that is tunable in color temperature and follows the black body locus with increasing or decreasing power to the light source (10).

[0057] The term substantially herein, such as in substantially all light or in substantially consists, will be understood by the person skilled in the art. The term substantially may also include embodiments with entirely, completely, all, etc. Hence, in embodiments the adjective substantially may also be removed. Where applicable, the term substantially may also relate to 90% or higher, such as 95% or higher, especially 99% or higher, even more especially 99.5% or higher, including 100%. The term comprise includes also embodiments wherein the term comprises means consists of. The term and/or especially relates to one or more of the items mentioned before and after and/or. For instance, a phrase item 1 and/or item 2 and similar phrases may relate to one or more of item 1 and item 2. The term comprising may in an embodiment refer to consisting of but may in another embodiment also refer to containing at least the defined species and optionally one or more other species.

[0058] Furthermore, the terms first, second, third and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequential or chronological order. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other sequences than described or illustrated herein.

[0059] The devices herein are amongst others described during operation. As will be clear to the person skilled in the art, the invention is not limited to methods of operation or devices in operation.

[0060] It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. Use of the verb to comprise and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The article a or an preceding an element does not exclude the presence of a plurality of such elements. The invention may be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer. In the device claim enumerating several means, several of these means may be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

[0061] The invention further applies to a device comprising one or more of the characterizing features described in the description and/or shown in the attached drawings. The invention further pertains to a method or process comprising one or more of the characterizing features described in the description and/or shown in the attached drawings.

[0062] The various aspects discussed in this patent can be combined in order to provide additional advantages. Furthermore, some of the features can form the basis for one or more divisional applications.