FIELD OF THE INVENTION

The invention relates to the field of color tunable light sources.

BACKGROUND OF THE INVENTION

Published patent application WO2009/045922 discloses a color tunable and color temperature tunable light emitting device. The tunable light emitting device comprises an excitation source which emits light of a first wavelength, and comprises a wavelength converting component for converting some light of the first wavelength to light of the second wavelength. The wavelength converting component has a wavelength converting property that varies in a lateral direction. The amount of converted light from the first wavelength to the second wavelength is controlled by moving the wavelength converting component within the light beam emitted by the excitation source. As such, the color and/or the color temperature of the light emitted by light emitting device are controlled.

WO2009/045922 discloses a plurality of inorganic phosphors which may be used in the wavelength converting component. Such inorganic phosphors may be based on silicate, aluminate, nitride, sulfate, oxy-nitrides and oxy-sulfate. Such inorganic phosphors are relatively expensive and, as such, the color and color temperature tunable light emitting device is relatively expensive.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a color temperature tunable lighting device which emits light that has a color point close to the black body line and has a high enough color rendering index, and which is also cost efficient.

A first aspect of the invention provides a color temperature tunable lighting device as claimed in claim 1. A second aspect of the invention provides a luminaire as claimed in claim 11. Advantageous embodiments are defined in the dependent claims.

A color temperature tunable lighting device for emitting light of a color close to or on a black body line in accordance with the first aspect of the invention comprises a light source and a luminescent element. The light source emits light of a color close to the black body line. The luminescent element comprises luminescent materials to tune the correlated color temperature of the light emission of the color temperature tunable lighting device. The luminescent materials comprise a first organic luminescent material and a second organic luminescent material. The first organic luminescent material absorbs a part of the light emitted by the light source or by a second organic luminescent material and converts a part of the absorbed light to a first light emission at least in the red spectral range. The second organic luminescent material absorbs a part of the light emitted by the light source or emitted by the first organic luminescent materials and converts a part of the absorbed light to a second light emission at least in the yellow spectral range.

The luminescent element absorbs a part of the light emitted by the light source and converts it to a combined light emission which comprises, besides the light emission of the light source, at least some additional yellow and red light. The combined light emission is such that the correlated color temperature of the light emitted by light source is tuned towards another color temperature. The light emission of the color temperature tunable lighting device remains close to or on the black body line and, thus, the color of the light emission of the color temperature tunable lighting device is experienced by the human eye as white light which may vary from cool white light to warm white light. It is to be noted that close to the black body line means that the color point is located within a MacAdam ellipse from the black body line. The size of the MacAdams ellipse is expressed in a predefined number of steps. The MacAdams ellipse presents a sub-area of a color space in which the color points are experienced by the human naked eye as similar colors. Thus, the larger MacAdams ellipse is, the more deviation from the black body line is allowed. Optionally, the MacAdam ellipse is a MacAdam ellipse of 7 steps. Optionally, the MacAdam ellipse is a MacAdam ellipse of 5 steps.

The use of organic luminescent materials decreases the cost of the color temperature tunable lighting device because organic luminescent materials are relatively cheap.

The skilled person who wants to lower the costs of the known color tunable light emitting devices may consider replacing the used red emitting inorganic phosphor of the known color tunable light emitting device by a single organic phosphor. Inorganic red phosphors have a broad excitation band, ranging from the deep blue up to its own emission band. The organic phosphors generally show a small stokes shift (absorption band close to the emission band). As such, it is not obvious which type or types of organic phosphor(s) have to be used. The most promising obvious solution would be to replace the (red emitting) inorganic phosphor by a single red emitting organic phosphor. When applying the red emitting organic phosphor in a color tunable lighting device, it was seen that the light emitted by the color tunable lighting device did not follow the black body line. When using such a red emitting organic phosphor, too much red light was generated.

Thus, the skilled person has to find another organic phosphor that emits less red light but still contributes to a light emission which is experienced as warmer white light. An orange emitting organic phosphor seems to be a good candidate, however, it was seen that in a certain range the combined light emission of the light sources and the orange emitting organic phosphor followed the black body line reasonable well, however, in another range the black body line was not followed well. But more importantly, it followed from a more in depth analysis of the light emissions that the color rendering properties of the emitted light were inferior. Thus, the results of using a single orange organic phosphor lead the skilled person away from using one or more organic phosphors which emit light of an orange color.

The inventors have found that the combination of the first organic luminescent material and the second organic luminescent material, surprisingly, provide an advantageous light emission. The emitted color, which is the combination of the light emission by the light source, the first organic luminescent material and the second organic luminescent material, is a color that is close to the black body line over a wide range of correlated color temperatures. Furthermore, the emitted light has a relative high color rendering index. As discussed above, the skilled person did not expect any advantageous result from combinations of organic luminescent material which comprise a red emitting organic luminescent material, and which combined light emission has an orange color. Not wishing to be held to any particular theory, it seems that the interaction between the absorption and emission spectra of the luminescent materials results in the advantageous effect.

It is noted that tuning of the correlated color temperature means in the context of the invention that it is possible to change the correlated color temperature of the light that is emitted by the color temperature tunable lighting device towards a desired or predefined correlated color temperature. Tuning may be done by the user of the device, and may be done at any moment in time, or, the tuning may be performed in the factory during the

manufacturing of the color tunable lighting device.

If a particular light emission comprises light in a specific spectral range, such as, for example, the first light emission at least comprises light in the red spectral range, it means that at least a full width half maximum spectral range of the particular light emission partly overlaps with the specific spectral range. It is known that organic luminescent materials may have a very long tail in their light emission distribution, however, most wavelengths of the emitted light are concentrated in the full width half maximum spectral range and is, as such, light of a specific color. Thus, if a luminescent material converts a part of the absorbed light to a specific light emission in at least a specific spectral range, a significant amount of the wavelengths of the specific light emission is within the specific spectral range.

The light emitted by color temperature tunable lighting device has a certain color point which is a color point in a specific color space. Different color spaces may be used to model the color of the light emitted by color temperature tunable lighting device, such as the CIE XYZ color space, Lab color space, CIE RGB, or, for example, CIE LUV color space. In each of these color space a black body line can be drawn which represents the color of a radiation of a black radiation body which has a specific temperature.

Optionally, the first organic luminescent material and second organic luminescent material comprise perylene derivative. Organic luminescent materials which comprise perylene derivatives are widely available and the different perylene derivatives have different light absorption spectra and light emission spectra thereby enabling almost any required color conversion. Further, the perylene derivatives are relatively cheap and as such they contribute to a cost efficient color temperature tunable lighting device.

Optionally, the light source comprises a light emitter emitting light in at least the blue spectral range and the light source comprises a further luminescent element. The further luminescent element absorbs a part of the light that is emitted by the light emitter and coverts a part of the absorbed light to a light emission which has, when combined with a remaining light emission of the light emitter, a color point close to or on the black body line. These light sources are available at reasonable costs and as such contribute to the cost efficiency of the color temperature tunable lighting device. The blue light emitting light emitter is, for example, a blue light emitting Light Emitting Diode (LED), or a cool white emitting diode or backlight LED.

Optionally, the luminescent element is arranged in between the light emitter and the further luminescent element. Optionally, the luminescent element is movable to adjust the amount of light from the light emitter that is being received by the luminescent element to tune the correlated color temperature of the light emission of the color temperature tunable lighting device. If the amount of light from the light emitter that is received by the luminescent element is increased or decreased the amount of blue light that is converted to the first light emission and the second light emission changes as well, and as such the total light emission of the color temperature tunable lighting device varies according to the movement of the luminescent element. Thus, the color temperature may be tuned. Moving includes movements along a vector or movements around a central point (rotation). It is to be noted that the movements may be performed by a user of the color temperature tunable lighting device, or that the movements may be performed in a factory immediately after the manufacturing of the color temperature tunable lighting device. Further, the movements may be performed automatically according to a particular algorithm.

Optionally, the luminescent element is arranged in a light emission path between the light source and a light exit window of the color temperature tunable lighting device. Optionally, the luminescent element is moveable to adjust the amount of light from the light source which impinges on the luminescent element to tune the correlated color temperature of the light emission of the color temperature tunable lighting device. If the amount of light from the light source that is received by the luminescent element is increased or decreased the amount of blue light that is converted to the first light emission and the second light emission changes as well, and as such the total light emission of the color temperature tunable lighting device varies according to the movement of the luminescent element. Thus, the color temperature may be tuned. Moving includes movements along a vector or movements around a central point (rotation). It is to be noted that the movements may be performed by a user of the color temperature tunable lighting device, or that the movements may be performed in a factory immediately after the manufacturing of the color temperature tunable lighting device.

Optionally, the luminescent element comprises a first layer comprising the second organic luminescent material and the luminescent element comprises a second layer comprising the first organic luminescent material. The first layer is arranged at a first position such that light emitted by the light source or emitted by the light emitter first impinges on the first layer before impinging on the second layer, and the second layer being arranged at a second position to receive also light emitted by the first organic luminescent material. The configuration of this optional embodiment provides an advantageous interaction between the first organic luminescent material and the second luminescent material such that the color point of the light emitted by the color temperature tunable lighting device remains close to the black body line.

Optionally, the luminescent element comprises a mix of the first organic luminescent material and the second organic luminescent material. If the organic luminescent materials are mixed, the interaction between the first organic luminescent material and the second luminescent material is optimal.

Optionally, the luminescent element has an increasing conversion characteristic in a specific direction, the conversion characteristic is a ratio between an amount of impinging light that is absorbed and converted and an amount of impinging light that is not converted. The luminescent element is at least movable in the specific direction. The tuning of the color temperature is performed by the luminescent element with the increasing conversion characteristic. By moving the luminescent element a specific part of the luminescent element may be positioned in the light transmission path such that a specific amount of light is converted to obtain a specific color temperature for the total light emission of the color temperature tunable device. The conversion characteristic may increase as the result of an increasing concentration of the first and the second organic luminescent material in the specific direction. Alternatively, the conversion characteristic may increase as the result of an increasing thickness in the specific direction of a layer with a fixed concentration of the first and the second organic luminescent material. The increase in concentration and the increasing thickness may also be combined.

According to a second embodiment of the invention, a luminaire is provided which comprises the color temperature tunable lighting device according to claim 1. The luminaire according to the second aspect of the invention provides the same benefits as the color temperature tunable lighting device according to the first aspect of the invention and has similar embodiments with similar effects as the corresponding embodiments of the system. If the color temperature of the light emitted by the luminaire is tunable, the luminaire may be used in a plurality of different environments each requiring a different color temperature for the emitted light.

These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.

It will be appreciated by those skilled in the art that two or more of the above- mentioned options, implementations, and/or aspects of the invention may be combined in any way deemed useful.

Modifications and variations of the device, the system, and/or the luminaire, which correspond to the described modifications and variations of the device, can be carried out by a person skilled in the art on the basis of the present description. BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

Fig. la schematically shows a top-view of a color temperature tunable lighting device according to a first aspect of the invention,

Fig. lb schematically shows a cross-section of the color temperature tunable lighting device of Fig. la,

Fig. 2a schematically shows a cross-section of another embodiment of color temperature tunable lighting device,

Fig. 2b schematically shows a cross-section of an alternative embodiment of the color temperature tunable lighting device,

Fig. 3a schematically shows a top-view of a color temperature tunable lighting device which comprises a rotatable luminescent element,

Fig. 3b schematically shows a cross-section of the color temperature tunable lighting device of Fig. 3a,

Fig. 3c schematically shows a cross-section of another color temperature tunable lighting device which comprises a rotatable luminescent element,

Fig. 4a schematically shows a top-view of an embodiment of a color temperature tunable lighting device comprising a luminescent element having an increasing conversion characteristic,

Fig. 4b schematically shows a cross-section of the color temperature tunable lighting device of Fig. 4a,

Fig. 5 schematically shows a chart with simulated color points of different color temperature tunable lighting device,

Fig. 6 schematically shows a chart with simulated color rendering indeces for different color temperature tunable lighting devices, and

Fig. 7 schematically shows a luminaire according to the second aspect of the invention.

It should be noted that items denoted by the same reference numerals in different Figures have the same structural features and the same functions, or are the same signals. Where the function and/or structure of such an item have been explained, there is no necessity for repeated explanation thereof in the detailed description.

The figures are purely diagrammatic and not drawn to scale. Particularly for clarity, some dimensions are exaggerated strongly DETAILED DESCRIPTION OF THE EMBODIMENT

A first embodiment is shown in Fig. la. A top-view of a color temperature tunable lighting device 100 is shown. A top-view means in the context of this document that a person looks towards the color temperature tunable lighting device 100 from a position at which he would receive light emitted by the color temperature tunable lighting device 100 when the color temperature tunable lighting device 100 is in operation. The color temperature tunable lighting device 100 comprises a housing 102 which comprises a light exit window 104. The color temperature tunable lighting device 100 further comprises a light source 106 inside the housing 102. The light source 106 emits white light, which means, that light is emitted with a color point in a color space that is close to or on a black body line. On top of the color temperature tunable lighting device 100 is provided a movable luminescent element 108. A smaller portion or a larger portion of the light exit window 104 is covered by luminescent element 108 by moving the luminescent element 108 to the right or to the left.

The black body line represents the color of the light emission of black body radiators which have a specific temperature (the so-termed color temperature). Light on the black body line is, in general, experienced by humans a white light. White light of a relatively high color temperature is more bluish and is experienced as cool white light, and light of a relatively low color temperature comprises more energy at yellow and/or red wavelength and is experienced as warm white light. A color temperature of light emissions, which do not have a color point exactly on the black body line but have color point close to the black body line, may be expressed in the so-termed correlated color temperature, which is an approximation of the color temperature.

The color temperature tunable lighting device 100 is configured to enable the tuning of the correlated color temperature of the light emitted by the color temperature tunable lighting device 100. This is done by the luminescent element 108 which comprises a first organic luminescent material and comprises a second organic luminescent material. The first organic luminescent material and the second organic luminescent material absorb light and convert a part of the absorbed light to light of another color. The first organic

luminescent material converts the absorbed light to a first light emission which comprises a substantial amount of energy in the red spectral range. The second organic luminescent material converts the absorbed light to a second light emission which comprises a

substantially amount of energy in the yellow spectral range. The luminescent element 108 comprises amounts of the first organic luminescent material and the second organic luminescent material such that, if a portion of light emitted by the light source 106 is absorbed and is converted to a combination of the first light emission and the second light emission, the total light emission of the remaining light of the light source 106 and the combination of the respective light emissions is still a light emission with a color point close to or on the black body line. If the amount of red and/or yellow increases in the total light emission, the correlated color temperature becomes lower. The luminescent element 108 may be moved, and by moving the luminescent element 108 to the left more red and yellow light are generated and thus the correlated color temperature of the total light emission of the color temperature tunable lighting device drops.

Fig. lb schematically presents a cross-section of the color temperature tunable lighting device 100 of Fig. la. Light rays 156 schematically represent the white light emission of the light source 106. Dashed light ray 152 schematically represents the first light emission which is obtained from the first organic luminescent material. Dotted light ray 154 schematically represents the second light emission which is obtained from the second organic luminescent material. It is to be noted that, although the arrows representing the different light emissions are drawn as arrows of the same length, the total amount of energy in the different light emissions may vary between the different light emissions.

Fig. 2a schematically shows a cross-section of another embodiment of a color temperature tunable lighting device 200. The color temperature tunable lighting device 200 comprises a housing 202 in which a light emitter 214 with a further luminescent element 216 are provided. The light emitter 214 emits at least light in a blue spectral range, and the further luminescent element 216 comprises a specific luminescent material which absorbs a part of the blue light emitted by the light emitter 214 and converts a part of the absorbed light to light of another color such that the combination of the remaining blue light and the light of another color has a color point in a specific color space close to or on the black body line. The specific luminescent material is, for example, Ce doped YAG and, thus, the light of the another color is yellow light. Light 204 is the combination of the remaining blue light and the light of the another color. Light 204 is experienced by the human naked eye as white light. A correlated color temperature of the light 204 is relatively high, for example, larger than 4500 Kelvin, or, alternatively, larger than 5500 Kelvin. The housing 202 further comprises a luminescent element 210 which has in the cross-sectional view a wedge shape. Thus, a thickness of the luminescent element 210 increases in a specific direction which is in the drawn cross-section a direction from the left to the right. The luminescent element 210 is provided in front of a light exit window of the color temperature tunable lighting device 200. Further, the luminescent element 210 is movable in the specific direction, which means, that, within the drawn cross-section, the luminescent element 210 may be moved to the left and to the right.

The luminescent element 210 comprises a mix of two different organic phosphors. A first organic phosphor of the mix absorbs light emitted by the light emitter 214, the further luminescent element 216 and/or a second organic phosphor of the mix and converts at least a part of the absorbed light towards light of a first light emission which is at least in the red spectral range. The second organic phosphor of the mix absorbs light emitted by the light emitter 214, the further luminescent element 216 and/or the first organic phosphor of the mix and converts at least a part of the absorbed light towards light of a second light emission which is at least in the yellow spectral range. Light 206, which is indicated by the dashed arrow, schematically indicates the light of the first light emission. Light 208, which is indicated by the dotted arrow, schematically indicates the light of the second light emission.

If the luminescent element 210 is moved to the left, the average thickness of a portion of the luminescent element which is in front of the light exit window increases, and thus, more of the first organic phosphor and more of the second organic phosphor is on the light transmitting path of the light 204 which is emitted by the light emitter 214 and the further luminescent element 216. Thus, if the luminescent element 210 is moved to the left, more light 204 is converted to light in the red and yellow spectral range, which results in a shift of the color point of the light that is emitted by the color temperature tunable lighting device 200. The amount of the first organic phosphor and the second organic phosphor in the luminescent element 210 is configured such that the shift of the color point is along the black body line. Further, the correlated color temperature of the emitted light by the color temperature tunable lighting device 200 decreases. If the luminescent element 210 is moved to the right, the opposite effect is obtained. A movement to the right results in less conversion of light towards the first and second light emission, and, thus, to a light emission which has a color point relatively close to the color point of the light emitted by the light emitter 214 and the further luminescent element 216.

Fig. 2b schematically shows a cross-section of an alternative embodiment of the color temperature tunable lighting device 250. The color temperature tunable lighting device 250 comprises a housing 202 in which a light emitter 214 is provided. The light emitter 214 is, for example, a blue light emitting Light Emitting Diode (LED). Light 256 schematically represents blue light and is indicated with a solid arrow. The housing has a light exit window which is covered with the further luminescent element 216. The further luminescent element 216 has been discussed in the context of Fig. 2a. The light emission 258 is the light which is emitted by the further luminescent element 216 as the result of absorbing a part of the light which impinges on the further luminescent element 216. The part of the impinging light that is not absorbed by the further luminescent element 216 is transmitted through the luminescent element 216.

The color temperature tunable lighting device 250 comprises inside the housing 202 another luminescent element which comprises a first layer 262 and a second layer 260. In a cross-sectional view, the another luminescent element is wedge shaped, which means that in a direction from the left to the right (in the drawn cross-sectional view) the thickness of the another luminescent element increases. As shown in Fig. 2b, the another luminescent element is subdivided in two layers. In Fig. 2b, half of the thickness of the another luminescent element is the first layer 262 and the other half of the thickness of the another luminescent element is the second layer 260. It is to be noted that embodiments of the invention are not limited to the subdivision of the another luminescent element in two layers as shown. In other embodiments, one layer has a constant thickness while the other layer has an increasing thickness, or the subdivision of the thickness of the another luminescent element has another ratio, for example, the first layer is one third of the thickness and the second layer two third. The first layer 262 is arranged at a position where it receives blue light 256 from the light emitter 214. The first layer 262 comprises a first organic luminescent material which absorbs a part of the blue light 256 and converts the absorbed light to yellow light 252. Blue light 256 which is not absorbed by the first organic luminescent material is transmitted through the first layer 262. The second layer 260 is arranged at a position in between the first layer 262 and the further luminescent element 216 such that it receives at least the yellow light 252 emitted by the first organic luminescent material. The second layer 260 comprises a second organic luminescent material which absorbs a part of the received yellow light 252 and/or the received blue light 256. The absorbed light is converted by the second organic luminescent material to red light 254. Light, which impinges on and is not absorbed by the second layer 260, is transmitted through the second layer. As such, as shown in Fig. 2b, the light emission from the another luminescent element towards the further luminescent element 216 comprises blue light 256, yellow light 252 and red light 254. It is to be noted that the amount of yellow light is probably relatively low compared to the amount of red and blue light. More yellow light is generated by the further luminescent element 216. Further, as shown in Fig. 2b, the total light emission of the color temperature tunable lighting device 250 comprises blue light 256, yellow light 252 as generated by the first organic luminescent material, red light 254 as generated by the second organic luminescent material, and yellow / orange light which is emitted by the luminescent material of the further luminescent element 216.

Fig. 3a schematically shows a top-view of a color temperature tunable lighting device 300 which comprises a rotatable luminescent element. Fig. 3b schematically shows a cross-section of the color temperature tunable lighting device 300 of Fig. 3a along line A- A'. The color temperature tunable lighting device 300 comprises a housing 306 which comprises three light sources 302 which each emit, in operation, white light which has a color point in a color space close to or on the black body line, and which has a relatively high correlated color temperature. The housing 306 further comprises a rotatable luminescent element 304. The rotatable luminescent element 304 is arranged at a position such that is may be arranged in between the light sources 302 and a light exit window 308 of the housing 306. The rotatable luminescent element 304 comprises a mix of a first organic phosphor and a second organic phosphor. The first organic phosphor is, for example, a perylene derivative which emits yellow light as the result of the absorption of blue light. Examples of such perylene derivatives are Lumogen F083 or Lumogen F170 which are sold by BASF. The second organic phosphor is, for example, a perylene derivative which emits red light as the result of the absorption of blue and/or yellow light. An example of such a perylene derivative is Lumogen F305 which is sold by BASF. The luminescent element 304 is rotatable around an axis 308 and by rotating the luminescent element 304 in a specific position, the amount of white light emitted by the light sources 302 which impinges on the luminescent element 304 may be controlled. The more light impinges on the luminescent element 304, the more light is converted to red and yellow light and as such the color point of the light emitted by the color temperature tunable lighting device 300 shifts. The luminescent element 304 comprises specific amounts of the organic phosphors such that the color point of the light emitted by the correlated color temperature tunable lighting device 300 moves along the black body line. Thus, by controlling the position of the luminescent element 304 relatively to the position of the light sources 302, the correlated color temperature of the emitted light may be controlled, in other words, the correlated color temperature is tuned. By converting more white light to red and/or yellow light, warmer white light of a lower correlated color temperature is obtained, and by converting less white light to red and/or yellow light a cooler white light emission is obtained.

Fig. 3c schematically shows a cross-section of another color temperature tunable lighting device 350 which comprises the rotatable luminescent element 304. The only significant difference between the color temperature tunable lighting device 300 of Figs. 3a and 3b and the another color temperature tunable lighting device 350 is that the light sources 302 of color temperature tunable lighting device 300 are replaced by blue light emitting LEDs 352 and a further luminescent element 354 is provided in front of the light exit window 308 of the housing 306. Without the luminescent element 304, the light emission of the combination of the blue light emitting LEDs 352 and the further luminescent element 354 is a white light emission which has a color point close to or on the black body line. The correlated color temperature of the white light is relatively high. Further, the luminescent element 304 may have slightly different amounts of the organic phosphors because the luminescent element does not receive white light, but blue light.

Fig. 4a schematically shows a top-view of an embodiment of a color temperature tunable lighting device 400 which comprises a luminescent element that has an increasing conversion characteristic. Fig. 4b schematically shows a cross-section of the color temperature tunable lighting device 400 of Fig. 4a along line A- A'. The color temperature tunable lighting device 400 is similar to the color temperature tunable lighting device 100 of Fig. la and Fig. lb. However, the color temperature tunable lighting device 400 has another luminescent element 402 which has a shape of a flat box. The another luminescent element 402 comprises two organic luminescent materials in accordance with previously discussed embodiments. However, the concentration of these materials is not constant. In a direction A- A' (which is from the left to the right in Fig. 4b), the concentration of the two organic luminescent materials increases and as such also the conversion characteristic. This is schematically indicated in Fig. 4b by the diagonal line drawn in the another luminescent element. The more material is available in a portion of the another luminescent element 402 which is in front of the light exit window 104, the more white light 156 emitted by the light source 106 is converted towards red light 152 and towards yellow light 154. By moving the another luminescent element 402 to the left (towards A in Fig. 4a), more white light 156 is converted and, consequently, the correlated color temperature drops. It is to be noted that the movement direction is described with reference to the orientation of Fig. 4a - moving to the left results in a coverage of the light exit window 104 with a portion of the another luminescent element 402 that has a higher conversion characteristic. A reverse effect of an increased correlated color temperature is obtained by moving the another luminescent element 402 to the right (towards A' in Fig. 4a).

Fig. 5 schematically shows a chart 500 in which the results of simulations are presented. The simulations determined color points of different color temperature tunable lighting device. The results of the color point simulations are presented in a CIE LUV color space. The x-axis represents the u variable of the color space and the y-axis the v- variable. The black body line is drawn with a dashed line indicated with BBL. The color points on the black body line BBL which have the respective correlated color temperatures (CCT) 6000, 5000, 4000, 3000 and 2700 Kelvin are indicated with a square sign. The light source of the used color temperature tunable lighting device emits white light with a correlated color temperature of about 5750 Kelvin.

For reference, the correlated color temperature of a color temperature tunable lighting device according to the state of the art comprising a red nitride (inorganic) phosphor is drawn with the solid line indicated with BRIO la.

If this inorganic phosphor is replaced by a red emitting organic phosphor based on perylene derivatives (Lumogen F305) it is seen that the color point of the emitted light strongly deviates from the black body line and thus that the emitted light is not white light. The color or the light tends to red at relatively low correlated color temperatures. This is undesirable. The simulations related to the use of the red emitting organic phosphor are indicated with F305 in the chart 500.

For reference, another simulation was performed wherein the inorganic phosphor was replaced by an orange emitting organic phosphor based on perylene derivates (Lumogen F240). Because the color of the light, when using Lumogen F305, is too red, an orange emitting organic phosphor could create a light emission which is closer to the black body line. The simulations are indicated with F240 in the chart 500. In the range of 5750 to 4000 Kelvin correlated color temperature, the color point of the emitted light is close to the black body line BBL. However, at lower correlated color temperatures (below 3500 Kelvin), the color point of the emitted light is not anymore close to the black body line BBL and as such the emitted light is not experienced by the human naked eye as white light. This is undesirable. Further, with reference to Fig. 6, the color rendering index of the light is too low when using the orange emitting organic phosphor.

The simulations of two different embodiments of the color temperature tunable lighting device according to the invention have been included in the chart 500. In both embodiment a white light emitting light source is used which comprises a blue light emitting LED and a further luminescent layer comprising YAG. The correlated color temperature of the light emitted by the light source is about 5750 Kelvin. The luminescent element which comprises two different organic phosphors is movable into the light emission path from the light source to the light exit window of the color temperature tunable lighting device. In a first embodiment the luminescent element comprises a red light emitting organic phosphor based on a specific perylene derivative (Lumogen F305) and a yellow light emitting organic phosphor based on another specific perylene derivative (Lumogen F083). The results of the color simulations of this embodiment are indicated in the chart 500 with F083/F305. It is seen that the color points are located close to or on the black body line BBL. In a second embodiment the luminescent element comprises the red light emitting organic phosphor based on the specific perylene derivative (Lumogen F305) and another yellow light emitting organic phosphor based on a further specific perylene derivative (Lumogen F170). The results of the color simulations of this embodiment are indicated in the chart 500 with F170/F305. It is seen that the color points are located very close to or on the black body line BBL. Thus, the light emitted by the color temperature tunable lighting device of the second embodiment is substantially white and has a well-tunable correlated color temperature.

In. Fig. 5, MacAdam ellipses around specific color points on the black body line BBL are further drawn. The dashed ellipses represent a 5 step MacAdam ellipse and the dotted ellipses represent a 3 step macadam ellipse. Color points within such ellipses have a color which is experienced by a human viewer as a similar color as the color point of the center of the MacAdam ellipse. The line related to the combination of the materials

F083/F305 is going through or is very close the 5 step MacAdam ellipses and the color points on this line have thus a color which will be experienced by most human viewers as a color close to white. The line related to the combination of the material F170/F305 is going through all drawn 3 step MacAdam ellipses and color points on this line have a color which is to a large extent experienced by human viewers as white light.

Assuming that the Lumogen F083 and the Lumogen F305 have the same molar extinction coefficient, the ratio between the F083 and the F305 material was 0.83:1.0 in the simulated color temperature tunable lighting device. In other words, the spectral contribution of F083 compared to F305 was 0.83: 1.0. Assuming that the Lumogen F170 and the Lumogen F305 have the same molar extinction coefficient, the ratio between the F170 and the F305 material was 2:3 in the respective embodiment of the simulated color temperature tunable device. In other words, the ratio between the spectral contributions of F170 and F305 was 2:3.

Fig. 6 schematically shows a chart 600 of color rendering index simulations. The x-axis of the chart 600 represents the correlated color temperature CCT and the y-axis represents the color rendering index CRI. The simulations are done for a subset of the color temperature tunable devices for which simulations were presented in Fig. 5. The first series of color rendering index simulations is for the color temperature tunable lighting device according to the state of the art which comprises a red nitride (inorganic) phosphor. The series of simulations is drawn with the solid line indicated with BRIO la. The second series of color rendering index simulations is for the color temperature tunable lighting device according to the state of the art wherein the red nitride (inorganic) phosphor is replaced by a orange emitting organic phosphor based on a specific perylene derivate (Lumogen F240). The series of simulations is drawn with the dotted line indicated with F240. The third series of color rendering index simulations is for the first embodiment of the color temperature tunable lighting device according to the invention which comprises two organic luminescent materials in the luminescent element which are based on perylene derivatives (Lumogen F083 and F305). The simulations of the third series are indicated with F083/F305 in the chart 600. The fourth series of color rendering index simulations is for the second embodiment of the color temperature tunable lighting device according to the invention which comprises two organic luminescent materials in the luminescent element which are based on perylene derivatives (Lumogen F170 and F305). The simulations of the fourth series are indicated with F170/F305 in the chart 600. It is seen that the color temperature tunable device comprising an orange emitting organic phosphor (F240) has a bad color rendering index at relatively low correlated color temperatures. The color rendering indices of the embodiments (F083/F305 - F170/F305) according to the invention are better or equal to the color rendering index of the color temperature tunable lighting device according to the state of the art (BRIOla). Thus, as concluded in the context of Fig. 5, the embodiments of the color temperature tunable lighting devices according to the invention have an advantageous color temperature tunable behavior wherein the color point of the emitted light is close to or on the black body line, and based on Fig. 6 it may be concluded that also the color rendering index of the emitted light it better than the color rendering index of the known color temperature tunable lighting devices.

Fig. 7 schematically shows a luminaire 700 according to the second aspect of the invention. The luminaire 700 may be used as the lighting device for a desk, or lighting a specific area of a room. The luminaire 700 comprises a color temperature tunable lighting device 702 according to the first aspect of the invention. It is to be noted that the second aspect of the invention is not limited to the presented luminaire of Fig. 7, and that other types of luminaires, like, for example, a ceiling luminaire, may also be embodiments of the luminaire according to the second aspect of the invention. 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 "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. 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.