WO2011158144A1

WO2011158144A1 - Light generating method

Info

Publication number: WO2011158144A1
Application number: PCT/IB2011/052233
Authority: WO
Inventors: Rifat Ata Mustafa Hikmet; Coen Theodorus Hubertus Fransiscus Liedenbaum
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2010-06-18
Filing date: 2011-05-23
Publication date: 2011-12-22
Also published as: TW201213498A

Abstract

The invention relates to a light generating apparatus for generating light. A primary light source (2) emits primary light, and a photoluminescent material (3) converts a part of the primary light into secondary light. Mixed light is generated by mixing non- converted primary light and the secondary light. An absorption cross section of the photoluminescent material is adapted such that, if the peak wavelength of the primary light would vary over a variation wavelength range of at least 20 nm, the color coordinate of the mixed light would vary by less than 10 SDCM. Different primary light sources emitting at different peak wavelengths within the variation wavelength range can therefore be used for generating mixed light having the same or similar color temperature. This leads to reduced requirements for choosing the primary light source to be used for manufacturing the light generating apparatus.

Description

Light generating method

FIELD OF THE INVENTION

The invention relates to a light generating apparatus and a light generating method for generating light. The invention relates further to a photoluminescent material which can be used by the light generating apparatus and a manufacturing method for manufacturing the light generating apparatus.

BACKGROUND OF THE INVENTION

US 7,646,032 B2 discloses a light generating apparatus comprising a light emitting diode emitting primary light which is directed to a shell coated with a phosphor blend. Upon illumination by the primary light, the phosphor blend converts the primary light into white secondary light. This light generating apparatus has the drawback that the manufacturing of the light generating apparatus is very complex and time consuming.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a light generating apparatus, which can be manufactured in a less complex way. It is a further object of the present invention to provide corresponding light generating and manufacturing methods, and a photoluminescent material which can be used for manufacturing the light generating apparatus in a less complex way.

In a first aspect of the present invention a light generating apparatus for generating light is presented, wherein the light generating apparatus comprises:

a primary light source for emitting primary light with an emission spectrum and a peak wavelength,

a photoluminescent material for converting a part of the primary light into secondary light, wherein the primary light source and the photoluminescent material are configured to generate mixed light by mixing a part of the primary light, which has not been converted to secondary light, and the secondary light, and

wherein an absorption cross section of the photoluminescent material is configured such that, if the peak wavelength of the primary light would vary over a variation wavelength range of at least 20 nm, the color coordinate of the mixed light would vary by less than 10 SDCM (Standard Deviation of Color Matching as defined by MacAdam).

In the above mentioned prior art the manufacturing of the light emitting apparatus is very complex and time consuming, because the phosphor blend has narrow band absorption characteristics and the emission spectrum of the light emitting diode must therefore be well matched with the thickness of the layer of the phosphor blend, in order to be able to obtain a desired color temperature. However, during the production of light emitting diodes it is almost impossible to be able to consistently produce light emitting diodes emitting the same emission spectrum with the same peak wavelength. As a result light emitting diodes are obtained, which emit primary light having different emission spectra and different peak wavelengths. From these several light emitting diodes the light emitting diode has to be chosen, which has an emission spectrum matching the narrow band absorption characteristics of the phosphor blend, and the thickness of the phosphor blend has to be adjusted, in order to obtain white light. Since according to the invention the absorption cross section of the photo luminescent material is configured such that, if the peak wavelength of the primary light would vary over a variation wavelength range of at least 20 nm, the color coordinate of the mixed light would vary by less than 10 SDCM, the photo luminescent material has a broad and preferentially flat absorption spectrum, and different primary light sources emitting at different wavelengths, which correspond to the broad and preferentially flat absorption spectrum of the photoluminescent material, can be used for generating secondary light having the same or a similar color temperature. This leads to a reduced requirement for carefully choosing the primary light source which matches the band absorption characteristics of the photoluminescent material and allows therefore

manufacturing the light generating apparatus in a less complex way and with a reduced manufacturing time.

The variation of the peak wavelength can be regarded as a shift of the emission spectrum together with the peak wavelength. This shift can be caused during the manufacturing process for manufacturing the primary light source, which may lead to different primary light sources having different emission spectra, which are shifted with respect to each other.

The absorption cross section which defines the fraction of light absorbed by the photoluminescent material depends preferentially on the integral of the product of the wavelength dependent absorption of the photoluminescent material and the wavelength dependent emission of the primary light source over the wavelength range of the emission spectrum. In particular, the absorption cross section can be defined by following equation:

( Α(λ)Ι(λ)άλ)/ψ(λ)άλ) (1)

x x wherein Α(λ) is the wavelength dependent absorption of the

photoluminescent material and Ι(λ) is the wavelength dependent intensity of the primary light. The integration is preferentially performed over the complete emission spectrum or only over a part of the emission spectrum.

The absorption at a given wavelength can be defined by following equation:

A(X) = \ - (λ)

(2)

(λ) wherein 7₀(λ) denotes the intensity of the primary light intensity before meeting the photoluminescent material and _α(λ) denotes the intensity of the primary light beam which has traversed the photoluminescent material and which has not been absorbed by the photoluminescent material. The expression _α(λ)/ 7₀(λ) can also be referred to as transmission.

It has been found that the combination of a variation wavelength range of 20 nm and a variation of less than 10 SDCM of the color coordinate of the mixed light allows to strongly reduce the complexity of manufacturing the light generating apparatus, without manufacturing light generating apparatuses having significantly deviating colors of the mixed light.

The absorption cross section defines how much of the primary light is absorbed and thus becomes converted to secondary light and how much of the primary light is transmitted. In this way, the absorption cross section determines the composition of the mixed light. The spectrum of the mixed light can then be calculated and positioned in the CIE colour space using known color matching functions. Variations in the absorption cross- section as a result of a shift of the peak wavlength of the primary light lead to changes in the emission spectrum of the mixed light and therefore to changes in the position of the color coordinate of the mixed light in the CIE colour space. This variation can expressed in terms of Standard Deviation of Color Matching (SDCM) as defined by MacAdam or in another way.

For example, in particular in an embodiment in which no total absorption of the primary light occurs, a photo luminescent material is used in combination with a primary blue light source at a absorption cross section of 0.92, i.e. at an emission spectrum of the primary light with a peak wavelength at which the absorption cross section is 0.92, giving rise to mixed white light with a colour temperature of 3000 K. If, for example, the absorption cross section is changed to 0.94 upon a shift in the position of peak wavelength of the primary light a change of 5 SDCM may be observed. Thus, about 2 percent change in the absorption cross section can give rise to a 5 SDCM change and about 4 percent variation in the absorption cross section can give rise to 10 SDCM change. Thus, in an embodiment, the relative variation of the absorption cross section of the photoluminescent material is not more than 4 percent and further preferred not more than 2 percent, if the peak wavelength of the primary light varies over a variation wavelength range of at least 20 nm, wherein a relative variation of the absorption cross section is preferentially defined as the difference between the maximum of the absorption cross section and the minimum of the absorption cross section relative to the maximum of the absorption cross section.

It is preferred that the absorption cross section has a maximum relative variation of not more than 10 percent, further preferred not more than 5 percent, and even further preferred not more than 2 percent, if the peak wavelength of the primary light would vary over a variation wavelength range of at least 20 nm.

The primary light source and the photoluminescent material are preferentially further adapted such that, if the peak wavelength of the primary light would vary over a variation wavelength range of at least 20 nm, the color coordinate of the mixed light would vary by less than 5 SDCM.

It is further preferred that the variation wavelength range is a wavelength range of at least 40 nm and even further preferred of at least 50 nm.

The primary light source is preferentially a light emitting diode or a laser. The primary light source is preferentially configured to emit blue light. Preferably the primary light source emits an emission spectrum having a peak wavelength in the range of 400 to 480 nm or in an ultraviolet wavelength range.

It is further preferred that primary light source is adapted such that the emission spectrum of the primary light has a full width at half maximum of at least 15 nm. It is also preferred that the full width at half maximum is at least 30 nm and even further 50 nm. A light generating apparatus using a primary light source having these bandwidths yields a good color rendering index.

The photoluminescent material is preferentially adapted to provide phosphorescence and/or fluorescence light as secondary light. The photoluminescent material comprises preferentially a phosphor.

The primary light source and the photoluminescent material are preferentially configured such that the mixed light is white light. For example, the primary light source can be adapted to emit blue light and the photoluminescent material can be adapted to emit yellow and/or orange and/or red light such that the mixed light is white.

It is preferred that the photoluminescent material comprises an organic photoluminescent material, in particular, organic phosphor. Organic photoluminescent materials are preferentially made of organic molecules. Organic photoluminescent materials are generally sustainable and relatively low cost materials which can be used in large volumes. The absorption and emission bands of organic luminescent materials can be chosen to be anywhere generally without any restrictions, in particular, such that the absorption cross section of the photoluminescent material is configured such that, if the peak wavelength of the primary light would vary over a variation wavelength range of at least 20 nm, the color coordinate of the mixed light would vary by less than 10 SDCM. This can be achieved, for example, by choosing the absorption and emission bands such that the photoluminescent material has a broad and flat absorption spectrum, wherein the absorption cross section of the photoluminescent material has a relative variation of not more than 10 percent, if the wavelength of the primary light varies over a range of 20 nm.

It is also preferred that the photoluminescent material is adapted such that more than 60 percent of the power of the secondary light has a wavelength below 650 nm. The photoluminescent material can comprise an organic phosphor with an emission band, wherein the integral power of the emission up to 650 nm is preferentially a fraction of the total integral power and wherein this fraction is preferentially larger than 60 percent, more preferred larger than 80 percent, and even more preferred larger than 90 percent. Thus, the light generating apparatus can be adapted such that a large part of the power of the secondary light is in the visible range and a small part is in the infrared range, where the human eye is very insensitive. The photoluminescent material can also be adapted such that the secondary light is not emitted in the infrared range. It is further preferred that the photolummescent material comprises different kinds of photolummescent elements showing a resonant energy transfer between them. The different kinds of photolummescent elements are preferentially different kinds of

photolummescent molecules, in particular, of phosphor molecules. Preferentially, if a photolummescent element of a first kind absorbs a part of the primary light, a part of the received energy is transferred to a photolummescent element of a second kind, wherein the photolummescent elements of both kinds emit secondary light, which is mixed with the primary light. The different kinds of photolummescent elements are preferentially located within the so-called Foerster radius and the emission band of the first kind of

photolummescent elements overlaps at least partly with the absorption band of the second kind of photolummescent elements. In general, a small fraction of a molecule of the second kind with respect to the molecule of the first kind is enough to receive all the energy. In such a system the absorption of the primary light by the second kind of molecules is much lower than the absorption of the primary light by the first kind of molecules. The extinction characteristics and/or the concentration of the molecules of the second kind, which emit secondary light, is preferentially adjusted so that the absorption of the second kind of molecules in the mixture is comparable to that of the first kind of molecules, in order to broaden the absorption band of the photolummescent material comprising the different kinds of the photolummescent elements, i.e. the different kinds of the photolummescent molecules. The different kinds of photolummescent elements and there concentrations are chosen such that the absorption cross section of the photolummescent material, which comprises the different kinds of photolummescent elements, is configured such that, if the peak wavelength of the primary light would vary over a variation wavelength range of at least 20 nm, the color coordinate of the mixed light would vary by less than 10 SDCM. This can be achieved, for example, by choosing different kinds of photolummescent elements and there concentrations such that the photolummescent material has a broad and flat absorption spectrum, wherein the absorption cross section of the photolummescent material has a relative variation of not more than 10 percent, if the wavelength of the primary light varies over a range of 20 nm.

Different kinds of photolummescent elements differ in their characteristics, for example, different kinds of photolummescent elements differ in at least one of the following: absorption band, emission band, structure, process of absorbing and/or emitting light, et cetera.

For example, a first kind of a photolummescent element can be Lumogen yellow 83 and a second kind of a photolummescent element can be anthracene, wherein these two phosphors can be mixed for generating the photolummescent material. The photolummescent material can also be made of another mixture of phosphors like Lumogen yellow 83 with other luminescent molecules such as Lumogen violet 570 and/or

polyfluorene. Lumogen yellow 83 and Lumogen violet 570 can be provided by, for example, the company BASF.

If the photolummescent material comprises Lumogen yellow 83 and anthracene as different kinds of photolummescent elements, the ratio of the weight percentage of Lumogen yellow 83 to the weight percentage of anthracene is preferentially 0.05 or larger, further preferred 0.1 or larger and even further preferred 0.15 or larger. If the photolummescent material comprises Lumogen yellow 83 and Lumogen violet 570 as different kinds of photolummescent elements, the ratio of the weight percentage of Lumogen yellow 83 to the weight percentage of Lumogen violet 570 is preferentially 0.05 or larger, further preferred 0.1 or larger and even further preferred 0.15 or larger. And, if the photolummescent material comprises Lumogen yellow 83 and polyfluerene as the different kinds of photolummescent elements, the ratio of the weight percentage of Lumogen yellow to the weight percentage of polyfluerene is preferentially 0.5 or larger, further preferred 1.5 or larger and even further preferred 2 or larger.

The photolummescent material can comprise two or more different kinds of photolummescent elements and is preferentially a mixture of different kinds of

photolummescent elements providing differently colored secondary light. These different kinds of photolummescent elements can also be regarded as different dyes.

The absorption and emission bands of the different kinds of photolummescent elements and their ratio within the photolummescent material are preferentially chosen such that the absorption spectrum of the overall photolummescent material is broad and flat, in order to produce a photolummescent material being configured such that, if the peak wavelength of the primary light would vary over a variation wavelength range of at least 20 nm, the color coordinate of the mixed light would vary by less than 10 SDCM.

It is further preferred that the different kinds of photolummescent elements are covalently attached to each other. If the different kinds of photolummescent elements are covalently attached to each other, a relatively broad and flat absorption spectrum can be obtained, in particular, by adjusting the relative positions of the absorption bands of the different kinds of photolummescent elements and their extinction coefficients such that, if the peak wavelength of the primary light would vary over a variation wavelength range of at least 20 nm, the color coordinate of the mixed light would vary by less than 10 SDCM. It is further preferred that the photolummescent material comprises a first layer of a first kind of a photolummescent element and a second layer of a second kind of a photolummescent element being located on the first layer. By using the layer structure, a desired absorption of the photolummescent material, in particular, such that, if the peak wavelength of the primary light would vary over a variation wavelength range of at least 20 nm, the color coordinate of the mixed light would vary by less than 10 SDCM, in particular, such that a broad and flat absorption spectrum is achieved, can easily be obtained by choosing the concentration of the molecules in the respective layer and the thickness of the individual layers.

It is further preferred that the primary light source and the photolummescent material are configured such that

a first part of the primary light is directed to the photolummescent material, a second part of the primary light is not directed to the photolummescent material, and

- the secondary light and at least one of a) the second part of the primary light, and b) an amount of the first part of the primary light which has traversed the

photolummescent material without having been transformed into secondary light, are mixed with each other for generating the mixed light. Option a) is preferentially used, if the primary light source and the photolummescent material are adapted such that the first part of the primary light is completely absorbed by the photolummescent material. For example, only parts of a light exit surface of the primary light source can be covered with the

photolummescent material for directing only a first part of the primary light to the photolummescent material.

It is further preferred that the photolummescent material comprises a first layer of a first kind of a photolummescent element and a second layer of a second kind of a photolummescent element being located on the first layer, wherein the primary light source and the photolummescent material are configured such that

a first part of the primary light is directed to the combination of the first layer and of the second layer for generating the secondary light,

- a second part of the primary light is not directed to the combination of the first layer and the second layer, and

the secondary light and the second part of the primary light are mixed with each other for generating the mixed light, wherein the combination of the first layer and the second layer is adapted such that the first part of the primary light is completely absorbed. A desired absorption of each layer can simply be chosen by choosing the concentration of photoluminescent molecules within the individual layers and the thickness of the individual layers accordingly, instead of mixing them together which would lead to a high amount of self absorption.

It is further preferred that the light generating apparatus comprises a region having at least one first area with the photoluminescent material and at least one second area without the photoluminescent material, wherein a first part of the primary light is directed to the at least one first area for generating secondary light and a second part of the primary light is directed to the at least one second area, wherein the secondary light and at least one of a) the second part of the primary light, and b) an amount of the first part of the primary light which has traversed the at least one first area without having been transformed into secondary light, are mixed with each other for generating the mixed light. Option a) is preferentially used, if the primary light source and the photoluminescent material are adapted such that the first part of the primary light is completely absorbed by the photoluminescent material. This configuration can be formed by a layer providing the second area, wherein this layer comprises a distribution of the first areas with the photoluminescent material included in the layer. In an alternative embodiment, one or all of the second areas can comprise

photoluminescent material, which does not totally absorb the primary light directed to these second areas, whereas the at least one first area comprises photoluminescent material which may totally absorb the primary light directed to the at least one first area.

It is further preferred that the photoluminescent material has an absorption band from 400 to 480 nm. In particular, the photoluminescent material comprises different kinds of photoluminescent elements, wherein the combination of the different kinds of photoluminescent elements yields an overall absorption band from 400 to 480 nm. This allows the light generating apparatus to use a primary light source emitting light having a wavelength in the blue range, which can be converted to other colors like yellow and/or orange for generating secondary light, which can be mixed with the blue primary light for obtaining white light. The variation wavelength range is therefore preferentially located within this absorption band. It is further preferred that the absorption band ranges from 410 to 460 nm and it is even further preferred that the absorption band ranges from 420 to 450 nm.

In an embodiment, the photoluminescent material is located on the primary light source. In particular, a layer comprising the photoluminescent material can be provided on a light exit surface of the primary light source. It is further preferred that the photoluminescent material is arranged with a distance to the primary light source. In this so-called remote configuration only a small fraction of light converted by the photoluminescent material gets back to the primary light source, which is preferentially a light emitting diode and which generally has a low reflectivity and gets a large extent of this light. By using the remote configuration the efficiency of the light generating apparatus can therefore be improved.

In a further aspect of the present invention a photoluminescent material for being used by the light generating apparatus is presented, wherein the photoluminescent material is adapted to convert a part of primary light with an emission spectrum and a peak wavelength emitted by a primary light source of the light generating apparatus into secondary light, wherein mixed light is generated by mixing a part of the primary light, which has not been converted to secondary light, and the secondary light, and wherein an absorption cross section of the photoluminescent material is adapted such that, if the peak wavelength of the primary light would vary over a variation wavelength range of at least 20 nm, the color coordinate of the mixed light would vary by less than 10 SDCM.

In a further aspect of the present invention a manufacturing method for manufacturing a light generating apparatus is presented, wherein the manufacturing method comprises:

providing a primary light source for emitting primary light with an emission spectrum and a peak wavelength,

providing a photoluminescent material for converting a part of the primary light into secondary light,

configuring the primary light source and the photoluminescent material to generate mixed light by mixing a part of the primary light, which has not been converted to secondary light, and the secondary light,

wherein an absorption cross section of the photoluminescent material is adapted such that, if the peak wavelength of the primary light would vary over a variation wavelength range of at least 20 nm, the color coordinate of the mixed light would vary by less than 10 SDCM.

In a further aspect of the present invention a light generating method for generating light is presented, wherein the light method comprises:

emitting primary light with an emission spectrum and a peak wavelength by a primary light source, converting a part of the primary light into secondary light by a photoluminescent material,

generating mixed light by mixing a part of the primary light, which has not been converted to secondary light, and the secondary light,

It shall be understood that the light generating apparatus of claim 1, the photoluminescent material of claim 13, the manufacturing method of claim 14 and the light generating method of claim 15 have similar and/or identical preferred embodiments, in particular, as defined in the dependent claims.

It shall be understood that a preferred embodiment of the invention can also be any combination of the dependent claims with the respective independent claim.

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

BRIEF DESCRIPTION OF THE DRAWINGS

In the following drawings:

Fig. 1 shows schematically and exemplarily an embodiment of a light generating apparatus for generating light,

Fig. 2 shows schematically and exemplarily absorption and emission spectra of two kinds of photoluminescent elements,

Fig. 3 shows exemplarily absorption and emission spectra of a combination of the two kinds of photoluminescent elements,

Fig. 4 shows exemplarily an absorption spectrum of Lumogen yellow 83,

Fig. 5 shows exemplarily an absorption spectrum of a combination of

Lumogen yellow 83 and anthracene,

Fig. 6 shows exemplarily an absorption spectrum of a mixture of Lumogen yellow 83 and Lumogen violet 570,

Fig. 7 shows exemplarily an absorption spectrum of a mixture of Lumogen yellow 83 with polyfluorene,

Fig. 8 shows schematically and exemplarily a further embodiment of a light generating apparatus for generating light, Fig. 9 shows schematically and exemplarily a further embodiment of a light generating apparatus for generating light,

Fig. 10 shows a flowchart exemplarily illustrating an embodiment of a light generating method for generating light,

Fig. 11 shows a flowchart exemplarily illustrating an embodiment of a manufacturing method for manufacturing a light generating apparatus,

Fig. 12 shows exemplarily an absorption spectrum of an inorganic YAG:Ce phosphor having narrow band absorption characteristics and emission spectra of different light emitting diodes,

Fig. 13 shows a CIE color space illustrating a significant blue shift of mixed light with decreasing primary light wavelength, and

Fig. 14 shows a CIE color space illustrating only a small shift of mixed light having a white color, if the primary light wavelength is modified and an embodiment of a photo luminescent material of the present invention is used.

DETAILED DESCRIPTION OF EMBODIMENTS

Fig. 1 shows schematically and exemplarily an embodiment of a light generating apparatus for generating light. The light generating apparatus 1 comprises a primary light source 2 for emitting primary light and a photoluminescent material 3 for converting a part of the primary light into secondary light, wherein the primary light source 2 and the photoluminescent material 3 are configured to generate mixed light by mixing the primary light and the secondary light. The absorption cross section of the photoluminescent material is configured such that, if the peak wavelength of the primary light would vary over a variation wavelength range of at least 20 nm, the color coordinate of the mixed light would vary by less than 10 SDCM. In particular, the photoluminescent material is preferentially adapted such that the absorption cross section has a maximum relative varation of not more than 10 percent, if the peak wavelength of the primary light would vary over a variation wavelength range of at least 20 nm. The photoluminescent material has therefore a broad and flat absorption spectrum. In this embodiment, the primary light source 2 comprises three light emitting diodes emitting blue light, preferably in the range of 400 to 480 nm. The light generating apparatus can also comprise less or more than three light emitting diodes.

Moreover, the light emitting diodes can be adapted to generate ultraviolet light, and, in addition or alternatively, the primary light source 2 can comprise a laser for providing the primary light. The light generating apparatus 1 further comprises a reflective mixing chamber 4 for mixing the primary light emitted by the primary light source 2. The primary light source 2 is located within the reflective mixing chamber 4 on a surface being opposite to the photoluminescent material 3. The primary light source 2 and the photo luminescent material 3 are arranged with a distance with respect to each other.

The photoluminescent material 3 is preferentially adapted to provide phosphorescence and/or fluorescence light as secondary light. In this embodiment, the photoluminescent material 3 comprises organic phosphor having a broad emission band for providing phosphorescence light as the secondary light.

The photoluminescent material 3 comprises different kinds of photoluminescent elements showing a resonant energy transfer between them. The different kinds of photoluminescent elements are different kinds of phosphor molecules, in particular, different kinds of organic phosphor molecules. Preferentially, if a photoluminescent element of a first kind absorbs a part of the primary light, a part of the received energy is transferred to a photoluminescent element of a second kind, wherein the photo luminescenet elements of both kinds emit secondary light which is mixed with the primary light. The different kinds of photoluminescent elements are located within the so-called Foerster radius and the emission band of the first kind of the photoluminescent element overlaps at least partly with the absorption band of the second kind of the photoluminescent element. This is schematically and exemplarily illustrated in Figs. 2 and 3.

Figs. 2 and 3 show absorption values a and emission values e in arbitrary units depending on the wavelength λ also in arbitrary units. In Fig. 2, an absorption spectrum 20 and an emission spectrum 22 of a first kind of a photoluminescent element, in this embodiment, of a first organic phosphor, and an absorption spectrum 21 and an emission spectrum 23 of a second kind of a photoluminescent element, in this embodiment, of a second organic phosphor, are shown. As it can be seen in Fig. 2, the emission spectrum 22 of the first kind of a photoluminescent element overlaps with the absorption spectrum 21 of the second kind of a photoluminescent element. Fig. 3 shows exemplarily and schematically an absorption spectrum 24 of the photoluminescent material 3 comprising a combination of the first kind of photoluminescent elements and the second kind of photoluminescent elements, and the emission spectrum 25 of the photoluminescent material 3. As it can be seen in Fig. 3, the absorption spectrum 24 of the photoluminescent material 3 has been broadened and is almost flat within a wide wavelength range. Preferentially, the absorption spectrum 24 is almost flat over a wavelength range of 400 to 480 nm. By adjusting the position of the bands and the ratio of the different kinds of photoluminescent elements, in particular, of the weighted average, a flat top excitation spectrum as schematically and exemplarily shown in Fig. 3 with almost no change in the emission spectrum 25 is obtained.

A first kind of a photoluminescent element, in this embodiment, of a phosphor, is for example Lumogen yellow 83 and a second kind of a photoluminescent element can be a second phosphor being anthracene, wherein these two phosphors can be mixed for generating the photoluminescent material. The photoluminescent material can also be made of another mixture of different kinds of photoluminescent elements, in particular, of different phosphors like Lumogen yellow 83 with other photoluminescent molecules such as Lumogen violet 570 and/or polyf uorene. The different kinds of photoluminescent elements can have different Stokes shifts. For example, a first kind of a photoluminescent element can have a larger Stokes shift and a second kind of a photoluminescent material can have a smaller Stokes shift. In an embodiment, Lumogen yellow 83 having a smaller Stokes shift is mixed with Alq3 having a larger Stokes shift. The absorption spectra of the first kind of

photoluminescent elements and of the second kind of photoluminescent elements are located at different wavelength positions. However, the wavelength positions of the emission spectra of both kinds of photoluminescent elements are preferentially almost the same. A Stokes shift is preferentially defined as the difference in wavelength between positions of the band maxima of the absorption and emission spectra of the same electronic transition.

Fig. 4 shows exemplarily an absorption spectrum of Lumogen yellow 83. If

Lumogen yellow 83 is mixed with anthracene, the absorption spectrum of the resulting photoluminescent material can be broadened and be relatively flat, in particular, in a range of about 400 to 480 nm as exemplarily shown in Fig. 5.

Fig. 5 shows an absorption spectrum of a photoluminescent material which is produced by dissolving Lumogen yellow 83 and anthracene together with

polymethylmethacrylate (PMMA) in dichloromethane. After the solution has been coated on a glass plate, the solution is dried. In this embodiment, the resulting layer of PMMA has a thickness of 25 micron and contains 0.13 weight percentage anthracene and 0.013 weight percentage Lumogen yellow 83.

In a further embodiment Lumogen yellow 83 and Lumogen violet 570 are dissolved together with PMMA in dichloromethane. After the solution has been coated onto a glass plate, the solution is dried for obtaining a layer of PMMA, which, in this embodiment, has a thickness of 25 micron and contains 0.46 weight percentage of Lumogen violet 570 and 0.036 weight percentage of Lumogen yellow 83. A broad and relatively flat absorption spectrum of this photoluminscent material is shown in Fig. 6.

In a further embodiment, Lumogen yellow 83 and polyfluerene are dissolved together with PMMA in dichloro methane, and the resulting solution is dried on a glass plate, after the glass plate has been coated by the solution, for obtaining a PMMA layer. In this embodiment, the PMMA layer has a thickness of 25 micron and contains 0.5 weight percentage of Lumogen yellow 83 and 0.33 weight percentage of polyfluerene. The broad and relatively flat absorption spectrum of this photolummescent material is shown in Fig. 7.

The absorption and emission bands of the different kinds of photolummescent elements and their ratio within the photolummescent material are chosen such that the absorption spectrum of the overall photolummescent material is broad and flat, in particular, in a blue range of 400 to 480 nm and further preferred in a blue range of 420 to 450 nm.

The different kinds of photolummescent elements are mixed with each other for forming the photolummescent material, with or without being covalently attached to each other. If the different kinds of photolummescent elements are mixed with each other without being covalently attached to each other, a broad and flat absorption spectrum, which can also be regarded as an excitation spectrum, can be obtained by adjusting the position of the absorption bands of the different kinds of photolummescent elements and the relative fraction of the different kinds of photolummescent elements within the photolummescent material accordingly. Since the different kinds of photolummescent elements preferentially provide differently colored secondary light, these different kinds of photolummescent elements can also be regarded as different dyes.

If the different dyes, i.e. the different kinds of photolummescent elements, are covalently attached to each other, the relative positions of the absorption bands of the different dyes as well as their extinction coefficients can be adjusted, in order to obtain a broad and flat absorption spectrum of the photolummescent material.

Fig. 8 shows schematically and exemplarily an arrangement of photolummescent material, which can be combined with a primary light source (not shown in Fig. 8) for emitting primary light, wherein a part of the primary light is converted into secondary light by the arrangement of photolummescent material. The arrangement of photolummescent material comprises a first layer 105 of a first kind of a photolummescent element, in particular, of a first organic phosphor, and a second layer 106 of a second kind of a photolummescent element, in particular, of a second organic phosphor, being located on the first layer 105. The first layer 105 and the second layer 106 form the photolummescent material 103 which is placed on a substrate 102 being transparent to the primary light. The primary light source and the photo luminescent material 103 are configured such that a first part of the primary light is directed to the photo luminescent material 103 and a second part of the primary light is not directed to the photo luminescent material 103. The secondary light generated by the photo luminescent material 103 and the second part of the primary light are mixed with each other for generating the mixed light. The photoluminescent material 103 with the first and second layers 105, 106 is adapted such that the first part of the primary light generated by the primary light source, which is preferentially a light emitting diode or a laser, is completely absorbed by the photoluminescent material 103. As can be seen in Fig. 8, only parts of a light exit surface 109 of the substrate 102 are covered with the photoluminescent material 103 for directing only a first part of the primary light to the first and second layers 105, 106. If the light exit surface 109 would be completely covered by the photoluminescent material 103, no leakage of the primary light, in particular, no blue leakage if the primary light is blue light, would occur, because in this embodiment the first and second layers 105, 106 totally convert the primary light into secondary light. Total conversion preferentially means that more than 99 percent of the primary light is converted into secondary light. In order to get a leakage of the primary light, in particular, blue leakage, for obtaining white light by mixing the primary light with the secondary light, the light exit surface 109 is only partially covered by the photoluminescent material 103.

Fig. 9 shows a further embodiment of an arrangement of photoluminescent material, which can be illuminated by primary light of a primary light source (not shown) for converting a part of the primary light into secondary light. The primary light source is preferentially a light emitting diode or a laser for emitting primary light, in particular, for emitting primary blue light. A layer 210 comprising photoluminescent material 203 is provided on a light exit surface 209 of a transparent substrate 202 being transparent to the primary light for converting a part of the primary light into secondary light, wherein the photoluminescent material 203 has a broad and flat absorption spectrum and preferentially comprises at least two different luminescent dyes, wherein one of which completely absorbs the primary light. The primary light source and the photoluminescent material 203 are configured to generate mixed light by mixing the primary light and the secondary light.

The layer 210 coated on the surface 209 of the substrate 202 has at least one first area 207 with the photoluminescent material 203 and at least one second area 208 without the photoluminescent material 203, wherein a first part of the primary light is directed to the at least one first area 207 and a second part of the primary light is directed to the at least one second area 208 for generating secondary light. The secondary light and the second part of the primary light are mixed with each other for generating the mixed light. Also in this embodiment, the photoluminescent material 203 in the first areas 207

preferentially completely absorbs the primary light directed to these first areas. Thus, first areas 207 comprising the totally absorbing photoluminescent material 203 are preferentially distributed within the layer 210 provided on the surface 209 such that a first part of the primary light is converted to secondary light in the first regions 207 and a second part of the primary light leaks through the layer 210 for mixing with secondary light. In this way a blue leakage of primary light can be obtained for generating white light by mixing with the secondary light. Total absorption preferentially means that more than 99 percent of the primary light is converted into secondary light. The layer 210 can be produced, for example, by mixing two non-miscible transparent polymers and providing one of of the polymers with the photoluminescent molecules. Such a system can lead to a formation of a dispersed phase in a continuous phase. In the above example, the dispersed phase would form the first areas 207 containing the photoluminescent material 203 and the continuous phase would form the transparent areas 208. It is also possible to use a material like a polymer in which

luminescent molecules can be disolved. The polymer can then be cut or ground to form small particles and then mixed into another transparant polymer which forms the continuous phase.

The substrates 102, 202 described above with reference to Figs. 8 and 9 can be a part of the primary light source, wherein the surfaces 109, 209 are light exit surfaces, or the substrates 102, 202 can be placed with a distance to the primary light source, wherein the primary light enters the substrate 102, 202 and leaves the substrate through the light exit surface 109, 209 for illuminating the photoluminescent material.

In order to have a maximum variation of the color point of the mixed light smaller than 10 SDCM and, in particular, smaller than 5 SDCM, if the peak wavelength of the primary light would vary over the variation wavelength range of at least 20 nm, in the above described embodiments in which the photoluminescent material totally absorbs the primary light, the absorption cross section of the the photoluminescent material varies preferentially between 0.99 and less then 1, if the peak wavelength of the primary light would vary over a variation wavelength range of at least 20 nm. Thus, in these embodiments, the maximal relative variation of the absorption cross section is preferentially smaller than 1 percent, if the peak wavelength of the primary light would vary over the variation wavelength range of at least 20 nm. Preferentially, the absorption spectrum and the emission spectrum of the photoluminescent material do not or do almost not overlap, in order to avoid a high degree of self absorption, which could lead to larger changes in the color point. Preferentially, the photoluminescent material is adapted such that the absorption cross section would vary over a range of 0.99 to 0.999, if the peak wavelength of the primary light would vary over a variation wavelength range of at least 20 nm.

In the following an embodiment of a light generating method for generating light will exemplarily be described with reference to a flowchart shown in Fig. 10.

In step 301, the primary light source emits primary light, and in step 302 a part of the primary light is converted into secondary light by the photoluminescent material, wherein the photoluminescent material has an absorption cross section being adapted such that, if the peak wavelength of the primary light would vary over a variation wavelength range of at least 20 nm, the color coordinate of the mixed light would vary by less than 10 SDCM. This absorption cross section is preferentially achieved by providing a

photoluminescent material having a broad and flat absorption spectrum such that the absorption spectrum has a maximum relative variation of not more than 10 percent, if the peak wavelength of the primary light would vary over a variation wavelength range of at least 20 nm. In step 303, mixed light is generated by mixing the primary light and the secondary light.

In the following an embodiment of a manufacturing method for manufacturing a light generating apparatus will exemplarily be described with reference to a flowchart shown in Fig. 11.

In step 401, the primary light source for emitting primary light is provided, and, in step 402, the photoluminescent material for converting a part of the primary light into secondary light is provided, wherein the photoluminescent material has an absorption cross section being adapted such that, if the peak wavelength of the primary light would vary over a variation wavelength range of at least 20 nm, the color coordinate of the mixed light would vary by less than 10 SDCM. This absorption cross section is preferentially achieved by providing a photoluminescent material having a broad and flat absorption spectrum such that the absorption spectrum has a maximum relative variation of not more than 10 percent, if the peak wavelength of the primary light would vary over a variation wavelength range of at least 20 nm. In step 403, the primary light source and the photoluminescent material are configured to generate mixed light by mixing the primary light and the secondary light. In particular, the thickness of the photoluminescent material along the light path through the photoluminescent material is adjusted such that the mixed light has a desired color temperature, in particular, such that the generated mixed light is white light. Moreover, the step of providing the photo luminescent material (step 102) can comprise generating a mixture of different kinds of photoluminescent elements, wherein absorption bands and emission bands of the photoluminescent elements, their ratio within the mixture, and optionally the extinction coefficients are adapted such that the generated mixed light has a desired color temperature. For example, phosphors having absorption and emission bands needed for generating the desired color temperature and optionally having certain extinction coefficients needed for obtaining the desired color temperature can be chosen and used for preparing the mixture of different kinds of photoluminescent elements.

In contrast to the photoluminescent material having the broad and flat absorption spectrum, in particular, in the blue wavelength range, inorganic phosphors like Ce doped LuAG and YAG or a single organic phosphor like Lumogen yellow 83 have only narrow band absorption characteristics.

Fig. 12 shows schematically the absorption band 30 of a single YAG:Ce phosphor together with three different emission spectra 31, 32, 33 of different light emitting diodes. As can be seen in Fig. 12, the absorption of the primary light of the primary light source, which is in this example a light emitting diode, changes, if the wavelength of the emitted primary light changes. This absorption change leads to a change in the conversion of the primary light into the secondary light and, thus, in a change of the color temperature of the finally generated mixed light. The color temperature of the finally generated light depends therefore strongly on the position of the absorption cross-section, if a phosphor with narrow band absorption characteristics would be used like the single Lumogen yellow 83 phosphor. This dependence of the color temperature of the finally generated mixed light on the wavelength of the primary light is exemplarily shown in a CIE color space in Fig. 13. As can be seen in this figure, if the wavelength of the primary light changes from 420 nm to 450 nm the color temperature changes from about 20000 K to 4000 K. The color temperature is therefore shifted to lower temperatures, if the primary light is changed to larger wavelengths. A decreasing primary light wavelength leads therefore to a strong blue shift of the generated mixed light. The same behaviour would also be observed, if a single component Lumogen yellow 83 is used.

However, if a mixture of Lumogen yellow 83 with anthracene is used as photoluminescent material, the color temperature of the generated mixed light is substantially unchanged, if the wavelength of the primary light is modified from 420 nm to 450 nm, because of the flat absorption spectrum of this mixture over this wavelength range. This is illustrated in the CIE color space shown in Fig. 14. The ratio of Lumogen yellow 83 to anthracene is preferentially 5 percent, further preferred 10 percent, and even further preferred 15 percent.

A variation of the color coordinate of the mixed light being smaller than 10 SDCM, if the peak wavelength of the primary light would vary over a variation wavelength range of at least 20 nm, preferentially means that, if the peak wavelength of the primary light would vary over a variation wavelength range having a width of 20 nm or if the peak wavelength of the primary light would vary over a variation wavelength range having a width of more than 20 nm, the color coordinate of the mixed light would vary by less than 10 SDCM.

If the photoluminescent material does not have a broad and flat absorption band, generally the emission spectrum of the primary light source must be well matched with the thickness of the layer of the photoluminescent material, in order to be able to obtain a desired color point. However, during the production of the primary light source, in particular, of the light emitting diode, it is almost impossible to be able to consistently produce the same wavelength emitting primary light sources. As a result, a distribution of primary light sources emitting in, preferentially, the blue range is obtained. This means that the primary light sources must be bound very carefully and the thickness of the photoluminescent material has to be adjusted so that the color temperature of the finally generated light remains constant. By using organic phosphors with broad emission band characteristics, wherein the absorption spectrum, i.e. the excitation spectrum, is relatively constant, it can be possible to produce a light generating apparatus providing white mixed light with color point consistency without the need for binning the primary light sources, in particular, the blue light emitting diodes. In particular, by providing a photoluminescent material having a broad and flat absorption spectrum the binning with respect to the wavelength variation, in particular, with respect to the blue light emitting diode wavelength variation, is preferentially not an issue anymore, which can result in a dramatically increased yield in producing the light generating apparatus and hence significant reduction in costs.

The photoluminescent material, in particular, the organic dyes, can be used in remote and proximity arrangements with respect to the primary light source. For example, the photoluminescent material shown in Figs. 8 and 9 can be located on a surface of the primary light source or these photoluminescent materials can be arranged with a distance to the primary light source as, for example, shown in Fig. 1. Thus, for example, the

photoluminescent materials shown in Figs. 8 and 9 can be used by the light generating apparatus described above with reference to Fig. 1. Other arrangements of the photoluminescent material and the primary light source are possible, as long as the primary light source and the photoluminescent material are arranged such that the light path of the primary light traverses at least a part of the photoluminescent material, wherein other inorganic or organic phosphor layers or compounds can be present in the light path of the primary light and/or of the secondary light for further converting the primary light and/or the secondary light. For example, tertiary light can be generated from the primary light and/or the secondary light and optionally further orders of light can be generated by the

photoluminescent material, wherein at least two of these different orders of light are mixed for generating the mixed light having preferentially a white color, and wherein preferentially a mixture of at least two different kinds of photoluminescent elements, in particular, of two different phosphors, comprises a broad and flat absorption spectrum preferably in the blue wavelength range.

In the above mentioned embodiments, measures are described to produce a photoluminescent material with a desired absorption cross section variation. In these embodiments the absorption cross section is adapted such that, if the peak wavelength of the primary light would vary over a variation wavelength range of at least 20 nm, the color coordinate of the mixed light would vary by less than 10 SDCM. Also other measures and corresponding photoluminescent materials can be used, as long as the absorption cross section is adapted such that, if the peak wavelength of the primary light would vary over a variation wavelength range of at least 20 nm, the color coordinate of the mixed light would vary by less than 10 SDCM.

Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims.

In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality.

A single unit or device may fulfill the functions of several items recited in the claims. 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.

Any reference signs in the claims should not be construed as limiting the scope.

The invention relates to a light generating apparatus for generating light. A primary light source emits primary light, and a photoluminescent material converts a part of the primary light into secondary light. Mixed light is generated by mixing non-converted primary light and the secondary light. An absorption cross section of the photoluminescent material is adapted such that, if the peak wavelength of the primary light would vary over a variation wavelength range of at least 20 nm, the color coordinate of the mixed light would vary by less than 10 SDCM. Different primary light sources emitting at different peak wavelengths within the variation wavelength range can therefore be used for generating mixed light having the same or similar color temperature. This leads to reduced requirements for choosing the primary light source to be used for manufacturing the light generating apparatus.

Claims

CLAIMS:

1. A light generating apparatus for generating light, wherein the light generating apparatus comprises:

a primary light source (2) for emitting primary light with an emission spectrum and a peak wavelength,

a photoluminescent material (3) for converting a part of the primary light into secondary light, wherein the primary light source (2) and the photoluminescent material (3) are configured to generate mixed light by mixing a part of the primary light, which has not been converted to secondary light, and the secondary light, and

wherein an absorption cross section of the photoluminescent material (3) is adapted such that, if the peak wavelength of the primary light would vary over a variation wavelength range of at least 20 nm, the color coordinate of the mixed light would vary by less than 10 SDCM.

2. The light generating apparatus as defined in claim 1, wherein the absorption cross section has a maximum relative variation of not more than 10 percent, if the peak wavelength of the primary light would vary over a variation wavelength range of at least 20 nm.

3. The light generating apparatus as defined in claim 1, wherein the primary light source (2) is adapted such that the emission spectrum of the primary light has a full width at half maximum of at least 15 nm.

4. The light generating apparatus as defined in claim 1, wherein the variation wavelength range is within a range of 400 nm to 480 nm.

5. The light generating apparatus as defined in claim 1, wherein the

photoluminescent material (3) comprises an organic photoluminescent material.

6. The light generating apparatus as defined in claim 1, wherein the photolummescent material (3) is adapted such that more than 60 percent of the power of the secondary light has a wavelength below 650 nm.

7. The light generating apparatus as defined in claim 1, wherein the

photolummescent material comprises different kinds of photolummescent elements showing a resonant energy transfer between them.

8. The light generating apparatus as defined in claim 1, wherein the

photolummescent material comprises a mixture of Lumogen yellow 83 and at least one of the following elements: anthracene, Lumogen violet 570, polyfluerene.

9. The light generating apparatus as defined in claim 1, wherein the

photolummescent material comprises a first layer (105) of a first kind of a photolummescent element and a second layer (106) of a second kind of a photolummescent element being located on the first layer (105).

10. The light generating apparatus as defined in claim 1, wherein the primary light source and the photolummescent material are configured such that

- a first part of the primary light is directed to the photolummescent material, a second part of the primary light is not directed to the photolummescent material, and

the secondary light and at least one of a) the second part of the primary light, and b) an amount of the first part of the primary light which has traversed the

photolummescent material without having been transformed into secondary light, are mixed with each other for generating the mixed light.

11. The light generating apparatus as defined in claim 10, wherein the

photolummescent material comprises a first layer (105) of a first kind of a photolummescent element and a second layer (106) of a second kind of a photolummescent element being located on the first layer (105), wherein the primary light source and the photolummescent material are configured such that

a first part of the primary light is directed to the combination of the first layer (105) and of the second layer (106) for generating the secondary light, a second part of the primary light is not directed to the combination of the first layer (105) and the second layer (106), and

the secondary light and the second part of the primary light are mixed with each other for generating the mixed light,

wherein the combination of the first layer (105) and the second layer (106) are adapted such that the first part of the primary light is completely absorbed.

12. The light generating apparatus as defined in claim 10, wherein the light generating apparatus comprises a region having at least one first area (207) with the photo luminescent material and at least one second area (208) without the photo luminescent material, wherein a first part of the primary light is directed to the at least one first area (207) and a second part of the primary light is directed to the at least one second area (208) for generating secondary light, wherein the secondary light and at least one of a) the second part of the primary light, and b) an amount of the first part of the primary light which has traversed the at least one first area (207) without having been transformed into secondary light, are mixed with each other for generating the mixed light.

13. A photo luminescent material for being used by the light generating apparatus (1) defined in claim 1, wherein the photo luminescent material (3) is adapted to convert a part of primary light with an emission spectrum and a peak wavelength emitted by a primary light source (2) of the light generating apparatus (1) into secondary light, wherein mixed light is generated by mixing a part of the primary light, which has not been converted to secondary light, and the secondary light, and wherein an absorption cross section of the

photoluminescent material (3) is adapted such that, if the peak wavelength of the primary light would vary over a variation wavelength range of at least 20 nm, the color coordinate of the mixed light would vary by less than 10 SDCM.

14. A manufacturing method for manufacturing a light generating apparatus, wherein the manufacturing method comprises:

- providing a primary light source (2) for emitting primary light with an emission spectrum and a peak wavelength,

providing a photoluminescent material (3) for converting a part of the primary light into secondary light, configuring the primary light source (2) and the photoluminescent material (3) to generate mixed light by mixing a part of the primary light, which has not been converted to secondary light, and the secondary light,

15. A light generating method for generating light, the light method comprising:

emitting primary light with an emission spectrum and a peak wavelength by a primary light source (2),

converting a part of the primary light into secondary light by a photoluminescent material (3),