WO2011004019A1

WO2011004019A1 - Lighting apparatus

Info

Publication number: WO2011004019A1
Application number: PCT/EP2010/059928
Authority: WO
Inventors: Frank Trebes
Original assignee: Fricke, Christian
Priority date: 2009-07-09
Filing date: 2010-07-09
Publication date: 2011-01-13
Also published as: GB0911941D0; GB2471836A

Abstract

The invention relates to a lighting apparatus for lighting a room with a light having a predetermined spectrum of wavelengths. The lighting apparatus comprises a plurality of positions to place light-emitting diodes and a plurality of light-emitting diodes. The light-emitting diodes are placed at the positions of the housing. A first subset of the plurality of light-emitting diodes emits light of a first colour and a second subset of the plurality of light-emitting diodes emits light of a second colour. The spectrum of the first colour is closer to the spectrum of the predetermined spectrum than the spectrum of the second colour to the predetermined spectrum. The light-emitting diodes of the first subset are placed such that at outer positions of the apparatus and the light-emitting diodes of the second subset are placed at the inner positions of the apparatus.

Description

Lighting apparatus

This disclosure relates to a lighting apparatus, particularly for rooms, and a method for manufacturing a lighting appara- tus . Artificial light in rooms often leads to an unpleasant impression of the light. Light bulbs and fluorescent tubes emit light that make objects and the skin of humans look pale. It is known that light-emitting diodes may be used to emphasize the colours of objects, e.g. fruits in convenience stores. However, the use of these diodes is restricted to certain colours. Improvements to the spectrum of artificial light are desirable.

It is an object of this disclosure to facilitate provision of a novel, particularly an improved, lighting apparatus. Preferably, a lighting apparatus is provided which emulates natural light, e.g. light at different times of the day, if applicable also taking into account different cloud coverings of the sky.

This object is achieved by the present disclosure, particularly by the subject matter of the independent claims. Advantageous embodiments and refinements are subject matter of dependent claims.

According to an aspect, a method for manufacturing a lighting apparatus comprising light-emitting diodes for emitting mixed colour light is provided, the lighting apparatus comprising:

- specifying a target value for the correlated colour tem- perature of the mixed colour light to be emitted by the lighting apparatus;

- choosing a plurality of light-emitting diodes from a set of light-emitting diodes, wherein the light-emitting diodes are chosen such that

a) the mixed colour light obtainable or obtained from the plurality of light-emitting diodes has a correlated col^¬ our temperature (CCT) which differs from the target value by 150K or less, preferably by IOOK or less;

b) the mixed colour light obtainable or obtained from the plurality of light-emitting diodes has a colour rendering index (CRI) of at least 88, preferably at least 91; and

- arranging the plurality of light-emitting diodes in a hous- ing or holding for the lighting apparatus.

According to a further aspect, a lighting apparatus for emitting mixed colour light is provided, the apparatus compris^¬ ing:

- a housing or holding;

- a plurality of light-emitting diodes arranged within the housing comprising light-emitting diodes which emit light of different colours; and

- the mixed colour light, in particular light which is emit- table from the lighting apparatus, has a colour rendering index of at least 91.

Features described in the following text in connection with different aspects, embodiments etc. may also apply for the other aspects, embodiments etc., even if not explicitly de^¬ scribed herein.

The plurality of light-emitting diodes may be chosen such that the total number of light-emitting diodes in the appara- tus is 21 or less, preferably 17 or less. Alternatively or additionally, the plurality of light-emitting diodes may be chosen such that the total number of light-emitting diodes (LED's) in the apparatus is 11 or more. It has turned out, that for achieving good colour rendering properties with as few LED' s as possible, 13 LED's are particularly advantageous. The apparatus may be an LED module. The mixed colour light generated by the lighting apparatus may be white light or non-white light. Preferably, the mixed colour light has an associated CCT. The CCT of the mixed colour light may be associated with the radiation of a Planck black body emitter emitted at a specific temperature. The mixed colour light may be or the black body emitter may emit radiation with colours from - with increasing CCT or temperature, respectively - deep orange, via yellow and then white, to almost bluish light. An odd number of LED's makes it possible to arrange the LED's in a row symmetrically with respect to a central LED. Particularly, the colours may be arranged symmetrically about the central LED. This arrangement may simplify to achieve that the mixed light of the LED's has a uniform colour im- pression over the extension of the row. Also, the CRI of the mixed colour light may be kept constant over the extension of the lighting apparatus more easily in this arrangement.

The lighting apparatus is preferably configured as an appara- tus for providing indirect light, i.e. light which is reflected by an external reflector, like the wall or ceiling of a room, before the light impinges on an object which should be illuminated. The LED's may be arranged invisibly for a user at all times. The lighting apparatus may provide light having a uniform colour impression without the need of providing additional internal optical elements within the lighting apparatus for mixing the light from the LED's. The LED's may be used as supplied by the manufacturer. According to a further aspect, a lighting apparatus for lighting a room with a light having a predetermined spectrum of wavelengths is provided. The lighting apparatus has hous- ings with a plurality of positions to place light-emitting diodes. The lighting apparatus further has a plurality of light-emitting diodes that are placed in the positions of the housing. A first subset of the plurality of light-emitting diodes emits light of a first colour and a second subset of the plurality of light-emitting diodes emits light of a second colour. The spectrum of the first colour is closer to the predetermined spectrum than the spectrum of the second colour to the predetermined spectrum. The light-emitting diodes of the first subset are placed at the outer positions of the ap- paratus and the light-emitting diodes of the second subset are placed at the inner positions of the apparatus.

Each LED comprises not one wavelength but a spectrum around its dominant colour. One of the possible colours is white be- cause blue LED's with a yellow coating emit white light. The spectrum of an LED to be closest to the predetermined spectrum is understood as having a good overlap between predetermined and dominant spectrum. The holding comprises outer and inner positions. The outer positions are understood as to be the positions having the largest distance to the centre of the holding.

If the spectrum is close, the correlation factor between the spectrums x (λ) and yl (λ) is large. The correlation factor is calculated by the following formula:

whereby λ is the wavelength in nm. The lighting apparatus provides a light that gives an impression of only one colour even though the colour results from mixing at least two colours. This makes it possible to generate light with desired colours without being restricted to the colours that lamp bulbs or single LED types originally provide. However, it is essentially that the outer positions are filled with LED's of a colour being closest to the predetermined spectrum. Otherwise, the light would look non- uniformly with clearly distinguishable colours other than the predetermined spectrum. It is believed that a colour at the outer positions is not completely mixed because, at the edges of the light cone emitted by the lighting apparatus, the light of the outer LED's misses at the outer ends light from other LED's. As discussed above, artificial light often makes objects look pale. One of the reasons is that the spectrum provided by artificial light misses specific wavelengths. E.g. artificial light often misses wavelengths for red light. By contrast, the spectrum of daylight emitted by the sun comprises these wavelengths including red. By combining a plurality of LED's having different wavelengths, LED's emitting red light may be included. Thus, the emitted light of the lighting apparatus comprised red light, which makes e.g. the skin of humans look more natural. Further, if a lighting apparatus is used in convenience stores to light fruit, the same lighting appara- tus may be used to light green lettuce as well as the meat, because the lighting apparatus may include green LEDs as well as red LEDs, which provide the wavelength to emphasize the natural colour of the respective objects.

In an embodiment of the lighting apparatus, the arrangement of the colours of the light emitting diodes is axis- symmetrical. This ensures that the light is equally distributed and mixed in the room.

Preferable the number of light-emitting diodes is odd, also ensuring a uniform distribution of the light.

Providing the positions of the housing in form of a row makes it possible to fix the lighting apparatus on a bar. Such bars are inconspicuous when distributed in rooms.

Alternatively, the positions for light-emitting diodes are placed concentrically. With this measure, the lighting appa- ratus may be fitted in a conventional lamp holder for a bulb lamp .

The distance d between neighboured light-emitting diodes preferably holds the following condition: 5mm < d < 30 mm, preferably d < 20 mm. A shorter distance would lead to insufficient heat dissipation, whereas a distance larger than 30 mm would mean that the light of the light cones do not sufficiently mix. An observer would get the impression that there is plurality of light sources having different colours.

Neighbour is understood as following: Neighbours of one LED are LED's that have the shortest distance to the one LED. An LED may have one neighbour or a plurality of neighbours. The distance is measured from the centre of the light-emitting part of the LED.

If the light cone of each of the light-emitting diodes has an opening angle of 160 and 170 degree, the light is broadly distributed without the need of a further optical apparatus like a lens. In an alternative embodiment, a lens is provided to focus the emitted light of the LED's. In a further embodiment, each light-emitting diode is hold in a holder forming an opening, whereby upper part of the light- emitting diode is 0 mm to lmm lower than opening end of the holder. This ensures that the light cones are completely mixed.

To summarize, eleven to twenty-one LED's are combined by placing them as close as possible together. The LED's produce a light that is well mixed and as homogeneous as possible. The LED module formed by these LED's is arranged in a lamp such that the LED's are not visible. The actual lighting of the room is done via reflections from walls and ceilings. The lamps may be mounted in short distance to an object to be illuminated, because the light source is not visible to the us- er and does not damage the object due to the low power consumption as well as by not emitting ultra-violet resp. infrared light.

In an embodiment, lenses and/or reflectors, preferably dif- fusely reflecting reflectors, like white reflectors, are provided to change the angle of the emitted light cone. The lamps may be realized in form of bars or in radial form as retrofit in existing lamps. The light is used to produce preset scenes in housing spaces in analogy to stage lighting. The atmosphere, the time of day and the location is identifiable by the light. The light fits to the scene and supports the general impression of the location. Different scenes are provided by the choice of modules. By combining different modules, atmospheres may be created. The modules may be adapted for lighting e.g. paintings and statues in museums.

A set of particularly advantageous aspects is described below by making use of numbers to facilitate making references to specific aspects. 1. Lighting apparatus (1) for lighting a room with a light having a predetermined spectrum of wavelengths,

the lighting apparatus (1)

- having a plurality of light-emitting diodes (101 to 111),

- and having a housing (1000) with a plurality of outer and inner positions, in which the light-emitting diodes are placed,

whereby a first subset of the plurality of light- emitting diodes (101, 111) emits light of a first colour (O) and a second subset (103,105,109) of the plurality of light-emitting diodes emits light of a second colour (R) , the spectrum of the first colour (O) being closer to the spectrum of the predetermined spectrum than the spectrum of the second colour (R) to the prede- termined spectrum,

whereby the light-emitting diodes (101, 111) of the first subset are placed at the outer positions of the apparatus and the light-emitting diodes of the second subset are placed at inner positions of the apparatus.

2. Lighting apparatus according to aspect 1,

whereby the arrangement of the colours of the light emitting diodes (101 to 111) is axis-symmetrical.

3. Lighting apparatus according to aspect 1 or 2,

whereby the central LED has the same colour as the outer ones.

4. Lighting apparatus according to aspect 1, 2 or 3,

whereby the number of light-emitting diodes is odd. 5. Lighting apparatus according to one of the aspects 1 to 4,

whereby the positions (101, 111) form a row.

6. Lighting apparatus according to one of the aspects 1 to 5,

whereby the positions are placed concentrically.

7. Lighting apparatus according to one of the aspects 1 to 6, whereby for the distance d between neighboured light- emitting diodes, the following condition holds: 5mm < d < 30 mm.

8. Lighting apparatus according to aspect 7,

whereby the light cone of each of the light-emitting di- odes has an opening angle of 160 and 170 degree.

9. Lighting apparatus according to aspect 8, whereby each LED is placed in a holder whereby upper part of the light-emitting diode is 0 mm to lmm lower than opening end of the holder. This ensures that the light cones are completely mixed.

10. Lighting apparatus according to one of the aspects 1 to

9, whereby the plurality of LED's consists of 11 to 21 LED' s. 11. Lighting apparatus according to one of the aspects 1 to

10, whereby the distance between two spectrums are defined by the correlation factors of the spectrums.

12. Lighting apparatus substantially as described herein

with reference to, and as illustrated in, the accompanying drawings .

13. Method to emulate of natural light, the method comprising the following steps:

- measurement of the spectrum of natural light,

- superimposing a plurality of LED's, the LED's being of differently coloured LED types,

- calculating the correlation factors between the spectrum of the natural light and LED types,

- positioning the LED having the largest correlation factor at the outer position of the LED's in a housing.

Embodiments will now be described with reference to the accompanying drawings. Further features and advantages will be- come apparent from the description in connection with the exemplary embodiments. Figure 1 illustrates a first embodiment of a lighting apparatus .

Figure 2 illustrates a second embodiment of a lighting appa- ratus .

Figure 3 illustrates a third embodiment of a lighting apparatus . Figure 4 illustrates a fourth embodiment of a lighting apparatus .

Figure 5 illustrates a fifth embodiment of a lighting apparatus .

Figure 6 illustrates a sixth embodiment of a lighting apparatus .

Figure 7 illustrates a seventh embodiment of a lighting ap- paratus .

Figure 8 illustrates an eighth embodiment of a lighting apparatus . Figure 9 shows a cross-section through a lighting apparatus.

Figure 10 illustrates the change of brightness during dimming. Figure 11 illustrates spectrums of artificial and natural light.

Figure 12 illustrates spectrum of natural light. Figure 13 illustrates spectrums of LED's.

Figures 14A to 14H show tables to illustrate exemplary em- bodiments for a lighting apparatus.

Figures 15A to 15G contain information about the emission spectrum of a lighting apparatus with a row of LED's in accordance with the associated figure in Figures 14A to 14G.

Figure 16 schematically illustrates the process flow for three methods for choosing LED's for a lighting apparatus.

Figure 1 shows a first embodiment of a lighting apparatus, illustrating a view on the lighting apparatus 1 being

switched on. Eleven light-emitting diodes (LED's) 101 to 111 are arranged in a row. Each of the LED's 101 to 111 is fixed in a holder 2 and emits a light cone, which is indicated by a circle 3 around the holder 2. In the holder 2 of each LED, a character indicates the colour of the emitted light. The first LED 101, the sixth LED 106 and the last LED 111 emit orange light. The second LED 101 and the tenth LED 110 emit yellow light, whereas the third LED 103, the fifth LED 105, the seventh LED 107 and the ninth LED 109 emit red light each. The fourth LED 104 emits white light. The fourth LED 104 is a blue LED having a yellow covering such that the emitted light appears to be white. The LED 101 has one neighbour, as well as the LED 111, the other LED's have two neighbours .

The lighting apparatus emits a light that gives an impression of a sunrise. A light of a natural sunrise is mainly characterized by a wavelength that is close to orange. Thus, the orange LED's are placed at the outer positions of LED row. Thus, an observer cannot differentiate between the colours of the inner LED's. He may not see the red light of LED 103, but a light being a mixture of the red colour with colours of the other LED's 102, 104, 106 and so on. The orange LED's 101 and 111 are placed at the outer positions because an observer would recognize that the colour of the outer LED is not close to the predominant colour. The light of the LED row 1 is preferably reflected by walls and the ceiling of the room. The reflections ensure that the light of the LED's 101 to 111 is mixed several times.

The emulation of natural light is done by the following steps. The spectrum of the natural light is measured. The spectrum x (λ) is the sum of all light intensities x at the respective wavelengths λ.

To emulate the natural spectrum, a spectrum is simulated by superimposing a plurality of LED's, the LED's being of differently coloured LED types. The superimposed spectrum is called predetermined spectrum.

The dominating impression of the colour of the predetermined spectrum is determined by an averaging step and is identified by measuring the spectral profile x (λ) of the predetermined spectrum.

The spectral profiles of the LED's are measured. The LED of a first type has an intensity-wavelength function yl (λ) , the LED of a second type has an intensity-wavelength function y2 (λ) , the LED of a third type has an intensity-wavelength function y3 (λ) . Then correlation coefficients are calculated.

The LED basic module comprises coloured LED's arranged in form of a chain having an odd number of LED's. The outer positions are at both ends of the chain. LED's having the larg- est correlation factors are positioned at the outer positions. At the middle position of the chain preferably the same type of LED is positioned as at the outer positions.

For the inner positions, LED types with decreasing correla- tion factors are chosen. The other types of LED types are ax- is-symmetrically arranged between outer positions.

Figure 2 shows a second embodiment of the lighting apparatus in form of an LED row. The lighting apparatus 1 comprises eleven LED's 201 to 211. The first LED 201, the third LED 203, the sixth LED 206, the ninth LED 209 and the eleventh LED 211 emit blue light, while the second LED 202, the fourth LED 204, the eighth LED 208 and the tenth LED 210 emit green light. The fifth LED 205 emits yellow light and the seventh LED 207 emits red light.

The LED' s having other colours than orange are placed at in- ner positions of the row 1. For a human observer, the light of the inner LED's is mixed with the light of the other

LED' s .

The lighting apparatus 1 emits the light of blue sky. The green and blue LED's are axis-symmetric in relation to a line perpendicular to the row in the middle of LED 206. The skylight is mainly blue. Accordingly, the blue LED's are arranged at the outer positions. The LED's 24 and 25, of which the wavelength is far away from blue, are arranged in the middle of the row.

Figure 3 shows a lighting apparatus according to a third embodiment. The first LED 301, the second LED 302, the fourth LED 304, the sixth LED 306, the eighth LED 308, the tenth 310 and eleventh LED 311 emit white light. The third LED 303 and the ninth LED 309 emit red light, while the fifth LED 305 emit yellow light and the seventh LED 307 emits green light. This light emitted by this lighting apparatus 1 appears to be perfect white light. Again, the outer LED's emit white light, whereby LED's of other colours are placed at inner positions of the row.

Figure 4 illustrates a fourth embodiment of a lighting apparatus. The first LED 401, the third LED 403, the sixth LED 406, the ninth LED 409 and the eleventh LED 411 emit orange light. The second LED 402, the fourth LED 404, the eighth LED 408 and the tenth LED 48 emit white light. The fifth LED 405 and the seventh LED 407 emits green light. The lighting apparatus according to the fourth embodiment emits light having a sand colour. Sand has a brown colour being close to orange. Accordingly, the orange LED's are placed at the outer positions.

Figure 5 illustrates a fifth embodiment of a lighting apparatus. The first LED 501, the sixth LED 506 and the eleventh LED 511 emit white light, while the second LED 502, the fourth LED 504, the eighth LED 508 and the tenth LED 510 emit yellow light. The third LED 503, the fifth LED 505, the seventh LED 507 and the ninth LED 509 emit green light. The light emitted by this lighting apparatus 1 appears to look like Caribbean water over sand.

Figure 6 illustrates a fifth embodiment of a lighting apparatus. The first LED 601, the third LED 503, the fifth LED 505, the seventh LED 507, the ninth LED 509 and the eleventh LED 511 emit white light, while the second LED 502, the fourth LED 504, the sixth LED 506, the eighth LED 508 and the tenth LED 510 emit green light. The light emitted by this lighting apparatus 1 appears to look like Caribbean water.

Figure 7 illustrates a seventh embodiment of a lighting appa- ratus. The first LED 701, the sixth LED 706 and the eleventh LED 711 emit white light, while the second LED 702, the fourth LED 704, the eighth LED 708 and the tenth LED 710 emit green light. The third LED 703, the fifth LED 705, the seventh LED 707 and the ninth LED 710 emit green light. The light emitted by this lighting apparatus 1 appears to look like dark blue sky. Figure 8 illustrates an eighth embodiment of a lighting apparatus. The LED' s of this lighting apparatus are arranged in circles around a white central LED 800. A first circle around the central LED 800 consists of six LED's 810, 811, 812, 813, 814 and 815. LED's with subsequent numbers are neighbours, whereby the LED's 810 and 815 are also neighboured. LED's 810, 812 and 814 are green, whereas LED's 811, 813 and 815 are red. A second circle of LED 820, 821, 822, 823, 824, 825, 826,

827, 829, 8291 and 8292 is arranged around the first circle. Again, LED's with subsequent numbers are neighboured, whereby LED's 829 and 8291 are neighboured as well as the LED's 8292 and 820. The LED's 820, 824 and 828 emit green light, whereas the LED's 821, 823, 825, 827, 829 and 8292 emit yellow light. The LED's 822, 826 and 8291 emit red light.

A third circle, which is the outer circle of the lighting apparatus 1 is arranged surrounding the second circle. The third circle comprises the LED's 830, 831, 832, 833, 834, 835, 836, 837, 838, 839, 840, 841, 842, 843, 844, 845, 846 and 847 which all emit white light. The lighting apparatus 1 emits a perfect white light. Thus, the white LED's are arranged in the outer positions of this apparatus.

The outer positions are those of the LED's 830, 845, 842, 839, 836 and 833 because they have the largest distance to the centre of the lighting apparatus which is the centre of LED 800.

The white LED is the predominant colour of the overall spectrum of the lighting apparatus. Accordingly, white LED's are placed at the outer positions and in the centre. As it will be seen in Figure 11, natural white light also comprises larger intensities of red and green light.

Figure 9 shows a cross-section through a lighting apparatus. There are shown two LED's having a distance d, which is 10 mm in this embodiment. The distance is measured from the centre of the light-emitting part of the first LED 101 to the centre of the light-emitting part of the second LED 102. The LED's are placed in holders 1001 and 1002 that form an opening each. The holders 1001 and 1002 are covered by a reflecting metal to reflect the light from the LED's. The light emitted by the LED's leaves the holder 1001 and 1002 through the opening, whereby the angle of the light cone depends on the characteristics of the LED's and the position of the open- ings. The upper ends of the holders are a length f above the upper end of the LED's. The length f is 1 mm in this case.

Figure 10 illustrates characteristics of 3 LED's having different colours. The colour generated by a string comprising these three LED's may vary with the brightness. In a first embodiment, this is achieved by equalling the current through all LED's, independent of the brightness. The brightness L is drawn in dependence of the current through the LED's. The left LED is orange, the LED in the middle is yellow and the right LED is red. For all LED's, the brightness of the emitted light increases along with increasing current. However, the degree of increase differs. At a first predetermined current II, the brightness of the orange LED is higher than the brightness of the red LED, which is brighter than the yellow LED. Accordingly, a current Il flowing through a string of these LED's gives an impression of a mainly red colour. However, at a second current 12 that is higher than II, the lightness of the orange LED is higher than the lightness of the yellow LED. The lightness of the yellow LED is larger than the lightness of the red LED. Accordingly, the light emitted by a string of these three LED's gives a main impression of yellow.

Figure 11 shows five examples for spectrums of white light, the spectrum being represented by the relative intensity in dependency of the wavelength. The examples are a white colour that looks cool, a light from a Sylvania Gro-Lux™ lamp, a colour generated by a tungsten filament, a natural white colour and the light of a low-pressure sodium lamp. All the spectrums of artificial light differ from the natural white significantly. Especially the wavelengths of green light around 550 nm are missing in the artificial light.

Figure 12 shows further spectrums of natural light. The examples are, on the left hand side, north sky light, noon day- light, noon sunlight, and a combination of sunset sky and sunlight. On the right hand side, spectrums of a blue sky and a red sunset are shown.

Figure 13 shows the spectrums of seven types of LED's. By combining LED's of different types as described above, the intensities of these LED's interpose to generate a spectrum that is close to one of the spectrums of natural light.

With the methods and apparatuses disclosed herein, it is pos- sible to provide lighting apparatuses, in particular LED modules, with very good colour rendering properties, which can be manufactured in a simple manner. Standard LED's with manufacturing tolerances may be used without the need for them to be sorted by the manufacturer, for example with respect to their wavelength or the emitted radiation power, to reduce the variations in the characteristics of the LED's, which may be intrinsically present even in the case of LED's of the same colour. Thus, binning of the LED's is not necessary for manufacturing the apparatus. LED's without a lens may be used. It is preferred that the LED's have similar radiation characteristics. Furthermore, it is, in particular, possible to provide LED modules with only a few LED's, e.g. as few as 13 LED's, with very good colour rendering index (CRI), e.g.

93 or more. Of course, more or less LED's may be used. It has turned out, that with more than 17 and, in particular, more than 21 LED's no significant improvements were possible, whereas the best results were achieved with eleven or more LED's. Furthermore, apparatuses may be provided which emit a mixed colour light with a desired correlated colour temperature (CCT) , which may be matched to the one of a natural light, with a small number of LED's and simultaneously good colour rendering properties.

The lighting apparatus is preferably provided for indirect lighting. Thus, light emerging from the lighting apparatus, when it is mounted in its operating position, impinges at first on a reflector, preferably a diffuse or non-specular reflector, such as a white reflector, like a wall or a ceiling of a room. Thereafter, the light impinges on the object which is to be illuminated.

Figures 14A to 14G show tables to illustrate exemplary em- bodiments for a lighting apparatus comprising an arrangement of, e.g. 13, LED's in a row, the LED's being specified by Dl to D13 in the table and being arranged in that order in the row, e.g. in a housing or holding. The respective table also contains information on the target value of the CCT, on the CRI, on the colour of the respective LED, on the forward voltage V_F of the respective LED in Volts, and on the product code of the respective LED which the manufacturer uses to identify that LED, the manufacturer being OSRAM Opto Semiconductors GmbH in this case. The table in Figure 14H shows more detailed information on the product code.

Each of Figures 14A to 14G has an associated Figure in Fig- ures 15A to 15G, whereby Figure 15A is associated with Figure 14A and so on. Figures 15A to 15G contain the emission spectrum of the lighting apparatus with the row of LED's of the associated Figure in Figures 14A to 14G and information on the LED's which were used, as well as on the achieved CCT and on the achieved CRI.

The very symmetrical arrangement of the LED's ensures that the colour of the light emitted by the lighting apparatus is very homogeneous .

Usually, the outer positions in the row have white LED's. White LED's may be warm white LED's (CCT, for example less than 3800K), neutral white LED's (CCT, for example 3800K to 5900K) or cold white LED's (CCT, for example greater than 5900K) . If the target CCT corresponds to a bluish colour, e.g. for a CCT greater than 10000K, the outer LED's may be chosen to be blue. The number of white LED's may be correspondingly reduced. If the target CCT corresponds to an orange or amber colour, the outer LED's may be amber, like for a target CCT of 1900K, for example.

For choosing the LED's for the lighting apparatus, e.g. from the LEDs specified in Figure 14H, one may proceed as follows: Firstly, the target CCT is specified. Then, the combined emission spectrum of five white LED's (CCT, for example

4500K) is determined, e.g. by measurement or by means of a computer which superimposes the known spectra of the individual LED's. This combined spectrum is superimposed with the spectrum of two red and green LED's. The changes in the combined spectrum, such as in the CCT and the CRI, are recorded. Afterwards, yellow, amber or orange, and blue LED's are added to the combined spectrum with the changes being recorded. The number of LED's is irrelevant at this stage. The combined spectrum is, e.g. by variation of the number of LED's and/or the colour of the LED's, optimized to have a CRI of 92 or more, preferably of 94 or more. Thereupon, the LED's are var- ied in colour and/or number, until the CCT of the combined spectrum is within ±IOOK of the target CCT and the mixed light has a CRI around 93, e.g. from 91 to 96, or of 93±1. Afterwards, the number of LED's is reduced, preferably to 13, - if applicable, the colours of the LED's may be varied - and the LED's are chosen so as to permit a symmetrical arrangement of the colours with respect to the central LED. That is to say, the number of LED's should be odd and preferably only an odd number of LED's of one colour, e.g. of the colour of the designated central LED of the row, should be included in the plurality of light-emitting diodes.

If the desired range of CCT and CRI cannot be reached with neutral white LED's, the process is started all over again with cold white or warm white LED's or a mixture of warm, neutral and/or cold white LED's. If this also does not return a plurality of LED's which permits a symmetric arrangement of the LED's of the same colour and/or white shade with respect to the central LED, two different colours may be included at symmetric positions in the row with respect to the central LED (see the amber and green LED in Figure 14B, for example) . Preferably, one of these two LED's is or both of these two LED' s are the only LED's of those colours in the plurality of LED's. The CRI achievable with this arrangement may be greater than the CRI achievable with any symmetric arrangement of the same number of LED's and having a CCT near the target CCT. The values for the CCT and the CRI mentioned and shown in the figures may be obtainable from the lighting ap- paratus in operation, e.g. if the lighting apparatus is provided with a predetermined amount of electrical power. However, if the lighting apparatus is dimmed during operation, the CCT and/or the CRI of the light emitted by the lighting apparatus may change with decreasing intensity. These quanti- ties often also changes when regular lighting apparatuses such as incandescent bulbs are dimmed, i.e. when intensity is lowered. Therefore, the proposed lighting apparatus may have dimming properties similar to the ones of conventional light sources. This may increase the acceptance by users as the lighting apparatus has familiar dimming properties although LED's and not conventional light sources are used.

It has turned out that all of the CCTs and CRIs in Figures 14A to 14G can also be achieved if only one type of white LED's, e.g. neutral white LED's, is used.

Furthermore, it has turned out that proceeding according to the method described below has proven to be advantageous for choosing the LED's for the lighting apparatus. The amount of time required for selecting the LED's may be kept small in this way. This method may be combined with or altered in accordance with features described further above, in particular with features described in connection with the method above. Figure 16 illustrates on the basis of method steps which may be carried out from top to bottom three methods for choosing LED' s, such as from the set in Figure 14H, to produce a lighting apparatus with high CRI and a CCT matched to the target CCT. The method may be used to choose LED's for the apparatuses as shown in Figures 14A to 14G, for example. Each of the three methods - one is specified in the left column, one in the middle column and one in the right column of Fig- ure 16 - is particularly suitable for a target CCT within the specified CCT range.

For selecting the LED's for a lighting apparatus comprising, e.g. 13, LED's as specified further above it has turned out to be advantageous to proceed as follows. Of course, other numbers of LED's are also possible, but 13 has turned out to have the best balance with respect to costs on the one hand and providing a platform which allows to realize desired target CCTs and high CRIs on the other hand.

It is started with an initial set of six LED's, i.e. just below half of the predetermined number of LED's. The initial set comprises six white LED's, preferably neutral white, if the target CCT is equal to or below 8000K or two blue and four green LED's, if the target CCT is greater than 8000K.

The CCT of the light emitted by the initial set is varied by adding further LED's to form an intermediate set which emits light of a CCT closer to the target CCT. If the target CCT is less than or equal to 4500K or greater than 8000K, two red LED's are added, if the target value is greater than 4500K and less than or equal to 8000K, two or one blue LED's are added. Afterwards, the CRI of the intermediate set is increased/improved by adding LED's to the intermediate set to form a further intermediate set. If the target value is less than 8000K, two green LED's are added. If the target value is greater than 8000K two white LED's are added, preferably neutral white ones. Thus, LED's of a colour are added to the in- termediate set, which colour was not present in the initial set. Finally, the remaining LED's are added to the further intermediate set, until the predetermined number of, e.g. 13 LED's, is reached for the final set of LED's which is used for the lighting apparatus. The LED's are added such that the CRI is optimized. The added LED's may comprise one, two, three, four or five different colours as specified in the last line of Figure 16. The added LED's may be non-white ones. If the target CCT is less than 8000K the added LED's are preferably free of blue LED's. If the target CCT is greater than 8000K the added LED's may comprise one, preferably one and only one blue LED.

Lighting apparatuses produced in accordance with the teachings herein may have various applications, e.g. in a hair sa- Ion. Modern colour pigments for human hair allow creating a multitude of different hair colours. With respect to indoor lighting blonde is especially critical since on the one hand there is a large variety of shades available (at least 30 to 40 different shades of blonde) and on the other hand wrong light, e.g. light of the wrong CCT or poor CRI, makes all colours look identical - most of the times yellowish due to the use of fluorescent tubes out of energy efficiency considerations. Therefore, customers have to leave the shop after or during the colourization process to check whether the de- sired colour shade is reached. Only outside of the salon, in full daylight and without any reflections from coloured buildings, the exact result is visible. Salon owners thrive therefore for a lighting solution which allows checking the result inside and provides the same results as if it were viewed under full natural light. LED modules with moderate CCT (3000 to 3500K) and a CRI greater than 91 were tested for this purpose and produced very convincing results.

Claims

1. A Method for manufacturing a lighting apparatus comprising light-emitting diodes for emitting mixed colour light, the method comprising:

- specifying a target value for the correlated colour tem^¬ perature of the mixed colour light to be emitted by the lighting apparatus;

a) the mixed colour light obtainable or obtained from the plurality of light-emitting diodes has a correlated col^¬ our temperature which differs from the target value by 150K or less, preferably by IOOK or less;

b) the mixed colour light obtainable or obtained from the plurality of light-emitting diodes has a colour rendering index of at least 91; and

- arranging the plurality of light-emitting diodes in a hous- ing for the lighting apparatus.

2. The method of claim 1,

wherein the set of light-emitting diodes comprises light- emitting diodes of different colours.

3. The method of any of the previous claims,

wherein the set of light-emitting diodes comprises white, red, amber, yellow, green and/or blue light-emitting diodes.

4. The method of claim 3,

wherein the white light-emitting diodes in the set comprise warm white, neutral white and/or cold white light-emitting diodes .

5. The method of any of the previous claims,

wherein the light-emitting diodes are chosen such that the white light-emitting diodes in the plurality of light- emitting diodes comprise only warm white, neutral white or cold white light-emitting diodes.

6. The method of any of the previous claims,

wherein the light-emitting diodes are chosen such that the number of light-emitting diodes in the plurality of light- emitting diodes is odd.

7. The method of any of the previous claims,

wherein the light-emitting diodes are chosen such that the number of light-emitting diodes in the plurality of light- emitting diodes is between 11 and 21, preferably between 11 and 17, e.g. 13.

8. The method of any of the previous claims,

wherein the light-emitting diodes are chosen such that the number of light-emitting diodes in the plurality of light- emitting diodes which emit light of a colour which is closest to a colour associated with the target value is even or odd and the number of light-emitting diodes of remaining colours of the plurality of the light-emitting diodes is odd or even.

9. The method of any of the previous claims,

wherein the light-emitting diodes are chosen such that the plurality of light-emitting diodes comprises an odd number of light-emitting diodes of one colour, preferably an odd number of light-emitting diodes of one and only one colour or an odd number of light-emitting diodes of three and only three col- ours, and an even number of light-emitting diodes of the remaining colours.

10. The method of any of the previous claims,

wherein the light-emitting diodes are chosen such that the plurality of light-emitting diodes comprises at least three light-emitting diodes of the same colour.

11. The method of any of the previous claims,

wherein the light-emitting diodes are chosen such that the number of white light-emitting diodes in the plurality of light-emitting diodes is two or more, e.g. three or more, preferably four or more, particularly preferably five or more, e.g. six or more.

12. The method of any of the previous claims,

wherein the light-emitting diodes are chosen such that the number of white light-emitting diodes in the plurality of light-emitting diodes is even or odd.

13. The method of any of the previous claims,

wherein the light-emitting diodes are chosen such that the number of non-white light-emitting diodes in the plurality of light-emitting diodes is odd or even.

14. The method of any of the previous claims,

wherein the light-emitting diodes are chosen such that the non-white light-emitting diodes in the plurality of light- emitting diodes comprise at least light-emitting diodes of three different colours, preferably at least four different colours, particularly preferably at least five different colours .

15. The method of any of the previous claims,

wherein the plurality of light-emitting diodes comprises at least three, preferably at least four, particularly preferably at least five, light-emitting diodes of different col- ours .

16. The method of any of the previous claims,

wherein the light-emitting diodes of the plurality of light- emitting diodes of different colours are arranged symmetri- cally in the housing.

17. The method of any of the previous claims,

wherein the light-emitting diodes of the plurality of light- emitting diodes are arranged in the housing in a row, one of the light-emitting diodes preferably being a central light- emitting diode of the row.

18. The method of claim 17,

wherein light-emitting diodes of different colours are ar- ranged in the row symmetrically with respect to the central light-emitting diode of the row.

19. The method of claim 17 or 18,

wherein light-emitting diodes, preferably all light-emitting diodes except the central light-emitting diode, of the same colour are arranged symmetrically with respect to the central light-emitting diode of the row.

20. The method of any claims 17 to 19,

wherein in two, preferably in two and only two, positions in the row, which positions are disposed symmetrically with respect to the central light-emitting diode of the row, light- emitting diodes of different colours are arranged.

21. The method of any claims 17 to 20,

wherein the outer light-emitting diodes of the row are light- emitting diodes of the same colour.

22. The method of any of claims 17 to 21,

wherein the central light-emitting diode of the row has the same colour as the outer light-emitting diodes.

23. The method of any of claims 17 to 21,

wherein the central light-emitting diode of the row has a colour different from the one of the outer light-emitting diodes of the row.

24. The method of any of the claims 17 to 23,

wherein the first two light-emitting diodes in the row, particularly at both ends of the row, comprise one, preferably one and only one, white light-emitting diode.

25. The method of any of the claims 17 to 24,

wherein white light-emitting diodes are arranged at the outer positions of the row.

26. The method of any of the claims 17 to 24,

wherein non-white light-emitting diodes are arranged at the outer positions of the row.

27. The method of any of the previous claims,

wherein for the distance d between neighbouring light- emitting diodes in the housing, the following condition holds: 5mm < d < 30mm.

28. The method of any of the previous claims,

wherein the target value is from and including 1800K up to and including 100000K, preferably from and including 1900K up to and including 15000K.

29. The method of any of the previous claims,

wherein the colour rendering index is at least 92, preferably at least 93, particularly preferably at least 94, most preferably at least 95, e.g. at least 96.

30. The method of any of the previous claims,

wherein the lighting apparatus has a predetermined number of light-emitting diodes and the plurality of light-emitting diodes is chosen such that the number of light-emitting diodes is equal to the predetermined number.

31. The method of any of the previous claims,

wherein choosing the plurality of light-emitting diodes from the set of light-emitting diodes comprises choosing an ini- tial set of light-emitting diodes, and wherein

- if the target value is equal to or below 8000K, the initial set comprises a plurality of white light-emitting diodes, preferably neutral white light-emitting diodes,

- if the target value is above 8000K, the initial set com- prises a plurality of non-white light-emitting diodes of different colours, preferably a plurality of green light- emitting diodes and a plurality of blue light-emitting diodes, particularly preferably more green light-emitting diodes than blue light-emitting diodes.

32. The method of claims 30 and 31,

wherein the number of light-emitting diodes in the initial set is less than one-half times the predetermined number.

33. The method of claims 30 and 31 or of claim 32,

wherein the number of light-emitting diodes in the initial set is greater than four, preferably greater than five, e.g. six .

34. The method of claim 33,

wherein the number of light-emitting diodes in the initial set is (N-I) /2, where N is the predetermined number.

35. The method of any of claims 31 to 34,

wherein, one or a plurality of additional light-emitting diodes, preferably of the same colour, is added to the initial set to form an intermediate set of light-emitting diodes, the correlated colour temperature of the light obtainable from the intermediate set being closer to the target value than the one of the light obtainable from the initial set, and wherein,

if the target value is above the correlated colour temperature of the light obtainable from the initial set, the additional light-emitting diodes comprise one or a plurality of blue light-emitting diodes, and

if the target value is below or equal to the correlated col- our temperature of the light obtainable from the initial set, the additional light-emitting diodes comprise one or a plurality of red light-emitting diodes.

36. The method of claim 35,

wherein one or a plurality of additional light-emitting diodes, preferably of a colour not included in the initial set, is added to the intermediate set to form a further intermediate set, the additional light-emitting diodes being prefera- bly of the same colour, wherein the additional light-emitting diodes are chosen such that the colour rendering index of the light obtainable from the further immediate set is greater than the one of the light obtainable from the intermediate set .

37. The method of claim 36,

wherein, if the initial set comprises white light-emitting diodes, the additional light-emitting diodes added to the in- termediate set comprise green light-emitting diodes, and, if the initial set is free of white light-emitting diodes, the additional light-emitting diodes added to the intermediate set comprise white light-emitting diodes.

38. The method of claim 36 or 37,

wherein a plurality of, preferably non-white, light-emitting diodes is added to the further intermediate set, e.g. three or more than three light-emitting diodes of equal or different colours, to form a final set of light emitting diodes, wherein the colour rendering index of the final set is greater than the one of the further intermediate set, and wherein a) the mixed colour light obtainable from the final set has a correlated colour temperature which differs from the target value by 150K or less, preferably by IOOK or less, and b) the mixed colour light obtainable from the final set has a colour rendering index of at least 91,

and wherein the plurality of light-emitting diodes of the final set is arranged in the housing to form the lighting apparatus .

39. A lighting apparatus for emitting mixed colour light, comprising :

- a housing; - a plurality of light-emitting diodes arranged within the housing comprising light-emitting diodes which emit light of different colours; and

- the mixed colour light has a colour rendering index of at least 91.

40. The lighting apparatus of claim 39,

wherein the light-emitting diodes are arranged in a row in the housing, the row having a central light-emitting diode.

41. The lighting apparatus of claim 40,

wherein the outer light-emitting diodes of the row are of the same colour.

42. The lighting apparatus of claim 40 or 41,

43. The lighting apparatus of any of claims 40 to 42, wherein in two, preferably in two and only two, positions in the row, which positions are disposed symmetrically with respect to the central light-emitting diode of the row, light- emitting diodes of different colours are arranged.

44. The lighting apparatus of any of claims 40 to 43, wherein the first two light-emitting diodes in the row, particularly at both ends of the row, comprise one, preferably one and only one, white light-emitting diode.

45. The lighting apparatus of any of claims 40 to 44, wherein white light-emitting diodes are arranged at the outer positions of the row.

46. The lighting apparatus of any of claims 40 to 44, wherein non-white light-emitting diodes are arranged at the outer positions of the row.

47. The lighting apparatus of any of claims 39 to 46, wherein the lighting apparatus is a lighting apparatus for indirect lighting.

48. The lighting apparatus of any of claims 39 to 47, wherein the lighting apparatus is manufactured or manufactur- able using the method of any of the previous method claims.