WO2013050918A1

WO2013050918A1 - Artificial daylight source

Info

Publication number: WO2013050918A1
Application number: PCT/IB2012/055247
Authority: WO
Inventors: Maarten Marinus Johannes Wilhelmus Van Herpen; Gabriel-Eugen Onac
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2011-10-05
Filing date: 2012-10-01
Publication date: 2013-04-11

Abstract

A light emitting arrangement (100), comprising: • - a light source (101) comprising a light out-coupling surface, adapted to emit light of a first wavelength range; and • - a wavelength converting member (102) capable of converting light of said wavelength range into light of a second wavelength range, the wavelength converting member (102) being arranged at a distance from the light source (101) and centrally aligned with the light out-coupling surface light source (101), and being arranged to receive and partially convert a central portion of the light emitted by the light source (101) while allowing light emitted by the light source (101) in a peripheral direction to pass beside the wavelength converting member (102). In this way the light emitting arrangement (100) provides light of first color in a normal direction and a light of another non-converted color in a peripheral direction, which may give a more realistic impression of daylight.

Description

Artificial daylight source

FIELD OF THE INVENTION

The invention relates to light emitting arrangements adapted to provide artificial light mimicking natural daylight.

BACKGROUND OF THE INVENTION

Daylight is of utmost importance to most living organisms, including humans. It has been established that daylight affects our biological rhythm, mood, productivity, and physical and mental health.

Daylight is a combination of direct and indirect sunlight, and includes diffuse sky radiation. It is an important light source for lighting of indoor environments, via windows and skylights. Benefits of utilizing daylight include, besides generally improved health and mood, also energy savings, improved working conditions, and improved esthetic appearance. For example, performing near work, such as reading, in daylight requires less

accommodative effort of the eye compared to traditional artificial light. Furthermore, it has been shown that the use of skylights in retail environments is positively correlated with higher sales.

Due to the great benefits of daylight, artificial light sources have been developed which are adapted to provide an impression of daylight. Such artificial daylight sources are today typically based on a fluorescent lamp with a strong diffuser in front, and are mainly focused on providing high and tunable light intensity, tunable color temperature, and slow dynamics (day/night rhythm). However, these light sources provide little or no diffuse blue light, which is an important component of daylight (the blue sky). Hence, there is a need in the art for improved artificial light sources which provide a more realistic impression of daylight.

SUMMARY OF THE INVENTION

It is an object of the present invention to overcome this problem, and to provide a light emitting arrangement that give a more realistic impression of daylight compared to known artificial daylight sources, in particular with respect to the realization of both direct and indirect light.

According to a first aspect of the invention, this and other objects are achieved by a light emitting arrangement, comprising:

a light source comprising a light out-coupling surface, said light source being adapted to emit light of a first wavelength range; and

a wavelength converting member capable of converting light of said wavelength range into light of a second wavelength range, said wavelength converting member being arranged at a distance from the light source, and being arranged to receive and partially convert a central portion of the light emitted by the light source, while allowing light emitted by the light source in a peripheral direction to pass beside the wavelength converting member. In this way said light emitting arrangement provides light of first color in a normal direction and a light of another non-converted color in a peripheral direction, which may give a more realistic impression of daylight.

In embodiments of the invention the wavelength converting member is centrally aligned with the light source, in particular with the light out-coupling surface.

Typically, light received by the wavelength converting member is partially converted and partially transmitted or scattered. Hence, a central portion of light exiting the light emitting arrangement comprises a mixture of light of said first wavelength range and light of said second wavelength range, and a peripheral portion of the light exiting the light source comprises only light of said first wavelength range.

In embodiments of the invention, the wavelength converting member or a wavelength converting region thereof has a width w and is arranged at a distance di from said light source, wherein di and w are selected such that light emitted by the light source from a position on the light out-coupling surface centrally aligned with the wavelength converting body at an angle a of at least 40° to the normal to the light-out-coupling surface of the light source may pass beside said wavelength converting plate, and light emitted at an angle smaller than said angle a may be received by the wavelength converting member. Thus, in embodiments of the invention, light of said first wavelength range emitted by the light source at an angle cc>40° is not incident on the wavelength converting member and is allowed to exit the light emitting arrangement as light of said first wavelength range, and wherein light of said first wavelength range emitted by the light source at angles < 40° is partially transmitted and partially converted by said wavelength converting member. In embodiments of the invention, di, w and a defined above fulfils the following relationships:

in which r₁ = w/2 , and 40° < a < 80°.

In embodiments of the invention, a may be in the range of from 40° to 60°.

In embodiments of the invention, the wavelength converting member may be planar. The wavelength converting member may typically be parallel to the light output surface of the light source.

The wavelength converting member or a wavelength converting region thereof may be circular. In such embodiments, x defined above corresponds to the radius of the circular wavelength converting member or region.

In embodiments of the invention, the light emitting arrangement may comprise a plurality of wavelength converting members. Alternatively, the wavelength converting member may comprise at least one converting region and a plurality of non-converting regions, or a plurality of converting regions and at least one non-converting region. Using a plurality of wavelength converting member or regions allows each member or region to be made small, thus decreasing the visibility of the wavelength converting members/regions, and also making them more easily masked with a diffuser.

In embodiments of the invention, the light emitting arrangement may comprise a plurality of light sources and wavelength converting members/regions, which may be arranged at same distance di, and wherein each light source is associated with a wavelength converting member or a wavelength converting region. Using a plurality of light sources enables a higher luminance, and also enhances the difference in direction between the peripheral, unconverted light and the central, mixed light, thereby providing more distinctly a "blue sky effect".

In embodiments of the invention, the light emitting arrangement may comprise a plurality of light sources or a large area light source, and a first wavelength converting member arranged at a distance di from the light source(s) and a second wavelength converting member arranged at a distance d₃ from the light source (s), and wherein di < d₃. In particular when using such an arrangement with a large area light source, a uniform light output can be obtained also when the wavelength converting regions are small. In embodiments of the invention, the light source may comprise a scattering member forming said light out-coupling surface.

In embodiments of the invention, a diffuser may be arranged in front of the wavelength converting member, to receive and diffuse the mix of converted and non- converted light leaving the wavelength converting member.

Examples of suitable light sources include a light emitting diode, or a large area light source such as an organic light emitting diode (OLED) or a light guide.

It is noted that the invention relates to all possible combinations of features recited in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

This and other aspects of the present invention will now be described in more detail, with reference to the appended drawings showing embodiment(s) of the invention.

Fig. 1 illustrates the basic principles of a light emitting arrangement according to the invention.

Fig. 2 illustrates the principle of using two light sources and two wavelength converting members, according to embodiments of the invention.

Fig. 3 illustrates the principle of using multiple light sources and a wavelength converting member comprising converting and non-converting regions, respectively, according to embodiments of the invention.

Fig.4 illustrates the principles of an embodiment using a large area light source and a dual wavelength converting member.

DETAILED DESCRIPTION

As used herein, "daylight" refers to the combination of direct and indirect sunlight during daytime, between sunrise and sunset. "Direct light" is light that produces sharp shadows, whereas "indirect light" is diffuse and generally less intense, and produces no sharp shadows.

As used herein, "artificial daylight source" refers to an artificial light source which is adapted to mimic the impression of daylight on a sunny day.

Fig. 1 illustrates the basic structure of a light emitting arrangement according to the invention. The light emitting arrangement 100 comprises a light source 101, typically a light emitting diode (LED) optionally arranged in a reflective cavity (not shown) and comprising a light out-coupling surface. The arrangement further comprises a wavelength converting member 102 in the form of a phosphor plate. The wavelength converting member 102 is arranged at a certain distance from the light source 101 in the light emitting direction. Hence, a portion of the light emitted by the light source is received by the wavelength converting member, to be either converted or transmitted (or reflected). Furthermore, a peripheral portion of the light emitted by the light source will pass beside the wavelength converting member without being obstructed by the wavelength converting member. Hence, the light exiting from this arrangement will have a central portion of light which is a mixture of converted light and non-converted (transmitted) light (shown as short dashed lines), and a peripheral portion which comprises only non-converted light (shown as longer dashed lines). In an example a blue LED is used as the light source, and the converting body converts blue light into yellowish light. As a result, when viewed from the front, the light appears white, whereas when viewed at a large angle or from the side the light appears blue. In this way, the light emitting arrangement according to the invention can provide a combination of white direct light (central portion) and indirect blue light (peripheral light), which may give a realistic impression of daylight.

The light source 101 is adapted to emit light of a first wavelength range, typically blue or bluish light, of the wavelength range of from 400 to 490 nm. Examples of suitable light sources include blue emitting LEDs such as GaN or InGaN based LEDs.

Inorganic LEDs may be suitable for use in the present invention also because they are essentially point light sources, having a small light emitting surface area compared to the area of the wavelength converting member.

As used herein, "light out-coupling surface" refers to the surface or area from which light is directed generally from the light source towards a viewer, to be received by or to pass by the wavelength converting member. For example, where the light source is an LED or an OLED, the light out-coupling surface may be the outermost layer of the layer stack, or a protective or optical structure surrounding or in front of said light source. For example, a light out-coupling surface may be represented by the surface of a light guide, or by a diffuser. A light out-coupling surface may also be represented by a reflective substrate on which an LED or OLED is arranged to provide recycling of light that is backscattered towards the substrate. Furthermore, it is envisaged that the bottom of a reflective cavity may represent said light out-coupling surface, for example where an LED is provided in a side wall of such a reflective cavity.

The wavelength converting member 102 is arranged to receive at least a central portion of the light emitted by the wavelength range and is capable of converting at least part of said light into light of a second wavelength range, typically yellow light.

However a part of the light received by the wavelength converting member is typically not converted, but only transmitted and optionally scattered by the wavelength converting member, such that light exiting the wavelength converting member is a mixture of light which may be perceived as white light.

The wavelength converting member is arranged at a certain distance di from the light source. The distance di is chosen with regard to the width of the wavelength converting member such that light emitted by the light source at an angle to the surface normal that is relatively small, is incident on the wavelength converting member, whereas light emitted at wider (larger) angles may pass beside the wavelength converting member. In particular, to provide a suitable balance between direct, partially converted light and unconverted peripheral light, the wavelength converting member, having a width w may be arranged at a distance di from the light source such that only light emitted at an angle a < 60° to the surface normal is incident on the wavelength converting member, and light emitted by the light source at larger angles may pass beside the wavelength converting member. If the angle a is larger than 60°, little peripheral unconverted light is obtained and can only be observed from a large distance, which is undesirable for many applications. However it is envisaged that for some applications, such as artificial daylight lighting of very large rooms or long corridors, a larger than 60°, for example up to 70° or 80°, may be acceptable.

Typically, the angle a is at least 40°.

By changing the position (di) and/or the size (w) of the wavelength converting member, it is possible to adjust the balance between direct light and indirect light. For example, in embodiments of the invention, the angle a may be in the range of 40° to 60°, or from 40° to 50° which is considered to provide a highly desirable combination of direct, partially converted light and peripheral, non-converted light.

Hence, mathematically, the light emitting arrangement fulfils the following relationship:

in which r₁ = w/2, and 40° < a < 80°, for example 40° < a < 60° or 40° < a < 50°. In embodiments of the invention, the wavelength converting member is arranged at distance di from the light source substantially corresponding to its width, i.e. di = w, and thus a may be about 45°.

The wavelength converting member comprises a wavelength converting material and optionally a carrier. For example, the wavelength converting material may be a ceramic phosphor plate, comprising only phosphor material. The wavelength converting member may be a planar plate having any suitable geometrical shape, in particular circular (disc-like) for reasons of symmetry, but also rectangular and other polygonal shapes may be possible. A phosphor plate may be mounted in front of the light source on a transparent carrier, e.g. a transparent plate or sheet for example a glass plate or a plate made of poly(methyl methacrylate) (PMMA) or polycarbonate, such that both peripheral and central light may pass through the carrier. Alternatively, the wavelength converting member may comprise elements of an inorganic phosphor material, for example particles, distributed in a binder and optionally deposited on a carrier. For example, the wavelength converting member may comprise a sheet of a transparent carrier material, e.g. a polymer material, onto which a continuous or discontinuous phosphor layer is deposited, comprising phosphor particles embedded in a binder material. Alternatively, phosphor particles may be embedded in a solid plate of a carrier material (e.g. a polymer material) which plate may be mounted as described above. Alternatively, the wavelength converting material may comprise an organic phosphor that is molecularly dissolved in a carrier material. As will be described in more detail below, such a sheet or plate may comprise wavelength converting regions covered by or comprising the phosphor, and non-converting regions that merely transmit the light without conversion (e.g. transparent regions).

Examples of suitable inorganic phosphors include cerium doped yttrium aluminum garnet (YAG:Ce). Examples of suitable organic phosphor materials include perylene based organic phosphors such as Lumogen^® F Yellow 083 and/or Lumogen^® F Yellow 170 (available from BASF).

Examples of suitable carrier materials include glass and polymeric materials such as polymethyl methacrylate (PMMA), polyetyhylene terephthalate (PET), polyethylene naphthalate (PEN), and polycarbonate (PC).

In embodiments of the invention, in particular where the light source originally does not have a lambertian light distribution, a scattering member, such as a scattering plate, film or foil, may be provided between the light source and the wavelength converting member. In such embodiments, the scattering member may be considered as the light out-coupling surface and thus di represents the distance from the scattering member to the wavelength converting member.

In other embodiments, the light emitting arrangement may further comprise a weak diffuser provided behind the light source, as seen from the wavelength converting member. Such an arrangement may improve light mixing and recycling.

Fig. 2 illustrates another embodiment of a light emitting arrangement 200 according to the present invention. The light emitting arrangement 200 comprises two light sources 201a, 201b arranged in one plane at a distance d₂ from each other as measured from the center of each light source. Each light source 201a, 201b is associated with a wavelength converting member 202a or 202b respectively, as described above with reference to Fig. 1. The wavelength converting members 202a, 202b are also arranged in one plane, and thus di is the same for both wavelength converting members. Each wavelength converting member has a width wi, w₂, respectively, and are arranged such that light emitted from the first light source 201a at an angle <¾ may pass beside the wavelength converting member 202a as peripheral light, light emitted from the second light source 201b at an angle <¾ may pass beside the wavelength converting member 202b as peripheral light. In this embodiment, wi and w₂ are the same (and therefore also ri and r₂), in the following simply denoted w, and di, d₂ and w are selected such that di=w and d₂=2*w. As can be seen in the Figure, with these relationships fulfilled, light emitted by each light source at large angles can escape both wavelength converting members. However, at very large emission angles, light from one light source 201a may be obstructed by the neighboring wavelength converting member 202b and vice versa, resulting in color converted light also at a large viewing angle (i.e. the arrangement seen from the side). This may be avoided by increasing the value of d₂ (larger separation between the light sources) or by decreasing the value of di (placing the

wavelength converting members closer to its respective light source). It is envisaged that in other examples wi and w₂ may differ, and/or that the wavelength converting members may be arranged at different distances from each respective light source, such that <¾ and <¾ may differ too.

In embodiments of the invention, in particular where the light emitting arrangement comprises a plurality of light sources, the wavelength converting member may be a sheet comprising wavelength converting regions and non-converting regions. For example, Fig. 3 illustrates a light emitting arrangement 300 in which the wavelength converting member comprises a sheet 302 comprising a carrier and a wavelength converting material, wherein the sheet comprises holes regularly spaced at a distance w. The sheet 302 is in front of an array of light sources 301a, 301b, 301c such that the holes are not positioned directly in front of a light source, but in front of areas between the light sources, and hence the portions comprising wavelength converting material is arranged in front of the light sources, with di, w, and d₂ being as described above. Alternatively, instead of holes the non- converting regions may be transparent regions of the film that contain no wavelength converting material and which only transmit light. Alternatively, the non-converting regions may comprise partially transmissive regions that contain a lower concentration or a lower amount of the wavelength converting material compared to the wavelength converting regions, and thus convert a substantially lesser amount of light. The non-converting regions (holes or at least partially transmissive regions) and the converting regions, respectively, may have any suitable shape. For example the converting regions may be circular, hexagonal or square.

Suitable dimensions of the converting and non-converting regions, respectively, as discussed above may be in the range of from 0.3 to 5 mm in diameter. Thus, w may be in the range of from 0.3 to 5 mm and r may be in the range of from 0.15 to 2.5 mm.

Advantageously, using a plurality of light sources and corresponding wavelength converting members or wavelength converting regions (e.g. as described with reference to Fig. 2 and Fig. 3), the daylight mimicking effect of the present invention is particularly strong, as a "blue sky effect" may be obtained when the light source is mounted in a ceiling, for instance. Using a plurality of light sources and wavelength converting members/regions may also provide a desirably high luminance and improved light uniformity.

Fig. 4 illustrates yet another embodiment of a light emitting arrangement 400 comprising a large-area light source 401 and a dual wavelength converting member.

Although only light rays emitted from a few positions on the large area light source are illustrated for reasons of clarity, it is understood that the large area light source 401 may emit light uniformly over the entire surface facing the wavelength converting member. Examples of large area light sources include organic light emitting diodes (OLEDs) and light sources comprising at least one LED and a large area side- emissive lightguide.

In the embodiment of Fig. 4, the wavelength converting member comprises two sheets 402, 403 comprising converting and non-converting regions as described above, a first sheet 402 being arranged at a distance di from the light source, and a second sheet 403 arranged at a larger distance d₃ from the light source. Converting regions of the first sheet 402 are aligned with non-converting regions of the second sheet 403, and non- converting regions of the first sheet 402 are aligned with converting regions of the second sheet 403. Using a plurality of relatively small wavelength converting regions may provide a more uniform light appearance, because small regions are less visible from a distance and are more easily masked by a weak diffuser.

In some embodiments, the dual wavelength converting member may be a multilayer sheet, comprising a transmissive core layer provided on one side with a wavelength converting layer representing the first sheet described above with reference to Fig. 4, and provided on the other side with a second wavelength converting layer representing the second sheet described above.

In embodiments using a large area light source, a wavelength converting member or region is not necessarily centrally aligned with the light source in its entirety. Instead the angle a may be defined for a ray of light emitted from a position on the large area light source that is aligned with the center of the respective wavelength converting member or region.

In embodiments of the invention, the light emitting arrangement may further comprise a weak diffuser, e.g. a 5-10° FWHM holographic diffuser, provided in front of the wavelength converting member as seen by a viewer, i.e. after the wavelength converting member in the light output direction. Hence, light converted or transmitted by the wavelength converting member may be additionally diffused, which may increase light distribution uniformity of both peripheral light and the mix of converted and non-converted light. In particular, the mix of converted and unconverted light, (e.g. white light) originates from the areas of the wavelength converting member, but using a weak diffuser it instead will appear to come from a larger area. The weak diffuser also ensures that the blue/white light directionality is generally preserved.

The person skilled in the art realizes that the present invention by no means is limited to the preferred embodiments described above. On the contrary, many modifications and variations are possible within the scope of the appended claims.

Claims

CLAIMS:

1. A light emitting arrangement (100), comprising:

a light source (101) comprising a light out-coupling surface, adapted to emit light of a first wavelength range; and

a wavelength converting member (102) capable of converting light of said wavelength range into light of a second wavelength range, said wavelength converting member being arranged at a distance from the light source, and being arranged to receive and partially convert a central portion of the light emitted by the light source while allowing light emitted by the light source in a peripheral direction to pass beside the wavelength converting member.

2. A light emitting arrangement according to claim 1, wherein the wavelength converting member or a wavelength converting region thereof has a width w and is arranged at a distance di from said light source, and wherein di and w are selected such that light emitted by the light source from a position on the light out-coupling surface centrally aligned with the wavelength converting body at an angle a of at least 40° to the normal to the light out-coupling surface may pass beside said wavelength converting member, and light emitted at an angle smaller than said angle a may be received by the wavelength converting member.

3. A light emitting arrangement according to claim 1, wherein a central portion of light exiting the light emitting arrangement comprises a mixture of light of said first wavelength range and light of said second wavelength range, and a peripheral portion of the light exiting the light source comprises only light of said first wavelength range.

4. A light emitting arrangement according to claim 2, wherein di = ri * tan(90-cc), in which n = w/2, and 40° < a < 80°.

5. A light emitting arrangement according to claim 2, wherein a is in the range of from 40° to 60°.

6. A light emitting arrangement according to claim 1, wherein the wavelength converting member is planar.

7. A light emitting arrangement according to claim 1, wherein the wavelength converting member or a wavelength converting region thereof is circular.

8. A light emitting arrangement according to claim 1, wherein the wavelength converting member is parallel to the light output surface of the light source.

9. A light emitting arrangement according to claim 1, comprising a plurality of wavelength converting members (202a, 202b), or wherein the wavelength converting member comprises at least one converting region and a plurality of non-converting regions, or a plurality of converting regions and at least one non-converting region.

10. A light emitting arrangement according to claim 1, comprising a plurality of light sources (201a, 201b, 301a, 301b, 301c), each light source being associated with a wavelength converting member or a wavelength converting region.

11. A light emitting arrangement according to claim 1 , wherein a first wavelength converting member (402) is arranged at a distance di from the light source and a second wavelength converting member (403) is arranged at a distance d₃ from the light source, and wherein di < d₃.

12. A light emitting arrangement according to claim 1, wherein the light source comprises a scattering member forming said light out-coupling surface.

13. A light emitting arrangement according to claim 1, further comprising a diffuser arranged in front of the wavelength converting member.

14. A light emitting arrangement according to claim 1, wherein the light source is a light emitting diode.

15. A light emitting arrangement according to claim 1, wherein the light source is a large area light source.