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Title:
A LIGHT EMITTING DEVICE
Document Type and Number:
WIPO Patent Application WO/2021/204581
Kind Code:
A1
Abstract:
A light emitting device (100) comprising at least one light source (10) adapted for, in operation, emitting light (11), at least one beam shaping element (12) arranged extending over the at least one light source and adapted for shaping the light emitted by the at least one light source such as to form shaped light, wherein the shaped light comprises an intensity distribution with a Half Width at Half Maximum (HWHM) angle and a Full Width at Half Maximum (FWHM) angle, and wherein the HWHM angle and the FWHM angle are defined in a cross-sectional plane in which an axis of symmetry of the light emitting device extends, and a light guide (200), wherein the light guide (200) is shaped and arranged to enclose the at least one light source (10) and the at least one beam shaping element (12) such as to shield at least a part of the light emitted by the at least one light source from direct view.

Inventors:
VISSENBERG MICHEL (NL)
VDOVIN OLEXANDR (NL)
IJZERMAN WILLEM (NL)
Application Number:
PCT/EP2021/058165
Publication Date:
October 14, 2021
Filing Date:
March 29, 2021
Export Citation:
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Assignee:
SIGNIFY HOLDING BV (NL)
International Classes:
F21K9/232; F21K9/233; F21K9/61; F21S8/06; F21V3/02; F21V8/00; F21V13/02
Attorney, Agent or Firm:
VAN DIJKEN, Albert et al. (NL)
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Claims:
CLAIMS:

1. A light emitting device (100) comprising: at least one light source (10) adapted for, in operation, emitting light (11), at least one beam shaping element (12) arranged extending over the at least one light source and adapted for shaping the light emitted by the at least one light source such as to form shaped light, wherein the shaped light comprises an intensity distribution with a Half Width at Half Maximum (HWHM) angle and a Full Width at Half Maximum (FWHM) angle, and wherein the HWHM angle and the FWHM angle are defined in a cross-sectional plane in which an axis of symmetry of the light emitting device extends, and a light guide (200), wherein the light guide (200) is shaped and arranged to enclose the at least one light source (10) and the at least one beam shaping element (12) such as to shield at least a part of the light emitted by the at least one light source from direct view, wherein a slope of the light guide (200) adjacent to the at least one light source comprises an angle (J) with respect to the vertical direction, wherein the angle (J) is larger than the HWHM angle, wherein the light guide (200) is shaped such that an angle of incidence (K) on the light guide of the shaped light propagating within the FWHM angle is less than 60 degrees, where the angle of incidence (K) is defined in a cross-sectional plane in which an axis of symmetry of the light emitting device extends as the angle between the direction of propagation of the shaped light incident in a point (Q) on a surface (202) of the light guide and an imaginary line (R) perpendicular to the surface of the light guide at the said point on a surface of the light guide, and wherein the at least one light source (10) and the light guide (200) are arranged in an edge-lit configuration.

2. The light emitting device according to claim 1, wherein the light guide (200) is shaped such that the angle of incidence (K) on the light guide of the shaped light propagating within the FWHM angle is less than 45 degrees.

3. The light emitting device according to claim 1 or 2, wherein the shaped light comprises an emission angle (a), and wherein the light guide (200) is shaped such that the angle of incidence (K) on the light guide of the shaped light propagating within the FWHM fulfils the relation K < 45 - a/2, where a < HWHM, and such that the angle of incidence (K) on the light guide of the shaped light propagating outside of the FWHM fulfils the relation K > 90 - (HWHM + a)/2, where a > 1.2*HWHM, the emission angle (a) being defined in a cross-sectional plane in which an axis of symmetry of the light emitting device extends as the angle between the vertical direction and the direction of propagation of the shaped light.

4. The light emitting device according to any one of the preceding claims, wherein the curved light guide (200) is transparent or partially transparent.

5. The light emitting device according to any one of the preceding claims, wherein the light guide (200) is a linear light guide and/or an extruded light guide, or wherein the light guide (200) is a curved light guide (200) which is delta-shaped or piriform or delta shaped with a cylinder segment.

6. The light emitting device according to any one of the preceding claims, wherein the curved light guide (200) has such a shape that the angle of incidence (K) on the light guide of the shaped light by the at least one light source is: less than 5 or zero for emission angles (a) of less than or equal to 40 degrees, and more than 85 or 90 for emission angles (a) of more than 65 degrees.

7. The light emitting device according to any one of the preceding claims, wherein the at least one light source (10) is any one of a straight or curved linear light source, a point light source, an area light source, a circular or circle segment shaped light source, and/or wherein the light emitting device (100) comprises one or more adjacent additional light sources.

8. The light emitting device according to any one of the preceding claims, wherein the light guide (200) is shaped and arranged to enclose the at least one light source (10) such as to shield all of the light emitted by the at least one light source from direct view.

9. The light emitting device according to any one of the above claims, wherein the at least one beam shaping element (12) is formed in one single piece with the curved light guide (200). 10. The light emitting device according to any one of the preceding claims wherein the at least one light source (10), and/or where present the one or more additional light sources, is tunable with respect to light color and/or correlated color temperature.

11. The light emitting device according to any one of the preceding claims, wherein the light output of the light emitting device (100), when the at least one light source (10), and where present the one or more additional light sources, are in operation, is composed of two parts: i) a functional beam that is emitted by the at least one light source (10); and ii) a secondary beam that is emitted by the adjacent additional light sources; wherein the functional beam is transmitted in the direction of the curved light guide (200), and the secondary beam is coupled into the curved light guide (200).

12. A luminaire comprising a light emitting device according to any one of the above claims and a fixation element (140) configured to support the light emitting device (100).

13. A luminaire comprising a light emitting device according to any one of the above claims, wherein the light emitting device (100) is configured for a suspended configuration of the light emitting device (100).

Description:
A light emitting device

FIELD OF THE INVENTION

The invention relates to a light emitting device and to a luminaire comprising such a lighting device.

It is noted that the term “beam shaping element” should, within the context of this application, be interpreted as any type and shape of beam shaping element (e.g. a lens, an array of lenses or a reflector) that may shape the emitted light and/or direct the emitted light towards a specific direction.

BACKGROUND OF THE INVENTION

Lens-based luminaires are efficient and allow for precise beam shaping. This optical architecture is often applied for functional lighting, like road lighting or industrial lighting. In office lighting such lens architectures are less used because of the uncomfortably high luminance contrasts due to the direct view to a light source. Instead, diffuse panels are a dominant architecture in office lighting. The drawback of these panels is that it is difficult to control the direction of the light in an efficient way. Additionally, other unwanted effects may rise from a panel luminaire as glare.

For use in an office environment, a luminaire should be office compliant. This for example means that the luminaire should have a Unified Glare Rating (UGR) below a certain limit. The UGR is a method of calculating glare by luminaires, defined by the International Commission on Illumination (CIE). The UGR helps to determine how likely a luminaire is to cause discomfort to those around it. In workplaces, glare is a common problem. The value of UGR may be different, depending on the type of work, though 19 is mostly used. For example, in office work areas, the UGR should typically be kept under 19, while in corridors or common spaces like break-out areas, it may vary between 19 to 25.

As discomfort of the direct view to a light source is known, some solutions have been provided in the state of art. For example, document US 9,829,618 B1 describes a curved lightguide used as a light-emitting lampshade. This solution improves visual comfort because the source light is spread over the large area of the light guide, which reduces the brightness. However, this solution does not allow for adjusting the direction of the light emitted, nor it is an energy efficient solution.

Another attempted solution is a luminaire having a light source with one or more beam shaping element(s). In this connection the light source is preferably a straight or curved linear light source, but it may also be a point light source or an area light source. The intensity distribution of such a luminaire can be precisely tuned by the beam shaping element to provide a sharp intensity cut-off for avoiding glare. However, a drawback of such a luminaire is that aesthetics and visual comfort suffer from the visibility of the light source.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide a directional and efficient light emitting device with which glare is avoided to keep the UGR low, with which unwanted Fresnel images are reduced or avoided, and which also has an improved visual comfort.

According to a first aspect of the invention, this and other objects are achieved with a light emitting device comprising at least one light source adapted for, in operation, emitting light, at least one beam shaping element arranged extending over the at least one light source and adapted for shaping the light emitted by the at least one light source such as to form shaped light, wherein the shaped light comprises an intensity distribution with a Half Width at Half Maximum (HWHM) angle and a Full Width at Half Maximum (FWHM) angle, and wherein the HWHM angle and the FWHM angle are defined in a cross-sectional plane in which an axis of symmetry of the light emitting device extends, and a light guide, wherein the light guide is shaped and arranged to enclose the at least one light source and the at least one beam shaping element such as to shield at least a part of the light emitted by the at least one light source from direct view, wherein a slope of the light guide adjacent to the at least one light source comprises an angle, J/2, with respect to the vertical direction, wherein the angle, J/2, is larger than the HWHM angle, and wherein the light guide is shaped such that an angle of incidence, K, on the light guide of the shaped light propagating within the FWHM angle is less than 60 degrees, where the angle of incidence, K, is defined in a cross- sectional plane in which an axis of symmetry of the light emitting device extends as the angle between the direction of propagation of the shaped light incident in a point, Q, on a surface of the light guide and an imaginary line, R, perpendicular to the surface of the light guide at the said point on a surface of the light guide.

By providing the light emitting device with a light guide shaped according to the above design rules, a light emitting device is provided with which LED images caused by Fresnel reflections off the surface of the light guide are reduced considerably or eliminated altogether. More particularly, these design rules ensure that Fresnel reflection images caused by the light guide are not visible in directions outside the beam. This does not mean that all Fresnel reflection images are removed, merely that remaining Fresnel reflection images should only be visible inside the beam (i.e. where the luminaire brightness is uncomfortably high for a viewer anyway bearing in mind that people should never look directly into a luminaire) or above the horizon (since a luminaire should always be placed above the viewer’s heads).

The condition J/2>HWHM reduces direct source light intensity, and source visibility, in directions outside the beam (i.e. nearly parallel guide outside the beam). The condition K<60 deg: Allow the beam to pass with minimal losses due to Fresnel reflections and scattering by the light guide (perpendicular inside the beam directions).

This in turn provides for a directional and efficient light emitting device with a light output having little or no glare and thus a low UGR, and thus a light emitting device that also has an improved visual comfort.

By furthermore providing a light emitting device with a light source that is covered by a curved light guide, the functional lighting may be done by emitting light from the light source through the beam shaping element at a high efficiency and with a good beam- cut-off at large angles. The curved light guide captures a fraction of the light, such that it lights up at an average luminance of about 500-3000 cd/m 2 . This is low enough to avoid glare, but high enough to appear as part of the light source and contribute significantly to the lighting in directions where the lens-beam is cut off. Thereby the UGR is kept sufficiently low to enable office application standards. The combination of a beam shaping element and a light guide allows for comfortable lighting with efficient beam control.

It is noted that the term “slope of the light guide” indicates the inclination of the terminal part of the light guide with respect to the geometric plane indicated with 222 on Fig. 1 of the at least one light source. The angle J/2 of the slope of the light guide is defined as the angle between an imaginary line perpendicular to the geometrical plane 222 of the light source and the surface of the light guide and is indicated on Figs. 4 and 5.

It has been shown that in particular if K is less than 60 degrees, the transmission of light through the light guide is above 80 % while the reflection is kept below 20 %. In this case it is assumed that the density of extraction features of the light guide is low. Also, these exemplary transmission values are for a clear light guide plate without extraction features. Thereby, a light emitting device is provided with which the transmission is improved, and the reflection is kept low.

In an embodiment, the light guide is shaped such that the angle of incidence, K, on the light guide of the shaped light propagating within the FWHM angle is less than 45 degrees..

Thereby, the transmission of light through the light guide has been shown to be above 90% while the reflection is kept below 10%. Thereby, a light emitting device is provided with which the transmission is improved event further and the reflection is kept particularly low. For example, with a slab of material with a refractive index of 1.5, the highest achievable transmission is 92%, where the perpendicular incidence gives about 4% reflection per interface. It is noted that most light guides have an index close to 1.5 (PMMA has index 1.49).

In an embodiment, the shaped light comprises an emission angle, a, and wherein the light guide is shaped such that the angle of incidence, K, on the light guide of the shaped light propagating within the FWHM fulfils the relation K < 45 - a/2, where a < HWHM, and such that the angle of incidence, K, on the light guide of the shaped light propagating outside of the FWHM fulfils the relation K > 90 - (HWHM + a)/2, where a > 1.2*HWHM, the emission angle, a, being defined in a cross-sectional plane in which an axis of symmetry of the light emitting device extends as the angle between the vertical direction and the direction of propagation of the shaped light.

In an embodiment, the curved light guide is transparent or partially transparent.

The transparency of the light guide is important for obtaining efficient light transmission. Therefore, a transparent light guide may give the optimal light transmission efficiency, while a partially transparent light guide may preferably improve the aesthetics of the light emitting device.

Suitable light transmissive materials may be selected from the group consisting of transmissive organic materials. Examples of suitable transmissive organic materials are PE (polyethylene), PP (polypropylene), PEN (polyethylene napthalate), PC (polycarbonate), polymethylacrylate (PMA), polymethylmethacrylate (PMMA) (Plexiglas or Perspex), cellulose acetate butyrate (CAB), polyvinylchloride (PVC), polyethylene terephthalate (PET), including in an embodiment (PETG) (glycol modified polyethylene terephthalate), PDMS (polydimethylsiloxane), and COC (cyclo olefin copolymer).

Especially, the light transmissive material may comprise an aromatic polyester, or a copolymer thereof, such as e.g. polycarbonate (PC), poly (methyl)methacrylate (P(M)MA), polyglycolide or polyglycolic acid (PGA), polylactic acid (PLA), polycaprolactone (PCL), polyethylene adipate (PEA), polyhydroxy alkanoate (PHA), polyhydroxy butyrate (PHB), poly(3-hydroxybutyrate-co-3 -hydroxy valerate) (PHBV), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polytrimethylene terephthalate (PTT), polyethylene naphthalate (PEN); especially, the light transmissive material may comprise polyethylene terephthalate (PET). Hence, the light transmissive material is especially a polymeric light transmissive material. In embodiments, the light transmissive material may comprise a silicone, such as in embodiments dimethyl silicone or methylphenyl silicone, and the like.

In an embodiment, the light guide is a linear light guide and/or an extruded light guide, or wherein the light guide is a curved light guide which is delta-shaped or piriform or delta-shaped with a cylinder segment.

The shape of the light guide is an important aspect when looking at the efficiency of the light source been transmitted outwards the light emitting device. Additionally, the angle of incidence K strictly depends on the shape of the light guide and therefore the shape of the light guide directly influences the efficiency of the light emitting device.

It has been shown that delta-shaped light guides as well as light guides being piriform or delta-shaped with a cylinder segment are the most suitable for the invention and provide for a light emitting device with a particularly high efficiency.

Light guides of other shapes are also feasible and include light guides shaped as a flat plate or a full cylinder. There are also many suitable shapes in between the full cylinder and the delta shapes that are suitable for this invention.

In an embodiment, the curved light guide has such a shape that the angle of incidence, K, on the light guide of the shaped light by the at least one light source is less than 5 or zero for emission angles, a, of less than or equal to 40 degrees, and more than 85 or 90 for emission angles, a, of more than 65 degrees.

Thereby a light emitting device is provided with which a substantial or full suppression of unwanted bright spots caused by Fresnel reflections when seen in a viewing direction outside the beam and below the horizon is obtained.

In an embodiment, the curved light guide has a thickness, d, within 0.5 mm and 20 mm.

In an embodiment, the light guide may contain light scattering particles (for instance particles with a deviating refractive index) to extract light from the guide. A given light guide may have a mean free path between scattering events that is larger than the light guide thickness. In this case most of the light incident perpendicular to the surface of the light guide will pass without scattering. For example, if the angle of incidence K is about 60 degrees, the pathlength of the light through the light guide is twice the thickness and it increases steeply at even higher angles. Therefore, the light at angles within the FWHM should pass the light guide at angles of incidence lower than 60 degrees. The light in the tail of the distribution, and especially the light at glary emission angles of -50-70 degrees should have an angle of incidence larger than 60 degrees, or even higher than 70 degrees, to suppress the light in these directions and hide the light source from direct view from these angles. This is achieved in embodiments with the above-mentioned light guide thicknesses.

As the light guide material may have a low density of scattering particles, the mean free path for scattering may be relatively long, such as at least 2 mm, like at least 5 mm, like especially at least 10 mm.

Hence, in embodiments the curved light guide and an optional coating thereon have a mean free path for scattering of the visible light of at least 5 mm. Amongst others, the mean free path may be determined with a laser and measuring the transmission of at least two different thicknesses of the light transmissive material.

Also, the mean free path should not be too long. Therefore, the mean free path length for scattering may in embodiments be configured or chosen to ensure the light that enters on one side of the light guide is out-coupled before it reaches the other side of the light guide. Typically, the scattering strength is tuned for an even distribution of the outcoupled light (i.e. constant luminance along the light guide).

Hence, the curved light guide may optionally comprise a coating at one or both sides. Especially, however, such coating is neither reflecting nor scattering. In embodiments, such coating is at least not essentially scattering, such as in embodiments having a mean free path for scattering of at least 5 mm.

In some embodiments the at least one light source is any one of a straight or curved linear light source, a point light source, an area light source, a circular or circle segment shaped light source, and/or wherein the light emitting device comprises one or more adjacent additional light sources.

The light source may for example be a LED, a diode laser, an OLED or an electroluminescent light source. Such technologies allow for different kind of shapes and geometries for the light source itself where the most common is linear. In an embodiment, the at least one light source and the curved light guide are arranged in an edge-lit configuration.

An edge-lit configuration is to be understood as a light guide configuration where the light source is placed along the edges of the terminal part or end of the light guide. This provides for a particularly efficient incoupling of light into the light guide.

In an embodiment, the light guide is shaped and arranged to enclose the at least one light source such as to shield all of the light emitted by the at least one light source from direct view.

Thereby, a light emitting device is provided in which the light guide completely covers the light source from direct view. This in turn provides for a light emitting device providing a particularly improved uniform light output.

In an embodiment, the at least one beam shaping element is arranged extending over the at least one light source and adapted for shaping the light emitted by the at least one light source.

The beam shaping element may for instance be a lens or array of lenses, for example with a thickness within 0.5 mm and 20 mm. The shape of the lens may be cylindrical or spherical or the lens may have a Fresnel lens structure.

Thereby a light emitting device is provided with which the lens may be used to direct the light from the light source in the forward direction and through the light guide with the smallest possible angle of incidence K. This in turn minimizes loss of light when coupling light through the light guide. It is noted that generally the shape of the light guide is chosen to fit the shape of the beam.

In an embodiment, the beam shaping element comprises two positive linear lenses or two linear lenses with an elliptical cross-section or spherical cross section or cylindrical cross section. In an additional preferred embodiment, the beam shaping element comprises a free-form lens.

Thereby a light emitting device with a light output having a pronounced batwing intensity distribution is provided for. Such a light emitting device further has a high efficiency and a low UGR. A batwing shape is typically used for uniform illumination of large areas with a relatively large spacing of the luminaires and is therefore a typical beam shape for general illumination purposes. For other purposes, other beam shapes may be more efficient or may provide a lower UGR. In an embodiment, the at least one beam shaping element is formed in one single piece with the curved light guide. The one single piece may for instance be fabricated by extrusion or molding.

Thereby, a light emitting device with fewer components and thus a particularly simple structure is provided for.

In an embodiment, the at least one light source, and/or where present the one or more additional light sources, is tunable with respect to light color and/or correlated color temperature.

Alternatively, or additionally, the at least one additional light source is tunable with respect to light color and/or correlated color temperature.

The possibility of tuning the light color and/or correlated color temperature provides a double effect of improving light comfort and light aesthetics for the viewer. Typically, the light color can span within the colors of visible light, while the correlated color temperature can vary between 1000 Kelvin and 20000 Kelvin and where normal office light is between 2700 Kelvin and 5500 Kelvin.

In an embodiment, the light output of the light emitting device, when the at least one light source, and where present the one or more additional light sources, are in operation, is composed of two parts, namely a functional beam that is emitted by the at least one light source and a secondary beam that is emitted by the adjacent additional light sources, and the functional beam is transmitted in the direction of the curved light guide, and the secondary beam is coupled into the curved light guide.

Thereby, a light emitting device with a high efficiency and a low UGR is obtained in a particularly simple manner and having a particularly simple construction.

In an embodiment, the light emitting device may further comprise a fixation element. Especially, the fixation element may be configured to keep the first terminal edges or ends of the light guide and the unit comprising the at least one light source and the at least one beam shaping element together.

In further embodiments the fixation element may be configured to have a suspension functionality or may be configured to allow such suspension functionality.

In any event, a light emitting device is thereby provided which is especially robust and durable in construction.

Furthermore, such a light emitting device may be integrated in a luminaire configured to be suspended, such as in office lighting, or the light emitting device may be suspended to a ceiling, without additional housing. The lighting device may be part of or may be applied in e.g. office lighting systems, household application systems, shop lighting systems, home lighting systems, accent lighting systems, spot lighting systems, theater lighting systems, fiber-optics application systems, projection systems, self-lit display systems, pixelated display systems, segmented display systems, warning sign systems, medical lighting application systems, indicator sign systems, decorative lighting systems, portable systems, automotive applications, outdoor road lighting systems, urban lighting systems, green house lighting systems, horticulture lighting, etc..

The invention thus also concerns a luminaire comprising a light emitting device according to the invention. In an embodiment, a support for the luminaire is provided for. In embodiments, the support may comprise the fixation element. Alternatively, in embodiments the fixation element may comprise the support. In yet further embodiments, the support and the lighting device are configured for a suspended configuration of the lighting device. In yet a further embodiment, the invention also provides an office lighting system comprising a luminaire, the luminaire comprising a lighting device according to the invention. The fixation element, of the luminaire, is configured to support the light emitting device. In yet a further embodiment, the light emitting device, of the luminaire, is configured for a suspended configuration of the light emitting device.

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 embodiments of the invention.

Fig. 1 shows a perspective view of a luminaire comprising a light emitting device according to the invention.

Figs. 2A-2B shows a cross-sectional view of a lighting emitting device according to the invention.

Figs. 3A-3H shows schematic cross-sectional views of eight alternative possible shapes of a light guide of a light emitting device according to the invention.

Fig. 4 shows a polar graph illustrating simulation results performed on a light emitting device according to Fig. 2B and comprising a beam shaping element.

Fig. 5 shows a graph illustrating the intensity distribution of simulation results performed on a light emitting device according to the invention and comprising a beam shaping element but without a light guide. Also illustrated is the Full Width at Half Maximum (FWHM) of the intensity distribution as well as the angle J.

Fig. 6 shows a schematic illustration of the emission angle, a, the angle of incidence, K, and the angle of reflection, r, of a light beam emitted by a light source and incident on a reflective surface.

Fig. 7 shows a cross-sectional view of a lighting emitting device according to another embodiment the invention.

Fig. 8 shows a polar graph illustrating simulation results performed on a light emitting device according to Fig. 7.

As illustrated in the figures, the sizes of layers and regions are exaggerated for illustrative purposes and, thus, are provided to illustrate the general structures of embodiments of the present invention. Like reference numerals refer to like elements throughout.

DETAILED DESCRIPTION

The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which currently preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided for thoroughness and completeness, and fully convey the scope of the invention to the skilled person.

Referring initially to Fig. 1, a luminaire 1000 comprising a light emitting device 100 according to the invention is shown in a perspective view. Generally, the light emitting device 100 according to the invention comprises a light source 10, which in operation is emitting light 11, a light guide 200 and a beam shaping element 12.

The emitted light 11 is shaped by the beam shaping element 12 such as to form shaped light, and the shaped light crosses the light guide 200. The shaped light comprises an intensity distribution with a Half Width at Half Maximum (HWHM) angle and a Full Width at Half Maximum (FWHM) and an emission angle, a. The HWHM angle, the FWHM angle and the emission angle a are defined in a cross-sectional plane in which an axis of symmetry of the light emitting device extends. The cross-sectional plane may by way of example correspond to the cross-sectional plane of the views in Figs. 2A and 2B. The axis of symmetry coincides with the imaginary line P shown in Fig. 2B. The light source is hidden from direct view by the light guide and glare is avoided by shaping the beam. The shaped beam is mainly directed towards the light guide 200 and in a direction parallel to the light guide 200. The part of the beam propagating outside of the FWHM tail of the intensity distribution is mostly guided in the lightguide or reflected in a much higher angle of incidence as compared to the part of the beam propagating within the FWHM tail of the intensity distribution when incident on the surface of the light guide 200.

Referring to Figs. 2A and 2B, a light emitting device 100 according to the invention is shown in a cross-sectional view. The light emitting device 100 according to the invention comprises a light source 10, which in operation is emitting light 11, a light guide 200 and a beam shaping element 12. Figs. 2A and 2B show two different shapes of light guides 200 provided on otherwise identical light emitting devices 100. The light guide 200 of the light emitting device 100 shown in Fig. 2A is delta-shaped with a cylinder segment. The light guide 200 of the light emitting device 100 shown in Fig. 2B is droplet or cylinder- segment-shaped.

The light guide 200 may be made of optically transmissive material having a refractive index greater than 1, preferably greater than 1.4. An end or terminal edge 201 is arranged to receive part of the light 11 emitted from the light source 10 and propagate light into the light guide 200 through total internal reflection within the walls of the light guide 200. The light guide 200 is primarily used as a visual comfort device that will hide the light source 10 and shields the direct view of the light source 10 to the viewer. The light guide 200 is configured as and arranged to be edge-lit with the light source 10. A fixation element 140 that keeps the light guide 200 in place near the light source 10 is provided. The angle J is measured with respect to the vertical direction, and the angle J/2 is larger than the HWHM angle - cf. Figs. 4 and 5. The vertical direction is illustrated by means of the imaginary line P shown in Fig. 2B. As shown, the angle J is the angle between an imaginary line P perpendicular to the geometrical plane 222 of the light emitting surface of the light source 10 and the surface 202 of the light guide 200.

Likewise, the emission angle, a, the angle of incidence, K, and the angle of reflection, r, of the light 11 emitted by the light source 10 and incident on the surface 202 of the light guide 200 is shown on Fig. 2 and further in a schematic manner in Fig. 6. The emission angle, a, is defined as the angle between the difference from an imaginary line P perpendicular to the light guide surface 202 and the direction in which the ray of light 11 propagates. The angle of incidence K of the emitted light 11 with the light guide 200 is defined in a cross-sectional plane in which an axis of symmetry of the light emitting device extends. The cross-sectional plane may by way of example correspond to the cross-sectional plane of the views in Figs. 2A and 2B. the angle of incidence K of the emitted light 11 with the light guide 200 is defined by measuring the angle between an imaginary line R perpendicular to the light guide surface 202 at a point of incidence Q of the light 11 incident on the surface and the direction of emission of the emitted light 11. This is a fundamental parameter for optimizing the shape and the efficiency of the light emitting device. Also, the light guide 200 has a thickness shown as d in Fig. 2.

Generally, the light guide 200 is shaped such that the angle of incidence K on the light guide 200 of a part of the shaped light propagating outside of the FWHM fulfils the relation K > 90 - (HWHM + a)/2, and such that an angle of incidence K on the light guide of a part of the shaped light propagating within of the FWHM fulfils the relation K < 45 - a/2, where the angle of incidence K is defined in a cross-sectional plane in which an axis of symmetry of the light emitting device extends as the angle between the direction of propagation of the shaped light incident in a point Q on a surface 202 of the light guide and an imaginary line R perpendicular to the surface of the light guide at the said point on a surface of the light guide.

The above general design rule for the light guide 200 of the light emitting device 100 according to the invention will now be explained with reference also to Fig. 6.

Fig. 6 illustrates schematically the emission angle, a, and the angle of incidence, K, and the angle of reflection, r, of the light 11 emitted by the light source 10 and incident on the surface 202 of the light guide 200. The angle of reflection, r, is defined as the direction of the reflected light with respect to the vertical direction and is illustrated in Figs. 2B and 6. The angle of reflection may be defined by the equation r = 180 - a - 2K.

To mask the Fresnel images generated when light 11 is incident on the surface 202 of the light emitting device 200, it has been shown to be advantageous when the angle of reflection, r, is either inside the beam of light 11, i.e. r < HWHM, in which case the Fresnel images are masked by the light source 10 itself, or larger than 90 degrees corresponding to the uplighting direction. With the above equation for the angle of reflection, r, these constraints yield the above defined design rules, namely

K > 90 - (HWHM + a)/2 for light 11 propagating outside the FWHM, and

K < 45 - a/2 for light 11 propagating within the FWHM. In this connection “outside the FWHM” may be defined as emission angles a > 1.2 * HWHM. Thereby, a too sharp kink in the lightguide at an emission angle a = HWHM may be avoided. The beam shaping element 12 may be any beam shaping element 12 that may shape the emitted light 11 and/or direct the emitted light towards a specific direction. The beam shaping element 12 may for instance be a lens, an array of lenses or a reflector. Suitable lenses are in particular cylinder lenses (cf. Figs. 1 and 2B) or spherical lenses. Beam shaping elements 12 in the form of lenses with varying shapes configured to optimize the intensity distribution are also feasible.

Referring to Fig. 7, the beam shaping element 12 may also comprise two positive linear lenses 12 and 12’ or even two linear lenses 12 and 12’ with an elliptical cross- section, thereby creating a pronounced batwing intensity distribution as shown in Fig. 8. The efficiency of such a light emitting device has been shown to be 85% and the UGR (more than 5000 lm output, 1200 mm long, 8 mm wide) is below 19. The average luminance at 60 degrees is about 3300 cd/m2, going down to about 1500 cd/m2 at 90 degrees viewing angle. The luminance distribution at 60 degrees is non-uniform, varying between 100 cd/m2 and 20000 cd/m2. This uniformity may be optimized by using different scattering strength of the light guide (less outcoupling) or even a variable scattering over position (less outcoupling in the brighter areas and vice versa).

Further advantageous design rules and embodiments, in so far as they have already been discussed in the introductory description, shall not be repeated here to avoid undue repetition.

Referring now to Fig. 3, light guides 200 of eight different feasible shapes are illustrated in schematic cross-sectional views. Fig. 3 A shows a light guide 200 shaped as a flat plate. Fig. 3B shows a semi-cylindrical light guide 200. Fig. 3C shows a delta-shaped light guide 200. Fig. 3D shows a block rectangle-shaped light guide 200. Fig. 3E shows a light guide 200 shaped as a full cylinder. Fig. 3F shows a light guide 200 shaped as a cylinder segment. Fig. 3G shows a delta-shaped light guide 200 with a cylinder segment. Fig. 3H shows a block square-shaped light guide 200. On all eight illustrations, the angle of emission is defined as a and the incident angle is defined as K. Simulations have been made on different variations of the shapes shown to find the optimal shape of the light guide 200 of a light emitting device 100 according to the invention.

By experimentation it has been show that that a delta-shaped light guide (Fig. 3C), a light guide shaped as a full cylinder (Fig. 3E) and a delta-shaped light guide with a cylinder segment (Fig. 3G) are good shapes for a light guide according to the invention. The experiments further showed that the other shapes shown in Figs. 3 A, 3B, 3D, 3F and 3H did not have the properties desired for a light guide according to the invention. Hence, the light guide shapes shown in Figs. 3 A, 3B, 3D, 3F and 3H are considered to not form part of the present invention, but only serve to illustrate the various light guide shapes explored.

Referring to Fig. 4 a polar curve illustrating such a simulation result of the intensity distribution of light emitted by a light emitting device 100 according to the invention and as shown on Fig. 2B after using a beam shaping element 12 is shown. The shown emitted light has an intensity distribution with a FWHM angle that is less than 35 degrees in the reference cross-section plane 222 of the light emitting device 100. The angle J is illustrated on the polar curve and shows that the light within the FWHM angle is contained within the light guide 200.

Referring to Fig. 5, a graph illustrating the intensity distribution of simulation results performed on a light emitting device according to the invention and comprising a beam shaping element but without a light guide is shown. This graph illustrates an emitted light beam with a FWHM angle below 35 degree. As may be seen, the angle J is bigger than the FWHM angle and contains most of the emitted light, leaving the edge intensity of the emitted light beam to hit the edge of the light guide 200 and be coupled with total internal reflection effect into the light guide 200.

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. For example, if the emitted light is highly directional it would not need a beam shaping element to fulfill the requirement of this invention.

Additionally, variations to the disclosed embodiments can be understood and effected by the skilled person 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. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage.