WO2008102287A1

WO2008102287A1 - A led luminaire

Info

Publication number: WO2008102287A1
Application number: PCT/IB2008/050549
Authority: WO
Inventors: Erik Boonekamp
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2007-02-23
Filing date: 2008-02-15
Publication date: 2008-08-28
Also published as: TW200905872A

Abstract

This invention relates to a LED luminaire (101) having a structure that provides indirect output light. The luminaire has a light transmissive front plate (103), a back plate (111), which is arranged opposite to the front plate, a plurality of LEDs (109), and a plurality of reflectors (113), which are arranged in an array at a front surface (115) of the back plate. The LEDs emit light towards the back plate. Each reflector comprises a collimator (114) surrounding a portion (117) of the back plate, which portion is diffusely reflective. The collimator is arranged to reflect light emitted from at least one of the LEDs towards the diffusely reflective portion, and the collimator is arranged to collimate light reflected from that portion towards the front plate.

Description

A LED luminaire

FIELD OF THE INVENTION

The present invention relates to a LED luminaire having a light transmissive front plate, a plurality of LEDs arranged on a rear surface of the front plate, a back plate, which is arranged opposite to the front plate, and a plurality of reflectors, which are arranged on a front surface of said back plate, wherein said LEDs emit light towards said back plate, and the reflectors reflect the light back through the front plate.

BACKGROUND OF THE INVENTION

A conventional LED luminaire has an arrangement of LEDs (Light Emitting Diodes) generating a particular beam shape. A common problem of the conventional LED luminaires is that at small viewing angles, for example less than 60 degrees, you are able to look directly into the LEDs, which have a high brightness that is uncomfortable to the eyes of the viewer.

EP 1221722 Al discloses a LED luminaire where a major part of the light emitted from the LEDs is reflected by rear reflectors. This indirect LED luminaire has a slightly decreased brightness, but the problem of unpleasant glare substantially remains.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a LED luminaire that is more comfortable for the eyes of a person than the prior art LED luminaries when the person happens to look right into the luminaire, regardless of viewing angle.

This object is achieved by a LED luminaire according to the present invention as defined in claim 1.

The invention is based on an insight that by assuring that only diffusely reflected light is output of the luminaire, it is possible to obtain an acceptable level of brightness while keeping the output amount of light at a good level.

Thus, in accordance with an aspect of the present invention, there is provided a LED luminaire comprising a light transmissive front plate, a back plate, which is arranged opposite to the front plate, a plurality of LEDs for emitting light towards the back plate, and a plurality of reflectors, which are arranged in an array at a front surface of said back plate. Each reflector comprises a collimator surrounding a portion of the back plate, wherein the portion is diffusely reflective. Each collimator is arranged to reflect light emitted from at least one of the LEDs towards said portion, and is arranged to collimate light reflected from said portion towards the front plate.

In accordance with an embodiment of the LED luminaire, as defined in claim

2, the LEDs are arranged on a rear surface of said front plate.

In accordance with an embodiment of the LED luminaire, as defined in claim

3, each LED emits collimated light. Thereby the capability of controlling the light distribution across the collimators is enhanced.

In accordance with an embodiment of the LED luminaire, as defined in claim

4, the LEDs emit light within a first collimation angle, and the collimators emit light within a second collimation angle. The second collimation angle is larger than or equal to the first collimation angle. When this condition is fulfilled, all light emtted from the LEDs hit the diffusely reflecting portion of the back plate only once.

In accordance with an embodiment of the LED luminaire, as defined in claim

5, the collimators constitute a CPC structure, where the CPCs are arranged adjacent to each other. Such a CPC structure allows an efficient use of the area of the back plate as diffuse reflector. In accordance with an embodiment of the LED luminaire, as defined in claim

6, each one of the LEDs is enclosed by a respective one of the collimators. This embodiment is advantageous in that it is has a compact structure that is mechanically strong.

In accordance with an embodiment of the LED luminaire, as defined in claim

7, the reflector array consists of a transparent polymer plate. This embodiment has a high efficiency.

In accordance with an embodiment of the LED luminaire, as defined in claim

8, the front surface of the back plate is provided with a phosphor layer. In addition to the light spreading effect of the portions within the collimators, the color is tunable by means of the phosphor layer. In accordance with an embodiment of the LED luminaire, as defined in claim

9, the front plate is provided with a rear surface layer consisting of an interference coating, which reflects a part of the light impinging thereon, and originating from the collimators, back towards said collimators. This embodiment has an enhanced LED light mixing. These and other aspects, features, and advantages of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described in more detail and with reference to the appended drawings in which:

Fig. 1 shows a cross-sectional view of a first embodiment of a LED luminaire according to the present invention;

Fig. 2 shows a cross-sectional view of a second embodiment of a LED luminaire according to the present invention;

Fig. 3 shows a cross-sectional view of a third embodiment of a LED luminaire according to the present invention; and

Fig. 4 shows a perspective view of a part of a LED luminaire according to an embodiment of the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to Fig. 1, in a first embodiment thereof the LED luminaire 101 comprises a front plate 103 having a front surface 105 and a rear surface 107, a number of LEDs 109, which are attached to the rear surface 107 of the front plate 103, a back plate 111, which is arranged in parallel with, and opposite to, the front plate 103, and a number of reflectors 113, which are arranged on a front surface 115 of the back plate 111.

The front plate 103 is made of glass and is light transmissive, and preferably transparent although other alternatives are employable as will be evident from below. The LEDs 109 are arranged in an array, either regularly or randomly, at a distance x from each other. The LEDs 109 are interconnected by a pattern of conductors, which are preferably transparent, such as ITO or SnO₂IF, but can be metal wires as well. The LEDs 109 can be of many different types. A type that is often useful is a group of a red, a green and a blue LED arranged as a single unit, i.e. an RGB package. More examples will follow. The LEDs emit light rearwards, i.e. towards the back plate 111, within a limited angle of 2α, where CC is a first collimation angle as measured from a normal to the plane of the front plate 103.

The reflectors 113 constitute an array of collimators 114, each surrounding a portion 117 of the back plate 111, and more particularly a portion 117 of the front surface 115 thereof. The front surface 115 of the back plate 111, and thus each portion 117, is a diffuse reflector, while the walls 116 of the collimators 114 are specular reflective. In this embodiment the collimators 114 are CPC (Compound Parabolic Concentrator) elements. The walls 116 are curved along the height thereof. The walls 116 are arranged in an advantageous form, preferably chosen to maximise the use of the diffusely reflective front surface 115 of the back plate 111. As shown in Fig. 4, according to one embodiment of a reflector array 401 , the collimators 403 are square and interconnected adjacent to each other forming a lattice covering the whole front surface 405 of the back plate, without any play between the collimators 403. Another example of a fully covering collimator structure is composed of hexagonal collimators. Returning to Fig. 1, in each collimator 114 the width d of a first end, or rear end, opening of the collimator 114, at the front surface 115 of the back plate 111, which width corresponds to the width of the diffusely reflective portion 117, is a fraction of the width w of a second end, or front end, of the collimator 114. The relation between the widths d and w in combination with the height h of the collimator 114, define a second collimation angle β. It is preferred that the first and second collimation angles CC and β fulfil the condition α < β, i.e. the first collimation angle is smaller than or equal to the second collimation angle. If this condition is fulfilled then all light emitted from the LEDs will reach the diffuse reflectors 117, either directly or via the walls 116, as shown by the arrows in Fig. 1.

In order to obtain the collimation angle CC, each LED 109 is provided with a collimating element, such as a lens or a reflector, 110, as known to a person skilled in the art. The distance H between the rear surface 107 of the front plate 103 and the front surface 115 of the back plate 111 and the distance x between the LEDs 109 are related to uniformity of LED illumination of the back plate 111. For example, by increasing the distance H between the front plate 103 and the back plate 111, the back plate will be more uniformly illuminated, via the collimators 114. On the other hand, it is generally desirable to keep the LED luminaire 101 as thin as possible.

The light that is emitted from a LED 109 enters one or more collimators 114 through the second end(s) thereof and is reflected by the collimator walls 116 onto the diffuse reflector(s) 117, or illuminate the diffuse reflector(s) 117 directly. The diffuse reflector(s) scatter the light, and reflect it back towards the front plate 103 either directly or via the collimator walls 117. Thus, the diffusely reflected light is collimated by the collimators 113. The diffusely reflected light passes through the front plate 103 and forms the light output of the LED luminaire 101. Due to the scattering step preformed by the diffuse reflectors the brightness is significantly reduced relative to a pure specular reflection. Because the support for the LEDs 109, i.e. the front plate 103 consists of glass, low-power LEDs 109 are preferred.

In an alternative embodiment an interference layer 125, as illustrated with the dashed line at the front plate in Fig. 1, is provided on the rear surface 107 of the front plate 103. The interference layer, or coating, 125 is semi reflective, for example it reflects 50% of the diffusely reflected light back towards the back plate 111, and thus towards the reflectors 113 where it is then scattered, or diffusely reflected, once more. By means of this interference layer 125, the light is spread across more reflectors 113 and the brightness is additionally decreased. In yet another alternative embodiment there is provided a layer of phosphor

127 on the front surface 115 of the back plate 111, as illustrated with the dashed line at the back plate in Fig. 1. More particularly, the phosphor layer 127 preferable is a mix of phosphor pigment, such as for example YAG:Ce, and a non-luminescent white pigment. The given example of phosphor pigment is combined with LEDs 109 emitting blue light. By adjusting the mix the color of the output light is tuned.

Referring to Fig. 2, another embodiment of the LED luminaire 201 basically has the same structural parts as the first embodiment described above in conjunction with Fig. 1. Thus, the LED luminaire 201 comprises a front plate 203, a back plate 211, a plurality of LEDs 209 arranged at the rear side 107 of the front plate 103, and a plurality of reflectors 213 arranged at the front side 215 of the back plate 211, and including collimators 214 each surrounding a respective portion 217 of the front side 215 of the back plate 211. However, the light emitted from each LED 209 is captured by a respective single collimator 214. Consequently, the number of LEDs 209 is equal to the number of collimators 214. Further, the front plate 203 is arranged close to the output openings, i.e. the front ends, 219 of the collimators 214, which results in a separate cavity for each LED 209. Each cavity is defined by a collimator 214, a portion 217 of the back plate 211, and a portion 221 of the front plate 103 where the LED 209 is attached. While having the advantages mentioned in the summary above, this embodiment is limited as to the mixing of light.

Another embodiment, as illustrated in Fig. 3, is similar to the embodiment shown in fig. 1, except for the reflector array, which is quite different. The collimators 303 of the LED luminaire 301 are integrated into, i.e. formed in, a plate 305 of a transparent polymer, such as PMMA (Polymethylmetaacrylat), PC (Polycarbonate), or COC (Cyclo- olefm-polymer). The polymer plate 305 ha a thickness that exceeds the height h of the collimators 303. There is a space between the LEDs 307 at the front plate 309 and a front surface 311 of the transparent polymer plate 305.

Above, embodiments of the LED luminaire according to the present invention as defined in the appended claims have been described. These should be seen as merely non- limiting examples. As understood by a skilled person, many modifications and alternative embodiments are possible within the scope of the invention.

For example as an alternative the collimators are wedge-shaped collimators having straight walls. Such collimators are easier to manufacture, but have a lower ratio d/w between the width of the first and second, i.e. entrance and exit, openings than the CPC elements.

Thus, as explained by means of the embodiments above, the brightness of a LED luminaire is decreased by means of an indirect light structure using diffusely reflective surface for scattering the LED light before it is output.

It is to be noted, that for the purposes of this application, and in particular with regard to the appended claims, the word "comprising" does not exclude other elements or steps, that the word "a" or "an", does not exclude a plurality, which per se will be apparent to a person skilled in the art.

Claims

CLAIMS:

1. A LED luminaire comprising a light transmissive front plate (103), a back plate (111), which is arranged opposite to the front plate, a plurality of LEDs (109) for emitting light towards the back plate, and a plurality of reflectors (113), which are arranged in an array at a front surface (115) of said back plate, characterized in that each reflector comprises a collimator (114) surrounding a portion (117) of said back plate, and in that the portion is diffusely reflective, wherein the collimator is arranged to reflect light emitted from at least one of said LEDs towards said portion, and wherein the collimator is arranged to collimate light reflected from said portion towards the front plate.

2. A LED luminaire according to claim 1, wherein the LEDs (109) are arranged on a rear surface (107) of said front plate (103).

3. A LED luminaire according to claim 1 or 2, wherein each LED (109) emits collimated light.

4. A LED luminaire according to claim 3, wherein each LED (109) emits light within a first collimation angle, wherein each collimator (114) emit light within a second collimation angle, and wherein the second collimation angle is larger than or equal to the first collimation angle.

5. A LED luminaire according to any one of claims 1-4, wherein said collimators (109) are comprised of a CPC structure, where the CPCs are arranged adjacent to each other.

6. A LED luminaire according to any one of the preceding claims, wherein each one of said LEDs (109) is enclosed by a respective one of said collimators (114).

7. A LED luminaire according to any one of the preceding claims, wherein the reflector array (113) consists of a transparent polymer plate (305).

8. A LED luminaire according to any one of the preceding claims, wherein said front surface (115) of the back plate (111) is provided with a phosphor layer (127).

9. A LED luminaire according to any one of the preceding claims, wherein the rear surface (107) of said front plate (103) is provided with an interference coating (125) reflecting a part of the light impinging thereon towards said collimators (114).