WO2009001272A1

WO2009001272A1 - Light output device with light guide with a truncated cone shape

Info

Publication number: WO2009001272A1
Application number: PCT/IB2008/052462
Authority: WO
Inventors: Erik Boonekamp; Johannes P. M. Ansems
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2007-06-27
Filing date: 2008-06-23
Publication date: 2008-12-31
Also published as: TW200918829A

Abstract

The present invention relates to a light output device (100) comprising a light source (101) and a light guide (102) for guiding light from the light source, the light guide having a concave inner side (103) and an outer side (104), which sides are tapered to each other, wherein the light guide is arranged to couple the light out of the light guide over an area of the inner side of the light guide after at least two total internal reflections on the inner side and/or the outer side of the light guide. A light output device is provided in which the perceived peak brightness of the coupled out light is decreased.

Description

LIGHT OUTPUT DEVICE WITH LIGHT GUIDE WITH A TRUNCATED CONE SHAPE

FIELD OF THE INVENTION

The present invention relates to a light output device comprising a light source and a light guide for guiding light from the light source, the light guide having a concave inner side and an outer side tapered to each other. The invention also relates to a light guide suitable for use in such light output device. The invention further relates to a luminaire provided with at least one such light guide.

BACKGROUND OF THE INVENTION

Light output devices utilizing light emitting diodes (LEDs) as their light sources have become increasingly popular. LEDs have high primary brightness, typically 1- 50 Mcd/m². If the angle distribution of the light from LEDs is not controlled properly, the light output device will cause glare. In order to reduce glare, it is known to place a simple optical element, such as a lens, CPC, reflector directly on a LED. However, the peak brightness of the LEDs is retained and the perceived brightness of the light is still too high when a person looks at the light output device.

US-A 7,181,378 discloses a refraction, reflection, internal reflection and refraction (RXIR) apparatus having a lens surrounding a LED. The lens has an upper surface comprising a reflectively coated central zone surrounded by a totally internally reflecting peripheral surface and extending to a mounting flange. The lower surface comprises a central cavity with an entry-surface and a surrounding reflective surface, such as a reflectively coated surface and/or a total internal reflection (TIR) surface. Rays propagate outward from a focal point to enter the lens through the entry surface, to be reflected by the upper surface to a reflector to be again reflected, then out the lens through the upper surface. In such a device the angle distribution of the light from the LEDs is relatively narrow and the peak brightness is relatively high. The viewer has the impression as if the light directly comes from the light source.

SUMMARY OF THE INVENTION It is an object of the invention to provide a light output device in which the perceived peak brightness of the coupled out light is decreased.

Accordingly, the present invention provides a light output device comprising a light source and a light guide for guiding light from the light source, the light guide having a concave inner side and an outer side tapered to each other, wherein the light guide is arranged to couple the light out of the light guide over an area of the inner side of the light guide after at least two total internal reflections on the inner side and/or the outer side of the light guide. The invention is based on the following observations:

In a light guide having two sides, a portion of the light coupled into the light guide will alternately be reflected by these two sides by total internal reflection. This can be achieved by the light guide having a refractive index higher than a refractive index of the medium outside the light guide. When these two sides are tapered, the angle at which the light is incident on the side relative to the normal of the side will become smaller by going through a reflection on the other side. After a number of reflections, this angle will eventually be below the critical angle of total internal reflection, and the light will be coupled out.

The light coupled into the light guide is viewed as a bundle of light rays in the form of a cone. The cross section of the cone will increase as the light travels through the light guide. When the cross section of the cone becomes larger, the light rays are distributed over a larger area, lowering the peak brightness. The number of times that the light experiences total- internal reflection before being coupled out of the light guide depends among others on the angle between the inner side and the outer side, the positioning of the light source with respect to the light guide, intensity distribution of the source, arrangement of the source(s) and the refractive indices of the light guide and the medium outside of the light guide. These parameters can be adjusted such that two or more total internal reflections occur for a large portion of the light coupled into the light guide.

By suitably arranging the above parameters of the light guide to couple the light out of the light guide after at least two total internal reflections, the peak brightness of the light coupled in will be significantly decreased when the light is coupled out. In this way an improved visual comfort of the luminaire is obtained.

According to a preferred embodiment of the invention, the light guide of the light output device is rotation symmetrical. However, the light guide of the light output device may also have an elliptical shape or a polygonal shape such as a hexagonal shape, an octagonal shape, or similar shapes. The light guide may have a shape formed by rotating a wedge type geometry over a central axis. Also the wedge surfaces can have a slight curvature to fine tune the intensity distribution.

According to a further embodiment of the invention, the light output device further comprises an optical body surrounded by the inner side of the light guide, the optical body having a refractive index lower than a refractive index at the inner side of the light guide. By selecting an optical body with a suitable refractive index, it is made possible to control the number of the total internal reflections even when the dimension of the light guide is limited.

According to a further embodiment of the invention, the outer side is covered by a reflecting surface. With this embodiment, the light coupled out from the outer side is reflected towards inner side. This results in that all light will leave the light output device from the inner side of the light guide.

According to a further embodiment of the invention, the light guide comprises a base side arranged for coupling the light into the light guide. According to a further embodiment of the invention, a phosphor layer is arranged on the base side. The phosphor layer may comprise e.g. YAG:Ce or a mixture of various phosphors. The phosphor layer converts the light from the light source into light of different wavelengths. The phosphor may be coated on a sheet and the sheet may be arranged on the base side. According to a further embodiment of the invention, the light source is mounted to a light source mounting element provided at a mounting part of the device, the light source being surrounded by the base side. The mounting part of the device may be the rim of the base side. The light source mounting element may be a plate which partly or wholly covers the space surrounded by the base side. This provides an alternative form of the light output device in which the light source is not directly mounted on the base side.

According to a further embodiment of the invention, the light coupled out from the light guide is emitted through a light emitting window defined by a free rim of the light guide.

According to a further embodiment of the invention, the device comprises a shielding element in the center of the light guide having a reflecting or absorbing surface for preventing the light from the light source from being directly emitted through the light emitting window. With this arrangement, more freedom of design is achieved for the tapering angle. The space surrounded by the base side of the light guide and the shielding element may act as a light mixing cavity for mixing the light from the light source to be mixed before being coupled into the light guide.

According to a further embodiment of the invention, the inner side and the outer side are tapered to each other in the direction from the base side to the free rim. According to a further embodiment of the invention, the outer side is tapered in the direction from the free rim to the base side. This allows the light to be emitted through the light emitting window which is larger than the base side where the light was coupled in.

According to a further embodiment of the invention, the light source comprises one or more LEDs. For instance, the light source may comprise a single LED (e.g. white or monochromatic) or several LEDs (e.g. R, G, B or R, G, B, A). LEDs are beneficial from many points of view, for instance power efficiency and long lifetime. Instead of LEDs, other appropriate light sources could be used, such as other semiconductor light sources (e.g. laser diodes). When a phosphor layer is arranged on the base side, the phosphor converts part of the light from one or more blue LEDs into yellow light. The combination yields white light.

According to another aspect of the invention, a light guide is provided suitable for use in the light output device according to the invention.

According to another aspect of the invention, a luminaire housing is provided comprising at least one light source mounting element and at least one light guide for guiding light from the light source, the light guide having a concave inner side and an outer side tapered to each other, wherein the light guide is arranged to couple the light out of the light guide over an area of the inner side of the light guide after at least two total internal reflections on the inner side and/or the outer side of the light guide.

According to another aspect of the invention, a luminaire is provided comprising a luminaire housing according to the invention and at least one light source.

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

BRIEF DESCRIPTION OF THE DRAWINGS These and other aspects of the invention will be apparent from and elucidated with reference to the drawings in which:

Figure Ia schematically illustrates a cross section of an embodiment of the light output device according to the invention; Figure Ib shows rotation of a wedge-shaped geometry which constitutes an embodiment of the light guide according to the invention;

Figure 2 is an angle distribution of the light coupled out of an embodiment of the light output device according to the invention; Figure 3 is a schematic diagram of the reflection of light in an embodiment of the light guide according to the invention;

Figure 4 schematically illustrates a cross section of a further embodiment of the light output device according to the invention and

Figure 5 schematically illustrates a cross section of a further embodiment of the light output device according to the invention.

Figure 6 schematically illustrates a cross section of an embodiment of the luminaire housing according to the invention.

DETAILED DESCRIPTION OF EMBODIMENTS Referring to Figure Ia, the cross section of a light output device 100 is shown comprising a light source 101 and a light guide 102. The light guide 102 has a rotation symmetrical form, which is formed by rotating a wedge-type geometry over a central axis Z as illustrated in figure Ib. The wedge can be described by the parameters (length a, length b, distance x, angle ψ) as indicated in this figure. The total height H of the construction is described by

H = a * sin (90 -ψ )+ b * sinψ The optical element has a light emission window 107 with a diameter d defined by d = (x + b * cosψ ) The light guide 102 comprises an inner side 103, an outer side 104, a base side 105 and a free rim 106. Since the light guide 102 is formed by rotating a wedge-type geometry over a central axis, the inner side 103 has a rotation symmetrical concave shape (truncated cone shape). The inner and the outer sides 103, 104 are tapered to each other in the direction from the base side 105 to the free rim 106 of the light guide where the two sides join and make an angle α. The free rim 106 defines a light emitting window 107 from which the light is emitted. The outer side 104 is tapered in the direction from the free rim 106 to the base side 105. The outer side 104 of the light guide 102 is covered by a mirror 108. The mirror 108 may be made from silver composition known per se. The space, being a cavity, surrounded by the inner side 103 and the light emitting window 107 is at least partly filled with an optical body 109 having a refractive index lower than a refractive index at the inner side 103 of the light guide 102.

In this embodiment, the light source 101 is provided on a phosphor layer 112 arranged on the base side 105 of the light guide 102. However, it is noted that the phosphor layer 112 is optional and that the light source 101 may be directly provided on the base side 105. The light source 101 comprises a plurality of LEDs 101 which emit lights of different colors, e.g., red, green and blue. The phosphor layer 112 may comprise e.g. YAG:Ce or a mixture of various phosphors. With this arrangement, the phosphor layer 112 converts part of the light from blue LEDs into yellow light. The combination of the yellow light and the non- converted blue light yields white light. The light guide 102 may be made of polymethylmethacrylate (PMMA), polycarbonate, glass or similar materials.

The light from the light source 101 is emitted in the light guide 102 between the inner and the outer sides 103, 104. The light transmitting in the light guide 102 will alternately be reflected on the inner and outer sides 103, 104. Through reflections, the angle at which the light is incident on the inner side 103 relative to the normal of the inner side 103 will eventually become lower than the critical angle of total internal reflection, and the light will be coupled out of the inner side 103. The light will be coupled out over an area of the inner side 103, to be emitted through the light emission window 107.

Figure 2 shows an angle distribution of the light coupled out of an exemplary light output device according to the present invention. In this figure, an angle distribution is shown for a light output device formed by rotating a wedge-type geometry having dimensions of length a=5mm, length b=30mm, distance x=0mm, angle ψ=35 degree. In this exemplary embodiment, three LEDs having a dimension of lmm² are located 2 mm from the central axis and are equally distributed around this axis. It will be appreciated that these dimensions and the arrangement of the LEDs are merely given as examples and the present invention is not limited to this embodiment. The cut-off in the angle distribution is determined by the angle ψ.

In figure 3, a cross section of the light guide according to the invention is shown, in which light rays travel through the light guide between the inner side 103 and the outer side 104. The light rays traveling in the light guide 102 are illustrated as a cone C of rays. The figure shows the change in the area of cross section of the cone of the light rays through reflections. The cone has light rays Al and Bl at the surface of the cone. The ray Al incident on the inner side 103 will be reflected thereby and becomes a ray A2. The ray A2 in turn will be reflected by the outer side 104 and becomes a ray A3. Similarly, the ray Bl incident on the inner side 103 will be reflected thereby to become B2, which in turn will be reflected by the outer side 104 to become B3. The distance between the points where the rays A3 and B3 hit the inner side 103 will be larger than the rays Al and Bl hit the inner side 103. The area of the cross-section of the cone of light rays is thus larger after reflections. This means that the light rays are incident on larger area, so the perceived brightness is decreased if the light rays are coupled out.

In figure 4, the light source is mounted to a light source mounting element 110 provided at a rim of the base side. The light source mounting element seals the space surrounded by the base side. The base side 105 of the light guide 102 extends to the center axis of the light guide. Substantially all the light from the light source will be coupled into the light guide through the base side.

In figure 5, the light source is mounted to a light source mounting element 110 provided at a rim of the bottom side, similarly to figure 4. The base side 105 of the light guide 102 does not extend to the center axis. The light output device comprises a shielding element 111 in its center having a reflecting surface. Substantially all the light from the light source will be coupled into the light guide through the base side. The shielding element 111 prevents direct light from the light source 101 without experiencing reflections between the inner and the outer sides 103, 104. It is thus prevented that the light from the light source 101 is directly emitted through the light emitting window 107. It is to be noted that the shielding element 111 may alternatively have an absorbing surface.

Figure 6 shows a luminaire housing 200 comprising a plurality of light source mounting element 110 and a plurality of light guide 102. The light guide 102 has a concave inner side 103 and an outer side 104 tapered to each other. The light guide 102 is arranged to couple the light out of the light guide 102 over an area of the inner side 103 of the light guide 102 after at least two total internal reflections on the inner side 103 and/or the outer side 104 of the light guide 102.

In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. Use of the verb "comprise" and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The article "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Claims

CLAIMS:

1. A light output device (100) comprising a light source (101) and a light guide

(102) for guiding light from the light source, the light guide having a concave inner side

(103) and an outer side (104), which sides are tapered to each other, wherein the light guide is arranged to couple the light out of the light guide over an area of the inner side of the light guide after at least two total internal reflections on the inner side and/or the outer side of the light guide.

2. A light output device as claimed in claim 1, wherein the light guide is rotation symmetrical.

3. A light output device as claimed in claim 1 or 2, further comprising an optical body (109) surrounded by the inner side, the optical body having a refractive index lower than a refractive index at the inner side of the light guide.

4. A light output device as claimed in claim 1, 2 or 3, wherein the outer side is covered by a reflecting surface (108).

5. A light output device as claimed in any one of the claims 1-4, wherein the light guide comprises a base side (105) arranged for coupling the light into the light guide.

6. A light output device as claimed in claim 5, wherein a phosphor layer (112) is arranged on the base side.

7. A light output device as claimed in claim 5 or 6, wherein the light source is mounted to a light source mounting element (110) provided at a mounting part of the device, the light source being surrounded by the base side.

8. A light output device as claimed in any one of the claims 5-7, wherein the light coupled out from the light guide is emitted through a light emitting window (107) defined by a free rim (106) of the light guide.

9. A light output device as claimed in claim 8, wherein the device further comprises a shielding element (111) in the center of the light guide, which shielding element has a reflecting or absorbing surface for preventing the light from the light source from being directly emitted through the light emitting window.

10. A light output device as claimed in claim 8 or 9, wherein the inner side and the outer side are tapered to each other in the direction from the base side to the free rim.

11. A light output device as claimed in claim 8, 9 or 10, wherein the outer side is tapered in the direction from the free rim to the base side.

12. A light output device as claimed in any one of the claims 1-11, wherein the light source comprises one or more LEDs.

13. A light guide suitable for use in the light output device according to any one of claims 1-12, characterized as defined in any one of claims 1-12.

14. A luminaire housing (200) comprising at least one light source mounting element (110) and at least one light guide (102) for guiding light from a light source, the light guide having a concave inner side (103) and an outer side (104), which sides are tapered to each other, wherein the light guide is arranged to couple the light out of the light guide over an area of the inner side of the light guide after at least two total internal reflections on the inner side and/or the outer side of the light guide.

15. A luminaire comprising a luminaire housing (200) as claimed in claim 14 and at least one light source (101).