Illumination system

FIELD OF THE INVENTION

The present invention relates to an illumination system comprising at least one light source and at least one redirection plate, which is arranged in front of said light source.

TECHNICAL BACKGROUND

There are many different illumination systems on the market, which use light sources such as for example halogen, fluorescent lamps, incandescent lamp or LED.

Today LEDs are used more and more in offices and professional environments and in apparatuses where light sources are needed, because they fulfill several important requirements. These requirements are for example long lifetime, robustness against dust and other dirt. They also have the advantage of being small, so that also a flat and thin illumination system may be created. A flat and thin illumination system makes it possible to use the illumination system in different applications for example as a back light in an apparatus with an LCD screen. An LCD is a non-emissive (i.e. passive) flat panel display that needs light to produce an image. A flat and thin illumination system may also be used as a normal lamp. Such a system may be mounted in a recess in a ceiling, or against a surface of the ceiling or wall, or be suspended from the ceiling. It is often a desire to focus the light in a certain area or direction. Sometimes illumination systems also need to fulfill an anti-glare requirement. However, LEDs systems are known to often show very bright spots caused by the small light-emitting area.

It is known to use waveguides for spreading and mixing light. One system without a waveguide is described in the following document. WO 2006/097859 shows a luminaire provided with a translucent lighting panel which is located in front of a light source present or to be accommodated in the luminaire, wherein the outer side of the lighting panel has a profiled surface, at least half of which is positioned at an angle between 20° and 50° to the plane of the lighting panel. The outer side of the lighting panel is the side facing away from the light source and may form the front side of the luminaire. The profiled surface directs the emitted light radiation mainly into a predetermined zone, which light radiation has relatively small angles to the direction perpendicular to the plane of the lighting panel.

It is known to arrange for example redirectional foil in front of a light source in order to redirect light from the light source in another direction. Conventional redirection foils work preferentially for only one direction of the incident beam and stacking several foils is required. These foils are usually quite expensive and they will have to be mounted on a transparent plate.

However, since the out-coupled light from foils causes at large angles a glare, there is still a need for an improved illumination system.

SUMMARY OF THE INVENTION The object of the present invention is to provide an illumination system, which is improved in order to reduce or even prevent glare.

This is achieved by an illumination system comprising at least one light source and at least one redirection plate, which is arranged in front of said light source, wherein, in operation, light from said light source is collimated essentially parallel to said redirection plate, and the redirection plate has at least one structured surface facing said light source and arranged to receive light from said light source, and an opposite light emitting surface arranged to emit said light after having passed through said redirection plate. Further, the redirection plate is arranged to redirect said light, so that light exiting said light emitting surface has a direction within a predetermined angular range. By ensuring that light is collimated essentially parallel to the redirection plate, the light in the illumination system is confined inside the system and is therefore distributed and, in case a plurality of light sources is present, mixed with the light from different light sources, e.g. LEDs. This prevents bright spots and results in a uniform light extraction, which also makes the system robust against differences between LEDs and individual LED failures. The system is also robust against small deviations from the ideal shape of the structured surface of the redirection plate when the light is collimated before it is incident on the redirection plate.

The expression "collimated essentially parallel to the redirection plate" is intended to mean that the light is collimated inside a certain angle range centered around a direction parallel to the redirection plate. Preferably, the light rays are collimated inside ±40°, preferably ±20° and more preferably ±10° from a direction parallel to the redirection plate. According to one embodiment, the redirection layer maintains essentially the same degree of collimation, so that light exiting the system is collimated essentially to the same degree as light inside the system.

The plate may be formed as one member, and the structured surface may e.g. be a structured pattern in a transparent substrate. Alternatively, a separate redirection layer, e.g. a redirection foil, may be arranged on a supporting substrate to form the plate with a structured surface. Advantageously, the structured surface comprises at least one top-shaped structure. A top-shaped structure may for example be a pyramid or a cone. The top-shaped structure directs the light rays in a second direction towards the outer side of the redirection plate i.e. on the side facing away from the light source. Since the light is collimated before it encounters the redirection plate, the system is robust against small deviations from the ideal shape for the top of the top-shaped structure such as a tapered top

The top angle for the top-shaped structure is determined by the refraction index of the material used and can be e.g. between 50° and 90°and preferably between 55° and 77. In ca 90% of the cases the light output is collimated to roughly ±60°. The remaining 10% of the light out-couple at angles larger than 60°. To eliminate rays which travel more through the material at skew angles, an absorbing material layer can be provided on the bottom of the redirecting plate. All top-shaped structures on the same redirection plate do not have to have the same top angle, and the top angle may be changed from the top to the base. For example, the side walls of the top-shaped structure may be slightly curved i.e. convex or concave. They may make one angle with the normal at the top and a different one at the base. Advantageously, at least one deflecting structure is arranged at a distance from the light source and extends at least partially in the area of the light rays. Such a deflecting structure prevents light emitted by one light source being absorbed by other light sources, especially the light rays going in the lateral direction. Preferably, the defecting structure is top-shaped. A top-shaped structure directs the light rays from the light source first in one direction and then in a second direction. Preferably, they are bended in the direction towards the redirection plate.

Advantageously, the base of the top-shaped deflecting structure is directed to the redirection plate. This way the light rays are directed towards the redirection plate.

The deflecting structure may be part of the redirection plate. By making it part of the redirection plate, no extra parts are needed and the assembly is easier.

Preferably the light source is a side emitting LED. LEDs have advantages which other light sources do not have, for example, they are small, robust, of low energy, they are rapidly decreasing in cost and have a long life. The LED can be a LED of any kind

for example a laser diode, a semiconductor device made of inorganic material or an organic LED (OLED).

The light source may be arranged in a box-like casing having an inside of which at least a part is reflective and the redirection plate is arranged at least partially in the opening of the box- like casing. By arranging the light sources this way, the illumination system is ready to mount on the wall or to a LCD screen.

The illumination system may comprise a plurality of light sources.

Preferably, at least one of the walls of the box- like casing having an inside of which at least a part is reflective is inclined to a line which is perpendicular to the redirection plate. In this way the light rays, which are traveling in the lateral direction towards the wall are prevented to bounce back and forth between the reflective walls or to be absorbed by the LEDs.

Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to "a/an/the [element, device, component, means, step, etc]" are to be interpreted openly as referring to at least one instance of the element, device, component, means, step, etc., unless explicitly stated otherwise. The steps of any method disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated.

Other objectives, features and advantages of the present invention will appear from the following detailed disclosure, from the attached dependent claims as well as from the drawings.

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

BRIEF DESCRIPTION OF THE DRAWINGS

The above, as well as additional objects, features and advantages of the present invention, will be better understood through the following illustrative and non- limiting detailed description of preferred embodiments of the present invention, with reference to the appended drawings, where the same reference numerals will be used for similar elements, wherein:

Fig. 1 shows a cross-section of one embodiment of an illumination system according to the invention.

Figs. 2a and 2b show two embodiments of a redirection plate according to the invention.

Fig. 3 shows a cross-section of a second embodiment of an illumination system according to the invention.

Fig. 4 shows a cross-section of the defecting structure. Fig. 5 shows how the deflecting structure may be arranged in relation to the light sources and a side wall in an illumination system according to the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

A currently preferred embodiment of the present invention will now be described in more detail, with reference to the accompanying drawings. Fig 1 shows an illumination system 1 as a box system where the bottom 3 and walls 4 of the box 2 is provided with reflecting surfaces. In a simple case the surfaces are mirrors. However, it is not necessary that the entire inside of the box 3 is covered with a reflecting material. The walls 4 are here perpendicular to the bottom 3. However, the walls 4 of the box 2 may be inclined in order to prevent the light rays traveling in the lateral direction towards the wall 4 to bounce back and forth between the mirrored walls 4 or to be absorbed by the LEDs 5.

On the bottom 3 of the box 2 two light sources 5 are arranged. Here in form of side-emitting LEDs. They may be both low and/or high-power LEDs. An optical plate 6 closes the opening of the box 2. The optical plate 6 is a redirection plate, which is so formed that when a ray 9 incident upon it, on the inside of the box 2 in a direction close to the parallel to the plate direction, it will exit the box 2 at an angle close to the perpendicular direction. The redirection plate has on one side a structured surface 7 and on the other side a flat surface 8, i.e. a light emitting surface. The structured surface 7 is facing the inside of the box 2 and the light sources 5 while the flat side 8 is facing the outside. The structured surface comprises small top-shaped structures, which are used to redirect the light. The top-shaped structure may for example be a pyramid or cone.

The working principle is that the light rays 9 are collimated in-plane i.e. they propagate mainly in the lateral direction inside the box 2, i.e. parallel to the plate 6. Preferably, inside ±20°, more preferably ±10°, from the horizontal direction by means of primary or secondary optics. The collimation may be done within the LED or by additional optical elements (not shown), for example through CPC shapes (compound parabolic concentrator) or collimation structures using total internal reflection. The light rays 9 (the number 9 symbolize all light rays) mix in air and as soon as a collimated ray 9a, 9b', 9c, 9d is incident on the structured redirection plate 6 it is redirected and extracted as a collimated ray

9a',9b", 9c', 9d' in a predetermined direction, which is mainly perpendicular to the flat side 8 of the redirection plate 9. However, the flat side may also have a structure, then the ray will be directed in another direction, for example where glare is not an issue. When the ray 9b hits the mirrored wall 4 it will be reflected in another direction 9b' until it reaches the redirection plate 6 or another LED.

Figs. 2a and 2b show two different variants of the redirection plate 6. Both have a top-shaped structured upper part 7 and a flat side 8. The structure 7 in Fig. 2a is square-base pyramids and the structure in Fig. 2b is conical pyramids with a round base. However, the base is not limited to this, it may have any kind of form. The pyramids/cones may be arranged on the plate in rows where the rows are arranged so that they also are in columns or the rows may be arranged offset as in fig. 2a and 2b. The number of top-shaped structures is preferably more then the number of LEDs. However it is not limited to this. The whole plate does not have to have top-shaped structures. The top-shaped structure 7 determines the direction of the out coupled collimated beam. By coupling out a collimated beam in a direction, which is substantially perpendicular to the flat side of the redirection plate glare is avoided. The top angle of the top-shaped structure is preferably between 50° and 90°, more preferably between 55° and 77°.

In fig. 3 a second embodiment of the illumination system 1 is shown. It shows a similar box 2 but with inclined walls 4 and with a redirection plate 6 in front of the box opening. Two LEDs 5 are arranged on the bottom 3 of the box 2. In order to prevent light emitted by one LED 5 being absorbed by other LEDs, especially the light rays 9a going in the lateral direction, i.e. parallel with the bottom 3 of the box 2, a deflecting structure 10 may be positioned at a distance from one LED or between two neighboring LEDs or between a LED and a side wall. The deflecting structure 10 is part of the redirection plate 6. However, this is not necessary. It may be a separate part, arranged either to the redirection plate 6 or the bottom 3 of the box 2 or in any other way, as long as it extends at least partially in the area of the light rays. The deflecting structure 10 extends from the redirection plate 6 to the light holding arrangement, here the bottom 3 of the box. However, the deflecting structure may be at a distance from one or the other or both. The light ray 9b is deflected by the slanted wall 4 in the direction of the light ray 9b'. The light ray 9a is redirected by the deflecting structure 10 in the direction of light ray 9a'.

The working principle of the deflecting structure 10 is described and shown in more detail in the cross section in fig. 4. The defecting structure 10 may be a pyramidal or a cone structure or be arranged all around a light source. The working principle is the

following: a collimated light ray 9' is propagating in the horizontal direction and hits the deflecting structure 10 under an incident angle, CC, to the normal, which is perpendicular to a facet 11 of the deflecting structure 10. The angle α is similar with the one between the structure facets and the vertical. The ray 9' changes direction depending on the refraction index of the material in the deflecting structure. After refraction at the first facet 11 (refraction angle β) the ray 9" undergoes a second refraction at the other facet 12 (new refraction angle γto the normal, which is perpendicular to the second facet 12 of the deflecting structure 10 ). The angles α,β,γmay be calculated using Snell's law.

The following mathematic calculus (where small angels approximations are used (sinα=α)) leads to a final angle between the ray and the horizontal direction: where n is the refraction index of the material of the deflecting structure 10. The final ray angle γ-cc is determined by the (smallest) distance between the LEDs and their height, so the top angle (the angle to a line which is perpendicular to said redirection plate ) may be calculated with the above formula i.e. the angle CC. The angels of the deflecting structure depend on, for example, the refraction index of the material and/or on the LED dimensions and/or the distance between LEDs.

Fig. 5 shows how the deflecting structures 10 with square base pyramids may be arranged in an illumination system 1 close to a wall 4, which comprises a matrix of four LEDs 5. The sides of the deflecting structures 10 are so arranged that they are facing the light sources 5 in order to direct the light beams.

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, the light sources of the illumination system are not limited to side-emitting LEDs. It may be all kind of light sources and the numbers of light sources may vary. It may be only one or several. In order to collimate the light from for example a top emitting LED, secondary optics known in the art may be used, this kind of optics is available, for example from companies such as Carclo Optics. The light sources may be electrically connected to each other in such a way that if one light source malfunctions the remaining light sources will still work. The light sources may be connected to be switched on and off by a switch known in the art.

The material of the redirection plate may be of a translucent material, for example a glass or a plastic material, which may for example be PMMA or polycarbonate.

The material may also contain a light-absorbing agent in order to achieve a better light collimation (by absorbing the light to be out coupled at large angles.) The material of the deflecting structure may be the same as the redirection plate and may also contain a light- absorbing agent.