WO2005073622A1

WO2005073622A1 - Light-emitting panel and illumination system

Info

Publication number: WO2005073622A1
Application number: PCT/IB2005/050216
Authority: WO
Inventors: Denis J. C. Van Oers; Stefan M. Verbrugh; Christoph G. A. Hoelen
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2004-01-30
Filing date: 2005-01-19
Publication date: 2005-08-11
Also published as: TW200540476A

Abstract

A light-emitting panel (1) has a light-emitting window (2) and a rear wall (3) situated opposite thereto. The light-emitting panel has at least one light-ingress edge (4) for coupling light from a light source (6B, 6G, 6R) into the light-emitting panel. The light-emitting panel is provided with a plurality of slits (8, 8', 8', ...) for coupling out light from the light-emitting panel. The slits extend from the rear wall to the light-emitting window. Material in the slits has a refractive index different from the refractive index of the light-emitting panel. Preferably, the slits are filled with a dielectric material, preferably, with air. Preferably, the slits are substantially flat planes and are substantially parallel with respect to each other. Preferably, the slits are arranged under an angle a with respect a normal on the light-emitting window. Preferably 30°≤α≤40°. Preferably, light is coupled out from the light-emitting panel by specular reflection.

Description

Light-emitting panel and illumination system

The invention relates to a thin light-emitting panel comprising a light-emitting window and a rear wall situated opposite thereto, and at least one light-ingress edge for coupling light from a light source into the light-emitting panel. The invention also relates to an illumination system provided with the above- mentioned light-emitting panel. Such light-emitting panels are known per se and are also denoted edge light- emitting panels. They are used, inter alia, as backlight-emitting panels in (picture) display devices, for example for TV sets and monitors. Such light-emitting panels are particularly suitable for use as backlights for non-emissive displays such as liquid crystal display devices, also denoted LCD panels, which are used in (portable) computers or (portable) telephones. Said display devices usually comprise a substrate provided with a regular pattern of pixels which are each controlled by at least one electrode. The display device utilizes a control circuit for achieving a picture or a data graphical display in a relevant field of a (picture) screen of the (picture) display device. The light originating from the backlight in an LCD device is modulated by means of a switch or modulator, various types of liquid crystal effects being used. In addition, the display may be based on electrophoretic or electromechanical effects. Such light-emitting panels are also used as luminaires for general lighting purposes or for shop lighting, for example shop window lighting or lighting of (transparent or semi-transparent) plates of glass or of (transparent) plates of glass or of (transparent) synthetic resin on which items, for example jewelry, are displayed. Such light-emitting panels are further used as window panes, for example for causing a glass wall to radiate light under certain conditions, or to reduce or block out the view through the window by means of light. A further alternative application is the use of such light-emitting panels for illuminating advertising boards. In the light-emitting panels mentioned in the opening paragraph, the light source used is usually a tubular low-pressure mercury vapor discharge lamp, for example one or several so-called cold-cathode fluorescent lamps (CCFL), wherein the light emitted by the light source during operation is coupled into the light-emitting panel, which acts as an optical waveguide. This waveguide usually constitutes a comparatively thin and planar panel which is manufactured, for example, from synthetic resin or glass, and in which light is transported through the optical waveguide under the influence of (total) internal reflection. As an alternative light source, such a light-emitting panel may also be provided with a plurality of light-emitting diodes (LEDs). These light sources are usually provided in the vicinity of or tangent to a light-transmitting edge surface of the light-emitting panel, in which case light originating from the light source is incident on the light- transmitting edge surface during operation and distributes itself in the panel. A light-emitting panel for illuminating an image display device is known from the international patent application WO-A 01/88 430. The known image display device comprises a light modulation panel, an illumination device arranged opposite the light modulation panel and a light source connected to the illumination device. The illumination device comprises at least one separating layer which has at least one flat surface extending parallel to the light modulation panel and a serrated surface situated on a side which faces away from the flat surface. Said serrated surface includes a number of saw-tooth-shaped, juxtaposed projections which are each bounded by a first and a second separating surface, the first separating surface enclosing an acute interior angle with the flat surface and the second separating surface enclosing an obtuse interior angle with the flat surface. A disadvantage of the known light-emitting panel is that the efficiency of the light-emitting panel is relatively low.

The invention has for its object to eliminate the above disadvantage wholly or partly. According to the invention, a light-emitting panel of the kind mentioned in the opening paragraph for this purpose comprises: a light-emitting window and a rear wall situated opposite to the light-emitting window, at least one light-ingress edge for coupling light from a light source into the light-emitting panel, the light, in operation, spreading in the light-emitting panel, the light-emitting panel being provided with a plurality of slits for coupling out light from the light-emitting panel, the slits extending from the rear wall to the light-emitting window, material in the slits having a refractive index different from the refractive index of the light-emitting panel. In this patent application, a slit is understood to be a relatively narrow opening in the light-emitting panel, the slit extending through the light-emitting panel from the rear wall to the light-emitting window. Because the material in the slits has a refractive index different from the refractive index of the material of the light-emitting panel, light is coupled out from the light- emitting panel. The change in refractive index at the interface between the light-emitting panel and the slit causes a reflection resulting the coupling of light out of the light-emitting panel. A particularly compact light-emitting panel is obtained through the measure according to the invention. A relatively high efficiency is realized thereby in particular in the case of (picture) display devices. A preferred embodiment of the light-emitting panel according to the invention is characterized in that the light is substantially coupled out from the light-emitting panel by specular reflection. In known light-emitting panels uniformity of light is normally obtained by using scattering of light. Light-scattering foils or other diffusing structures create a translucent but not transparent light-emitting window of the light-emitting panel. Because of the use of scattered light, the efficiency of the known light-emitting panels used as (backlight) luminaires is limited to approximately 50% which is too low for general illumination applications. Another disadvantage of the known light-emitting panel based on the scattering of light is that the light is emitted as a so-called Lambertian emitter (with a large surface area). Lambertian emission makes the light-emitting panel unsuitable and inefficient for general lighting purposes. In addition, information about the direction of the light can not be used if the mechanism for coupling light from the light-emitting panel is diffuse. When light is diffusely scattered the directions of the emitted light are randomized, making no discrimination between the directions towards the front and back surface of the panel. As a consequence, the known light-emitting panel based on scattering of light emits more or less equally from the light-emitting window as well as from the rear wall unless a mirror is used at the location of the rear wall to block the emission of light in that direction. By employing substantially specular reflection, the light source or the individual light sources can be seen when looking to the light-emitting panel. When a discrete number of light sources are used, such as LEDs, the brightness of such light sources is "reflected" in the various slits, resulting in a sparkling effect. This sparkling effect gives the light-emitting panel a dynamic character because the light sources seem to move when the observer changes his/her position relative to the light-emitting panel. This sparkling effect is substantially enhanced by applying discrete light sources with various colors. Another advantage of employing specular reflection upon coupling light out from the light-emitting panel is that a substantially transparent luminaire is created that emits light substantially in only one direction, i.e. the light-emitting panel only emits light from the light-emitting window and practically no light is emitted from the rear wall of the light- emitting panel. In this manner a kind of "light shield" or "light curtain" is created. For a spectator it is possible to look through the light-emitting panel in a direction from the rear wall towards the light-emitting window, i.e. in the direction of area illuminated by the light- emitting panel, while in the opposite direction the spectator mainly observes reflection of the light sources at the various slits in the light-emitting panel. If the luminance of the light sources is relatively high, these reflections may have the result that the spectator can not see what is behind the light-emitting panel when he tries to look through the light-emitting panel in a direction from the light-emitting window towards the rear wall. Light-emitting panels based on scattering of light are only partly transparent.

The light-emitting panel based on specular reflection is substantially transparent. The light- emitting panel according to the invention can be seen through with clarity. A light-emitting panel being transparent enables the design and manufacture of a new class of luminaries with a novel and attractive appearance. The light-emitting panel based on an optically transparent light-emitting panel according to the invention is a simple, robust and easy to use single- member light tile. Preferably, the slits of the light-emitting panel are filled with a dielectric material. Because the refractive index of the dielectric material in the slits differs from the refractive index of the material of the light-emitting panel, light is coupled out from the light- emitting panel. The change in refractive index at the interface between the light-emitting panel and the dielectric material in the slit causes a reflection resulting the coupling of light out of the light-emitting panel. A very favorable embodiment of the light-emitting panel according to the invention is characterized in that the slits are filled with air. By definition, the refractive index of air is 1. The refractive index of the light-emitting panel is normally around 1.5, depending on the material of the light-emitting panel. The change in refractive index at the interface between the light-emitting panel and the air in the slits causes a reflection resulting the coupling of light out of the light-emitting panel. A preferred embodiment of the light-emitting panel according to the invention is characterized in that the slits are substantially flat planes. Light-emitting panels with planar slits are easily manufactured, e.g. by laser cutting. Preferably, the slits are substantially parallel with respect to each other. Light-emitting panels with plan parallel slits are easily manufactured, e.g. by laser cutting. The orientation of the slits in the light-emitting panel determines where the light is coupled out from the light-emitting panel. To this end a preferred embodiment of the light-emitting panel according to the invention is characterized in that the slits are oriented such in the light-emitting panel that the distance of a slit to the light-ingress edge decreases from the light-emitting window towards the rear wall. In this manner light incident at the light-ingress edge reflects at the slit and is coupled out from the light-emitting panel at the light-emitting window. In a preferred embodiment of the light-emitting panel, the slits are arranged under an angle with respect to a normal on the light-emitting window, the angle being in the range from 10°< α <80°. Preferably, the angle is in the range from 30°< <40°. This latter range of angles is particularly suitable when the slits are filled with air. Selecting an angle in the range from 30° to 40° enables obtaining a substantially symmetric light distribution. In a favorable embodiment of the light-emitting panel a substantially uniform flux density of the light emitted by the light-emitting window is obtained. To this end a preferred embodiment of the light-emitting panel according to the invention is characterized in that the length of the slits and the pitch between the slits varies with respect to the light- ingress edge. Preferably, the length of the slits increases and the pitch between the slits decreases with respect to the light-ingress edge. Close to the light-ingress edge the coefficient of coupling light out of the light-emitting panel is chosen to be relatively low because there is relatively much flux available in this part of the light-emitting panel. At distances further away from the light-ingress edge the coefficient of coupling light out of the light -emitting panel is chosen to be larger because part of the light flux has already been coupled out. In an alternative embodiment of the light-emitting panel according to the invention, the number of slits in a line parallel to the light- ingress edge increases with distance from the light- ingress edge, such that the total effective length of the slits that are at the same distance from the light- ingress edge, increases. In this manner, the number of slits and the length of the slits increase as a function of the distance from the light-ingress edge. A further preferred embodiment of the light-emitting panel according to the invention is characterized in that the light-emitting panel comprises a further edge situated opposite to the light-ingress edge, the further edge being provided with a specular or diffuse reflector. Specular reflection enables the coupling of light out from the light-emitting panel also at the rear wall of the light-emitting panel. Diffuse reflection enables the diffusely coupling of light out from the light-emitting panel at the rear wall of the light-emitting panel. In the latter situation the light at the light-emitting window is crystal clear whereas the light emitted by the rear wall is diffusely emitted. In a preferred embodiment of the light-emitting panel the light-emitting panel is provided with a sensor for measuring the optical properties of the light emitted, in operation, by the light-emitting panel. In this manner a dynamic control of the light output of the light-emitting panel is obtained. In addition, the sensor can also be employed to adjust the light emitted by an individual light source, for instance, to compensate for effects of temperature. A further preferred embodiment of the light-emitting panel according to the invention is characterized in that the light-emitting panel comprises a further ingress edge for coupling light from a further light source into the light-emitting panel, the further ingress edge being situated opposite to the light-ingress edge, the rear wall functioning as a further light-emitting window. If light is coupled in the light-emitting panel at two ingress edges with separate light sources, the light from one edge can be directed to only one side (light emitting window) of the light-emitting panel and the light from the further ingress edge is emitted by the rear wall functioning as the further light-emitting window. If by way of example the light-emitting panel is mounted horizontally, then part of the light is directed towards the ceiling (indirect ambiance lighting) and another part of the light is directed away from the ceiling (functional direct lighting). In addition, the light directed in both directions can be controlled separately (intensity, color). The invention also relates to an illumination system provided with the above- mentioned light-emitting panel. According to the invention, an illumination system provided with a light-emitting panel of the kind mentioned in the opening paragraph is characterized in that: the light-ingress edge is associated with a single light source or with a plurality of light sources. Preferably, the plurality of light sources comprises at least two light-emitting diodes (LEDs) with different light emission wavelengths. Colors may be mixed in a desired manner through a suitable use of LEDs, for example for making white light of a desired color temperature. The LEDs preferably comprise the combinations of red, green, and blue LEDs known per se, or, for example, combinations of red, green, blue, and amber LEDs. LEDs with three light emission wavelengths may also be realized by means of two LEDs with different light emission wavelengths, wherein the LEDs of one of the types are (partly) provided with a phosphor, such that the light emission of the LED is converted by the phosphor into light of a third, desired light emission wavelength. A combination, known per se, of the red, green, and blue LEDs renders it possible to realize color changes independently of the status of the display device. The use of LEDs has the further advantage that dynamic lighting possibilities are obtained. For this purpose, a sensor is present at one of the edge surfaces for measuring the optical properties of the light emitted by the light source during operation. The quantity of light emitted by the LEDs is adjusted in that the luminous fluxes of the light-emitting diodes are varied. This control of the luminous flux usually takes place in an energy-efficient manner. The LEDs can be dimmed without an appreciable loss in efficacy. Preferably, the intensity of the light emitted by the light-emitting diodes is variable in response to the desired illumination level or in response to the level of the ambient light. Preferably, each of the light-emitting diodes has a luminous flux of at least

10 lm. LEDs with such a high output are also denoted LED power packages. The use of such high-efficiency, high-output LEDs has the specific advantage that the number of LEDs required for a desired, comparatively high light output can be comparatively small. This benefits the compact construction and the efficiency of the light-emitting panel to be manufactured. Further advantages of the use of LEDs are a comparatively very long useful life, the comparatively low energy cost, and the low maintenance cost for a light-emitting panel with LEDs.

The invention will now be explained in more detail with reference to a number of embodiments and a drawing, in which: Figure 1 is a side elevation of an illumination system comprising a light- emitting panel according to an embodiment of the invention; Figure 2 is a cross-sectional view of a detail of the illumination system as shown in Figure 1 showing a number of light rays, and Figure 3 is a typical arrangement of the slits in a light-emitting panel according to an embodiment of the invention. The Figures are purely diagrammatic and not drawn true to scale. Some dimensions are particularly strongly exaggerated for reasons of clarity. Equivalent components have been given the same reference numerals as much as possible in the Figures.

Figure 1 schematically shows a side elevation of an illumination system comprising a light-emitting panel 1 according to an embodiment of the invention. The light- emitting panel 1 is made of a light-transmitting material. The light-emitting panel 1 is manufactured, for example, from a synthetic resin, from acryl, from polycarbonate, from PMMA, for example Perspex, or from glass. During operation light is transported through the light-emitting panel 1 utilizing total internal reflection (TIR). The panel has a front wall or light-emitting window 2 and a rear wall 3 opposite thereto. Between the light-emitting window 2 and the rear wall 3 of the light-emitting panel 1, furthermore, there is at least one light-ingress edge 4 for coupling light from a light source 6B, 6G, 6R into the light-emitting panel 1, the light, in operation, spreading in the light-emitting panel 1. In the embodiment of Figure 1, the light- ingress edge 4 is associated with a plurality of light sources 6B, 6G, 6R, for example a number of light-emitting diodes (LEDs). The LEDs are, preferably, mounted on a (metal-core) printed circuit board. When power LEDs are provided on such a (metal- core) printed circuit board (PCB), the heat generated by the LEDs can be readily removed by the PCB through thermal conduction. In an alternative embodiment the light-ingress edge 4 is associated with only one light source (not shown in Figure 1), for example a tubular low- pressure mercury vapor discharge lamp, for example, one or several so-called cold-cathode fluorescent lamps (CCFL). According to the invention, the light-emitting panel 1 is provided with a plurality of slits 8, 8', 8", ... for coupling out light from the light-emitting panel (1). The slits 8, 8', 8", ... extend from the rear wall 3 to the light-emitting window 2. To stimulate the coupling of light out of the light-emitting panel 1, material in the slits 8, 8', 8", ... has a refractive index different from the refractive index of the light-emitting panel 1. In the example of Figure 1, the slits 8, 8', 8", ... are filled with air. In an alternative embodiment the slits are filled with a dielectric material. The change in refractive index at the interface between the light-emitting panel and the dielectric material in the slit causes a reflection resulting the coupling of light out of the light-emitting panel. The orientation of the slits in the light-emitting panel determines where the light is coupled out from the light-emitting panel 1. In the example of Figure 1, the slits 8, 8', 8", ... are substantially parallel with respect to each other. In addition, the slits 8, 8% 8", ... are oriented such in the light-emitting panel 1 that the distance of a slit 8, 8% 8", ... to the light-ingress edge 4 decreases from the light-emitting window 2 towards the rear wall 3. In this manner light incident at the light-ingress edge 4 reflects at the slit 8, 8', 8", ... and is coupled out from the light-emitting panel 1 at the light-emitting window 2. Preferably, the light is substantially coupled out from the light-emitting panel 1 by specular reflection. In this manner the light-emitting panel is substantially transparent. The light-emitting panel 1 in Figure 1 can be seen through with clarity. A light-emitting panel being transparent enables the design and manufacture of a new class of luminaries with a novel and attractive appearance. In order to obtain a substantially uniform flux density of the light emitted by the light-emitting window 1, the length of the slits 8, 8', 8", ... and the pitch between the slits 8, 8', 8", ... varies with respect to the light-ingress edge 4. In Figure 1, it can be seen that the length of the slits 8, 8', 8", ... increases and the pitch between the slits 8, 8', 8", ... decreases as a function of the distance with respect to the light-ingress edge 4 (also see Figure 3). Close to the light-ingress edge 4 the coefficient of coupling light out of the light-emitting panel 1 is chosen to be relatively low because there is relatively much flux available in this part of the light-emitting panel 1. At distances further away from the light-ingress edge 4 the coefficient of coupling light out of the light-emitting panel 1 is chosen to be larger because part of the light flux has already been coupled out. In an alternative embodiment, the light-emitting panel is provided with a further light-ingress edge 5 situated opposite to the light-ingress edge 4, the further edge 5 being provided with a specular or a diffuse reflector. In yet another embodiment, the light- emitting panel comprises a further ingress edge for coupling light from a further light source (not shown in Figure 1) into the light-emitting panel, the further ingress edge being situated opposite to the light-ingress edge. In the latter embodiment, the rear wall 3 functioning as a further light-emitting window. The light-emitting panel 1 may further be provided with a sensor 10 for measuring the optical properties of the light emitted, in operation, by the light-emitting panel 1. The sensor 10 is coupled to control electronics (not shown in Figure 1) for suitably adapting the luminous flux of the light source 6B, 6G, 6R. A feedback mechanism can be realized by means of the sensor 10 and the control electronics for influencing the quality and quantity of the light coupled out from the light-emitting panel 1. In this manner a dynamic control of the light output of the light-emitting panel 1 is obtained. In addition, the sensor 10 can also be employed to adjust the light emitted by an individual light source, for instance, to compensate for effects of temperature. In an alternative embodiment, the sensor is provided on the light-emitting window. Figure 2 schematically shows a cross-sectional view of a detail of the illumination system as shown in Figure 1 showing a number of light rays. The light rays emerge from the light source 6G. A guiding chamber 15 is shown in Figure 1 for mixing and guiding light rays in the direction of the light-ingress edge 4. An inside wall of the guiding chamber 15 may be provided with a reflecting surface, the reflecting surface being preferably specularly reflecting. To facilitate the tracking of the light rays in Figure 2, the light rays are referenced with a number of arrows. In Figure 2 light is coupled out from the light-emitting panel 1 at the location of the slits 8, 8', .... The slits 8, 8', ... extend from the rear wall 3 to the light-emitting window 2. In Figure 2 the slits 8, 8', ... are arranged under an angle α with respect to a normal 18 on the light-emitting window 2, the angle being in the range from 10°< α <80°. Preferably, the angle is in the range from 30°< oc <40°. This latter range of angles is particularly suitable when the slits 8, 8', ... are filled with air. Selecting an angle in the range from 30 to 40° enables obtaining a substantially symmetric light distribution. In case the light-emitting panel 1 is manufactured from PMMA (n=1.49) and the slits 8, 8', ... are filled with air (n=l), the angle is preferably selected to be approximately 36°. At this value of the angle oc the light emitted by the light-emitting window 1 is substantially symmetric around the normal 18 to the light-emitting window 2. A maximum intensity of the light is obtained around the normal 18 and at the left-hand and the right-hand side of the normal 18 a comparable amount of illuminative flux is emitted by the light-emitting window 2 of the light-emitting panel 1. Figure 3 schematically shows a typical arrangement of the slits 8, 8', 8", ... in a light-emitting panel 1 according to an embodiment of the invention. In the proximity of the light-ingress edge 4 a relatively high number of slits 8, 8', 8", ... is provided with a relatively short length. At distances further away from the light-ingress edge 4 the number of slits 8, 8', 8", ... decreases whereas the length gradually increases. Close to the light- ingress edge 4 the coefficient of coupling light out of the light-emitting panel 1 is relatively low whereas at distances further away from the light- ingress edge 4 the coefficient of coupling light out of the light-emitting panel 1 is relatively large. In this manner a substantially uniform flux density of the light emitted by the light-emitting window 2 is obtained. In the embodiment of the light-emitting panel as shown in Figure 3, the number of slits in a line parallel to the light-ingress edge increases with distance from the light- ingress edge, such that the total effective length of the slits that are at the same distance from the light-ingress edge, increases. In this manner, the number of slits and the length of the slits increase as a function of the distance from the light-ingress edge. In an alternative embodiment of the light-emitting panel, the slits are filled with materials with different refractive indices. This is another method of controlling the coupling of light out of the light-emitting panel. For instance by selecting appropriate materials with suitable refractive indices, the length of the slits can be the same as a function of the distance to the light-ingress edge, the variation in the refractive index determining the percentage of light coupled out from the light-emitting panel. The light-emitting panel according to the invention is a particularly compact light-emitting panel with a relatively high luminous efficacy. It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. 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 invention may be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer. In the device claim enumerating several means, several of these means may be embodied by one and the same item of hardware. 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-emitting panel (1) comprising: a light-emitting window (2) and a rear wall (3) situated opposite to the light- emitting window (2), at least one light-ingress edge (4) for coupling light from a light source (6B, 6G, 6R) into the light-emitting panel (1), the light, in operation, spreading in the light- emitting panel (1), the light-emitting panel (1) being provided with a plurality of slits (8, 8', 8", ...) for coupling out light from the light-emitting panel (1), the slits (8, 8', 8", ...) extending from the rear wall (3) to the light-emitting window (2), , material in the slits (8, 8', 8", ...) having a refractive index different from the refractive index of the light-emitting panel (1).

2. A light-emitting panel as claimed in claim 1, characterized in that the light is substantially coupled out from the light-emitting panel (1) by specular reflection.

3. A light-emitting panel as claimed in claim 2, characterized in that the light- emitting panel (1) is substantially transparent.

4. A light-emitting panel as claimed in claim 1 or 2, characterized in that the slits

(8, 8', 8", ...) are filled with a dielectric material.

5. A light-emitting panel as claimed in claim 1 or 2, characterized in that the slits (8, 8', 8", ...) are filled with air.

6. A light-emitting panel as claimed in claim 1 or 2, characterized in that the slits (8, 8', 8", ...) are substantially flat planes.

7. A light-emitting panel as claimed in claim 1 or 2, characterized in that the slits (8, 8', 8", ...) are substantially parallel with respect to each other.

8. A light-emitting panel as claimed in claim 1 or 2, characterized in that the slits (8, 8', 8", ...) are oriented such in the light-emitting panel (1) that the distance of a slit

(8, 8', 8", ...) to the light- ingress edge (4) decreases from the light-emitting window (2) towards the rear wall (3).

9. A light-emitting panel as claimed in claim 7, characterized in that the slits (8, 8', 8", ...) are arranged under an angle α with respect a normal (18) on the light-emitting window (2), the angle being in the range from 10°< oc <80°.

10. A light-emitting panel as claimed in claim 7, characterized in that the angle oc is in the range from 30°< <40°.

11. A light-emitting panel as claimed in claim 1 or 2, characterized in that the length of the slits (8, 8', 8", ...) and the pitch between the slits (8, 8', 8", ...) varies with respect to the light-ingress edge (4) such that the average flux density of the light emitted by the light-emitting window (2) is substantially uniform.

12. A light-emitting panel as claimed in claim 1 or 2, characterized in that the number of slits (8, 8', 8", ...) in a line parallel to the light-ingress edge (4) increases with distance from the light- ingress edge (4), such that the total effective length of the slits (8, 8', 8", ...) that are at the same distance from the light-ingress edge, increases (4).

13. A light-emitting panel as claimed in claim 1 or 2, characterized in that the length of the slits (8, 8', 8", ...) increases and the pitch between the slits (8, 8', 8", ...) decreases with respect to the light-ingress edge (4).

14. A light-emitting panel as claimed in claim 1 or 2, characterized in that the light-emitting panel (1) comprises a further light- ingress edge (5) situated opposite to the light-ingress edge (4), the further light-ingress edge (5) being provided with a specular or a diffuse reflector.

15. A light-emitting panel as claimed in claim 1 or 2, characterized in that the light-emitting panel (1) is provided with a sensor (10) for measuring the optical properties of the light emitted, in operation, by the light-emitting panel (1).

16. A light-emitting panel as claimed in claim 1 or 2, characterized in that the light-emitting panel (1) comprises a further ingress edge (5) for coupling light from a further light source into the light-emitting panel (1), the further ingress edge (5) being situated opposite to the light-ingress edge (4), the rear wall (3) functioning as a further light-emitting window.

17. An illumination system provided with a light-emitting panel (1) according to claim 1 or 2, characterized in that the light-ingress edge (4) is associated with a single light source or with a plurality of light sources (6B, 6G, 6R).

18. An illumination system according to claim 15, characterized in that the plurality of light sources (6B, 6G, 6R) comprises at least two light-emitting diodes with different light emission wavelengths.