WO2013156905A1

WO2013156905A1 - A lighting device with a light guide

Info

Publication number: WO2013156905A1
Application number: PCT/IB2013/052872
Authority: WO
Inventors: Erik Paul Boonekamp; Johannes Maria Thijssen
Original assignee: Koninklijke Philips N.V.
Priority date: 2012-04-19
Filing date: 2013-04-11
Publication date: 2013-10-24

Abstract

A lighting device (1) comprises a light guide (2) comprising a front surface (3), a back surface (4) opposite said front surface and an edge surface (7), at least one point light source (8) arranged to emit light into said light guide through said edge surface, and a reflective element (6) adjacent to said back surface of said light guide, such that light (12) travelling within said light guide is reflected by said back surface and/or said reflective element and is emitted through said front surface. The lighting device (1) further comprises a pattern of light scattering structures (5a, 5b, 5c) in optical contact with said front surface (3), which allows a range of different beam shapes to be created. The smallest dimension or size, d, of the light scattering structures is important for obtaining the desired effect. The lighting device provides improved illumination of large building areas.

Description

A LIGHTING DEVICE WITH A LIGHT GUIDE

FIELD OF THE INVENTION

The invention relates to a lighting device comprising a light guide comprising a front surface, a back surface opposite the front surface and an edge surface, at least one point light source arranged to emit light into the light guide trough the edge surface, and a reflective element adjacent to the back surface of the light guide, such that light travelling within the light guide is reflected by the back surface and/or the reflective element and is emitted through the front surface.

BACKGROUND OF THE INVENTION

Light guides of the above-mentioned type are widely used for office lighting applications, particularly in the USA and Europe. Furthermore, such lighting devices, particularly in embodiments wherein the point light source is one or more LEDs, are also widely used in schools. Other relevant uses include but are not limited to such applications as large, decorative light emitting surfaces such as walls, ceilings, floors, thin (e.g. architectural) LED lines in specific buildings and integrated in tiles, e.g. ceiling tiles (embedded lighting), or placed behind translucent textile surfaces.

In the prior art extraction of light from a solid light guide (e.g. poly(methyl methacrylate) (PMMA) or glass) is generally established by dots painted on the surface of the light guide that is provided with a reflective sheet. Especially for large area light guides (LED backlights and LED luminaires), this technique offers a relatively low cost way to apply these dots. Light injected at the edge(s) of the light guide is extracted from the light guide by scattering at the dot pattern. A Lambertian intensity profile is generated, independent of dot size, dot arrangement or way of incoupling the light. A lighting device with such a light guide is described in US-2004/0218376-A1.

However, the prior art light guides have the disadvantage of offering little or no possibility of tuning the shape of the light beam emitted by the light guide, and thereby tend to provide insufficient illumination particularly of large areas such as office buildings. Hence, and particularly for general lighting applications, an option to tune the beam shape is highly desired. SUMMARY OF THE INVENTION

It is an object of the present invention to overcome this problem, and to provide a lighting device in which the shape of the light beam emitted may be tuned and/or controlled, and with which improved illumination of large areas, particularly large building areas, may be obtained.

According to the invention, this and other objects are achieved by a lighting device of the type mentioned initially and wherein the lighting device further comprises a pattern of light scattering structures in optical contact with the front surface such that light that is scattered back into the light guide by the pattern of light scattering structures is reflected by the reflective element.

With such a lighting device it becomes possible to create a range of different beam shapes (e.g. wide, batwing, asymmetric etc.) by means of the pattern of light scattering structures. The huge variety of possible print patterns allows the creation of lighting devices having highly decorative light emitting surfaces and having well defined beam shapes.

Thereby optimization and improvement of the illumination capabilities of the lighting device is obtained, which in turn provides for an improved illumination when used in large building areas.

In relation to the pattern of light scattering structures the following parameters are used herein:

L, being the distance between the reflective element adjacent to the back surface of the light guide and the pattern of light scattering structures on the front surface of the light guide,

d, being the smallest dimension or size of the light scattering structures, - p, being the pitch of the light scattering structures, in other words the center- to-center distance between two adjacent light scattering structures, and

R, being the reflectance of the light scattering structures.

In an embodiment, the light scattering structures have a smallest dimension d in the range 0.1 < d/L < 1, L being the distance between the reflective element adjacent to the back surface of the light guide and the pattern of light scattering structures on the front surface of the light guide.

Thereby the smallest dimension of the light scattering structures are chosen such that the lower boundary of the above-mentioned range enables beam shaping by preventing a Lambertian intensity profile at the front surface of the light guide. The upper boundary ensures that the device has sufficient efficiency. In this way a lighting device having a particularly well defined beam shape is obtained.

In an embodiment, the lighting device has a ratio p/L, L being the distance between the reflective element adjacent to the back surface of the light guide and the pattern of light scattering structures on the front surface of the light guide and p being the pitch of the light scattering structures, the ratio p/L being equal to or larger than <i/L+2-tan[arcsin(l//?)], n being the refractive index of a material that is comprised in the light guide.

Thereby a lighting device is provided with which a particularly well defined beam shaping may be obtained in that the light scattering structures are spaced apart sufficiently to not influence each other.

On the other hand, embodiments in which the ratio p/L is smaller than

<i/L+2-tan[arcsin(l//?)] are also feasible. Such embodiments provide for a lighting device with which a further beam shaping possibility is enabled because the light scattering structures may influence each other.

In one embodiment, the pattern of light scattering structures comprises a pattern of paint printed or painted on the front surface. Suitable printing methods include e.g. inkjet printing and screen printing.

Thereby a lighting device is obtained in which the pattern of light scattering structures may be added in a particularly cost efficient and simple manner.

In an embodiment, the pattern of light scattering structures has a reflectance of

80 % to 90 %, thereby ensuring a particularly good efficiency of the lighting device.

In an embodiment, the pattern of light scattering structures is partly

transmissive, thereby providing for a yet further beam shaping possibility in virtue of the amount of light transmitted by the light scattering structures.

In an embodiment, the lighting device further comprises at least one optical film and/or at least one scattering film. Using an additional optical film provides the advantage that glare can be accurately controlled in all angular directions, whereby effective glare control may be obtained. Using an additional scattering film provides the advantage of smoothening the light distribution of the emitted beam.

In an embodiment, the reflective element is chosen from the group comprising any one or more of a reflective layer or coating, a reflective plate, an at least partly specularly reflective layer or plate, a perforated or semi-transparent reflective layer or coating, a perforated or semi-transparent reflective plate and a mirror layer or plate. A perforated or semi-transparent reflective layer, coating or plate provides the particular advantage of allowing part of the incident light to lighten the area above the lighting device, e.g. a ceiling, and thereby of enabling the use of the lighting device according to the invention in e.g. a suspended luminaire.

In an embodiment, the lighting device further comprises a plate arranged adjacent the front surface of the light guide such as to allow an air gap between the front surface of the light guide and the plate. Such a plate may in further embodiments be a glass plate or a polycarbonate plate, and/or the plate may support an optical film.

Thereby a lighting device is provided which is protected such as to prevent dust and other impurities from being deposited on the front surface and/or pattern of light scattering structures.

In a further embodiment, the plate comprises a front surface and a back surface opposite the front surface, and a pattern of light scattering structures are provided in optical contact with the front surface of the plate. Thereby a lighting device having even more possibilities for shaping the beam emitted by the light guide is provided.

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

BRIEF DESCRIPTION OF THE DRAWINGS

This and other aspects of the present invention will now be described in more detail, with reference to the appended drawings showing embodiment(s) of the invention.

In the drawings:

Figure 1 shows a cross sectional view of a planar lighting device according to a first embodiment of the invention,

Figure 2 shows a cross sectional view of a lighting device according to a second embodiment of the invention,

Figure 3 shows a cross sectional view of a lighting device according to a third embodiment of the invention,

Figure 4 shows a cross sectional view of a curved lighting device according to the first embodiment of the invention,

Figure 5 shows a perspective and exploded view of a plate shaped lighting device according to the first embodiment of the invention, Figures 6 to 8 show examples of intensity profiles of light emitted by three different lighting devices according to the invention having a pattern of light scattering structures with mutually different pitches but otherwise being identical,

Figures 9 to 13 show examples of intensity profiles of light emitted by five different embodiments of a lighting device according to the invention all having a regular pattern of light scattering structures, and

Figures 14 to 16 show examples of intensity profiles of light emitted by three different embodiments of a lighting device according to the invention all having a closely spaced pattern of light scattering structures.

Furthermore, in all of Figures 6 to 16 the sub-figures show the following:

Sub-figure a shows the pattern of light scattering structures used,

Sub-figure b shows the far field intensity profile, Ι(θ,φ), of the emitted light, where light areas correspond to a high intensity, while dark areas correspond to a low intensity,

Sub-figure c shows the far field intensity as a function of the angle with respect to a normal to light guide, and thereby to the pattern of light scattering structures, measured in degrees, and

Sub- figure d shows the far field intensity in candela (cd) seen in a vertical cross section through the center of the intensity profile of sub figure b as a function of an angle such that minus 90 degrees corresponds to the uppermost peripheral point of the intensity profile, 0 degrees corresponds to the center of the intensity profile and plus 90 degrees corresponds to the lowermost peripheral point of the intensity profile.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Figure 1 shows a first embodiment of a lighting device 1 according to the invention. The lighting device 1 comprises a light guide 2, the light guide 2 comprising a front surface 3, a back surface 4 opposite said front surface 3 and an edge surface 7. The lighting device 1 furthermore comprises at least one point light source 8 arranged at the edge surface 7 such as to emit light 12 into the light guide 2 trough the edge surface 7. The lighting device 1 furthermore comprises a reflective element 6 adjacent to the back surface 4 of the light guide 2 and a pattern of light scattering structures 5a, 5b, 5c in optical contact with the front surface 3.

The point light source 8 may in principle be any feasible type of point light source, such as e.g. a light source with a pin hole arranged between the light source and the light guide or an array of point light sources. Alternatively, a linear light source such as a linear Chip-On-Board LED, may be used. In the embodiments shown in the drawings the at least one point light source 8 is, however, a light emitting diode (LED), but may also be two or more LEDs or an array of LEDs. Furthermore, irrespective of the embodiment of the lighting device, the lighting device 1 may comprise an additional light source (not shown) arranged such as to inject light through the edge surface opposite the edge surface 7.

The reflective element 6 may be a reflective layer or coating, a reflective plate, an at least partly specularly reflective layer, coating or plate, a perforated or semi-transparent reflective layer or coating, a perforated or semi-transparent reflective plate or any

combination thereof. In embodiments where the reflective element 6 is a specularly reflective plate or layer, a MIRO 27 plate from Alanod or ESR films from 3M may be used. The reflective element 6 may be provided in optical or physical contact with the light guide 2. Alternatively, a space or gap may be provided between the light guide 2 and the reflective element 6. Such a space or gap may be used to create specific optical effects. The reflective element 6 may also be a mirror film or plate. The mirror film or plate may be provided with a (Gaussian) spreading effect on the reflected light, such as provided in a MIRO 20 plate from Alanod. An optical film, possibly slightly scattering, arranged between the reflective element 6 and the light guide 2 may also be provided to create such a (Gaussian) spreading effect. The reflective element 6 may in another alternative be perforated or semi-transparent to allow some light to pass through, which is of interest when the lighting device 1 is used in suspended luminaires.

The light guide 2 may in principle be any feasible type of light guide. In one embodiment the light guide 2 comprises a thickness of 1 mm to 5 mm. In the embodiment shown in Figure 1 the light guide 2 is a planar light guide.

Figure 4 shows a lighting device 1 ' comprising a curved light guide 2'. Apart from the shape of the light guide 2', the lighting device 1 ' Figure 4 comprises the same features as the planar light guide 2 shown in Figure 1. Preferably, and as shown on Figure 4, the whole lighting device 1 ' is curved such that the wall thickness of the light guide 2' is constant and the reflective element 6 follows the curvature of the light guide 2'. The curvature may be convex, concave or any other shape.

Figure 5 shows a lighting device 1 " comprising a plate shaped light guide 2". Apart from the shape of the light guide 2", the lighting device 1 ' ' of Figure 5 comprises the same features as the planar light guide 2 shown in Figure 1. The light guide 2" is provided with approximately the same thickness over its whole area. In one embodiment the luminaire area size of the plate shaped light guide 2" is 600 mm by 600 mm. In embodiments where the light guide is a plate shaped light guide 2", such as the one shown in Figure 5, the point light source may advantageously be an array of LEDs.

It is noted, however, that the embodiment of a lighting device with a plate shaped light guide shown in Figure 5 is merely one exemplary and non-limiting embodiment. Particularly the shape of the plate shaped light guide may take any feasible form other than the generally square shape shown in Figure 5, such as circular, rectangular, elliptical, shapes having any number of edges etc. Also, the plate shaped light guide may additionally be curved. Furthermore, the lighting device 2" may comprise one or more additional light sources (not shown) arranged such as to inject light through one or more of the edge surfaces perpendicular to and/or opposite the edge surface 7.

The pattern of light scattering structures 5a, 5b, 5c may in principle be applied to the front surface 3 of the light guide 2 using any feasible method. In one embodiment, the pattern of light scattering structures 5a, 5b, 5c are printed, e.g. inkjet printed or screen printed, on the front surface 3 of the light guide 2. In another embodiment, the pattern of light scattering structures 5a, 5b, 5c are painted on the front surface 3 of the light guide 2.

In other not shown embodiments it is also feasible that the pattern of light scattering structures may be provided as a structuring of the front surface of the light guide, e.g. as protrusions or indentations. Such a structuring may be provided by means of molding or casting or engraving, e.g. laser engraving.

The pattern of light scattering structures 5 a, 5b, 5 c may in principle comprise any suitable material capable of blocking or reflecting incident light at least partially without or essentially without absorbing any light. In one embodiment, the pattern of light scattering structures 5a, 5b, 5c is a pattern of paint, e.g. white paint, reflective white paint or even highly reflective white paint. In a supplementary embodiment, one or more coloured and/or luminescent pigments or dyes are added to the paint.

The pattern of light scattering structures 5 a, 5b, 5 c may in principle comprise any suitable geometrical or other shape. For instance, the light scattering structures may be dots, stripes, circular structures, elliptical structures and structures with two or more edges or any combination thereof. The light scattering structures may furthermore have uniform or varying dimensions and/or distribution. Likewise, in other embodiments, the pattern of light scattering structures may be a continuous pattern of light scattering structures, or an abstract pattern of light scattering structures. The reflectance R of the pattern of light scattering structures 5a, 5b, 5c is typically 80 % to 90 %. In case no absorption of light occurs in the pattern of light scattering structures 5a, 5b, 5c, the amount of transmitted light is consequently 10 % to 20 %. The transmitted light has been shown to have a Lambertian character and to add up to the light distribution created by the reflected part of the light at the pattern of light scattering structures 5a, 5b, 5c. In other embodiments the reflectance R of the pattern of light scattering structures may be as high as 100 %.

As shown in Figure 1 light 12 is emitted by the point light source 8 through the edge surface 7 and into the light guide 2. The light 12 then travels within the light guide where it is reflected by the back surface 4 and/or the reflective element 6, scattered by at least one of the light scattering structures 5b of the pattern of light scattering structures 5a, 5b, 5c and finally emitted by the light guide 2 through the front surface 3.

Referring to Figure 1 , when a light scattering structure 5b on the front surface 3 of the light guide 2 is hit by incident light 20, scattered light 21, 22 is reflected from the light scattering structure 5b via the back surface 4 and/or reflective element 6 to the front surface 3. The light scattering structure 5b acts as a mask or obstruction by partially blocking the passage of the light to the front surface 3. By adjusting the size of the light scattering structure with respect to the thickness L of the light guide 2, the blocking of the light and consequently the shape of the beam, which is eventually emitted from the light guide 2, is defined. In contrast to the conventional use of printed dots in light guides, a number of variables is thereby created, which variables can be used to balance the required intensity pattern and visual (decorative) effect. Such variables include but are not limited to:

Size, e.g. smallest dimension d, and shape of the printed pattern,

Distance, L, between the reflective element adjacent to the back surface of the light guide and the pattern of light scattering structures on the front surface of the light guide, or alternatively the thickness of the light guide.

Reflectance, R, and transmittance of the printed pattern.

Distribution of the printed features on the surface, e.g. dot-dot distance or pitch p), and

Relations between such variables.

The light scattering structures according to the invention have a smallest dimension or size d in a range fulfilling 0.1 < d/L < 1, wherein L is the distance between the reflective element adjacent to the back surface of the light guide and the pattern of light scattering structures on the front surface of the light guide. In the case where the reflective element is in contact with the back surface of the light guide, L thus denotes the thickness of the light guide. If the scattering structures are stripes, d refers to their width. If they are circles or ellipses, the d refers to their diameter or conjugate diameter, respectively. The smallest dimension or size, d, of the light scattering structures has been shown to be essential for obtaining the desired optical effect.

When the size, d, of the light scattering structures is very small with respect to the distance, L, between the reflective element adjacent to the back surface of the light guide and the pattern of light scattering structures on the front surface of the light guide, corresponding to the ratio d/L < 0.1, a Lambertian intensity distribution is obtained on the average. When the size of the light scattering structures becomes larger, i.e. the ratio between d and L becomes 0.1 < d/L < 1, the intensity distribution is strongly influenced by the light scattering structures, which enables obtaining all kinds of intensity distributions.

However, it is noted that when the size of the light scattering structures becomes too large, i.e. in practice when the ratio d/L becomes larger than 1, it has been shown that the optical efficiency of the lighting device may decrease because of multiple reflections in the cavity of the light guide, i.e. between the light scattering structures and the back surface and/or reflective layer.

The center-to-center distance or pitch, p, between the light scattering structures 5a, 5b and 5c is very important. When e.g. a regular pattern of light scattering structures 5a, 5b, 5c has a small pitch p, i.e. a narrow gap between two light scattering structures, the light emitted from one light scattering structure is hindered or screened by the adjacent light scattering structure or even by multiple light scattering structures. It is noted that light is not blocked in the traditional sense but rather reflected back into the light guide 2 and emitted in other directions. The inventors have shown that this effect may be used to create sparkling effects and even to create a cut-off in certain directions of the light distribution. In

embodiments where no interaction between the light scattering structures is required, it has been found that the ratio p/L should satisfy the relation:

(1)

In equation (1), n is the refractive index of the material of the light guide. As an example, for a light guide material having a refractive index of 1.50, and a desired ratio d/L is equal to 0.5, the minimal distance between two light scattering structures becomes 2.3. For lower values of the pitch p, the light scattering structures optically interact, which leads to a variety of beam shapes depending on the size of the ratios d/L and p/L.

Turning now to Figure 2, a second embodiment of a lighting device 1 according to the invention is shown. The lighting device 1 of Figure 2 differs from that shown in Figure 1 only in that a film 9 has been added adjacent to the front surface 3. The film 9 may be an optical and/or a scattering film. Optical films, such as a linear prism film, may be added to control glare. Scattering films, such as symmetric or asymmetric light- shaping diffusers from Luminit or LCD diffusers may be used to smoothen the light distribution. In the embodiment shown in Figure 2, the film 9 is arranged such that a gap 16 is present between the film 9 and the front surface 3 of the light guide 2. In other

embodiments the film may be arranged in optical and/or physical contact with the front surface and/or the pattern of light scattering structures of the light guide.

Turning to Figure 3, a non- limiting example of a third embodiment of a lighting device 1 according to the invention is shown. The lighting device 1 of Figure 3 differs from that shown in Figure 1 only in that a plate 10 comprising a front surface 13 and a back surface 14 is provided in front of the light guide 2, that is adjacent the front surface 3 of the light guide 2, to protect the light guide. The plate 10 is placed in front of the light guide 2 allowing an air gap 11 between the light guide 2 and the front surface 3. Preferably the air gap 11 has a size of less than 10 μιη. The plate 10 may in further embodiments support a thin optical film, such as the film 9 shown in Figure 2, present between the light guide 2 and the plate 10. The plate 10 is in one embodiment a clear plate and may be made of glass or a polycarbonate with flame retarding properties. In other embodiments the plate may be arranged in optical and/or physical contact with the front surface and/or the pattern of light scattering structures of the light guide. In yet other embodiments the plate 10 may be provided with one or more additional optical features such as prisms.

As shown in Figure 3, the lighting device 1 has a pattern of light scattering structures 5a, 5b, 5c on the front surface 3 of the light guide 2 as well as another pattern of light scattering structures 15a, 15b, 15c on the back surface 4 of the light guide 2. Such a pattern of light scattering structures 15a, 15b, 15c can be useful to obtain certain light distributions. In an alternative, not shown embodiment, the pattern of light scattering structures 15a, 15b, 15c on the back surface 4 of the light guide 2 are arranged displaced with respect to the pattern of light scattering structures 5a, 5b, 5c on the front surface 3 of the light guide 2. Likewise the pattern of light scattering structures 15a, 15b, 15c on the back surface 4 of the light guide 2 may be identical to the to the pattern of light scattering structures 5a, 5b, 5c on the front surface 3 of the light guide 2 or it may be different in size and/or shape from the pattern of light scattering structures 5a, 5b, 5c on the front surface 3 of the light guide 2. Particularly, with a light guide according to the embodiment shown in Figure 3, the shape of the emitted light beam and the flux of the emitted light may be controlled on both sides of the light guide 2 in that the respective patterns of light scattering structures 5a, 5b, 5c and 15a, 15b, 15c interact optically with each other to create an upward and a downward intensity profile. This specific configuration may in an alternative embodiment be used without the reflective element 6, which is of interest e.g. in the case of suspended systems where an uplight and a downlight is integrated in a single luminaire. Likewise, the lighting device according to Figure 3 may be used in embodiments without the plate 10 or even without both the reflective element 6 and the plate 10.

In an alternative or addition to the embodiments shown in Figure 3, a pattern of light scattering structures (not shown) may also be present on the back surface 14 and/or the front surface 13 of the plate 10. Likewise, in alternatives or additions to any one of the above described embodiments, a pattern of light scattering structures (not shown) may also be present on the reflective element 6 of the light guide 2.

In the following a number of examples will be described in order to describe the invention in further details. Example #1

In Figures 6 to 8 the front surface 3 of a light guide 2 of a lighting device 1 according to the invention is provided with circular dots of paint having a ratio d/L of 0.5 and having a variable ratio p/L. The paint used has a reflectance R of 100 %. The dots are arranged in a hexagonal arrangement. Decreasing the ratio p/L from 2 (Figure 6a) to 1.3 (Figure 7a) and further to 1 (Figure 8a), the far field intensity profile of the emitted light gradually transforms from a smooth rotational symmetric distribution to a profile dominated by the interaction between the paint dots, as can be seen in Figures 6b to 6d, 7b to 7d and 8b to 8d. The "hole" in the center of the intensity profile has been shown to increase when d/L increases. Similar intensity distributions to that shown in Figure 6 may be obtained when printing the pattern of light scattering structures as concentric paint lines or spirals. The intensity pattern (and consequently the visual appearance of the light emitting surface, i.e. the front surface 3 of the light guide 2) may be modified in almost infinite ways using free-shape paint features and various regular or random arrangements of the painted dots. Example #2

In Figures 9 to 12 the front surface 3 of a light guide 2 of a lighting device 1 according to the invention is provided with a regular pattern of paint lines, and in Figure 13 with a regular pattern of oval dots. In all cases the ratio d/L is 0.5. In Figures 9 to 11 the paint used has a reflectance R of 100 %, while the paint used in Figures 12 and 13 has a reflectance R of 85 %.

Figure 9 shows a strong batwing shaped intensity profile. The ratio p/L is 3, i.e. there is no interaction between the paint lines.

When the ratio p/L is decreased to 1.6 - see Figure 10 - a cut-off is created in the direction perpendicular to the lines.

In Figure 11, the lighting device used in connection with Figure 10 was combined with a linear prism sheet containing prisms with a top angle of 138 degrees. The prism direction is perpendicular to the direction of the paint lines on the front surface of the light guide of the lighting device, i.e. the prisms are arranged such as to point away from the paint lines. This combination limits the intensity at all high angles larger than approximately 65 degrees with respect to the normal to the light guide plane. An advantage is that this type of prism film has a low reflectance and that the printed features can easily be seen through the prism film. The prism plate can be produced at dramatically lower costs than currently used Micro -Lens Optics (MLO).

Figure 12 shows a case otherwise identical to that shown in Figure 11, but where the reflectance of the paint used is 85 %, allowing some light leakage through the paint dots of 15 %. This increases the intensity at angles smaller than 65 degrees significantly, as can be seen in Figures 11c and 12c, respectively. However, a cut-off about 70 degrees still remains due to the specific structure of the prism film. The advantage is the possibility to create various beam shapes while the glare keeps within limits.

In the case shown in Figure 13 a more symmetric beam is created based on the combination of oval shaped paint dots and a linear prism film. The oval paint dots used has a ratio p/L of 3 and a ratio d/L of 1.5 in the direction of the major axis of the ovals, and a ratio p/L of 1.6 and a ratio d/L of 0.5 in the direction of the minor axis of the ovals.

Example #3

In Figures 14 to 16 some examples are given for closely spaced dot and line patterns, i.e. cases in which there is an interaction between the light scattering structures. In all cases the ratio p/L is 0.3, while the reflectance of the paint used is 100 %. In Figure 14 the front surface of a light guide of a lighting device according to the invention is provided with small circular dots having a ratio d/L of 0.2 in a hexagonal arrangement. A "sparkling" light distribution is obtained which can be glare protected by a prism sheet as discussed in example #2, as can be seen in Figure 15 showing a pattern of dots with the ratio d/L being 0.2. In a similar way, line patterns can be generated as shown in Figure 16, the lines shown having a ratio d/L being 0.1.

Visual and/or decorative aspects of the invention

Uniform light output over the entire surface of the lighting device may be realized e.g. by gradual increase of the spacing of the light scattering structures as a function of the distance from the light source(s) at the edge surface(s). This may influence the required intensity distribution. An alternative is to make clusters/groups of equally spaced light scattering structures (the ratio d/L being constant) and vary the distance between these clusters. Another alternative is to vary the smallest dimension d at constant pitch p (ratio p/L being constant).

The pattern of light scattering structures may consist of multiple domains, e.g. like a chessboard structure. Each domain may have a different pattern of light scattering structures (e.g. line patterns rotated with respect to each other). In this way two or more light distributions may be combined in a single lighting device. In this way different visual effects may be created depending on the viewing angle. At certain angles, some domains are dark while other domains light up.

Various additional optical effects may be created such as Moire effects, optical illusions, depth/3D illusions etc.

Printed color patterns may also be used. For example, a white pattern may be printed with a color print on top. If no transmission of light is allowed, a dark (black) pigment may be printed on top of the white pattern.

Applications

Lighting devices according to the invention may, as also mentioned initially, be used in office lighting applications, particularly in the USA and Europe. For the US market, where glare norms are less strict than in Europe, lighting devices according to the invention may be used even in embodiments without the prism foil. For the European market, the glare performance is slightly lower than the current MLO solution. However, the costs of the new concept are much lower and the beam shaping possibilities helps to illuminate an office more efficiently.

Lighting devices according to the invention may furthermore, and particularly in embodiments where the point light source is one or more LEDs, be used in schools. In school lighting applications, a wide range of color settings from very cool- white (almost bluish) to warm- white are used. The lighting devices according to the invention fit with the need for excellent color mixing and new decorative/design possibilities desired in school applications.

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.

Claims

CLAIMS:

1. A lighting device (1) comprising:

a light guide (2) comprising a front surface (3), a back surface (4) opposite the front surface (3), and an edge surface (7),

at least one point light source (8) arranged to emit light into the light guide (2) through the edge surface (7),

a reflective element (6) adjacent to the back surface (4) of the light guide (2), and

a pattern of light scattering structures (5a, 5b, 5c) in optical contact with the front surface (3).

2. A lighting device according to claim 1, having a ratio d/L in a range from 0.1 to 1, d being the smallest dimension of the light scattering structures (5 a, 5b, 5 c), and L being the distance between the reflective element (6) and the pattern of light scattering structures (5a, 5b, 5c).

3. A lighting device according to any one of the above claims, having a ratio p/L equal to or larger than <i/L+2-tan[arcsin(l//?)], p being the pitch of the light scattering structures (5a, 5b, 5c), L being the distance between the reflective element (6) and the pattern of light scattering structures (5a, 5b, 5c), d being the smallest dimension of the light scattering structures (5a, 5b, 5c), and n being the refractive index of a material that is comprised in the light guide (2).

4. A lighting device according to any one of the above claims, wherein the pattern of light scattering structures (5a, 5b, 5c) comprises a pattern of paint printed or painted on the front surface (3).

5. A lighting device according to claim 4, wherein the paint is white paint, optionally comprising one or more coloured and/or luminescent pigments or dyes.

6. A lighting device according to any one of the above claims, wherein the pattern of light scattering structures (5a, 5b, 5c) comprises any one or more of dots, stripes, circular structures, elliptical structures, and structures with two or more edges, having uniform or varying dimensions and/or distribution.

7. A lighting device according to any one of the above claims, wherein the pattern of light scattering structures (5a, 5b, 5c) has a reflectance of 80 % to 90 %.

8. A lighting device according to any one of the above claims, wherein the pattern of light scattering structures (5a, 5b, 5c) is partly transmissive.

9. A lighting device according to any one of the above claims, wherein the light guide is a planar light guide (2), a curved light guide (2'), a curved and plate-shaped light guide, or a plate-shaped light guide (2").

10. A lighting device according to any one of the above claims, further comprising at least one optical film (9) and/or at least one scattering film.

11. A lighting device according to any one of the above claims, wherein the reflective element (6) is chosen from the group comprising any one or more of a reflective layer or coating, a reflective plate, an at least partly specularly reflective layer, coating or plate, a perforated or semi-transparent reflective layer or coating, a perforated or semi- transparent reflective plate and a mirror layer or plate.

12. A lighting device according to any one of the above claims, further comprising a plate (10) arranged adjacent to the front surface (3) of the light guide (2) such as to allow an air gap (11) between the front surface (3) of the light guide (2) and the plate (10).

13. A lighting device according to claim 12, wherein the plate (10) is a glass plate or a polycarbonate plate, and/or wherein the plate (10) supports an optical film, and/or wherein the plate (10) comprises a front surface (13) and a back surface (14) opposite the front surface, and wherein a pattern of light scattering structures are provided in optical contact with the front surface (13) and/or the back surface (14) of the plate (10).

14. A lighting device according to any one of the above claims, further comprising a pattern of light scattering structures (15a, 15b, 15c) in optical contact with the back surface (4)· 15. A lighting device according to any one of the above claims, wherein the at least one point light source (8) is at least one light emitting diode (LED) or an array of LEDs.