FIELD OF THE INVENTION

The invention relates to an optical element for use in a lighting system, wherein the lighting system is capable of producing light beams that have low-glare and/or that are symmetrical.

BACKGROUND OF THE INVENTION

Solid light guides are applied to transport light which is injected at the edges by one or more light sources (e.g. LEDs). Out-coupling structures are applied on one side of the light guide to extract the light. These out-coupling structures can be arranged in such a way that light is extracted uniformly over the whole light guide area. The most important application areas are LCD-TV, smart-phones and tablets. However, in the last decade, light guides are used in a number of lighting system designs.

A major issue in light guide design is to control uniformity over the complete light guiding surface and the intensity distribution of the extracted light. In case of screen- printed 2D-structures, e.g. white, light scattering paint dots, the light output is always

Lambertian. Refractive 3D-microstructures are used in light guides for more efficient direct light extraction but cannot be used to make low-glare, symmetrical light beams.

SUMMARY OF THE INVENTION

It is an object of the present invention to overcome at least some of the above problems.

According to a first aspect, this and other objects are achieved by providing an optical element. The optical element comprises a light guide arranged to guide light by means of total internal reflection. The light guide has a front side and a back side connected via a circumferential edge. Each of the front side and the back side is arranged to transmit the light. The optical element further comprises a light scattering 3D-structure arranged directly onto the front side of the light guide and partly covering the front side of the light guide. The light scattering 3D-structure is arranged to scatter the light such that at least a part of the scattered light is escaping the light guide at the back side thereof. The light scattering 3D-structure comprises a plurality of light scattering 3D-elements. The light scattering 3D-elements are separated by distances in a range of 0.1 mm to 20 mm. The light scattering 3D-elements have an extension in a direction away from the light guide in a range of 0.1 mm to 20 mm, a width in a range of 0.1 mm to 20 mm, and a length in a range of 10 mm to 1000 mm.

The light scattering 3D-structure provides for efficient light extraction from the light guide and at the same time allows for shaping of the light beam. The modeling of the light scattering 3D-structure influences the shape of the light beam. Moreover, the shaping of the beam may be achieved such that the glare is low. The light scattering 3D-structure may be applied onto the front side of the light guide using 3D printing. Fused Deposition Modeling, FDM, or Fused Filament Fabrication, FFF, are two preferred techniques for 3D-printing the 3D-structures. However, any other technique for 3D printing may be used as well. This allows 3D-printing of an optical element for use in an office compliant lighting system. In addition, the 3D-printed light scattering 3D- structure can be customized to the preferences of the user.

The light scattering 3D-structure may comprise a polymer filled with reflective particles. The reflective particles are preferably white. The reflectivity of the light scattering 3D-structure is preferably more than 95 %.

The light scattering 3D-structure has an extension in a direction away from the light guide in a range of 0.1 mm to 50 mm, preferably in a range of 0.1 mm to 20 mm. The term "in a direction away from the light guide" may also be referred to as the thickness of the 3D-print.

The light scattering 3D-structure may be arranged to cover 5 % to 50 % of the front side of the light guide.

The light scattering 3D-structure comprises a plurality of light scattering 3D- elements. The light scattering 3D-elements are separated by distances in a range of 0.1 mm to 50 mm, preferably in a range of 0.1 mm to 20 mm. The light scattering 3D-elements have an extension in a direction away from the light guide in a range of 0.1 mm to 50 mm, preferably in a range of 0.1 mm to 20 mm. The light scattering 3D-elements have a width in a range of 0.1 mm to 50 mm, preferably in a range of 0.1 mm to 20 mm and a length in a range of of 10 mm to 1000 mm.

The optical element may further comprise an additional light scattering 3D- structure arranged directly onto a part of the back side of the light guide, wherein the additional light scattering structure is arranged to scatter light interacting with the same such that at least a part of the scattered light is escaping the light guide at the front side thereof.

The light guide may have a uniform thickness in a range of 1 mm to 5 mm. The light guide may have an area of in a range of 0.01 m ² to 1 m ² .

According to a second aspect, the above and other objects are achieved by providing a lighting system. The lighting system comprises the optical element according to the above, an array of light sources arranged at least a portion of the circumferential edge of the light guide, wherein the array of light sources is arranged such that light emitted from the array of light sources is injected into the light guide and guided therein, and a reflecting element facing at least the back side of the light guide, wherein the reflecting element is arranged to direct at least part of the scattered light escaping from the light guide back towards the light guide such that at least part of the directed light is transmitted through the light guide and is exiting the front side of the light guide.

Hence, an indirect lighting system is provided. The light scattering 3D- structure provides for efficient light extraction from the lighting system and at the same time allows for shaping of the light beam. The modeling of the light scattering 3D- structure influences the shape of the light beam. Moreover, the shaping of the beam may be achieved such that the glare of the lighting system is low. The uniformity of the light outputted from the lighting system may be optimized by adjusting the local density of the light scattering 3D-structure and a gap between the optical element and the reflecting element.

The reflecting element may be arranged at a distance from the light guide. The distance may be in a range of 10 μηι to 100 mm.

The light sources may be LEDs.

A further scope of applicability of the present invention will become apparent from the detailed description given below. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the scope of the invention will become apparent to those skilled in the art from this detailed description.

Hence, it is to be understood that this invention is not limited to the particular component parts of the device described or steps of the methods described as such device and method may vary. It is also to be understood that the terminology used herein is for purpose of describing particular embodiments only, and is not intended to be limiting. It must be noted that, as used in the specification and the appended claim, the articles "a", "an", "the", and "said" are intended to mean that there are one or more of the elements unless the context clearly dictates otherwise. Thus, for example, reference to "a unit" or "the unit" may include several devices, and the like. Furthermore, the words "comprising", "including",

"containing" and similar wordings does not exclude other elements or steps.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects of the present invention will now be described in more detail, with reference to the appended drawings showing embodiments of the invention. The figures should not be considered limiting the invention to the specific embodiment; instead they are used for explaining and understanding the invention.

Figs, la and lb illustrate a cross-sectional side view and a bottom view of a lighting system.

Figs. 2 and 3 illustrate a particular design of a light scattering 3D-structure. Fig. 4 illustrates a lighting system comprising the light scattering 3D- structure of Fig. 3.

Fig. 5 illustrates the intensity distribution of the lighting system of Fig. 4. Fig. 6 illustrates the uniformity of the lighting system of Fig. 4.

Fig. 7 illustrates an alternative design of a light scattering 3D-structure.

Fig. 8 illustrates an alternative optical element having light scattering 3D- structures applied both onto the back and front sides of the light guide.

As illustrated in the figures, the sizes of layers and regions are exaggerated for illustrative purposes and, thus, are provided to illustrate the general structures of

embodiments of the present invention. Like reference numerals refer to like elements throughout.

DETAILED DESCRIPTION

The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which currently preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided for thoroughness and completeness, and fully convey the scope of the invention to the skilled person. In connection with Figs, la and lb an optical element 10 is illustrated. The optical element 10 is in Fig. 1 comprised as a part of a lighting system 1. The optical element comprises a light guide 1 1 and a light scattering 3D-structure 18.

The light guide 1 1 has a front side 12, a back side 13 opposite to the front side 12 and a circumferential edge 14 connecting the front side 12 and the back side 13. The light guide 1 1 is sheet shaped. The light guide 1 1 is optically clear. The light guide 1 1 may e.g. be made of polymethylmethacrylate (PMMA), polycarbonate (PC), silicones or glass.

Preferably, all sides and edges of the light guide 1 1 are optically smooth. Hence, the sides and edges of the light guide 1 1 are preferably polished. The light guide has preferably a uniform thickness d. The thickness d is typically in the range of 1 mm -5 mm. The area of the light guide 1 1 is preferably in the range of 0.01 m ² - 1 m ² . The shape of the light guide 1 1 may be a rectangular shape, an oval shape, a circular shape, or any other suitable shape.

The light scattering 3D-structure 18 is arranged directly onto the front side 12 of the light guide 1 1. The light scattering 3D-structure 18 is arranged to partly cover the front side 12 of the light guide 1 1. The light scattering 3D-structure is arranged to cover 5% - 50% of the front side of the light guide 1 1. The light scattering 3D-structure 18 is arranged to scatter light within the light guide interacting with the light scattering 3D-structure 18 such that at least a part of the scattered light is escaping from the light guide 1 1 at the back side 13 thereof. This will be discussed in more detail below. Preferably the light scattering 3D- structure 18 is applied onto the light guide 1 1 using fused deposition modeling (FDM) or any other suitable 3D-printing technique. The light scattering 3D-structure 18 is preferable made out of polymer. Suitable polymers may e.g. be polylactic acid (PLA), acrylonitrile butadiene styrene (ABS), polyethylene terephthalate (PET), polycarbonate (PC) or

polymethylmethacrylate (PMMA). The polymer may be filled with pigment particles to enhance the reflectivity and light scattering properties of the light scattering 3D-structure 18. The reflectivity of the light scattering 3D-structure 18 is preferably more than 95%. The pigment particles may be white. The pigment particles may be inorganic. Non-limiting examples of pigment particles are T1O2, A1 ₂ 0 ₃ and Zr0 ₂ . The size of the pigment particles is preferable less than 10 μηι. The light scattering 3D-structure 18 comprises wall portions having a wall thickness w a mutual spacing x and an extension h in a direction away from the light guide. The wall thickness w, the mutual spacing x and/or the extension h may vary along the area of the light guide 1 1. Preferably w, x and h are in a range of 0.1 mm - 50 mm, preferably 0.1 mm - 20 mm. Preferably h>w and h>x. Accordingly, the light scattering 3D- structure 18 may have a variety of shapes and orientations on the light guide 1 1. Some examples are discussed below. The light scattering 3D-structure 18 may be modeled differently in order to create different desired light properties of the lighting system 1.

The lighting system 1 further comprises an array of light sources 20 and a reflecting element 30.

The array of light sources 20 is arranged at the circumferential edge 14 of the light guide 1 1. The light sources 20 may be arranged around the full circumference of the light guide 1 1. Alternatively, the light sources 20 are arranged only at a part or parts of the circumferential edge of the light guide 1 1. For example, in case of a rectangular light guide 1 1 , light sources 20 may be arranged at only one or two of the four edges or surfaces of the light guide 1 1. This example is illustrated in FIG. lb. The arrangement of the light sources influences the uniformity of the light emitted from the lighting system 1 to a large extent. The light sources 20 may e.g. be LEDs. However, it is contemplated that other suitable light sources may be used as well. The array of light sources 20 is arranged such that light emitted from the array of light sources 20 is injected into the light guide 1 1 and guided therein by total internal reflection (TIR). Light distributed by TIR may efficiently propagate within the light guide 1 1. Hence, a larger portion of light injected into the light guide 11 may efficiently be transported therein until it reaches the light scattering 3D-structure 18.

The reflecting element 30 is facing the back side 13 of the light guide 1 1. The reflecting element 30 is arranged over essentially the whole area of the back side 13 and optionally also over the circumferential edge 14 of the light guide 1 1. The reflecting element 30 is arranged to direct at least part of the scattered light escaping from the light guide 1 1 at the back side 13 thereof back towards the light guide 1 1 such that at least part of the directed light is transmitted through the light guide 1 1 and thereafter exiting the front side 12 of the light guide 1 1.

The reflecting element 30 is arranged at a distance D from the light guide 1 1.

The distance D is in the range of 10 μηι - 100 mm. Hence, a gap is formed between the reflecting element 30 and the light guide 1 1. The gap may e.g. be an air gap. There is no direct mechanical contact between the reflecting element 30 and the light guide 1 1. The reflecting element 30 may at least in part be specular reflective. A non-limiting example of a material to be used for a specular reflective reflecting element 30 is Miro Silver (R~98%) from Alanod. Alternatively or in combination, the reflecting element 30 may at least in part be diffusive. Non-limiting examples of materials to be used for a diffusive reflecting element 30 are white paint on an arbitrary substrate or MCPET sheets (R~98%) from

Furukawa. The reflecting element 30 may be arranged to direct, or reflect back, substantially all light to the back side 12 of the light guide 1 1. Alternatively, the reflecting element 30 may be a partially reflective element arranged to transmit a part of the light emitted from the light guide 1 1. This may be achieved by using a suitable material that directs, or reflects, part of the light emitted from the light guide 1 1 back towards the back side 13 of the light guide 1 1 and transmits another part of the light emitted from the light guide 1 1 through the, in this case partially, reflective element. Alternatively or in combination a perforated reflecting element 30 may be used. The reflecting element 30 arranged to transmit part of the light emitted from the light guide 1 1 may be advantageous in suspended lighting systems 1 which have an up-lighter and a down-lighter function.

With reference to Fig. la the working principle of the optical element 10 will now be discussed. Light from the light sources 20 is injected at the circumferential edge 14 of the light guide 1 1. The injected light is transported by TIR until the light scattering 3D- structure 18 is hit. Light scatters at the light scattering 3D-structure 18 in the direction of the back side 13 of the light guide 1 1. At least part of the scattered light exits the light guide 1 1 through the back side 13 thereof. Hence, at least part of the light scattered at the light scattering 3D-structure 18 is exiting the light guide in a direction towards the reflecting element 30. The reflecting element 30 directs, or reflects back, the light in a direction towards the light guide 1 1. Now the optical element 10 acts as a beam shaping component for the light that is reflected back by the reflective element 30 towards the light guide 1 1. In FIG. la (x-z plane), the light rays perpendicular to the light guide 1 1 plane are transmitted with less scattering events than skewer light rays. This results in a peak-shaped intensity profile of the light outputted from the lighting system 1 in the x-z direction.

The local density of light scattering 3D-structure 18, i.e. the contact surface area at the interface between the light scattering 3D-structure 18 and the light guide 1 1, determines the effectiveness of the light out-coupling. The higher the local density of light scattering 3D-structure 18, the more light is coupled out within a certain transportation length AL of the light guide 1 1. The visual uniformity of the optical element 10 can be tuned by local density of light scattering 3D-structure 18 and the gap between the light guide 1 1 and the reflecting element 30. A larger gap allows spreading the light more effectively in the x-y plane. A density gradient of the light scattering 3D-structure 18 may be applied to improve uniformity and to optimize optical efficiency. The intensity profile of the light outputted from the lighting system 1 can be tuned by adjusting h, x and w of the light scattering 3D- structure 18. Note that optimizing uniformity and optical efficiency, and optimizing intensity profile of the light outputted from the lighting system 1 are not independent. For example, a narrow intensity profile of the light outputted from the lighting system 1 requires equally spaced structures (x=constant) while, to get a perfect uniformity, a gradient in x is required.

As mentioned above the design of the light scattering 3D-structure 18 may be varied in order to tune the optical efficiency, the uniformity and the intensity profile of the light outputted from the lighting system 1. Below some examples of light scattering 3D- structure 18 designs will be discussed.

In connection with Figs. 2-6 a light scattering 3D-structure 18 used for creating a symmetric intensity profile of the light outputted from the lighting system 1 will be discussed. With reference to Figs. 2 and 3, it will be discussed how to design this particular design of the light scattering 3D-structure 18. A triangular like shape is defined in two steps. The triangular shape is chosen as an example, many other shapes are possible. The outer contour of this specific shape is defined by a function r(Q), r being the radius as a function of the angle Θ: 1/nl

[1] in which q=2, A=\, B=\, m=3, n\=5, «2=14 and «3=14. The function r(Q) is defined by J. Gielis, "A generic geometric transformation that unifies a wide range of natural and abstract shapes", American Journal of Botany 90(3), page 333-338 (2003), which is hereby incorporated by reference. The inner contour is defined by the function r(6)-0.5. To create an symmetric intensity profile, this shape is combined with a conical shape having a radius defined by the maximum radius r _m a of the contour r(Q) [Eq. 1]. The maximum height of the structures is determined by the height of the cone h = [2]

tan in which a is the top half angle of the cone. The cone shape is subtracted from the triangular shape as is illustrated in Fig. 2. The value of a determines, in combination with the shape r(Q), the final shape of portions of the light scattering 3D-structure 18. According to one non- limiting example the portions of the light scattering 3D-structure 18 are defined by mm, «=6.57 mm, =35 degrees. The full light scattering 3D-structure 18 may then be designed by arranging a plurality portions of the light scattering 3D-structure 18 in a matrix as illustrated in Fig. 3.

In Fig. 4 a partial cross-section of a lighting system 1 comprising an optical element 10 comprising the light scattering 3D-structure 18 of Fig. 3 is illustrated. The light guide area is 0.25 m x 0.25 m and the gap D is ~40 mm. The thickness d of the light guide is 4 mm. Light sources 20 in the form of LEDs are placed on two sides of the square light guide 1 1 and produce 694 lm in total (equivalent to a typical 4000 lm per 0.6mx0.6m tile, which is a standard luminaire size in Europe). FIG. 5 shows the intensity distribution of the lighting system of Fig. 4. Curves 50 and 60 show the intensity profile of the emitted light in the x-z and y-z plane respectively. A maximum intensity value of 420 cd/klm was found. The lighting system has an Unified Glare Rating (UGR) of 19. The glare can be decreased further to lower (UGR) values when the aspect ratio (h/r _max ) of the printed structures is higher (i.e. lower a). FIG. 6 shows the uniformity of the lighting system as a result of calculations of the illuminance E (lm/m ² , or Lux at a small distance from the light scattering 3D-structure 18. The graph at the bottom of the x-y graph is the calculated luminance as a function of the x- direction at y=0, and the graph at the right side of the x-y graph the calculated luminance as a function of the xy-direction at x=0, both graphs being extracted from the x-y graph. The measured uniformity is perceived by the eye as perfectly uniform.

In Fig. 7 yet another design of a light scattering 3D-structure 18 is illustrated. According to this design rectangular 3D-sub-structures are used for producing a symmetric beam profile.

Asymmetric beam shapes may e.g. be made by linear 3D-sub-structures 19 as illustrated in Fig. 1.

Because, the light scattering 3D-structure 18 may be designed in many different ways having similar intensity profiles of the light outputted from the lighting system 1 , the perception of light emitted from the lighting system 1 may be customized depending on the aesthetic preferences of the user. Moreover, the appearance of the lighting system 1 may be customized depending on the aesthetic preferences of the user.

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, as illustrated in Fig. 8 an additional light scattering 3D-structure 22 may be arranged directly onto the back side 13 of the light guide 1 1. The additional light scattering 3D-structure 22 is arranged to scatter light within the light guide 1 1 interacting with the additional light scattering 3D-structure 22 such that at least a part of the scattered light is escaping the light guide 1 1 at the front side 13 thereof. The additional light scattering 3D-structure 22 may comprise any feature that the light scattering 3D-structure 18 comprises. The application of the additional light scattering 3D-structure 22 may be useful in suspend lighting systems 1 where both up-light and a down-light is desirable.

Moreover, the linear 3D-sub-structures may be tilted. This will produce asymmetric tilted light beams.

Furthermore, as discussed above the light scattering 3D-structure 18 comprises pigment particles. The pigmented particles closest to the light guide 1 1 are presentably white. Having white pigmented particles closest to the light guide 1 1 optimizes the reflectively for light at the interface between the light guide 1 1 and the light scattering 3D-structure 18. Moreover, the pigmented particles being present at a distance from the light guide 1 1 may be having a color different from white. This may create attractive visual effects depending on the viewing angle without affecting light output and color too much.

Moreover, optical films may be added in the gap between the reflecting element 30 and the light guide 1 1 to manipulate the optical performance.

Furthermore, after applying the light scattering 3D-structure 18 onto the light guide 1 1, the optical element 10 may be heated and curved into a desired shape.

Moreover, the light scattering 3D-structure 18 may be provided with slits. These slits relieve the mechanical stess which may be present in a single 3D-printed wall of material.

Furthermore, the light scattering 3D-structure 18 may be provided with sections wherein the light scattering 3D-structure 18 is not in direct contact with the light guide 1 1. This will introduce gaps between the light guide 1 1 and the light scattering 3D- structure 18. Hence, the light scattering 3D-structure 18 is not in mechanical contact with the light guide 1 1 everywhere. This approach may be used for adjust the rate of out-coupling and the beam shaping properties.

Additionally, variations to the disclosed embodiments can be understood and effected by the skilled person in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage.