FIELD OF THE INVENTION

The present invention relates to an optical device that can be used to collimate the luminous output of a lighting device such as a luminaire.

The present invention further relates to a lighting device comprising such an optical device.

The present invention yet further relates to a lighting system comprising a plurality of such lighting devices.

BACKGROUND OF THE INVENTION

In certain lighting applications, e.g. certain types of luminaires, it is desirable for the lighting device to generate a certain type of luminous output, such as a homogeneous or collimated luminous output. A non-limiting example of such an application is ceiling lighting in office spaces, where a homogeneous or collimated output is considered desirable to create a suitable working environment. Other application domains will be apparent to the skilled person.

Known techniques for homogenizing light make use of arrayed micro-lenses, diffractive diffusers, ground glass diffusers, and holographically generated diffusers. Micro- lens arrays homogenize light by creating an array of overlapping diverging cones of light. Each cone originates from a respective micro-lens and diverges beyond the focal spot of the lens. In the known arrays, the individual lenses are identical to each other. Ground glass diffusers are formed by grinding glass with an abrasive material to generate a light-scattering structure in the glass surface.

It is known to provide a luminaire in which a plurality of solid state lighting elements such as a light emitting diodes (LEDs) are arranged to direct their luminous output towards a micro-optical foil that comprises a plurality of collimating elements with each of the collimating elements being associated with one of the solid state lighting elements. The ability of shaping the light by the foil depends on the angular spread of the light entering the foil at any location. The smaller the beam width of the incoming light, the more precise the light can be redirected by the foil. For this reason, it is desirable that each collimating element of the micro-optical foil is substantially associated with a single solid state lighting element only, because an increasing number of solid state lighting elements associated with such a collimating element equates to an increasing angular spread in the incident light, which therefore equates to a reduced efficiency of the coUimation.

The consequence of such an arrangement is that when looking at a single location of the foil, the light originating from that single location will originate from a single solid state lighting element, i.e. the light of the solid state lighting elements in the luminaire is not mixed before exiting the luminaire through the micro-optical foil. This can lead to binning issues; wherein different solid state lighting elements produce light of a slightly different color, these color differences are reproduced in the output of the luminaire, which is undesirable. Also, such an arrangement obviously is unsuitable in applications where color mixing is required, such as a lighting device comprising LEDs having different colors.

It is known to mix the light produced by multiple LEDs in a light guide. This is for instance disclosed in US 2006/0146573 Al, which discloses a light guide plate including a first light guide layer made of a material having a refractive index n 1 , and a scattering light guide layer hav ing a function o scattering light. A light emitting diode is arranged alongside the light guide layer for coupling light into the light guide layer sideways. The scattering light guide layer comprises a plurality of scattering elements to generate an angular light distribution below the critical angle of the light guide, i.e. the angle above which total internal reflection occurs.

However, due to the large angular distribution of light emanating from the scattering elements, it is far from trivial to produce a collimated luminous output from such a lighting device. It is known that a degree of coUimation can be controlled by controlling the diameter of the scattering element; a smaller scattering element achieves a higher degree of coUimation, i.e. generates light having a smaller beam angle. However, this comes at the cost of reduced extraction efficiency, such that a trade-off has to be made between the degree of coUimation and efficiency of the lighting device.

SUMMARY OF THE INVENTION

The present invention seeks to provide an optical device that can be used to produce a highly collimated luminous output in such lighting devices.

The present invention further seeks to provide a lighting device comprising such an optical device. The present invention yet further seeks to provide a lighting system

comprising a plurality of such lighting devices.

The invention is defined by the independent claims. Dependent claims define advantageous embodiments.

According to an aspect, there is provided an optical device comprising a first major surface and a second major surface opposite the first major surface, wherein the second major surface comprises a grid of tessellated optical elements, each tessellated optical element comprising concentrically arranged around the center of the optical element a plurality of polygonal rings having reflective facets

The optical elements of such an optical device may be accurately aligned with scattering elements of a light guide-based lighting device such that each optical element aligns with a separate scattering element, wherein the reflective facets, e.g. total internal reflection facets, of the optical elements are arranged to receive a different portion of the angular spread of light generated by such a scattering element, and are shaped to produce a collimated luminous output for such a portion of the incident light. Furthermore, by arranging the optical elements in a tessellated fashion, bleeding of light through spaces in between the optical elements is avoided, thereby further improving the collimation of the light generated by the optical device. Tessellation of a flat surface is the tiling of a plane using one (or more) geometric shapes, called tiles, with no overlaps and no gaps, or in other words, tessellation is a repeating pattern of similar or identical shapes which fills a surface. The polygonal rings, of which the reflective facets of an outer rings forms the complete perimeter of the tessellation optical element, are bounded by straight lines which together, for example, form a triangular, square, rectangular or hexagonal shape.

In a particularly advantageous embodiment, each tessellated optical element further comprises a refractive element in said center. This arrangement yields a particularly high degree of collimation because the refractive element can effectively collimate incident light at relatively low angles of incidence, whereas the reflective facets can effectively collimate incident light at higher angles of incidence.

The refractive element may be spherical or aspherical. Aspherical for instance may be beneficial if the optical device has a different refractive index then the light guide, wherein the aspherical degree of the refractive element, e.g. a lens element, can be chosen to compensate for the refraction between the light guide and the optical device, in particular to correct for spherical aberrations originating from the short distance between a scattering element and the refractive element, i.e. a low f number, which causes the focal point to smear out. This may for instance be compensated by the inclusion of a modification near the rim of a refractive element, e.g. lens, which causes the curvature of the refractive element to deviate from a spherical shape.

The tessellated optical elements are typically identical to ensure the desired homogeneous output although theoretically it is possible to have different optical elements, for instance if a lighting device comprises an irregular pattern of scattering elements.

In an embodiment, the tessellated optical elements have a hexagonal shape. Although any suitably shaped optical element may be used to form a tessellated grid, a hexagonal shape is particularly preferred because it more closely approximates a circular shape compared to e.g. a square optical element, which therefore gives a better performance when combined with circular scatter elements.

The optical device according to embodiments of the present invention can be kept extremely thin; in an embodiment, the optical device is a foil having a thickness of less than 1.0 mm. Moreover, each reflective facet may have a height not exceeding 0.15 mm.

in accordance with another aspect, there is provided a lighting device comprising a light guide having a first major surface and a second major surface opposite the first major surface, the first major surface being connected to the second major surface by respective side surfaces; a plurality of solid state lighting elements arranged along at least one of said side surfaces, wherein each solid state lighting element is arranged to emit its luminous output into the light guide via said side surface; and the optical device according to one or more of the aforementioned embodiments, wherein the optical device faces the second main surface of the light guide; and wherein the first main surface of the light guide comprises a pattern of scattering elements, each scattering element being aligned with the center of one of said optical elements.

Such a lighting device, e.g. a luminaire, can achieve a high degree of collimation in its luminous output due to the alignment of the scattering elements with the respective optical elements of the optical film of the present invention.

Preferably, the solid state lighting elements are arranged alongside opposite side surfaces of the light guide to increase the luminous output intensity generated by the lighting device.

The scattering elements may be Lambertian scatter dots.

In an embodiment, the distance between neighboring scattering elements is at least 2h* tan(9), wherein h is the distance between the first major surface and the second major surface of the light guide and Θ is the critical angle of the light guide. This ensures that each optical element is associated with scattered light predominantly originating from a single scattering element only, i.e. the optical element does not suffer from significant interference from neighboring scattering elements, which interference reduces the efficiency of the collimation.

The optical device may contact the light guide to reduce such interference as much as possible.

Alternatively, the optical device may be spatially separated from the light guide. This for instance may be advantageous for improving intermixing of the light originating from different solid state lighting elements such as LEDs.

According to yet another aspect, there is provided a lighting system comprising a plurality of the lighting devices according to one or more embodiments of the present invention. Such a lighting system may for instance be a modular system for integration in a (suspended) ceiling, or may form part of a suspended ceiling system. BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention are described in more detail and by way of non- limiting examples with reference to the accompanying drawings, wherein

Fig. 1 schematically depicts an optical device according to an embodiment of the present invention;

Fig. 2 schematically depicts a top view of an aspect of the optical device of

Fig. 1;

Fig. 3 schematically depicts a cross-section of the aspect of the optical device of Fig. 1;

Fig. 4 schematically depicts a cross-section of an aspect of an optical according to another embodiment of the present invention;

Fig. 5 schematically depicts a cross-section of a lighting device according to an embodiment of the present invention;

Fig. 6 schematically depicts a top view of a part of the lighting device of Fig. 5; Fig. 7 schematically depicts the operating principle of the lighting device of Fig. 5;

Fig. 8 schematically depicts a cross-section of a lighting device according to another embodiment of the present invention;

Fig. 9 depicts the simulation result of a degree of collimation achieved with a lighting device according to an embodiment of the present invention; and Fig. 10 depicts the simulation result of a degree of collimation achieved with a lighting device according to another embodiment of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

It should be understood that the Figures are merely schematic and are not drawn to scale. It should also be understood that the same reference numerals are used throughout the Figures to indicate the same or similar parts.

Fig. 1 schematically depicts a top view of an optical device 10 in accordance with an embodiment of the present invention. The optical device 10 comprises a plurality of optical elements 20, which are tessellated to form a tessellated grid of optical elements 20, i.e. a grid without gaps between the optical elements 20.

In Fig. 1, the optical elements 20 are shown to have a hexagonal shape although it should be understood that other shapes suitable to form such a tessellated grid may also be used, e.g. squares. However, the advantage of using hexagonally shaped optical elements 20 is that these shapes more closely resemble a circular shape than for instance square shaped optical elements, which improves the optical performance of the optical elements when used in conjunction with a circular scattering element as will be explained in more detail later.

In an embodiment, the optical elements 20 are all identical. This for instance is a suitable embodiment when each optical element 20 receives a substantially similar, e.g. identical, luminous distribution. It should however be understood that in embodiments where different optical elements 20 receive light of a different nature, e.g. light having a different angular distribution, some of the optical elements 20 may be different to some other optical elements 20. In such a scenario, each optical element 20 may be individually designed as a function of the luminous distribution the optical element 20 is expected to receive.

Only 64 optical elements 20 are shown in Fig. 1 by way of non-limiting example only. It should be understood that generally an optical device 10 may comprise any suitable number of optical elements 20, such as for example at least 100 optical elements, typically 5,000 or 10,000 optical elements, even up to 100,000, 1,000,000 optical elements or more.

The optical device 10 may be made of any suitable optical material, e.g. a material having suitable transparency. In an embodiment, the optical device 10 is made of a material having a high refractive index, i.e. a refractive index of at least 1.45 respective to air at 589 nm. Suitable high refractive index materials include for example glass, poly (methyl methacrylate) (PMMA), polyethylene (PE), and polycarbonate (PC), although other suitable materials will be apparent to the skilled person. This has the advantage that incident light over relatively large angles of incidence can be effectively collimated due to the high refractive index of the material of the optical device 10.

In an embodiment, the optical device 10 is made in one piece such as a foil or plate. Such embodiments are relatively easy to handle, and relatively easily adaptable in shape and size to substrates and/or lighting devices. The advantage of being in one piece is that cumbersome mutual attachment of the plurality of optical elements 20, as is done in some prior art optical devices, is avoided.

The optical elements 20 are easily obtainable in sheet, plate or foil material via laser ablation, thus the plurality of optical elements 20 can be formed in sheet, plate of foil material made in one piece. Said one piece material could easily be shaped into a desired shape if required.

A top view of an optical element 20 is shown in Fig. 2, whereas Fig. 3 schematically depicts a cross-section of the optical element 20 along the dashed line shown in Fig. 2. Each optical element 20 is designed to operate as a micro-collimator. To this end, the optical element 20 comprises a central element 22 that is centered around the central (symmetry) point 23 of the optical element 20 and a plurality of reflective facets 24 that are arranged as a polygonal ring, the plurality of rings being arranged in a concentric pattern around the central element 22. In other words, the optical element 20 comprises a plurality of hexagonally shaped faceted rings 24, with a first of said rings being arranged around the central element 22 and each next ring being arranged around the previous ring of said plurality. Each optical element has a perimeter 21 which is completely bounded or formed by reflective facets 24-1, 24-2, 24-3, 24-4, 24-5, 24-6 of the outer faceted ring 24. Such collimator designs are known per se. However, it has not been previously reported to miniaturize such designs into micro-optical elements 20 and to combine such elements into an optical device 10.

The facets 24 preferably are total internal reflection facets and typically define a second major surface 26 of the optical device 10, which second major surface 26 is arranged opposite a first major surface 25. In at least some embodiments, the first major surface 25 is arranged as the light entry surface of the optical device 10, wherein the second major surface at least in part defined by the facets 24 is arranged as the light exit surface of the optical device 10. However, in some alternative embodiments, the first major surface 25 is arranged as the light exit surface of the optical device 10, wherein the second major surface at least in part defined by the facets 24 is arranged as the light entry surface of the optical device 10.

As will be understood by the skilled person, each facet 24 will have a different angle relative to the optical axis (i.e. the axis perpendicular to the first main surface 25 that extends through the central point 23), which angle is selected based on the angle of incidence of the incident light relative to the optical axis; for instance, facets 24 closer to the central point 23 may have a sloped surface under a smaller angle with the optical axis than facets 24 further away from this central point, because the latter facets 24 are arranged to receive light at a higher angle of incidence. As the optimization of individual facets 24 to generate the desired degree of collimation is considered to be a routine skill for the person skilled in the art, this is not explained in further detail for the sake of brevity only.

The optical device 10 can be kept extremely thin, especially when using the aforementioned high refractive index materials. For instance, the total height or thickness H _TOT can be as little as 1.0 mm or less, whereas the total height H _FAC of each facet 24 can be as little as 0.15 mm or even 0.05 mm for a facet 24 having a 45° slope. At this point, it is noted that only three facets 24 are shown by way of non-limiting example only. It should be understood that such facets 24 typically have a width that is comparable to their height, e.g. a width of around 0.15 mm or 0.05 mm. Consequently, each optical element 20 may comprise a substantially larger number of such facets 24, e.g. 100 or more of such facets. It is furthermore noted that the optical elements 20 typically are micro-optical elements, i.e. optical elements having dimensions of only a few, e.g. 5 or 10 centimeters or less. For instance, in at least some embodiments the length of a side of an optical element 20 may be chosen in the range of 5-25 mm.

The central element 22 is shown as a reflective element in Figs. 2 and 3.

However, in at least some embodiments of the present invention, the central element 22 may be a refractive element such as a lens. Such an embodiment is shown in Fig. 4. In an embodiment, the refractive element 22 is implemented as a Fresnel lens. As will be understood by the skilled person, this for instance is useful if a central portion of the incident light has a relatively small angle of incidence, e.g. 45° or less, in which case the refractive element 22 is particularly suited to generate a collimated luminous output for this central portion of the incident light, with the reflective facets 24 being arranged to receive light at angles of incidence of greater than 45°.

In an embodiment, the refractive element 22 is an aspherical lens. Such an aspherical lens may be used to correct for spherical aberrations originating from the short distance between a scattering element and the refractive element 22, i.e. a low f number, which causes the focal point generated by a (spherical) lens to smear out. This may for instance be compensated by the inclusion of a modification near the outer edge or rim of the refractive element 22, e.g. lens, which modification causes the curvature of the refractive element to deviate from a spherical shape.

At this point it is noted that the refractive element 22 and the reflective elements 24 are shown as part of the same surface of the optical element 20, i.e. the second major surface 26. It should however be understood that it is equally feasible to provide an optical device 10 in which the (refractive and) reflective portions of the optical elements 20 are divided between the first major surface 25 and the second major surface 26, i.e. at least some of the (refractive and) reflective elements 22 and 24 are located on the first major surface 25. As will be immediately understood by the skilled person, the appropriate configuration for such optical elements 20 will be governed by design requirements, e.g. the nature of the light source to be collimated.

Fig. 5 schematically depicts a lighting device 100 according to an embodiment of the present invention. In Fig. 5, the lighting device 100 is shown to be a luminaire, although it should be understood that the lighting device 100 may take any other suitable shape. The lighting device 100 comprises a plurality of solid state lighting elements 110, e.g. LEDs, which may be any suitable LEDs, e.g. LEDs comprising an organic or inorganic semiconductor layer. The LEDs may be mounted on any suitable carrier, e.g. a printed circuit board, and may comprise additional layers or components, e.g. a phosphorus layer, to provide the LED with the desired characteristics, e.g. color point, color temperature and so on.

The solid state lighting elements 110 are typically arranged along one or more side surfaces 126 of a light guide 120, which side surfaces 126 connect a first major surface 122 to the second major surface 124 of the light guide 120. In other words, the first major surface 122 is arranged opposite to the second major surface 124. The solid state lighting elements 110 are arranged to direct their luminous output into the light guide 120, i.e. the luminous surfaces of the respective solid state lighting elements 110 face the corresponding side surface 126. In an embodiment, solid state lighting elements 110 are arranged alongside at least two opposing side surfaces 126. Solid state lighting elements 110 may be arranged along each of the side surfaces 126. The solid state lighting elements 110 may be mounted alongside the side surfaces 126 in any suitable manner, for instance by attaching them to the housing (not shown) of the lighting device 100. The light guide 120 may be made of any suitable optical material, i.e. any material suitable for providing total internal reflection of incident light originating from the solid state lighting elements 110 under angles above the critical angle Θ of the light guide 120. For instance, the light guide 120 may be made of a suitable glass or a suitable polymer material such as PMMA, PE, PET, PC and so on.

In at least some embodiments, the light guide 120 has a thickness in the range of 0.5 - 5 mm, e.g. 1mm. As will be understood, the dimensions of the optical elements 20 may be tailored to the thickness of the light guide 120, as an increased thickness corresponds to a larger optical path from the first major surface 122 to the second major surface 124, which therefore increases the width of the luminous profile generated by each scattering element 130. For instance, for a light guide 120 having a thickness of 1 mm and a scattering element 130 with a radius of 1 mm, each optical element 20 may have sides of 12.5 mm length.

The light guide 120 comprises a pattern of scattering elements 130 on the first major surface 122, with the optical device 10 according to an embodiment of the present invention facing the second major surface 124. In at least some embodiments, the second major surface 26 of the optical device 10 faces the second major surface 124 of the light guide 120. However, it should be understood that it is also feasible for the first major surface 25 of the optical device 10 to face the second major surface 124 for instance if so dictated by optical requirements. The scattering elements 130 may for instance be Lambertian scatter dots. The scattering elements 130 generate light that can escape the light guide 120 by scattering incident light under angles smaller than the critical angle Θ of the light guide 120. It will be understood that a ray of light may have to be scattered by one of the scattering elements 130 multiple times before the light is scattered under an angle that is small enough to escape the total internal reflection of the light guide 120. Scattering elements are known per se and it suffices to say that any suitable scattering element 130 may be selected, e.g. dots of white paint by way of non-limiting example.

Each scattering element 130 is aligned with the central point 23 of one of the optical elements 20 of the optical device 10 as indicated by the dashed lines in Fig. 5. This is shown in more detail in Fig. 6, which depicts a top view of a part of the lighting device 100. The scattering elements 130 can be seen through the optical elements 20 with which they are aligned. Consequently, the scattering elements 130 are laid out in a hexagonal pattern, as mandated by the hexagonal shape of the optical elements 20. The light guide 120 ensures that light generated by different solid state lighting elements 110 is mixed before exiting the light guide 120, as each array of light is typically reflected several times by one of the first major surface 122 and the second major surface 124 before escaping the light guide 120 by scattering through one of the scattering elements 130.

In accordance with an embodiment of the present invention, each optical element 20 is arranged relative to a scattering element 130 such that each optical element 20 predominantly receives the scattered luminous output of a single scattering element 130, i.e. from the scattering element 130 that is aligned with the optical element 20. This therefore provides the appearance of each optical element 20 being associated with a single light source (e.g. a single LED) without the disadvantages of color differences between different optical elements 20 due to the mixing of the luminous output from different LEDs by the light guide 120. This furthermore avoids the appearance of glary spots when looking at the lighting device 100.

In an embodiment, the distance between neighboring scattering elements 130 is at least 2h* tan(9), wherein h is the distance between the first major surface and the second major surface, i.e. the thickness of the light guide 120, and Θ is the critical angle of the light guide 120. This ensures that each optical element 20 is associated with scattered light predominantly originating from a single scattering element 130 only, i.e. the optical element 20 does not suffer from significant interference from neighboring scattering elements 130, which interference reduces the efficiency of the collimation achieved by the optical element 20, as long as the optical elements 20 are placed at a distance from the light guide 20 such that the individual luminous distributions produced by the respective scattering elements 130, or at least the highest intensity parts of such luminous distributions, do not overlap at this distance.

Based on the size (radius) of the scattering element 130, the refractive index and thickness of the light guide 120 and the refractive index of the optical device 10, each optical element 20 may be designed to collimate the luminous distribution received from the corresponding scattering element 130. This is schematically shown in Fig. 7. As previously explained, the central element 22 and the reflective facets 24 are shaped as a function of the angle of incidence of the light originating from scattering element 130 travelling through light guide 120 in order to ensure a high degree of collimation of the incident light indicated by the dashed lines. In this respect it is noted that it is particularly advantageous that the facets 24 can be dimensioned in the micron domain, e.g. having a width and height of around 50 microns, such that a relatively small range of angles of incidence are received by a single facet 24, thereby achieving a high degree of collimation.

At this point, it is furthermore noted that in the arrangement shown in Fig. 5, the optical elements 20 of the optical device 10 may not collect the luminous distribution produced by a scattering element 130 over its full area, but may instead only receive this distribution on a central portion of the optical element 20. As will be understood by the skilled person, each scattering element 130 generates light exiting the light guide 120, which light has a luminous distribution having an angular spread determined by the critical angle of the light guide 120, i.e. the angle of which total internal reflection occurs. In other words, each scattering element 130 will create a luminous distribution in a range from -Θ to Θ. This distribution will fan out upon exiting the light guide 120 at the second major surface 124 (as shown in Fig. 7 and 8), such that the area of each optical element 20 illuminated by the (highest intensity part of the) luminous distribution generated by its corresponding scattering element 130 will be determined by the distance between the optical element 20 and the second major surface 124 of the light guide 120.

It may be desirable to harvest the luminous distribution produced by a scattering element 130 over a larger area of the corresponding optical element 20, for instance to improve the degree of collimation produced by the optical device 10. To this end, the spatial separation between the light guide 120 and the optical device 10 may be increased, as is shown in Fig. 8, which schematically depicts an alternative embodiment of a lighting device 100 in which the optical device 10 is separated from the second major surface 124 of the light guide 120 by a distance D. The aforementioned fan-out of the luminous distribution exiting the light guide 120 at the second major surface 124 increases the width of the luminous distribution generated by each scattering element 130, as is indicated by the dashed lines, such that each optical element 20 may collect the (highest intensity part of the) luminous distribution generated by each scattering element 130 over a larger region of the optical element 20, i.e. the luminous distribution illuminates a larger area of the optical element 20. Consequently, the beam portions received by respective facets 24 become narrower because each facet 24 receives a smaller (angular) portion of the luminous distribution, such that a higher degree of collimation can be obtained. It is noted that optical elements 20 in Fig. 8 are shown to be larger than the optical elements 20 in Fig. 5 for demonstrative purposes only; it should be understood that it is equally feasible that the optical elements 20 in these different embodiments have the same dimensions. Preferably, overlap between (the highest intensity parts of the) luminous distributions from neighboring scattering elements 130 exiting the second major surface 124 of the light guide 120 at a single optical element 20 of the optical device 10 should be avoided as this can severely reduce the quality of the collimation achieved by the optical device 10. It is noted that Fig. 8 schematically depicts the maximum distance D at which the optical device 10 may be placed relative to the second major surface 124, as beyond this distance the luminous distributions generated by neighboring scattering elements 130 will start to overlap such that optical elements 20 will receive light generated by multiple scattering elements 130, e.g. from the scattering element 130 aligned with such an optical element 20 as well as from neighboring scattering elements 130.

As previously explained, the radius of the scattering elements 130 may also be varied in order to control the degree of collimation produced by the lighting device 100. Obviously, a higher degree of collimation already can be achieved by the addition of the optical device 10 and by varying the distance between the optical device 10 and the light guide 120 as explained above, but a further degree of control can be achieved by variation of the aforementioned radius. This is demonstrated in the simulations shown in Figs. 9 and 10, which depict the luminous intensity distribution produced by a luminaire 100 comprising an optical device 10 having optical elements 20 with sides having a length of 12.5 mm and scattering elements having a radius of 1 mm (Fig. 9) and 2 mm (Fig. 10) respectively. The optical device 10 was simulated to be 2.5 mm above the light guide 120 having a thickness of 1 mm.

As can be seen in Figs. 9 and 10, the full- width half-maximum (FWHM) of these light distributions, which is an expression of the degree of collimation achieved by the luminaire 100, is 12° and 20° respectively.

It is noted that in embodiments of the lighting device 100, the loss of efficiency as a trade-off against the improved collimation when reducing the size of the scattering elements 130 may be less critical when the arrangement of the solid-state lighting elements 110 around the edges or side surfaces 126 of the light guide 120 ensures that a high density of such solid state lighting elements 120 can be achieved. In any event, the use of a micro-structured optical device 10 when aligned with the pattern of scattering elements 130 as described above ensures an improved control over this trade-off, which therefore may yield an overall performance increase of the lighting device 100. The lighting device 100 may be included in a lighting system that comprises a plurality of such lighting devices 100, for instance as tiles in a modular system such as a suspended ceiling system, a suspended wall system and so on.

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. The word "comprising" does not exclude the presence of elements or steps other than those listed in a claim. The word "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. The invention can be implemented by means of hardware comprising several distinct elements. In the device claim enumerating several means, several of these means can 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.