FIELD OF THE INVENTION

The present invention relates to a lighting strip for use in a modular panel system such as a suspended ceiling. The present invention further relates to a lighting system comprising such a lighting strip, to a support element comprising such a lighting strip, and to a modular panel system comprising such a lighting strip.

BACKGROUND OF THE INVENTION

In construction, modular panel systems are commonly used to reduce build cost and construction time. Modular panel systems typically allow for the rapid construction of floors, walls and ceilings, albeit often at the compromise of reduced aesthetic appearance. A prime example of such a modular panel system is a suspended ceiling, which can be found in most professional environments, such as for example office spaces. A suspended ceiling typically comprises a metal or plastic grid defining rectangular or square recesses, which are filled with tiles to form a continuous ceiling.

In such modular systems, e.g. a suspended ceiling, lighting may be integrated into the system, typically by replacing one or more tiles with a lighting unit such as a luminaire. Most suspended ceilings comprise luminaires in which a number of fluorescent light tubes are present. For a number of reasons, such luminaires are not ideal. Firstly, such luminaires are considered aesthetically displeasing, i.e. obtrusive. Secondly, in order to improve light efficiency from such luminaires, they usually contain a reflector, which commonly has a parabolic shape. This however can cause glare for an occupant of the office space if the reflected light exits the luminaire under shallow angles to the plane of the modular system. Glare can be very disturbing in an office environment, as it can obscure the image on a computer monitor and can cause physical discomfort, e.g. headaches or sight problems, to the occupant when being subjected to the glare for a prolonged period of time. This is why health and safety standards such as the IEC60598-1 :2008 standard in Europe require lighting solutions to comply with stringent requirements for preventing excessive glare levels. Solutions exist to overcome glare. A straightforward solution is shown in Figure 1, in which a suspended ceiling luminaire 10 is shown. The luminaire 10 comprises a plurality of chambers defined by respective parabolic reflectors 12, with a fluorescent light tube 14 fitted in each of the chambers. Each light tube 14 is offset from the light exit plane 10a of the luminaire 10 by a distance d such that light emitted by the fluorescent light tube 14 under a shallow angle is reflected by the parabolic reflector 12, as indicated by the dotted broken arrow. This increases the exit angle of the light, thus reducing glare. A drawback is that this solution leads to relatively bulky luminaires, which can be considered aesthetically unsatisfactory.

Another solution is shown in Figure 2, in which a luminaire 10 for integration in a suspended ceiling is fitted with a micro-lens optical plate or diffuser 20, which has the function of preventing shallow angle light beams from exiting the luminaire 10. As this causes a substantial amount of light being reflected back into the chamber 11 of the luminaire 10, the luminaire 10 may comprise a reflector 22 to recycle such reflected light. Commonly, the micro-lens optical plate takes the form of a prism plate.

The company SwitchMade offer a light emitting diode based (LED) luminaire marketed under the name Paneos ^® for integration in a suspended ceiling. This has the advantage of lower energy consumption compared to fluorescent light tube-based luminaires. However, as these luminaires replace tiles in the ceiling, they still disrupt the visual appearance of the suspended ceiling.

The Gemino Company (www . gemino.it) markets a suspended ceiling solution in which the light fittings can be integrated into the band raster of the ceiling. The band raster consists of the main structural beams of the suspended ceiling. With smaller form factor lighting such as LED lighting this is a feasible solution, and has the advantage of the improved appearance of the ceiling due to the fact that no tiles need replacing with luminaires. This however increases the manufacturing complexity of the band raster, and thus the cost of the overall design. Moreover, this solution is unsuitable for retrofitting purposes, and is difficult to maintain as the band raster cannot be easily disassembled for maintenance purposes.

SUMMARY OF THE INVENTION

The present invention seeks to provide an aesthetically acceptable lighting solution that can be easily (retro-) fitted to existing modular panel systems in a cost-effective manner whilst being capable of being compliant with health and safety standards. In accordance with a first aspect of the present invention, there is provided a lighting strip for mounting in or on a panel support element of a modular panel system, the strip comprising a first optical waveguide and a plurality of solid state lighting elements placed adjacent to the first optical waveguide in a length direction of the lighting strip and arranged to emit light in the length direction, wherein the first optical waveguide comprises an alternating pattern of first mixing regions and first outcoupling regions. The lighting strip further comprises a first plurality of light entry structures for coupling at least some of the emitted light into the first optical waveguide.

This arrangement has a number of notable advantages. Firstly, because the solid state lighting elements, e.g. light emitting diodes (LEDs) are placed on the sides of the lighting strip, the overall thickness of the lighting strip can be limited to only a few mm. Secondly, the combination of the solid state lighting elements being arranged to emit their light in the length direction of the strip, i.e. in parallel with the side surface of the optical waveguide, and the periodic light entry structures ensure that a relatively small number of solid state light elements suffice to create a homogeneous light output from the lighting strip as the mixing regions of the optical waveguide are dimensioned to ensure that the entered light is evenly distributed throughout the mixing region, i.e. spreads over the entire width of the optical waveguide through complete internal reflection. This for instance makes it possible to produce light of a homogenous color composition in case of solid state elements generating different colors.

In a preferred embodiment, the lighting strip further comprising a glare reducing element, wherein the first outcoupling regions of the first optical waveguide are arranged to couple light into the glare reducing element, such that the lighting strip can more easily comply with health and safety requirements concerning glare.

The first optical waveguide may comprise the first plurality of light entry structures in the form of protrusions on a side surface facing the solid state light elements. In an embodiment, the light entry structures may be separated by respective recesses each housing a solid state lighting element. This has the advantage that the entry structure can be realized in a particularly cost-effective manner.

Alternatively, said entry structure may comprise a protrusion extending into the light path of said emitted light. This has the advantage that the emitted light can be more effectively coupled into the optical waveguide compared to an opening as an entry structure. Said protrusion preferably has a saw tooth shape. In an embodiment, each first mixing region comprises a plurality of said protrusions including a terminal protrusion having a portion extending over an adjacent first outcoupling region. It has been found that such overlap between the protrusion and the outcoupling region improves the yield of the emitted light coupled into the optical waveguide as lateral outcoupling is less pronounced in outcoupling regions.

In an embodiment, the plurality of solid state lighting elements comprises a first group of solid state lighting elements for emitting light having a first color and a second group of solid state lighting elements for emitting light having a second color different to the first color such that the color temperature of the light strip can be controlled by varying the respective emission intensities of the first and second group.

Preferably, the plurality of solid state lighting elements comprises a first group of solid state lighting elements and a second group of solid state lighting elements, each of said groups comprising respective subsets of solid state lighting elements for emitting light of different colors. This achieves a high homogeneity in color output as well as controllability over the color temperature of the lighting strip.

In an embodiment, the lighting strip further comprises a second optical waveguide comprising an alternating pattern of second mixing regions and second outcoupling regions , and a second plurality of light entry structures, wherein the first plurality of light entry structures is arranged to collect the light emitted by the first group of solid state lighting elements, wherein the second plurality of light entry structures is arranged to collect the light emitted by the second group of solid state lighting elements, and wherein the first optical waveguide and the second optical waveguide form a stack in which the first mixing regions are aligned with the second outcoupling regions and the first outcoupling regions are aligned with the second mixing regions.

This also ensures a homogenous color output as each color component is properly distributed over the full width of its respective optical waveguide. This yields a light guide having less complex optical waveguides at the penalty of a small increase in thickness of the lighting strip.

The first and second groups of solid state elements may be arranged such that they are positioned in the plane of the optical waveguide into which their light is coupled, i.e. facing the sidewalls of the relevant optical waveguide.

Alternatively, the first and second groups of solid state lighting elements both face the first optical waveguide, in which case the lighting strip may further comprise a plurality of reflecting elements for reflecting the light emitted by the second group of solid state lighting elements towards the second optical waveguide. Such an arrangement has the advantage that the manufacturing complexity of the lighting strip is reduced. As an alternative to the reflecting elements, the second optical waveguide may comprise a plurality of protrusions extending out of the plane of the second optical waveguide such that an alternating pattern of protrusions of the first optical waveguide and of the second optical waveguide is formed.

According to a further aspect of the present invention, there is provided a lighting system including a plurality of lighting strips of the present invention, the lighting system further comprising a controller for setting the light output of individual lighting strips as a function of at least one of incident daylight, room layout and room occupancy. This allows for the output of the lighting strip to be adapted to localized needs, e.g. in areas such as corridors, office spaces, printing areas and so on, and/or adapted in the presence of an occupant of the room. To this end, the lighting system may further comprise a presence sensor for detecting the presence of an individual in said room, the controller being responsive to the presence sensor.

According to yet another aspect of the present invention there is provided a support element for a modular panel system comprising a lighting strip of the present invention. The lighting strip may be attached to or integrated into the support element.

According to yet another aspect of the present invention there is provided a modular panel system comprising a support grid comprising support members for attaching to a building structure and support elements for extending between support members and a plurality of panels dimensioned to be supported by the support grid, wherein the support grid comprises a plurality of lighting strips of the present invention.

The lighting strips preferably are integrated in or attached to the support elements.

More preferably, the lighting strips are attached to the support elements by a fastener that shields the solid state lighting elements, thus preventing stray light from being generated whilst facilitating retrofitting of the strips to existing modular panel systems at the same time.

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: Figure 1 schematically depicts a prior art luminaire for a modular panel system;

Figure 2 schematically depicts another prior art luminaire for a modular panel system;

Figure 3 schematically depicts a part of a modular panel system including a solid state lighting strip according to an embodiment of the present invention;

Figure 4 schematically depicts a non-limiting example of a glare reducing element suitable for use in the solid state lighting strip of the present invention;

Figure 5 depicts the simulation results of the light distribution of the glare reducing element of Figure 4 when used in conjunction with a Lambertian light source;

Figure 6 schematically depicts an optical waveguide according to an embodiment of a lighting strip of the present invention;

Figure 7 schematically depicts a lighting strip according to a further embodiment of the present invention;

Figure 8 schematically depicts an optical waveguide stack according to an embodiment of a lighting strip of the present invention;

Figure 9 schematically depicts an optical waveguide stack according to another embodiment of a lighting strip of the present invention;

Figure 10 schematically depicts part of an optical waveguide stack according to yet another embodiment of a lighting strip of the present invention;

Figure 11 schematically depicts part of an optical waveguide stack according to yet another embodiment of a lighting strip of the present invention;

Figure 12 schematically depicts a part of a modular panel system including a solid state lighting strip according to a further embodiment of the present invention;

Figure 13 schematically depicts a room with a modular panel system according to an embodiment of the present invention; and

Figure 14 schematically depicts a modular panel system according to an embodiment of the present invention in more detail.

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. Figure 3 depicts a panel support element 210 of a modular panel system 200, which is designed to support panels or tiles 220. To this end, the panel support element 210 may comprise a lip 212 onto which the panel or tile 220 rests. Examples of such modular panel systems 200 include suspended ceilings and floors. The panel support element 210 comprises a lighting strip 100 in which LEDs 110 or other suitable solid state lighting elements are placed adjacent to a light extraction layer 120 such as an optical waveguide (from hereon referred to as a light guide), which at regular intervals couples the light received from the LEDs 110 out to a glare reducing element 130, which ensures that the intensity of light emitted from the lighting strip 100 under shallow angles with the plane formed by the panels 220 is kept sufficiently low to suppress glare for inhabitants of a room in which the modular panel system 200 is fitted.

The provision of light from the LEDs 110 to the glare reducing element 130 through a light guide layer 120 further ensures that the glare reducing element 130 is illuminated in a uniform manner, thus improving the uniformity of the light output of the solid state lighting strip 100, as will be explained in more detail below.

In the lighting strip 100, the LEDs 110 or the other suitable solid state lighting elements are arranged to emit light in a length direction of the lighting strip 100, i.e. parallel to the side surface of the light guide 120, with the side surface of the light guide comprising a plurality of light entry structures (not shown) to divert the emitted light into the light guide 120. The light guide 120 has an upper surface, i.e. the surface opposite the surface facing the glare reducing element 130, which at regular intervals comprises light extraction structures that cause the scattering of incident light such that a light exit window is created in the light guide 120 located opposite the light extraction structures for light that is scattered such that it exceeds the critical angle of the light guide 120. In other words, the light extraction elements act as a diffuser for the light travelling through the light guide 120. Such principles are well-known in optics, in particular from optical waveguide design, and will therefore not be explained in further detail for the sake of brevity.

The light extraction elements may comprise a plurality of preferably regularly spaced grooves. Such grooves may for instance be formed in the upper surface by laser ablation or sand blasting techniques. Alternatively, the light extraction elements may comprise a plurality of protrusions facing the glare reducing element 130. In a cost-effective embodiment, the protrusions may take the form of dots of (white) paint. The protrusions are preferably arranged in a regular pattern such that a homogeneous light extraction is obtained. The glare reducing element 130 preferably is a micro-lens optical (MLO) plate or prism plate, which preferably is in the form of a thin foil having a thickness of less than 5 mm, and more preferably no more than 1 mm. It has been found that such a foil can be used when the LEDs 110 can be operated in the so-called Safe Extra Low Voltage Domain, for which the fire proof requirements as for instance tested by the 5VA and glowwire tests are less stringent. Such a mode of operation may for instance be achieved if the density of light strips 100 in the modular panel system 200 is high enough to ensure sufficient lighting levels in the room when operating the light strips 100 in the safe extra low voltage domain, as will be explained in more detail below.

The glare reducing element 130 typically reflects substantial amounts of light originating from the light extraction layer 120 back into this layer. It is therefore

advantageous to provide a reflective layer 140 such that the light guide 120 is sandwiched between the reflective layer 140 and the glare reducing element 130. The reflective layer 140 may for instance be a reflective foil, a layer of white paint applied to the surface of the panel support element 210 facing the solid state lighting strip 100 or to the surface of the light extraction layer 120 facing the panel support element 210. Other embodiments of such a reflective layer 140 are equally feasible.

The use of a MLO plate or foil 130 makes it possible to keep the overall thickness of the solid state lighting strips 100 to less than 5 mm, in particular because LEDs 110 are used that are placed at the sides of (i.e. adjacent to) the light extraction layer 120. This reduces the weight and cost of the luminaire as fewer materials need to be used to realize the desired lighting levels.

The dimensions of the MLO layer 130, or more precisely, the width W of the exit window of the MLO layer 130, are preferably chosen to be 0.04 times the pitch P of the panel support elements 210 as shown in Figure 3, as at this ratio the optimal tradeoff between light output and minimalistic appearance is achieved. A commonly used width for panel support elements 210 is 2.4 cm. By defining the pitch of the support elements 210 (in the width direction) at 60 cm, sufficient luminance levels can be achieved in a room whilst driving the light strips 100 in the safe extra low voltage domain, as previously mentioned. Satisfactory results are still obtained if the aforementioned ratio lies in the range of 0.02 - 0.08.

If the value of this ratio exceeds 0.08, the density of panel support elements 210 becomes too high for the modular system 100 to be cost-effective, and its appearance becomes unsatisfactory. If the value of this ratio falls below 0.02, the spacing between luminaires 100 increases to such an extent that the output levels of each luminaire 100 have to increased to ensure homogeneous lighting of the room to such a level that glare can no longer be avoided; i.e. the luminaires 100 can no longer be operated in the safe extra low voltage domain.

Hence, an important insight of the present invention is that by dimensioning the lighting strips 100 in a modular panel system 200 such that the width of the light exit window of the luminaire 100 falls within the W/P ratio of 0.02 - 0.08, the output levels per luminaire 100 can be reduced to such an extent that the intensity of emitted light that is perceived as glare can be kept to allowable levels as dictated in the various legislatory standards.

The solid state lighting strip or luminaire 100 preferably has a light output in lumen in excess of 500 times the pitch (in meters) of the elements used per meter of solid state lighting strip or luminaire 100. This leads to typical lumen outputs per meter luminaire of more than 150 lumen up to 600 lumen. This typically ensures that no additional lighting has to be present in a room fitted with the modular panel system 200 of the present invention, i.e. a modular panel system including the lighting strips 100. It is also preferred that the pitch of the panel support elements 210 in the modular panel system 200 is chosen in the range of 0.3 - 3.0 meters for indoor use as this ensures sufficiently uniform lighting levels in the room.

In Figure 3, the solid state lighting strip 100 is integrated into the panel support elements 210, which preferably are secondary support elements of the modular panel system 200, i.e. elements that are not fixed to a floor, wall or ceiling of a room, such that the panel support elements 210 can be easily replaced in case of, e.g. an end of life malfunction of the solid state lighting strip 100, i.e. without having to remove the primary support elements from the ceiling, wall or floor. This also has the advantage that the solid state lighting strip can be easily retrofitted in existing modular panel systems by replacing the support elements of the modular panel system with the panel support elements 210 of the present invention.

The support element 210 including luminaire 100 may have a light exit window of 24 mm (width) by 60 cm (length). Such a luminaire would have a light output of around 250 lm. Such a support element 210 may be used in a modular panel system 200 such as a suspended ceiling, by setting these luminaires in lines of panel support elements 210 having a pitch of 60 cm as previously explained. Other dimensions are equally feasible.

Upon assembly of the modular panel system 200 in a room, the support elements 210 are preferably aligned parallel to the window(s) in the room that receive most daylight. As the solid state lighting strips 100 in support elements 210 typically are individually controllable, this makes it straightforward to adjust the lighting levels in the room to the incident daylight, i.e. by dimming the solid state lighting strips 100 in support elements 210 near to the window by a larger extent than the solid state lighting strips 100 in support elements 210 further away from the window.

To this end, the plurality of lighting strips 100 on different support elements 210 typically form part of a lighting system that further comprises a controller (not shown) for adjusting the output of the individual solid state lighting strips 100 in accordance with lighting requirements in the room. Such control signals may for instance be configured to adjust the lighting levels in accordance with a layout of a room comprising the modular panel system 200. For instance, the room may be partitioned into work areas connected to each other by one or more corridors, in which the work areas are to receive higher lighting levels than the corridors. To this end, the controller may increase the light output of the lighting strips 100 located over the work areas whilst reducing the light output of the lighting strips 100 located over the corridors.

Similarly, the controller may adapt the light output of the lighting strips 100 to compensate for disruptions in the regularity of the grid of the modular panel system 200, for instance if the modular panel system 200 is fitted around air ventilation shafts or air- conditioning units. The controller may be adapted to increase lighting levels in the vicinity of such disruptions to compensate for the absence of lighting underneath such disruptions.

The lighting system may further comprise one or more sensors, such as daylight sensors and/or presence sensors for detecting the presence of an individual in the room, with the controller being responsive to these sensor(s) such that the lighting levels can be adapted accordingly.

At this point it is noted that MLO-based glare reducing elements 130 can have a metallic appearance at relatively high light output levels. This is sometimes considered unappealing. A presence sensor may be used to adjust the lighting levels in the presence of one or more individuals in the room to sufficient levels to allow the individuals to perform their activities, with these light levels being sufficiently low to avoid the metallic appearance of the MLO-based glare reducing element 130 such that overall appearance of the modular panel system 200 is improved.

Figure 4 shows a non-limiting example of a MLO-based glare reducing foil 130, which consists of a rectangular array of cones 132 with a top angle of 108° and a pitch of 50 micrometers. The small dimple in the top of each cone 132 is present for manufacturing reasons. The pitch of the cones 132 may be varied without departing from the scope of the present invention. The cones may be made of any suitable transparent material, e.g. PMMA or polycarbonate. As such MLO-based glare reducing elements 130 are known per se, their manufacture will not be explained in further detail for the sake of brevity. Alternative embodiments of the glare reducing element 130 are also feasible; an example of another suitable glare reducing element 130 can for instance be found in WO2008058585 Al .

Figure 5 depicts the simulated angular light intensity distribution of the MLO foil 130 shown in Figure 4 when a Lambertian light source is placed below the MLO foil 130. The arrow indicates the direction of the light rays that have passed through the MLO foil 130. The light transmitted in the opposite direction is light that has been reflected by the MLO foil 130. Such reflected light can be recycled by the use of diffuse reflectors as previously explained.

As can be seen in Figure 5, the light intensity at a 65° angle relative to the vertical light emission axis is very low. As light emitted at around these angles typically causes glare, it can be seen that the MLO foil 130 can effectively suppress glare. For the MLO foil 130 in a light strip 100 in a modular panel system 200 at a W/P ratio of 0.04 and operating at a luminance output of 380 lm/m luminaire, luminance levels at 65° have been found to be at around 1100 cd/m ² , which is well below the requirement of such levels not exceeding 1500 cd/m ² . This equated to a glare evaluation rating according to the UGR method of less than 17, which is well under the UGR requirement of 19 or less.

As previously mentioned, the lighting strip 100 comprises solid state lighting elements 110, e.g. LEDs, that emit light in a length direction of the strip, i.e. parallel to the (principal portion of the) sidewall of the light guide 120. Consequently, measures have to be taken to couple the light generated by the solid state lighting elements 110 into the light guide 120.

A first embodiment of such an arrangement is shown in Figure 6. In this embodiment, the light guide 120 comprises a plurality of recesses 121 that house the solid state lighting elements 110. As indicated by the arrows, the solid state lighting elements 110 are arranged to emit light in a length direction L of the light guide 120 and the lighting strip 100. The protrusions 122 that separate the recesses 121 couple the light emitted by the solid state elements 110 into the light guide 120.

The light guide 120 is divided into mixing regions 124 where the light emitted by the solid state lighting elements 110 is homogeneously distributed over the full width W of the light guide 120 and outcoupling regions 126, where a surface of the light guide 120 perpendicular to its side surfaces comprises light extraction elements, e.g. grooves or protrusions such as white paint dots as previously explained for coupling the internally reflected light out of the light guide 120 towards the glare reducing element 130 as shown in Figure 3. An important advantage of this arrangement over for instance an arrangement where the solid state lighting elements are arranged to emit light in the width direction W is that for the present arrangement complete internal reflection is obtained in the mixing regions 124, such that the distance between the solid state lighting elements 110 may be increased. Consequently, fewer solid state lighting elements 110 are required to obtain a homogenous light output from the lighting strip 100.

In a preferred embodiment, the lighting strip 100 comprises (at least) two groups of solid state lighting elements, i.e. groups of solid state lighting elements 110 and 110' are present. This may for instance be used to define color groups, i.e. different groups of solid state lighting elements are arranged to generate different (white) colors, e.g. a warm (white) and a cold (white) color. By making these groups individually controllable, the light temperature of the light output of the solid state lighting strip 100 can be controlled.

However, it is important that the respective colors emitted by the solid state lighting elements 110 and 110' is properly mixed prior to emission from the lighting strip 100 to ensure a homogenous color experience for the occupant of the space in which the lighting element 100 is used.

This may be achieved in a number of ways. In a first embodiment as shown in

Figure 7 the lighting strip 100 comprises a stack of light guides including a light guide 120 for receiving the light for the first group of solid state lighting elements (not shown) and a further light guide 120' for receiving the light for the second group of solid state lighting elements (not shown) with the a further light guide 120' having outcoupling regions 126', e.g. grooves, paint dots and so on, on its upper surface, i.e. the surface facing the light guide 120, for coupling the mixed light of the first and second groups of solid state lighting elements into the glare reducing element 130. In a preferred embodiment, the first and second groups of solid state each comprise subsets of solid state elements of different color, such that each light guide 120 and 120' is arranged to mix (blend) different colors originating for these subsets.

The lighting strip 100 optionally may further comprise a reflective layer 140 as previously explained. Fastening means 180 may be used to fit the light strip 100 to a panel support element 210. The fastening means, e.g. clips, 180 preferably shield the solid state lighting elements of the lighting strip 100 such that the light emitted by the solid state lighting elements can only exit the lighting strip 100 through the stack of light guide 120 and further light guide 120'. To this end, the surface of the fastening means 180 facing the solid state lighting elements is preferably reflective to optimize the yield of the emitted light coupled into the corresponding light guide.

To ensure that the different colors are fully mixed, the light guide 120 and the further light guide 120' must be stacked in a particular manner. This is shown in Figure 8. Here, two light guides as shown in Figure 6 are used to define the stack of light guides as shown in Figure 7. The upper light guide 120 comprises recesses 122 housing solid state lighting elements 110. The solid state lighting elements 110 may comprise at least two subsets, each generating a different color, e.g. a different white. The recesses 110 are placed the mixing regions 124 that are separated from each other by outcoupling regions 126 as previously explained.

The lower light guide 120' comprises recesses 121 ' separated by light entry structures, i.e. protrusions, 122' and housing solid state lighting elements 110' that may also comprise at least two subsets for generating different colors, e.g. different shades of white, which may be the same colors as generated by the subsets of the solid state lighting elements 110. The recesses 121 ' are placed against the mixing regions 124' that are separated from each other by outcoupling regions 126'. The outcoupling regions 126 are aligned with the mixing regions 124' in the stack (as indicated by the dashed lines). The (different colors of) light coupled into the light guide 120 are properly mixed (i.e. blended) prior to being coupled into the further light guide 120'. Similarly, the light coupled into the lower light guide 120' is mixed in the mixing regions 124' prior to being coupled out via the outcoupling regions 126' towards the glare reducing element 130. The outcoupling regions 126 are aligned with the mixing regions 124' in the stack.

In other words, the outcoupling regions 126 of the top light guide 120 are stacked onto the mixing regions 124' of the bottom light guide 120' and the mixing regions 124 of the top light guide 120 are stacked onto the outcoupling regions 126' of the bottom light guide 120', thus providing a stack that has an alternating pattern of outcoupling regions and mixing regions in the height direction H shown in Figure 7.

One potential drawback of the type of light guide 120 and 120' as shown in

Figure 6 and 8 is that light emitted by a solid state light element 110 (or 110') can be partially absorbed by a neighboring solid state lighting element 110 (or 110') due to the emission of the light in the length direction L as shown in Figure 6. This can cause a reduction in the light intensity produced by the lighting strip 100. Figure 9 shows an embodiment of a light guide stack in which this problem has been addressed. In this embodiment, each light guide comprises a plurality of protrusions having an angled outer surface, such as saw tooth-shaped protrusions 122 for the top light guide 120 (top part of Figure 9) and saw tooth- shaped protrusions 122 for the bottom light guide 120' (middle part of Figure 9). Such an angled outer surface increases the amount of emitted light that is deflected into the light guide, as will be readily understood by the skilled person. The bottom part of Figure 9 shows a top view of the light guide stack of the lighting strip 100 comprising the top light guide 120 and the bottom light guide 120' with saw tooth- shaped protrusions 122 and 122' respectively.

As will be appreciated, some outcoupling of light may occur from the light guide 120 at the protrusions 122 (and at the light guide 120' from the protrusions 122'). As these protrusions 122 are typically placed along the mixing regions 124 of the light guide 120, such outcoupling is undesirable, despite the fact that this generally will not lead to a significant loss in brightness of the lighting strip 100, especially when the solid state lighting elements 110 (and 110') are placed between their respective light guides and a reflective surface, e.g. a reflective inner surface of the fastening means 180 or housing of the lighting strip 100 as the thus outcoupled light can be reintroduced into the light guides by these reflections. Nevertheless, it may be desirable to further reduce the unwanted outcoupling of light from a light guide in the lighting strip such as the light guide 120.

An embodiment of such a light guide 120 is shown in Figure 10. As can be seen, the protrusions 122 are arranged along the sides of mixing regions 124, with each mixing region 124 comprising several protrusions 122. The terminal protrusion 122 of each mixing region is designed such that at least a part of this terminal protrusion overlaps with the neighboring outcoupling region 126. Now, part of the light emitted by solid state lighting elements 110 that is coupled out by the protrusions 122 is coupled out in an intended region, i.e. outcoupling region 126, thus reducing the amount of light coupled out at the mixing regions 124. Obviously, in case of a lighting strip 100 comprising a stack of light guides 120 and 120' as shown in Figure 7, the further light guide 120' may have the same shaped protrusions as the light guide 120 shown in Figure 10.

In Figure 6 and 8-10 the solid state elements 110 and 110' have been placed at the same height in the strip as their corresponding light guides 120 and 120', i.e. are not vertically displaced with respect to these light guides. In other words, in case of lighting strip 100 comprising a stack of light guides as shown in Figure 7, the solid state lighting elements 110 are positioned adjacent to the top light guide 120 and the solid state lighting elements 110' are positioned adjacent to the bottom light guide 120'.

In case of the light strip 100 further comprising a control circuit, e.g. in the form of a printed circuit board (PCB), the PCB may be placed at the top of the light strip 100, i.e. above the reflector 140 shown in Figure 7. It will be immediately apparent that the solid state lighting elements adjacent to the top light guide 120 can easily be connected to this PCB. For a straightforward connection of the bottom solid state lighting elements 110' to the PCB, the top light guide 120 preferably should not overlap, i.e. cover, the solid state elements 110' of the bottom light guide 120'. As for instance can be seen in the top view of the light guide stack in Figure 9 (bottom drawing), the light guides in Figure 9 and 10 obey this design preference.

In an alternative arrangement, the solid state lighting elements 110' are placed on the same height as, i.e. adjacent to, the top light guide 120, in which case the lighting strip 100 comprises deflection means to direct the light from the solid state lighting elements 110' to the bottom light guide 120. Reflectors for instance are a suitable

embodiment of such deflection means.

Another strategy to deflect the light from the solid state lighting elements 110' to the bottom light guide 120' is shown in Figure 11, which depicts a variation to the design of the bottom light guide 120' as shown in Figure 9. The top part of Figure 11 provides a top view and the bottom part of Figure 11 provides a side view of this design variation. The bottom light guide 120' shown in Figure 11 comprises a small incision 125 in the saw tooth protrusion 122 at the side of the light guide 120' where the light must be coupled in. This allows for an out-of-plane bending of the protrusion 122' such that the light can be coupled in from the vertically displaced solid state element 110', i.e. the solid state light element 110' positioned above the bottom light guide 120' and adjacent to the top light guide 120, as is demonstrated in the side view of Figure 11. Consequently, an alternating pattern of protrusions of the top light guide 120 and of the bottom light guide 120' is formed

substantially in the plane of the top light guide 120.

It will be appreciated that in the previous embodiments, a stack of two light guides 120 and 120' has been shown by way of non-limiting example only. Further light guides may be added to the stack if so desired, e.g. in case the lighting strip 100 comprises more than two groups of solid state lighting elements. Such light guides preferably are stacked with a mixing region of an upper light guide facing the outcoupling region of the light guide underneath and vice versa as previously explained. Although in Figure 6-11 the light guides 120 and 120' comprise solid state lighting elements 110 and 110' in each of its side walls, it is equally feasible that solid state lighting elements are present on only one of the side walls in the length direction of the light guide 120, in which case only this side wall may carry the protrusions 122 and 122'.

Figure 12 shows an alternative embodiment of a solid state lighting strip 100, which is particularly suited for retrofitting purposes. The solid state lighting strip 100 is fitted to the exposed surface of a panel support element 210 rather than being integrated into the panel support element 210. The panel support element 210 is T-shaped by way of non- limiting example only. The solid state lighting strip 100 comprises a housing 150 including a bottom surface 152 including a light exit window 154, a top surface 158 facing the T-shaped panel support element 210 and side surfaces 156 extending from the bottom surface 152 to the top surface 158 in a length direction of the housing 150. The glare reducing element 130 is fitted, e.g. fixated over the light exit window 154, with the light guide(s) 120 being located in between the top surface 158 and the glare reducing element 130. The glare reducing element 130 may be kept in place by a reflective surround, which improves the light output of the solid state lighting strip 100. Solid state lighting elements, e.g. LEDs, 110 and 110' are located in between the light extraction layer 120 and one of the side walls 156 as previously explained.

The material of the housing 150 may be flexible, e.g. made of a plastics material. The housing 150 may be reflective on the inside to maximize the light output of the solid state lighting strip 100. Any suitable reflective material may be used. The material of the housing 150 may be reflective or the inner surfaces of the housing 150 may be coated with a reflective material. In addition, a reflective layer may be present between the upper surface 158 of the housing 150 and the light extraction layer 120.

The outer surface of the upper surface 158 may contain an adhesive for fixing the solid state lighting strip 100 to the panel support element 210. Alternatively, the solid state lighting strip 100 may be clamped to the panel support element 210 using clamps 180 or other suitable fasteners. The fasteners may be designed to shield the solid state lighting elements 110 and 110', in which case the housing 150 may be omitted. In such an

embodiment, the inner surface of the fasteners, i.e. the surface facing the solid state lighting elements 110 and 110' preferably is reflective to increase the amount of emitted light coupled into the light guide 120. Although the embodiment of the solid state lighting strip 100 in Figure 12 is shown separate to the panel support element 210, it should be understood that it is equally feasible to integrate this embodiment into a panel support element 210 as shown in Figure3.

Figure 13 shows a simulation of the appearance of a room fitted with a modular panel system, here a suspended ceiling, comprising lighting strips or luminaires 100. The modular panel system may have primary support beams, e.g. band rasters, which run perpendicularly to the luminaires 100, with support elements extending between adjacent primary support beams being extended with the luminaires 100. Simulations have

demonstrated that for a solid state lighting strip 100 having a width of 24 mm and a length of 60 cm in a ceiling having lines of panel support elements 210 at a pitch P of 60 cm, a luminance of 500 lux can be achieved at the working surfaces in the room by having each solid state lighting strip 100 having a luminous output of 230 lumen, i.e. 380 lumen/m of the strips 100.

As shown in Figure 14, which depicts a non- limiting example of a modular panel system 200, the primary support beams 205 of the modular panel system 200 are suspended from the ceiling 300 of a room, with the panel support elements 210 carrying the panels or tiles 220 extending between primary support beams 205. The luminaire 100 is fitted to the panel support elements 210, for instance by integration into the panel support elements 210 or by attachment thereto, as previously explained. As the panel support elements 210 can be easily removed from the modular panel system 200 without dismantling the whole system, e.g. removing it from the ceiling 300, it is straightforward and cost-effective to retrofit the luminaire 100 into the modular panel system 200, either by replacing a prior art panel support element 210 with a panel support element 210 of the present invention, or by attaching the luminaire 210 to an existing panel support element 210. It is of course also feasible to integrate a luminaire 100 of the present invention into a primary support beam 205 or retrofit it thereto, although this is likely to be less straightforward and not as cost-effective as the (retro -)fitting to panel support elements 210. It is reiterated that for a modular panel system 200 in accordance with the present invention, it is preferred that the ratio of the width W of the exit window the solid state lighting strips 100 and the pitch P of the panel support elements 210 is chosen in the range 0.02-0.08, and W/P preferably is 0.04 for the reasons already explained above.

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.