Patent Publication Number: US-9897279-B2

Title: Reflector device and lighting device comprising such a reflector device

Description:
TECHNICAL FIELD 
     The field relates to a reflector device comprising a reflector, having an inner surface, and at least one solid state light emitting element, and to a lighting device comprising such a reflector device. 
     BACKGROUND 
     Recent years traditional fluorescent tubes have been modernized in that the outer features of the tube and the electric connection parts have been kept but the light engine has been replaced with modern technology of one or more solid state light emitting elements, such as LEDs (Light Emitting Diodes), and OLEDs (Organic Light Emitting Diodes), etc. One example thereof is EnduraLED T8 manufactured by Philips. Typically, several solid state light emitting elements are mounted in a line on a carrier, which is introduced into a glass tube, and the inside of the glass tube is provided with a light diffuser, which diffuses the spot shaped light from the solid state light emitting elements into a homogeneous light output. Present light diffusers obtain the diffusing effect by a combination of reflection and scattering transmission of the light. However, in order to obtain a good uniformity of light distribution the solid state light emitting elements have to be densely mounted or the light diffuser has to be reflective to a high extent. A high reflectivity causes a low optical efficiency. Densely mounted solid state light emitting elements cause a high cost. 
     SUMMARY 
     It is an object to provide a lighting device that alleviates the above-mentioned problems of the prior art, and provides a homogeneous light output with high optical efficiency at less densely mounted solid state light emitting elements than the prior art lighting devices. 
     The object is achieved by a reflector device according to the present invention as defined in the claims and the description herein. 
     The disclosure is based on the insight that avoidance of a direct light path from the solid state light emitting elements to the viewer creates a basis for solving the prior art problems. 
     Thus, in accordance with an aspect, there is provided a reflector device comprising a reflector, having an inner surface, and at least one solid state light emitting element. The inner surface of the reflector comprises first and second surface portions, which extend in planes intersecting at an angle. The at least one solid state light emitting element is/are mounted at at least one of said first and second surface portions such that a major part of the light emitted from said at least one solid state light emitting element illuminates the other one of said first and second surface portions. The first and second surface portions may be flat. 
     By arranging the solid state light emitting elements at the reflector, and arrange them to emit light towards the reflector inner surface, the light is being more diverged before being outlet to the surrounding environment, which results in that, when using several solid state light emitting elements the distance between them can be larger than in the prior art lighting device, while still obtaining a uniform light output. Additionally, the freedom of positioning the solid state light emitting elements is increased. The mounting of the at least one solid state light emitting element together with the emission direction of the generated light ensures that the generated light, or at least a major part of it, leaves the reflector after being reflected at least once by the reflector. The amount of light, if any, that is not reflected by the reflector has a negligible effect on how the light is perceived by a viewer. 
     In accordance with an embodiment of the reflector device, the first and second surface portions define a V-shaped groove. This is an efficient shape. 
     In accordance with an embodiment of the reflector device, it further comprises an intermediate inner surface portion interconnecting the first and second inner surface portions, wherein the intermediate inner surface portion extends non-parallel to said planes. The intermediate inner surface portion further increases the efficiency. 
     In accordance with an embodiment of the reflector device, each one of the first and second surface portions has a free side edge, wherein the free side edges define a reflector opening, said at least one solid state light emitting element being mounted at a distance from the free side edge of the surface portion at which it is mounted. In other words, the side edges constitute the rim of the reflector. This positioning of the at least one solid state light emitting element ensures that no shadow effect is caused by the at least one solid state light emitting element, which could be the case in some applications if mounted at the very edge. 
     In accordance with an embodiment of the reflector device, at least one of said first and second surface portions comprises a diffuse reflective portion. The diffusion arranged already at the reflector further increases the homogeneity of the outlet light. 
     In accordance with an embodiment of the reflector device, wherein the at least one solid state light emitting element extends through the reflector, such that a light emitting portion of each solid state light emitting element protrudes from an inner surface of the reflector while a support portion of each solid state light emitting element is positioned at an outer surface of the reflector, wherein the support portion supports the light emitting portion. This is an advantageous mounting where the reflector surface is maximized. 
     In accordance with an embodiment of the reflector device, at least one of said first and second surface portions constitutes a top surface of a printed circuit board. 
     In accordance with an embodiment of the lighting device, the at least solid state emitting element comprises a solid state light emitting element, which is arranged to have a centre emitting direction which is non-perpendicular to the intersection axis between the first and second surface portions. Thereby, the optical path length within the reflector device is increased. 
     In accordance with an embodiment of the lighting device, it comprises a light diffuser, which includes the light outlet portion. Thus, since the light outlet portion is provided with light diffusing properties, no separate light diffusing means has to be arranged. 
     For the purposes of this application it should be noted that by “light diffusing”, and similar expressions, is meant different kinds of light diffusing properties, such as for instance diffuse and specular transmission, and diffuse or specular reflection. Typically, the light diffusing means provides a combination of several different kinds. Furthermore, the light diffusing means can be a separate part, a coating or a stack of one or more photo-luminescent materials integrated in the light outlet portion, etc. As regards the reflector, it can be specular reflective, diffuse reflective or a combination thereof. Furthermore, the reflector may constitute a film of one or more photo-luminescent materials such as remote phosphor. 
     These and other aspects, and advantages of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be described in more detail and with reference to the appended drawings in which: 
         FIG. 1  is a schematic perspective view of a part of an embodiment of a reflector device according to the present invention; 
         FIG. 2  is a cross-sectional view of another embodiment of the reflector device; 
         FIGS. 3-16  are schematic views of further embodiments of a reflector device according to the present invention. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     A first embodiment of the reflector device  400 , as shown in  FIG. 4 , comprises a reflector  406 , and at least one solid state light emitting element  414 . For the purposes of the present application, in the following description the solid state light emitting elements  414  will be exemplified by LEDs (Light Emitting Diodes), while any other kind of solid state light emitting element is applicable as well. In this embodiment a single LED  414  is shown. The LED may emit light of one or more wavelengths. 
     An inner surface  420  of the reflector  406  comprises first and second surface portions  422 ,  424 , which are flat and which extend in planes intersecting at an angle α of approximately 90°. Thus, the first and second surface portions  422 ,  424  define a V-shaped groove  426 . The angle can differ from 90°. For instance, down to about 80° will also work as well as up to 100° or even 110°, but preferably it is about 90°. The LED  414  is mounted at the first surface portion  422 , such that a major part of the emitted light illuminates the second surface portion  424 . In this embodiment, this is obtained by having an emitting side of the LED  414  face the second surface portion  424 . The inner surface  420  of the reflector  406  is diffuse reflective, i.e. the reflector  406  is provided with a diffusively reflective inner surface  420 . Thereby the spreading of the generated light is maximized while obtaining a good efficiency, providing a homogeneous light output. The diffuse reflective surface  420  can be obtained by, for instance, providing the surface with a diffusing pattern, such as a series of dots and/or strips, brushing or any mix of either one or more diffuse and/or specular reflective materials at a diffuse and/or specular reflective surface of the reflector  406 . Either one or both of the first and second surface portions  422 ,  424  can be provided with one or more diffuse reflective portions, or they can both be fully specular reflective. The diffuse reflective portions provide an increase in the homogeneity of the light leaving the reflector  420 . When several LEDs are arranged in a row, the distance between the LEDs can be increased compared to prior art bottom-up lit devices while keeping the same homogeneity of the light output. Thereby the manufacturing cost is lowered. 
     As an additional alternative, one half of the reflector can comprise a metal plate, e.g. tin coated with diffuse white paint, and the other half a highly reflective surface of for example barium sulfate (BaSO4) and/or titanium dioxide (TiO2) coated plastic or paper, MCPET (Micro Cellular PET), etc. 
     Each one the first and second surface portions  422 ,  424  has a free side edge  428 ,  430  wherein the free side edges  428 ,  430  define a reflector opening. The flat first and second surface portions  422 ,  424  can be elongated, such that the free side edges  428 ,  430  are long side edges. The LED  414  is mounted at a distance from the free side edge  428  of the first surface portion  422 . In other words, the LED  414  is mounted recessed in the groove  426 . The solid state light emitting elements  414  can be mounted non-recessed as well, i.e. at the free edge  428 ,  430  of the first and/or second flat surface portions  422 ,  424 , but then there is a risk of causing LED self-shadows in the light output of the reflector device, at least in some lighting device applications. 
     It should be noted that the single LED  414  embodiment is possible, while it is common to have several LEDs arranged at the reflector, on one or both flat surface portions, as will be exemplified below. 
     A first embodiment of a lighting device  100 , as shown in  FIGS. 1 and 2 , comprises a reflector device  101  and a light transmissive light outlet portion  104 . Furthermore, the lighting device  100  has an elongated tubular portion, which is an outer tube,  102 , and which includes the light outlet portion  104 . The reflector device  101  comprises a reflector  106  and LEDs  114  mounted at the reflector  106 . In fact, in this embodiment, more particularly, the whole outer tube  102  is light transmissive, such as a glass tube, but due to a reflector  106  mounted within in the outer tube  102 , and covering about half the outer tube  102 , there is left the light outlet portion  104 , thus constituting about half the outer tube  102 , for the light output of the lighting device  100 . Furthermore, a semi-cylindrical light diffuser  108  is arranged inside of the outer tube  102 . More particularly, the extension of the light diffuser  108  corresponds with the extension of the light outlet portion  104 . The light diffuser  108  is a diffusing layer deposited on the inner surface of the tube  102 . Alternatively, the light diffuser can be an individual element, i.e. a separate diffuser, mounted in the tube  102  at a reflector opening, or as a shrink wrap applied at the exterior of the tube  102 , or between a reflector opening and the light outlet portion  104 . As a further alternative, the diffusing properties can be provided by the light outlet portion  104  itself, thereby saving steps when manufacturing the lighting device  100 . On the other hand it can be economically advantageous to be able to use standard transparent glass or plastic tubes. Furthermore, as exemplified above, the reflector  106  can include a diffusing surface, which cooperate with the light diffuser  108  in spreading the light before outlet thereof. The reflector  106  is generally V-shaped, and can be formed like a bent plate. Alternatively, it can be comprised of two portions that can be unfolded after insertion into the tube  102 . The reflector  106  has an inner surface, which comprises first and second surface portions  120 ,  122 , which are elongated and flat and extend in planes intersecting at an angle, here a right angle. Thus, the first and second surface portions  120 ,  122  are rectangular in this embodiment. More particularly, the first and second surface portions  120 ,  122  preferably extend in orthogonal planes, while other intersection angles are feasible as well although not optimum. The reflector  106  further has first and second free long side edges  124 ,  126  of the respective first and second surface portions  120 ,  122  extending longitudinally along the length of the tube  102 . The free long side edges  124 ,  126  define a reflector opening. 
     The LEDs  114  are mounted at a distance from the free long side edges  124 ,  126 . In other words, the LEDs  114  are mounted recessed in the reflector  106 . The LEDs  114  are mounted at both the first surface portion  120 , and at the second surface portion  122 . They emit light towards the inner surface  120 ,  122  of the reflector  106 . More particularly, the LEDs  114  mounted at one surface portion  120 ,  122  emit light towards the other surface portion  122 ,  120 . The emitted light is reflected by the reflector  106  and directed out of the reflector opening towards the light outlet portion  104 , and passes the light diffuser  108  on its way out. However, the light diffuser  108  is typically reflecting a minor part of the light back towards the interior of the tube  102 . Thus, generally, all or the greater part of the emitted light is reflected at least once by the reflector  106  before leaving it through the reflector opening. Alternatively, the LEDs  114  may be mounted only at the first surface portion  120  or only at the second surface portion  122 . 
     The LEDs  114  are mounted such that the centre direction of the emitted light is parallel with the major surface portion  120 ,  122  at which the LEDs  114  are mounted, and perpendicular to the other surface portion  122 ,  120 . Thus, the emitting side of each LED  114  is facing the opposite inner surface portion of the reflector  106 . Alternatively, as shown in  FIG. 14 , the LEDs  1414  may be mounted at the reflector  1406  such that the centre direction of the emitted light is parallel with the major surface portion  1420 ,  1422  of the inner surface  1424  at which the LEDs  1414  are mounted, and under an incident angle of 45 degrees to the other surface portion,  1422 ,  1420 . Other centre incident angles between 30 to 60 degrees are also possible. 
     A common type of tubular lighting devices  100  has a diameter of 25.4 mm and wall thickness of 1 mm. In order to obtain a good uniformity of the distribution of the light output and a high optical efficiency, for such a lighting device  100 , in one example the LEDs  114  were mounted at a spacing, also called pitch, of 30 mm, i.e. the distance between two adjacent LEDs  114 . 
     According to a second embodiment of the lighting device  200 , and of the reflector device  201 , the reflector  206  is generally semi-cylindrically shaped, and comprises a portion  216 , having a semi-cylindrical outer surface  218  abutting against the inside of the tube  202 , and an opposite inner surface, which defines a V-shaped groove  224 , having first and second surface portions  220 ,  222 , like in the first embodiment of the lighting device  100 . The LEDs  214  are arranged on the inner surface  220 ,  222  similarly as in the first embodiment. 
     According to a third embodiment of the lighting device, and of the reflector device  300 , see  FIG. 3 , the LEDs  314  extend through respective holes  312  of the wall of the reflector  306 . For reasons of simplicity, only the reflector and the LEDs are illustrated in  FIG. 3 . A light emitting portion  316  of each LED  314  protrudes from the inner surface  320  of the reflector  306 , while a support portion  318  of the LED  314 , which carries the light emitting portion  316 , is positioned at an outer surface  330  of the reflector  306 . In this embodiment the area of the inner surface  320  of the reflector  306  has been maximized. Alternatively, a small PCB may be mounted at the inner surface  320 , and, in order to optimize its reflective properties, be coated with a highly reflective material such as white paint, MCPET, etc. Like above, the light emitting surface of each light emitting portion  316  is turned into the V-shaped groove, i.e. it is facing an opposite inner surface portion of the reflector  306 . 
     According to a fourth embodiment of the lighting device  500 , and reflector device  501 , as shown in  FIGS. 5 a  and 5 b   , the lighting device  500  is useful e.g. as a retrofit light bulb. It comprises a cylindrical enclosure  502  including a light outlet portion, a socket  503  attached to the enclosure  502 , and a reflector device  501  mounted inside of the enclosure  502 . The reflector device  501  comprises a reflector, which has a plus (+) shaped cross section, and which embodies four V-shaped grooves, defined by respective pairs of surface portions  522 ,  524 ;  526 ,  528 ;  530 ,  532 ;  534 ,  536 , arranged circumferentially adjacent to each other. The reflector  506  could be regarded as made by two square plates extending in orthogonal planes intersecting at the middle of the plates. LEDs  514  are mounted on at least one of the flat major surface portions of each pair, thereby creating an omnidirectional lighting device. Alternatively, three V-shaped reflectors having an opening angle of 120 degrees may be deployed, and for larger diameter half-tubes, two adjacent V-shaped reflectors, i.e. a half plus (+) arrangement. 
     According to a fifth embodiment of the lighting device  600  and the reflector device  601 , it comprises a V-shaped primary reflector carrying at least one LED  614 , and a secondary reflector  604 . The primary reflector  606  is arranged with the inside facing the inside of the secondary reflector  604  and at a distance from the secondary reflector  604 . The secondary reflector has a flat centre portion  608  and two flat side portions  610 ,  612 , which are integral with the centre portion  608  and are inclined to the centre portion  608 . The side portions extend at the respective sides of the primary reflector  606 . Light leaving the primary reflector  606  is reflected by the secondary reflector  604  before being outlet from the lighting device  600 . This embodiment of the lighting device could typically be used as a ceiling lamp or a wall lamp. The secondary reflector  604  can be made diffuse reflective or specular reflective or any mix thereof. Preferably, the primary reflector  606 , just like the secondary reflector  604 , is provided with a flat centre portion and two flat side portions, which are integral with the centre portion and are inclined to the centre portion. Then the amount of light that is reflected back to the very LEDs  614  from the secondary reflector  604  and is absorbed by the LEDs  614  is minimized. 
     If blue LEDs are used a remote phosphor element can be arranged in the lighting device, such as to cover the primary reflector opening or in some other suitable way, such as at the inner surfaces of the reflector itself, in order to transform the blue light into white light. This is illustrated by a sixth embodiment in  FIG. 7 , similar to the fifth embodiment, but additionally comprising a remote phosphor element. Thus, the lighting device  700  has a reflector device  701  comprising a V-shaped primary reflector  706 , and an opposite secondary reflector  704 . The primary reflector  706  is arranged with the inside, carrying at least one LED  714 , facing the inside of the secondary reflector  704  and at a distance from the secondary reflector  704 . The remote phosphor element  716  is arranged at the opening of the primary reflector  706 , covering the opening thereof. Thus, the light leaving the primary reflector towards the secondary reflector  704  passes the remote phosphor element  716 . Of course, any other embodiment presented herein can be provided with a remote phosphor element as well. 
     Referring to  FIGS. 8 a  and 8 b   , according to a seventh embodiment  800 , the reflector device  801  comprises LEDs  814  constituting protrusions of a plate shaped substrate  818 . The protrusions  814  extend through holes  812  of the reflector  806 , which is V-shaped and has two flat inner surface portions  820 ,  822 . This embodiment is similar to the first embodiment, the only difference being the shape and arrangement of the LEDs  814 . Thus, the reflector device  801  is arranged in a cylindrical outer tube  802 , which is provided with a semi-cylindrical diffuser  808  on its inner surface. More particularly, as illustrated in  FIG. 8 b   , the substrate  818  is elongated and castle-nut shaped, where the “nuts” are the above mentioned protrusions  814 . Each protrusion, or LED,  814  has a light emitting area  816 . The central emission direction is about perpendicular to the major extension of the substrate  818 . Thus, by mounting the substrate  818  at the outer, or rear, side of the reflector  806  such that the “nuts”, or protrusions  814  extend through the holes  812  of the reflector  806 , perpendicular to the inner surface  820 ,  822 , the LEDs  814  on one inner surface portion  820  emit light towards the other inner surface portion  822 . Additionally, by mating the height of the substrate  818  with the distance between the outer surface of the reflector  806  and the inner surface of the outer tube  802  the substrate  818  is supported by the outer tube  802 . 
     According to further embodiments, the reflector is formed with additional surface portions, as will now be exemplified. According to an eighth embodiment shown in  FIG. 9 , there is provided a lighting device  900  comprising an outer tube  902  and a reflector device  901  arranged within the outer tube  902 . The reflector device  901  comprises a reflector  906  having a body portion  916  with a semi-cylindrical outer surface  918  abutting against the inside of the tube  902 , and first and second inner surface portions  920 ,  922 , which are engaged at an angle at a centre of the reflector  906 , thereby forming a V-shaped groove  923 . Furthermore, it comprises third and fourth inner surface portions  924 ,  926  engaged with a respective one of the first and second inner surface portions  920 ,  922 , and extending perpendicular to the respective first and second inner surface portions  920 ,  922  to a low height. Finally, fifth and sixth inner surface portions  928 ,  930  are engaged with a respective one of the third and fourth inner surface portions  924 ,  926 , and extend slopingly relative to the first and second inner surface portions  920 ,  922  to the inner surface of the outer tube  902 . The LEDs  914  are mounted at the third and fourth inner surface portions  924 ,  926  with their respective emitting surface  932  facing the opposite second and first inner surface portion  922 ,  920 , respectively. Thus, an additional LED mounting portion is arranged on either half of the reflector inner surface. 
     An alternative to the LED mounting of the eighth embodiment is shown in  FIGS. 10 a  and 10 b   , where one half of the reflector inner surface is provided with the additional mounting portion  1024 , the other one being a single flat portion  1020 . However, in this alternative the reflector  1006  is basically plate shaped and the substrate  1018  with the LEDs  1014  is mounted at the outer surface of the mounting portion  1024  of the reflector  1006 . The emitting surfaces of the substrate  1018  emit light through holes  1028  of the mounting portion  1024  thereby facing the other half  1020  of the inner surface. 
     According to a tenth embodiment, as shown in  FIGS. 11 a  and 11 b   , the reflector device  1100  comprises a reflector  1106  of a presently preferred shape. The reflector  1106  is shown as such, but of course LEDs and other additional elements will be added as desired, as well as additional shaping of the reflector as exemplified with other embodiments herein. The inner surface of the reflector  1106  has a flat inner surface centre portion  1124  arranged between and engaged with flat first and second inner surface side portions, respectively, at a first angle, typically an obtuse angle. The first and second inner surface side portions  1120 ,  1122  extend in planes intersecting at a second angle, such as about 90° as described above. Thus, in a sense, the reflector  1106  is tray shaped. When arranged in an outer tube  1102 , the reflector can be arranged closer to the tube inner wall than a strictly V-shaped reflector. 
     According to an eleventh embodiment, as shown in  FIG. 12 , the reflector device  1200  comprises a reflector  1206 , which is basically shaped like the tenth embodiment having a flat inner surface centre portion  1224  arranged between and engaged with flat first and second inner surface side portions  1220 ,  1222 . However, the first inner surface side portion  1220  is provided with an additional portion  1226 , shaped like an angular U in cross-section, which protrudes from the rest of the first inner surface portion  1220 , i.e. the basic flat part of it, and which has first and second side walls  1228 ,  1232  extending in parallel and extending perpendicular to the basic flat part of the first inner surface portion  1220 , and a top portion  1230  extending in parallel with the basic flat part of the first inner surface portion  1220 . The top portion  1230  is provided with holes  1234 . A castle-nut shaped PCB  1218 , similar to the one describe above in conjunction with the seventh embodiment, has been received in the groove defined by the U-shaped portion  1226  on the outer surface of the reflector  1206 . The LED portions  1214  of the PCB  1218  have been inserted through the holes  1234  and protrude from the top portion  1230 , the emitting surfaces facing the central portion and the second inner surface side portion  1222 . The U-shaped additional portion, at which the PCB is arranged, is applicable to other basic reflector shapes as well, such as a pure V-shape, as will be understood by the person skilled in the art. 
     According to a twelfth embodiment  1300 , as shown in  FIGS. 13 a  and 13 b   , the reflector device  1301  comprises a similar reflector  1306  as in the third embodiment and as in the second embodiment, respectively. Thus, either the LEDs  1314  extend through holes of the reflector walls, or they are attached to the inner surface of the reflector. However, in both cases the LEDs  1314  are top emitting LEDs  1314 , thus emitting light along a centre axis perpendicular to the inner surface portion  1320 ,  1322  of the V-shaped reflector  1306  where they are arranged. In order to avoid direct illumination of the surroundings of the reflector  1306  each LED  1314  is covered by a tongue  1315  attached to the flat inner surface portion  1320 ,  1322  at one end thereof, and extending above the LED  1314 . Thus, the light emitted from the LED  1314  is directed to, and illuminates, the other inner surface portion. Preferably, the surface of the tongue  1315  that faces the LED  1314  is diffuse reflective, and thereby the emitted light is scattered shortly after leaving the LED  1314  and reaches the opposite inner surface portion of the reflector  1306  more scattered than in other embodiments where the LEDs face the opposite inner surface portion. The inner surface portion, or at least a part thereof, at which the LEDs  1314  are arranged can be the printed circuit board that carries the emitting material. In that case, the surface of the printed circuit board has been made reflective in the desired way. 
     As an alternative to the fourth embodiment, the lighting device  1500 , in a thirteenth embodiment as shown in  FIGS. 15 a  and 15 b   , comprises a spherical enclosure  1502  instead of the cylindrical enclosure  502  of the fourth embodiment, attached to the socket  1503 . The reflector device  1501  comprises a reflector  1506 , which has a plus (+) shaped cross section, and which embodies four V-shaped grooves, defined by respective pairs of flat surface portions  1520 . If the reflector  1506  is instead regarded as consisting of two plates arranged perpendicular to each other and intersecting each other at the middle of each plate, the LEDs  1514  are arranged on one of the plates, and aligned in pairs with the LEDs of each pair being arranged on the opposite sides  1528 ,  1530  of the plate. Furthermore, the pairs are arranged in one line on one side of the middle and another line on the other side of the middle. Thereby, each V-shaped groove houses one line of LEDs  1514 . 
     When arranging LEDs on both inner surface portions of the reflector as described in various embodiments above, one line on each inner surface portion, it is advantageous to arrange the LEDs as most schematically illustrated in  FIG. 16 . The LEDs of both lines are mounted with the same spacing S, but the LEDs  1602  of one line are displaced by half the spacing S relative to the LEDs  1604  of the other line. 
     Above embodiments of the lighting device according to the present invention as defined in the appended claims have been described. These should only be seen as merely non-limiting examples. As understood by the person skilled in the art, many modifications and alternative embodiments are possible within the scope of the invention as defined by the appended claims. 
     For instance alternative mounting positions of the LEDs are possible in all embodiments, as understood by the person skilled in the art in light of the description. However, the alternative mounting positions may be less favorable than those disclosed herein. 
     It is to be noted that for the purposes of this application, and in particular with regard to the appended claims, the word “comprising” does not exclude other elements or steps, and the word “a” or “an” does not exclude a plurality, which per se will be evident to a person skilled in the art.