Abstract:
The invention relates to a luminaire that enables the use of more sparsely distributed light sources, having a uniform fixture surface brightness and good color mixing. The luminaire comprises first and second light guide layers ( 10, 11 ) optically coupled together by at least one optical coupler ( 12   a,    12   b ), which allows the passage of light from the first ( 10 ) to the second light guide layer ( 11 ). The luminaire also comprises a plurality of light sources ( 16   a,    16   b ), each of which being arranged such that light emitted by the light source ( 16   a,    16   b ) is coupled into the first light guide layer ( 10 ) at an interface surface ( 17 )arranged substantially perpendicularly to a longitudinal axis of the first light guide layer ( 10 ). The luminaire further comprises a scattering element ( 14 ) adapted to cause light to be emitted from an emitting surface of the second light guide layer ( 11 ). By providing two optically coupled light guide layers ( 10, 11 ), one for receiving light emitted by the light sources ( 16   a,    16   b ) and the other for emission, greater opportunity for dispersion and color mixing of the light is provided.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to luminaires, such as luminaires suitable for mounting in or on a panel support element of a modular panel system. It also relates to a panel support element for a modular panel system comprising such luminaires. 
       BACKGROUND OF THE INVENTION 
       [0002]    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. 
         [0003]    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. 
         [0004]    Solutions exist to overcome glare. One solution involves luminaires comprising a plurality of chambers defined by respective parabolic reflectors, with a fluorescent light tube fitted in each of the chambers. Each of the light tubes is offset from the light exit plane of the luminaire such that light emitted by the fluorescent light tube under a shallow angle is reflected by the parabolic reflector, which 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. 
         [0005]    Another solution involves fitting luminaires for integration in a suspended ceiling with a micro-lens optical (MLO) plate or diffuser, which has the function of preventing shallow angle light beams from exiting the luminaire. As this causes a substantial amount of light being reflected back into the chamber of the luminaire, the luminaire may comprise a reflector to recycle such reflected light. Commonly, the micro-lens optical plate takes the form of a prism plate. Again, this style of luminaire is relatively bulky and obtrusive. 
         [0006]    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. 
         [0007]    However, as these luminaires replace tiles in the ceiling, they still disrupt the visual appearance of the suspended ceiling. 
         [0008]    One important option for unobtrusively integrating lighting fixtures into the ceiling system is shown in  FIG. 1 . In this system thin lighting fixtures  1  are fixed onto support strips  2 . The width of such a fixture  1  would typically be 25 mm with a thickness of a few millimeters. A set of such fixtures  1  would typically span the whole width or length of a room, yielding continuous “lightlines”. A typical office ceiling would be equipped with multiple “lightlines” spaced approximately 60 cm apart. Ceiling tiles  3  can be suspended on the lighting fixtures  1  as shown. 
         [0009]    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. 
         [0010]    It is important with this kind of lighting fixture that the brightness is more or less uniform over the emission surface. If the surface contains high brightness non-uniformities, this can lead to glare and discomfort in the case of direct viewing. Since strip mounted lighting fixtures have a relatively small emitting area, the glare requirements are quite strict and an evenly distributed emission over the fixture surface is needed. Besides this, an even distribution is aesthetically more pleasing. 
         [0011]    One successful design of strip mounted lighting fixture is shown in  FIG. 2 . It consists of a transparent light guide  4  (made from PMMA, for example), coated with a diffuser  5  and, optionally, with a reflector  6 . The thickness of the light guide  4  is typically 1-2 mm. Light is generated by LEDs  7  at the sides and is coupled into the light guide  4 . If the diffuser  5  were absent, the light would be guided by total internal reflection (TIR) towards the opposite side of the guide, and exit. The diffuser  5  serves to scatter the light out of the light guide  4 . Once outside the light guide  4 , the light encounters a micro lens optics (MLO) plate  8 . This MLO plate  8  contains pyramid-like structures. It has the property that only light that exits the plate within a half-cone-angle of approximately 60° is transmitted. Light that is not transmitted is recycled; it returns towards the light guide  4  where it is redirected by the diffuser  5 , and gets a second chance to escape. 
         [0012]    If the LEDs are closely packed along the edges of the light guide  4 , this design shows a pleasing, homogeneous light output over the surface of the MLO plate  8 . In practice however, a high density closely packed LED distribution will generate too much light for the applications envisioned. With today&#39;s low power packages, the LEDs have to be spaced 1 to 2 cm apart. It is envisioned that lumen output of LEDs will increase significantly in the near future, and the spacing between LEDs will increase accordingly. Besides this, cost issues will probably stimulate the use of medium power or even high power LEDs. It has been found that when spacing the LEDs further apart, the homogeneity rapidly diminishes and becomes unacceptable. 
         [0013]    This issue is exacerbated if the fixture is made color-tunable by using a mix of different color LEDs, like for instance warm white and cold white. In that case, the spacing per color increases even further. It has been found that using warm white and cold white LEDs at a LED-to-LED distance of 1 cm results in a surface light distribution that is very inhomogeneous. The positions of LED sources become clearly visible, having relatively high brightness, and mixing between the colors is very poor. 
       SUMMARY OF THE INVENTION 
       [0014]    In accordance with one aspect of the invention, there is provided a luminaire comprising first and second light guide layers optically coupled together by at least one optical coupler, which allows the passage of light from the first to the second light guide layer, and a plurality of light sources, each of which is arranged such that light emitted by the light source is coupled into the first light guide layer at an interface surface arranged substantially perpendicularly to a longitudinal axis of the first light guide layer. The luminaire further comprises a scattering element adapted to cause light to be emitted from an emitting surface of the second light guide layer. 
         [0015]    By providing two optically coupled light guide layers, one for receiving light emitted by the light sources and the other for emission, greater opportunity for dispersion and color mixing of the light is provided. Thus, the invention provides a luminaire suitable for strip mounting that enables the use of more sparsely distributed light sources, having a uniform fixture surface brightness and good color mixing. 
         [0016]    Furthermore, by providing interface surfaces that are substantially parallel to the longitudinal axis, the light form the light source is coupled into the first light guide layer such that it may be coupled between the first and second light guide layers using total internal reflection alone without any light escaping. 
         [0017]    The luminaire is typically one suitable for mounting in or on a panel support element of a modular panel system. 
         [0018]    Preferably, the refractive index of the material from which the first and/or second light guide layers and/or the at least one optical coupler are made is greater than the square root of 2. After passage of the light from a light source across an interface surface into the first light guide layer, the maximum angle of the light rays within the first light guide layer with respect to the normal to the interface surface (i.e. the longitudinal axis of the first light guide layer) can be seen from Snell&#39;s law to be arcsin (1/n) degrees, where n is the refractive index of the first light guide layer. This assumes that the refractive index of air is 1 and that the light ray impinges on the interface surface at 90 degrees. Since the normals to the surfaces of all the other surfaces (except other interface surfaces) within the luminaire are perpendicular to the normal to the interface surface, the light will never hit any of those surfaces at an angle less than 90-arcsin (1/n) degrees. Provided that this angle exceeds 45 degrees, the light will undergo total internal reflection. Thus, the practical lower limit for refractive index, n, is the square root of 2, which ensures that 90-arcsin (1/n) always exceeds 45 degrees. 
         [0019]    Normally, the light sources will be solid state light sources, such as light emitting diodes (LEDs). 
         [0020]    The first and second light guide layers are typically planar. Preferably, they are parallel or substantially parallel. 
         [0021]    The first and second light guide layer may each have an inwardly facing surface (i.e. the facing surface of each of the first and second light guide layer may lie opposite the facing surface of the other of the first and second light guide layer). In this case, the emitting surface will usually be the opposite surface of the second light guide layer to the inwardly facing surface. 
         [0022]    The first and second light guide layers may be spaced apart by an opaque layer, which could be the scattering element. 
         [0023]    In a preferred embodiment, the at least one optical coupler comprises an edge coupler for coupling adjacent edges of the first and second light guide layers. 
         [0024]    Preferably, the at least one edge coupler is simply an extension of the first and second light guide layers. In essence, it provides a bend for joining the two layers together. Normally, the at least one edge coupler extends along the whole of the adjacent edges of the first and second light guide layers. 
         [0025]    The at least one optical coupler may comprise two spaced apart mirrored surfaces. 
         [0026]    In some embodiments, the at least one optical coupler comprises at least one coupling element passing between inwardly facing surfaces of the first and second light guide layers. 
         [0027]    The luminaire typically further comprises an optical structure associated with the second light guide layer adapted to couple light into a desired angular range. This may be an external MLO. 
         [0028]    Alternatively, the optical structure may be formed in the emitting surface, or an inwardly facing surface, opposite to the emitting surface, in the second light guide layer. 
         [0029]    Preferably, the first and second light guide layers and the optical coupler are integrally formed. In this case, the whole assembly of the first and second light guide layers and the optical coupler may be formed by extrusion, for example. This is a very cost effective way of manufacture. 
         [0030]    The luminaire may further comprise a gap in the first light guide layer, the gap extending between two edges of the first light guide layer opposed along a longitudinal axis of the first light guide layer. This structure can be made by folding a sheet of the light guide material, the outer edges of the sheet being brought together adjacent the gap. Again, this results in an integral structure of the first and second light guides and the optical coupler, and is cheap to manufacture. 
         [0031]    Typically, the luminaire further comprises a plurality of cavities in the first light guide layer, at least one of the plurality of light sources being disposed within each cavity. There may be two light sources disposed in each cavity, in which case they will normally face in opposing directions. 
         [0032]    Each cavity preferably has a side wall forming the interface surface and extending substantially perpendicularly to the longitudinal axis of the first light guide layer, a light source within each cavity being disposed to emit light towards the side wall. 
         [0033]    If there are two light sources in a cavity then preferably each cavity has two opposed side walls extending substantially perpendicular to a longitudinal axis of the first light guide layer, each light source within each cavity being disposed to emit light towards a respective one of the side walls. 
         [0034]    Each cavity may comprise two converging side walls, a light source within each cavity facing away from the two converging side walls. The two converging side walls typically meet at an apex, which will normally be located opposite to the side wall perpendicular to the longitudinal axis of the first light guide layer. 
         [0035]    The plurality of light sources may comprise first and second sets of light sources, each set adapted to emit light of different colors. 
         [0036]    The luminaire may further comprise an array of optical structures in the first light guide layer, each of the optical structures disposed on a straight line path between two of the plurality of light sources and adapted to reflect light emitted by one or other of the light sources along the path. The optical structures are typically cavities formed, for example in rectangular or rhomboid shapes, which cause reflection of light emitted by one light source before it impinges on a cavity containing another light source. 
         [0037]    The scattering element is typically disposed between the first and second light guide layers. 
         [0038]    In accordance with a second aspect of the invention, there is provided a panel support element for a modular panel system comprising a luminaire according to the first aspect of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0039]    Embodiments of the invention are described in more detail and by way of non-limiting examples with reference to the accompanying drawings, wherein: 
           [0040]      FIG. 1  shows a prior art strip mounted luminaire; 
           [0041]      FIG. 2  shows the luminaire of  FIG. 1  in more detail; 
           [0042]      FIGS. 3   a  and  3   b  show side and top views respectively of a first embodiment of a luminaire according to the invention; 
           [0043]      FIG. 4  shows an example of a light ray traversing through the light guide layers in the embodiment of  FIG. 3 ; 
           [0044]      FIG. 5  shows examples of how the optical coupler may be formed in the embodiment of  FIG. 3 ; 
           [0045]      FIGS. 6   a  and  6   b  show examples of integral scattering elements that may be used with the invention; 
           [0046]      FIG. 7  shows a second embodiment of a luminaire according to the invention; 
           [0047]      FIG. 8  shows a third embodiment of a luminaire according to the invention; 
           [0048]      FIGS. 9   a - h  show examples of how the light sources may be distributed and of additional optical structures that may be provide in the first light guide layer in each of the first to third embodiments; and 
           [0049]      FIGS. 10   a  and  10   b  show additional ways of optical coupling between the first and second light guide layers. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0050]    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. 
         [0051]    A first embodiment of the invention is shown in  FIGS. 3   a  and  3   b . In this a first or top light guide layer  10  is optically coupled to a second or bottom light guide layer  11  via optical couplers  12   a  and  12   b.  The optical couplers  12   a  and  12   b  are simply bends to allow the passage of light from the top light guide layer  10  to the bottom light guide layer  11 , which lie parallel and spaced apart from each other. The top light guide layer  10 , the bottom light guide layer  11  and optical couplers  12   a  and  12   b  are contiguous and form an integral structure, which may be manufactured for example by extrusion. They may be made from suitable materials such as PMMA or polycarbonate. 
         [0052]    In the space between the top and bottom light guide layers  10 ,  11  there are an opaque reflector layer  13  and a diffusing layer  14 . The opaque reflector layer  13  ensures that the LEDs mounted in the first light guide layer  10  cannot be seen through the bottom light guide layer  11 . If the diffusing layer  14  is sufficiently thick, the opaque reflector layer  13  may be omitted. If the diffusing layer  14  is not sufficiently thick to obscure sight of the LEDs through the second light guide layer  11 , but is sufficiently thick to reflect substantially all the light entering from the second light guide layer  11  then the opaque reflector layer  13  may be replaced by a simple non-reflecting opaque layer, which may be light-absorbing, for instance black. An MLO plate  15  is provided adjacent an emission surface  16  of the second light guide layer  11 . This MLO plate  15  only transmits light within a certain angular range and helps to prevent glare. 
         [0053]    Two LEDs  16   a,    16   b  are shown placed in a cavity  17  in the first light guide layer  10 . There are several such cavities, each having two LEDs within them. The LEDs  16   a ,  16   b  are placed such that they emit in a direction parallel to a longitudinal axis of the first light guide layer  10 . The LEDs  16   a,    16   b  face away from each other and emit light in opposing directions. By arranging the LEDs  16   a,    16   b  in this way, the light on average travels in the longitudinal direction of the first light guide layer  10 , resulting in the light spreading out considerably before entering the second light guide layer  11 . This facilitates homogenization and color mixing. 
         [0054]    The LEDs  16   a,    16   b  as shown in  FIG. 3   b  emit light so that it is incident on a side wall of the cavity  17 . The side wall of the cavity  17  is perpendicular to the longitudinal axis of the first light guide layer  10 . The light emitted by the LEDs  16   a,    16   b  is refracted at the interface between the boundary and the side wall towards the longitudinal axis. Thus, the light emitted by the LEDs  16   a,    16   b  does not need to be emitted at right angles to the side wall to be properly transported through the system. 
         [0055]    The LEDs  16   a  and  16   b  may be placed on a PCB, which is preferably located in the space between the top and bottom light guide layers  10 ,  11 . This has the advantage that the PCB is electrically insulated from the outside world as the light guide layers are not electrically conductive. If the LEDs  16   a  and  16   b  are placed on a PCB that is located in the space between the top and bottom light guide layers  10 ,  11 , the opaque reflector layer  13  (or the non-reflecting opaque layer) and/or the diffusing layer  14  may be part of the PCB. The PCB may further be in optical contact with the bottom light guide layer  11 , thereby providing a means to extract light from this layer. 
         [0056]    The light in the first light guide layer  10  is propagated within an angle relative to the longitudinal axis of α, which equals arcsin (1/n), n being the refractive index of the material from which first light guide layer  10  is made. For practical materials with n=1.5 or higher, the angle a typically equals 42° or less. This ensures that as long as the emitted light rays do not encounter disturbing elements like cavities, LEDs or scattering elements or layers, they will always remain in the first or second light guide layers  10 ,  11  or the optical couplers  12   a,    12   b  by virtue of total internal reflection. This is true even if the optical couplers  12   a,    12   b  are bent very sharply, and even if the path of the optical couplers  12 ,  12   b  is not along an arc, but angular. 
         [0057]    An example of a light ray traversing the system is shown in  FIG. 4 . The light from the LED  16   a  or  16   b  traverses the first light guide layer  10  in the forward direction and to the sides, and then bends around the optical coupler  12   b  and enters the second light guide layer  11 . The numeral “ 1 ” in a circle indicates where the light ray undergoes total internal reflection; the numeral “ 2 ” in a circle indicates where it is scattered; and the numeral “ 3 ” in a circle indicates where it is refracted. As can be seen, after being totally internally reflected several times, the light ray is eventually scattered by the diffusing layer  14  at a sufficiently steep angle for it to be transmitted by MLO plate  15 . 
         [0058]      FIG. 5  shows three different types of optical coupler that may be used. As stated above, if the light rays are undisturbed whilst travelling through the first light guide layer  10  they will not be emitted from the optical couplers  12   a,    12   b,  irrespective of the shape of these. However, an important aspect is that preferably all the light generated in the first light guide layer  10  can travel through the optical couplers  12   a,    12   b  towards the second light guide layer  11 . For some geometries this may not be the case, and part the light remains in the first light guide layer  10  and, in practice, ultimately gets lost. The most preferable shape is that shown at the far left where the optical couplers  12   a,    12   b  are semi-circular with the centers of the semi-circular inner and the outer walls coinciding. In this case, all light is transmitted from the first  10  to the second light guide layer  11 . Deviations are allowed, but may give rise to some losses. The other two shapes shown (i.e. approximately semi-circular as in the middle illustration and facetted as in the right hand illustration) provide acceptable results. 
         [0059]      FIGS. 6   a  and  6   b  show examples of how the MLO plate  15  may be dispensed with. In these, micro-optic structures  18  are formed on or in either the emitting surface  16  ( FIG. 6   a ) or the inwardly facing surface  19  of the second light guide layer  11 . The micro-optic structures  18  couple the light into the desired angular range without the need for the MLO plate  15 . A reflecting layer between the opaque layer and the second light guide layer  11  is optional. 
         [0060]    As mentioned above, the light guide structures discussed so far and shown in  FIGS. 3 to 6   b  may be made in one piece, for instance by extrusion. However, alternative embodiments are possible, allowing different fabrication technologies.  FIG. 7  shows such an alternative embodiment, which can be made by bending a flat strip of the light guide material. This results in a gap  20  in the first light guide layer  10 . Now the LEDs  16   a - 16   d  can be placed in off-centre cavities  17   a,    17   b.  If the gap  20  is made very narrow or even completely closed, the LEDs could be placed in a central cavity as with the first embodiment. The gap  20  may be not straight, but may be curved and the size of the gap  20  may vary along the longitudinal direction of the device. The shape of the gap may be chosen to perform optical effects in the first light guide layer  10 . 
         [0061]      FIG. 9  shows yet another embodiment, consisting of two separate parts with a gap  20  in the first light guide layer  10  and another gap  21  in the second light guide layer  11 . The gap  21  in the second light guide layer is preferably very thin, or it will become visible.  FIGS. 9   a  to  9   h  show different layouts for the light sources in the first light guide layer  10 . In  FIG. 9   a , the cavities  17   a  to  17   c  are centered along the longitudinal axis of the first light guide layer  10 . In  FIG. 9   b , the cavities  17   a  to  17   d  are distributed on either side of the longitudinal axis.  FIGS. 9   c  and  9   d  show cavities  17   a  to  17   c  suitable for use with one single LED  16   a  to  16   c  per cavity  17   a  to  17   c.  In this case the shape of the cavities is triangular, and is chosen such that light from a certain LED  16   a  to  16   c  will not directly encounter the next LED in sight (and then be lost). By tapering the cavities as shown, the light will be deflected towards the bends as is shown by the dashed arrows in  FIG. 9   c.    
         [0062]      FIGS. 9   d  and  9   e  show additional optical elements integrated in the first light guide layer  10 . In  FIG. 9   d , the optical elements consist of rhombus-shaped cavities  22   a ,  22   b  which prevent light emitted from any of LEDs  16   a - 16   f  from impinging on any of the other LEDs  16   a - 16   f  and being scattered prematurely. In  FIG. 9   e  thin rectangular slits  23   a - 23   c  are used for the same purpose. The rectangular slits are well-suited to the distributed layout shown in  FIG. 9   e.    
         [0063]    Rhombuses, rectangular slits, and in fact cavities with any other shape can be used not only for shielding neighboring LEDs from each other but also to shape the beams travelling through the first light guide layer  10 . Care must be taken however, since any cavity (except thin narrow slits extending in the longitudinal direction) will alter the angular distribution of the rays inside the first light guide layer  10 . If this occurs, a portion of the light may escape prematurely from the cavities themselves or in the optical couplers  12   a,    12   b.    
         [0064]      FIGS. 9   g  and  9   h  show examples of embodiments for combining different color-temperature LEDs in the top guide. In  FIG. 9   g , the LEDs  16   a,    16   c,    16   e,    16   g  facing in a first direction are of one color, for example warm white, and the LEDs  16   b,    16   d,    16   f,    16   h  facing in a second direction are of another color, for example cold white. In  FIG. 9   h , the triangular cavities  17   a,    17   c  face in the opposite direction to cavities  17   b,    17   d.  The LEDs  16   a ,  16   c  in cavities  17   a,    17   c  are of one color, for example cold white, whereas the LEDs  16   b,    16   d  in cavities  17   b,    17   d  are of another color, for example warm white. 
         [0065]      FIGS. 10   a  and  10   b  show alternatives for optical coupling between the first and second light guide layers  10 ,  11 . In  FIG. 10   a , the integral optical couplers  12   a,    12   b  are replaced with curved mirrors  24   a - 24   d.  Also segmented mirrors could be used. This embodiment is less preferred because of possible losses at the mirrors  12   a  to  12   d.  However, it might be cheaper and easier to manufacture since only flat light guide layers need to be made and handled. 
         [0066]      FIG. 12   b  contains an extra optical coupling element  25  is provided to transport the light from the first light guide layer  10  to the second light guide layer  11 . Multiple optical coupling elements may be provided transversely across the first and second light guide layers  10 ,  11 . The side walls of the optical coupling element  25  have a curved shape, but they may be straight or segmented. Preferably, the optical coupling element extends longitudinally along the whole length of the first and second light guide layers  10 ,  11 . This variation could be made by, for instance, extrusion. 
         [0067]    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.