Abstract:
The invention relates to an optical module comprising two or more segments positioned around an axis of symmetry of the module. Each segment includes a light collimating structure for providing a predefined light distribution of light exiting the module and a light source assembled in a cavity within the light collimating structure. The center of the cavity coincides with the optical axis of the light collimating structure and is at a distance (d) from the axis of symmetry of the module. Including two or more segments where each segment comprises its own light source allows obtaining higher lumen output compared to prior art luminaires having only one light source while arranging the segments so that the center of each cavity coincides with the optical axis of the collimating structure of the segment allows preserving narrow beamwidth collimation of the light exiting the module.

Description:
FIELD OF THE INVENTION 
       [0001]    Embodiments of the present invention relate generally to the field of illumination systems, and, more specifically, to optical modules for providing light output having narrow beam size and high lumen output. 
       BACKGROUND OF THE INVENTION 
       [0002]    As the efficacy (measured in lumens per Watt) of light emitting diodes (LEDs) increases and prices go down, it is expected that LED illumination and LED based luminaires soon will be serious alternatives to and at a competitive level with until now predominant tube luminescent (TL) based luminaires. 
         [0003]    WO 2008/126023 describes a luminaire comprising a light source positioned within a source cavity in a collimating structure arranged for providing predefined light distribution from the luminaire. The light source includes a plurality of LEDS. The number of LEDS that can be included within the source cavity depends on the size of the cavity. In turn, the intensity of the light produced by the luminaire depends on the number of LEDS included. Thus, in order to increase lumen output of such a luminaire, a larger source cavity capable of accommodating a larger number of LEDs should be used. 
         [0004]    One drawback of the proposed structure is that increasing the size of the source cavity also increases the beamwidth of the output light.  FIG. 1  illustrates a relationship between the diameter of the source cavity and the beamwidth of the output light. As can be inferred from  FIG. 1 , in order to obtain light output having a narrow beamwidth, only a few LED dies can be placed within the source cavity of such a structure. For example, the narrowest beamwidth that can be achieved has an angular extent of 2×5°. The corresponding source cavity then has a diameter of 2×3.5 mm Because LED dies typically measure 1 mm×1 mm, such a cavity has just enough space to accommodate four dies. Typically, present day LED dies can deliver 100 lumen per die for a color temperature of warm white and up to 160 lumen per die for a color temperature of neutral white to cold white. With an approximate efficiency of the described luminaire structure being about 85%, this means a maximum of about 340 to 540 output lumens in absolute numbers. 
         [0005]    These absolute light levels can be too low for a range of applications where narrow beam spotlights with high light output are needed, such as surgical lighting, outdoor lighting, entertainment, etc. Therefore, it is desirable to provide a luminaire capable of providing light having both a narrow beamwidth and a high lumen output. 
       SUMMARY OF THE INVENTION 
       [0006]    According to one aspect of the invention, an optical module is disclosed. The module includes two or more segments positioned around an axis of symmetry of the module. Each segment includes a light collimating structure for providing a predefined light distribution of light exiting the optical module and a light source, preferably a LED or a laser diode, assembled in a cavity within the light collimating structure. A center of the cavity coincides with the optical axis of the light collimating structure and is at a distance d from the axis of symmetry of the optical module. 
         [0007]    As used herein, the term “center of a cavity” refers to a point of symmetry (e.g. the center of a circle or a regular polygon, or the axis of symmetry), or a focus point lying on such an axis of symmetry (e.g. one of the foci of an ellipse or parabola). 
         [0008]    Providing an optical module that includes two or more segments where each segment comprises its own light source allows obtaining higher lumen output compared to prior art luminaires having only one light source. Within each segment, a light source is positioned within its own source cavity. Arranging the segments in such a manner that the center of each source cavity coincides with the optical axis of the collimating structure of the segment allows preserving narrow beamwidth collimation of the light exiting the optical module. 
         [0009]    According to another aspect of the invention, a light output device or a luminaire comprising such an optical module is provided. 
         [0010]    Embodiments of claims  2 - 5  advantageously allows guiding light provided by each of the light sources towards the light collimating structure of the corresponding segment. Placing specular mirrors at certain key positions, such as e.g. in the back of the cavities, may aid in directing the light from each light source into the proper corresponding collimating optics, resulting in a dramatic increase of the luminaire efficiency. 
         [0011]    Embodiment of claim  6  specifies that the collimating structure may comprise a light guide, such as e.g. a wedge-shaped light guide, and a re-direction layer, such as e.g. a redirection foil. In one embodiment, the light guide may be substantially rotational symmetric in a plane, with the center of symmetry of the light guide coinciding with the center of the cavity. Rotational symmetry enables for provision of a symmetric light beam which often is desirable in lighting applications, such as in downlighting applications. 
         [0012]    Embodiment of claim  7  specifies an advantageous structure for the light guide. 
         [0013]    Embodiment of claim  8  provides that the optical module may further include a light transmitting layer adapted to transmit light diffusively and arranged to cover at least a portion of the light-entry surface of the light guide. The light transmitting layer allows for controlled and efficient incoupling of diffuse light transmitted from a comparatively large area into the light guide. Dimensioning of the light guide allows for forming the incoupled light into a light beam having predetermined properties when leaving the light guide, which properties allow for fulfillment of luminaire requirements, e.g. as regards to angular distribution and glare. The light transmitting layer may be a light transmissive layer adapted to diffuse incident light and output the diffused light from the side of the layer facing the light-entry surface. Hence, problems related to light source brightness can be remedied or alleviated without using a diffuser at the luminaire output. 
         [0014]    Embodiment of claim  9  provides that the light transmitting layer may also be a light emitting layer adapted to emit light in response to excitation. The light emitting layer may thus be a layer that can generate light and not a translucent layer that merely forwards light through the layer. The light emitting layer may be a layer adapted to emit light in response to excitation by light, preferably a phosphor layer. It has been found that increased efficiency is particularly desirable/needed in slim luminaires (large light output area compared to thickness) from which a uniform and “non-glare” light is desirable to provide. In such luminaires the active phosphor area for re-generating the light will be relatively small compared to the total light output area of the luminaire (in order to be able to provide collimated light within glare requirements and still keep the luminaire thin). 
         [0015]    Embodiment of claim  10  specifies that the light source may be arranged to directly or indirectly illuminate the light transmitting layer and the optical module may further include a re-transmitting light source arranged to illuminate the light transmitting layer in response to illumination by the light source. The re-transmitting light source may be adapted to emit light in response to excitation by light, preferably by comprising a phosphor material. This e.g. allows a phosphor layer to be used to generate light, e.g. by illumination from a LED, without arranging the phosphor to cover the light-entry surface, and thus the phosphor can be shielded from being visible via the light-exit surface. One advantage from this is that a colored appearance, such as yellow, can be avoided when e.g. a luminaire comprising the optical arrangement is in a off-state. 
         [0016]    Embodiment of claim  11  provides that the light transmitting layer may be arranged less than 1 mm, preferably substantially equidistantly, from the light-entry surface, and more preferably as close as possible to the light-entry surface without being in optical contact. An advantage from non-optical contact is that light rays, emitted by the light emitting layer and coupled into the light guide, will be refracted with a collimating effect. 
         [0017]    Alternatively, the light transmitting layer may be in optical contact with the light-entry surface. This has another advantage, viz. that light more efficiently can be coupled into the light guide since reflections in the light-entry surface can be avoided. 
         [0018]    Hereinafter, an embodiment of the invention will be described in further detail. It should be appreciated, however, that this embodiment may not be construed as limiting the scope of protection for the present invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    In all figures, the dimensions as sketched are for illustration only and do no reflect the true dimensions or ratios. All figures are schematic and not to scale. In particular the thicknesses are exaggerated in relation to the other dimensions. In addition, details such as LED chip, wires, substrate, housing, etc. have been omitted from the drawings for clarity. 
           [0020]      FIG. 1  illustrates relationship between the beamwidth of the light output and the size of a source cavity of one type of prior art luminaire. 
           [0021]      FIG. 2A  shows a cross-sectional side view of one luminaire arrangement, a segment of which may be used in an optical module according to an embodiment of the present invention; 
           [0022]      FIG. 2B  shows a top view of the luminaire arrangement in  FIG. 2A ; 
           [0023]      FIG. 3A  shows a cross-sectional side view of another luminaire arrangement, a segment of which may be used in an optical module according to an embodiment of the present invention; 
           [0024]      FIG. 3B  shows a top view of the luminaire arrangement in  FIG. 3A ; 
           [0025]      FIG. 4  sets forth a flow diagram of method steps for designing an optical module using segments of either the luminaire arrangement illustrated in  FIGS. 2A-2B  or the luminaire arrangement illustrated in  FIGS. 3A-3B , according to an embodiment of the present invention; 
           [0026]      FIGS. 5A-5D  provide schematic illustrations of the design steps set forth in  FIG. 4 ; and 
           [0027]      FIGS. 6A-6D  show various embodiments for directing light emitted by each of the light sources towards the collimating structure of the corresponding segment. 
       
    
    
     DETAILED DESCRIPTION 
       [0028]    In the following description, numerous specific details are set forth to provide a more thorough understanding of the present invention. However, it will be apparent to one of skill in the art that the present invention may be practiced without one or more of these specific details. In other instances, well-known features have not been described in order to avoid obscuring the present invention. 
         [0029]      FIGS. 2A-2B  show a cross-sectional side view and a top view of a luminaire arrangement  100 , a pie-shaped section of which may be used in an optical module according to an embodiment of the present invention. The shown luminaire arrangement comprises a light guide  101 , here circle symmetric in a plane y-x. The light guide  101  has a cylindrical through-hole  102 , which inner side is a light-entry surface  105  covered by a light emitting layer  113 , here a layer that emits light upon illumination, preferably a phosphor layer. The light emitting layer  113  is not in direct contact with the light-entry surface  105 , instead there is a small, equidistant air gap between the light-entry-surface  105  and the light emitting layer  113 . The gap is preferably as small as possible without there being any optical contact between the surface  105  and the layer  113 , preferably the gap is less than 1 mm. The layer  113  may even be in mechanical contact with the surface  105 , as long as there is no optical contact. Note that in  FIG. 2A  the gap shown between the layer  113  and the surface  105  is exaggerated. In most implementations the light emitting layer may be considered to be located at the same distance from the central axis CA of the through-hole  102  as the light-entry surface. 
         [0030]    In the shown embodiment there is a second light guide  157  shaped as a tube, or cylinder with a cylindrical through-hole  132  in the center, concentrically located in the cylindrical through hole  102 . The second light guide  157  has a light input surface  158  facing the center of the through-hole  132  and a light output surface  168  facing the light emitting layer. The second light guide further has lateral surfaces  159 , i.e. the end surfaces of the cylinder which are perpendicular to the light input and output surfaces  158 ,  168 . These surfaces are preferably not in optical contact with neighboring objects, but instead interfacing an optically less dense medium, preferably air, i.e. are in optical contact with a medium of lower refractive index than the second light guide  157 . The light emitting layer  113  is shown at a distance from the light output surface  168  i.e. in non-optical contact with the second light guide, but may in alternative embodiments be in optical contact. 
         [0031]    The second light guide  157  provides a collimating effect which increases efficiency. However, it can be noted that the second light guide is not required for the function as such of the luminaire arrangement in  FIGS. 2A-2B . Hence, in alternative embodiments, the second light guide may be omitted. 
         [0032]    At the lower or bottom part of the cylindrical through hole  132  there is a light source  117 , preferably a light emitting diode (LED), which may be omnidirectional. The light source may be attached to a substrate (not shown), such as a PCB. In other embodiments there may be one or many light sources also at other positions, such as at various positions in the mixing cavity  132 . For example, to produce white light a blue LED or LEDs  117  can be used in combination with a yellow or orange phosphor layer  113 . 
         [0033]    Opposite to the light source  117 , at the top end of the cylindrical hole  102 , there is a mirror  115  covering the opening of the cylinder. The mirror  115  presents an inclined surface for reflecting light from the light source  117  towards the light emitting layer  113 , light which else would escape via the cylinder opening. Since the light source is arranged so that it also illuminates the light emitting layer directly, the mirror  115  is not necessary, although it increases efficiency. Alternatively the mirror may be flat (not inclined) and/or may have diffusely reflective properties for light spreading. In  FIG. 2A , when the light source  117  directly or indirectly provides light to the light input surface  158  of the second light guide  157 , the light is passing an air interface owing to the through hole  132  and will therefor be refracted into an optically denser medium being the second light guide. As a result there will be a collimating effect of the light entering the second light guide  157  and the amount of light that can be guided to the light output surface by total internal reflection (TIR) in the lateral surfaces  159  increases. Preferably the refractive index of the second light guide is at least about 1.4 since that allows for TIR in the lateral surfaces  159  for light incident on the light input surface  158  virtually independent on an angle of incidence, provided that the lateral surfaces are also interfacing air or other medium with similar or lower refractive index. It should be understood that the second light guide  157  also is helpful and efficient for guiding back-scattered light from the light emitting layer entering via the light output surface  168  so that the light, at lower loss, can be incident on the light emitting layer  113  at another location, e.g. at an opposite side of the through hole  132 . In an example implementation it was found that with a second light guide  157  present in the center of the luminaire there was an increase from 70% of light passing the light emitting layer to 87%. Since efficiency drops when the thickness of a luminaire of this kind decreases (because more reflections, causing losses, are required in a thin structure), adding a second light guide  157  can be used to reduce thickness at maintained efficiency. When the light emitting layer  113  emits light as a response from illumination by the light source  117 , it emits light towards the outer side of the light-entry surface  105  of the light guide  101 . Owing to that the light emitting layer  113  covers the light-entry surface  105  and is arranged very close to it, light will, via the small air gap, be incident on the light-entry surface  105  at virtually all possible angles of incidence, i.e. from about +90 degrees to −90 degrees in relation to the normal of the light-entry surface  105 . The air gap means there will be an interface of lower refractive index to higher refractive index and Snells law will determine a largest entry angle (&lt;90 degrees) of the light entering the light guide  101 , i.e. the situation is similar as for the light entering the second light guide. This provides some control of the light entering the light guide  101  and will, for example, make it easier to fulfill requirements related to angular distribution of the light, which will be explained in some detail below. 
         [0034]    The light entering the light guide  101  via the light-entry surface  105  is first guided in a light-entry portion  103  of constant thickness, here equal to the thickness t 1g  of the light guide  101 . Light that fulfills the conditions of TIR in inner surfaces  109 ,  110  of the light guide  101  will be guided towards a tapering portion  107  of the light guide  101 , which portion  107  presents a reflecting surface  111  that is inclined and facing in the direction of the light-entry surface  105 . The reflecting surface  111  is arranged with an angle [beta] in relation to the normal direction of the light-entry surface  105  and the plane x-y of the light guide. 
         [0035]    The reflecting surface  111  reflects light incident from the light-entry portion  103 , i.e. from the x-direction in  FIG. 2A , towards a light-exit surface  109 , which is in a perpendicular relationship to the light-entry surface  105 . In other words, owing to the enclosing light-entry surface  105 , light entering via the light-entry surface  105  and traveling in the plane x-y of the light guide  101  is being redirected by the reflecting surface  111  and thus escapes the light guide  101  “out-of-plane” (in the z-direction in  FIG. 2A ) via the light-exit surface  109 . Owing to the “refractive” collimating effect when the light enters the light guide  101  via the light-entry surface  105  and/or the “reflective” collimating effect when the light is guided in the first portion  103  of constant thickness, the reflecting surface  111  can be designed to only handle incident light in a limited angular range, i.e. with a predetermined degree of collimation. The angle [beta] is selected so that a uniform light beam with a desirable beam width (at full-width-at-half-maximum, FWHM) can be achieved. In most practical applications the angle [beta] will be relatively small, such as in the range of 1 degree to 15 degrees. 
         [0036]    To ensure that light does not leave the reflecting surface  111  via refraction, a mirror layer  119  may be provided to cover the outside of the reflecting surface  111 . Preferably the mirror layer  119  is arranged at a small distance from the light guide surface so that there is no optical contact. 
         [0037]    In the plane (x-y) of the light guide  101  there is an angular distribution of the light. Owing to that the light emitting layer  113  will emit light into the light guide via the light-entry surface  105  at a distance of about R 1  from the central axis CA, not all light will be incident on the reflecting surface  111  at 90 degrees in the x-y plane as would have been the case without the cylindrical hole and instead a “point like” light source on the central axis CA of the light guide. Note that this applies in the shown x-y plane and not when light is incident on the reflecting surface from directions that are not in this plane. When light from the light emitting layer is entering the light guide at the distance R 1  from the center, a largest angle [phi] of light incident on the reflecting surface in the plane of the light guide occurs where the tapering portion  107  and the reflecting surface  111  begin, i.e. at a distance R 2  from the central axis CA. It can be noted that non-optical contact between the light emitting layer  113  and the light-entry surface  105  typically will make the largest angle smaller than the angle [phi] indicated in the figure when light is refracted into the light guide  101  via the light-entry surface  105 . 
         [0038]    Still referring to  FIGS. 2A-2B , a transmissive re-direction layer  121  is arranged to cover the light-exit surface  109  of the light guide  101 . The re-direction layer  121  may take care of the final adjusting and tuning of the light distribution. The re-direction layer  121  comprises triangular elements  123  formed in the surface of the layer facing the light-exit-surface  109  of the light guide  101 . The triangular elements  123  are in the form of protrusions, or ridges, encircling the central axis CA of the light guide in the x-y plane. Each triangular element  123  presents a first surface  125  facing in the direction of the center of the light guide  101 , i.e. where light enters the light guide via the light-entry surface  105 , and a second surface  127  facing away from the light-entry surface  105 . The first surface  125  is arranged at a first angle [alpha 1 ] in relation to the normal to the plane of the layer and the second surface  127  at a second angle [alpha 2 ]. The surfaces  125 ,  127  meet and form the tip of the triangular element  123 , which tip may be in contact, but preferably not in optical contact, with the light-exit surface  109 . It should be noted that mechanical contact not necessary results in optical contact, as will be recognized by the skilled person. It is mainly “air-pockets” in the form of the valleys between the triangular elements  127  that are directly facing the light guide. 
         [0039]    A light ray leaving the light-exit surface  109  of the light guide  101  will thus first be refracted at a light guide to air interface, pass the air filled “valley” between adjacent triangular elements, be refracted in the first surface  125  of a triangular element  123  at an air to re-direction layer interface, and then be reflected by TIR in the second surface  127  of the triangular element  123  at a re-direction layer to air interface. The last reflection directs the light ray towards the opposite surface of the redirection layer  121 , which it passes by refraction at a re-direction layer to air interface. The re-direction layer may thus have a collimating and/or focusing effect on the light from the light guide. 
         [0040]    It may be noted that the redirection layer  121  shown in  FIG. 2A  has a cavity formed above the mirror  115 . However, the exact design of the redirection layer in that area is typically of less significance since it is not participating in the re-direction of light. 
         [0041]    Moreover, in  FIG. 2A  trace  143  shows the path of an exemplary light ray emitted by the light emitting layer  113  in response to illumination by the light source  117 . In a first detailed example based on the first embodiment, the light guide  101  is of PMMA and has a refractive index of about 1.5 and the re-direction layer is of PC and has a refractive index of about 1.6. 
         [0042]    The material of the light guide  101  and the second light guide  157  may in general and advantageously have an optical absorption less than 0.3/m, provide low haze and scattering, contain particles smaller than 200 nm and be able to sustain an operational temperature higher than 75 degrees Celsius. Since the optical path in the light guide typically is relatively large (e.g. about 50 mm), the material should preferably have high optical transparency and be of good optical quality so that absorption still can be low. The material of the re-direction layer  121  may generally and advantageously have an optical absorption of less than 4/m, provide low haze and scattering, contain particles smaller than 200 nm, be able to sustain an operational temperature higher than 75 degrees Celsius. The redirection layer may be similar to a so-called re-direction foil, such as the transmissive right angle film (TRAF) as currently is available under the name Vikuti™ from 3M. Furthermore, in the first detailed example the light guide  101  has a thickness t 1g =5 mm and the re-direction layer  121  a thickness t 11 =3 mm. The light-entry surface  105  is located at a distance R 1 =20 mm from the central axis CA of the light guide, the tapering portion  107  and the reflecting surface  111  begin at a distance R 2 =30 mm from the central axis CA, and the light guide  101  and the reflecting surface  111  end at a distance R 3 =55.5 mm. The angle [beta] of the reflecting surface  111  is thus about  11  degrees and the area of the light-entry surface  105  and the light emitting layer covering it, is about 600 mm 2 . The light source  117  is a LED of less than 10 W having an area of 3 mm 2 . The light emitting layer is a phosphor layer, such as YAG:Ce (Cerium-doped Yttrium Aluminum Garnet) which is arranged as close as possible to the light-entry surface  105  without optical contact. There are about 100 adjacent triangular elements concentrically arranged about the central axis CA of the light guide  101 . The first angle [alpha 1 ] of each triangular element  123  is 9 degrees and the second angle [alpha 2 ] is 31 degrees. The first detailed example results in a light beam with a beam width of about 2*30 degrees. 
         [0043]    A second detailed example differs from the first detailed example in that R 2 =80 mm and R 3 =151 mm, whereby [beta] is about 4.0 degrees. The second detailed example results in a light beam with a beam width of about 2*10 degrees. A third detailed example differs from the first detailed example in that the first angle [alpha 1 ] of each triangular element  123  is 2 degrees and the second angle [alpha 2 ] is 36 degrees. In comparison with the light beam of the first detailed example, the third detailed example results in a light beam with a reduced “tail”, i.e. with less light flux at angles between half the beam width (at FWHM) and the cut-off angle. Furthermore, in linear systems it has been found that, at least in the range of a reflecting surface having an angle [beta] in the interval 2 degrees-15 degrees, the beam angle being provided is, as a design rule of thumb, about 5 times the angle [beta]. 
         [0044]    The number of triangular elements  123  disposed between the center and the perimeter of the light guide  101 , i.e. along any radial direction in the x-y plane, is typically not crucial, however, more elements  123  (at constant layer thickness t 1g ), means smaller dimensions of the elements  123 , which has the advantage that the elements will be more discrete and virtually invisible. On the other hand, when the dimensions become too small, there is a risk that imperfections in the triangular surfaces  125 ,  127 , e.g. caused by manufacturing, will have increasing and eventually detrimental impact on the light beam to be provided. Hence, care should be taken when increasing the number of and downsizing the triangular elements. 
         [0045]    In another embodiment there is a transmissive diffuser layer  113  instead of the light emitting layer  113 . Light that pass through the diffuser is being diffused, i.e. here light incident on the inner side becomes diffused light that leaves from the side facing the light-entry surface. The diffuser may diffuse light in directions corresponding to those being provided by the light emitting layer and the diffuser layer may be arranged in relation to the light-entry surface similarly to the light emitting layer. In yet another embodiment, there is a light emitting layer, such as a phosphor layer, instead of the mirror  115 , and instead of the light emitting layer  113  covering the light-entry surface there is a diffuser layer arranged to cover the light-entry surface  105 . In this embodiment, the light source  117  emits light that is converted with a re-emitting effect by the light emitting layer at the top end of the cylindrical hole  102  thus forming a re-transmitting light source. The re-transmitted light then is incident on the diffuser layer. The diffuser layer may be shielded from direct light from the light source  117 . 
         [0046]      FIGS. 3A-3B  show a cross-sectional side view and a top view of another luminaire arrangement  169 , a pie-shaped section of which may be used in an optical module according to an embodiment of the present invention. 
         [0047]    Most is the same in luminaire arrangements  100  and  169 . However, a difference is that there is no second light guide  157  present and also that the mirror layer  119  has been replaced by a reflecting layer  118  covering not only the outer side of the reflecting surface  111  of the light guide, but also an outer surface side of the surfaces  110 ,  112  in the light-entry portion  103  and the bottom opening of the cylindrical hole  102 . It is understood, however, that a second light guide may be used also with the luminaire arrangement  169 . Furthermore the light source  117  is here arranged on the side of the reflecting layer  118  facing the through-hole  102 . The reflecting layer  118  has a mirror or a specularly reflecting surface facing the light guide  101 , and is preferably not in optical contact with the light guide  101 . 
         [0048]    Another difference between the embodiments of  FIGS. 2 and 3  is that the light-entry portion  103  in the luminaire arrangement  169  has a first sub-portion  106  which has a slope and increases in thickness from the light-entry surface  105  towards the tapering portion  107 . The slope of the sub-portion  106  is preferably in the range of 35 degrees-45 degrees in relation to the normal to the light-entry surface  105 . If the slope angle is too small, this may lead to leakage of light, however, some leakage may be permitted. A slope angle substantially greater than 45 degrees is typically not desirable. One approach may be to start with a slope angle of about 45 degrees, depending on the index of refraction, and use lower angles farther from the light-entry surface. 
         [0049]    When the sub-portion  106  reaches the thickness t 1g  of the light guide  101 , at a distance R 2 ′ from the central axis CA, there is a second sub-portion  108  of constant thickness, between distances R 2 ′and R 2  from the central axis CA, before the tapering portion  107  begins. The reason for the first sub-portion  106  of increasing thickness is to reduce the risk of undesirable refraction out from the light guide. The sloped surfaces  112  of the sub-portion  106  reduce the angle of light incident directly from the light-entry surface  105 , and thus facilitate TIR. A sloped first sub-portion  106  may be particularly advantageous when the light-emitting layer is in optical contact with the light-entry surface. (In a situation with optical contact and without the sloped first sub-portion  106 , some light would be incident by approximately 90 degrees in surfaces  109 ,  110 .) 
         [0050]    Some relations regarding the angular distribution in the plane of the light guide will now be given with reference to the two embodiments disclosed in the foregoing. With optical contact between the light-entry surface and the light emitting layer, the following equations may be used in the design of the light guide: 
         [0000]      sin [phi]= R 1/ R 2  (Eq. 2A)
 
         [0051]    The angle [phi] may be considered a good approximation for the cut-off angle for rule-of-thumb estimates. R 1 , R 2  and [phi] are in accordance with  FIG. 2A  and  FIG. 3A . 
         [0052]    Without optical contact between the light-entry surface and the light emitting layer, the following equation replaces Eq. 2A: 
         [0000]      sin [phi]= R 1/( n   1g   *R 2)  (Eq. 2B)
 
         [0053]    with n 1g  being the refractive index of the light guide. 
         [0054]    However, since the re-direction layer  121  may give a small but adverse contribution to the cut-off angle, it may be advised to have some margin when designing the light guide using the equations above. 
         [0055]    For example, in a design with a cut-off of 10 degrees in air, a light guide with a refractive index of 1.5 and a light-entry surface arranged at R1=20 mm from the center, Eq. 2B results in that R 2  should be about 77 mm. In practice R 2  may need to be larger than this to accomplish a cut off not exceeding 10 degrees. It should be noted that the angle [beta] can be considered to determine the beam width in the direction orthogonal to the direction of [phi] and that thus both [phi] and [beta] must be considered in order to have a narrow beam, i.e. for a narrow beam both [phi] and [beta] should be small. In the foregoing the refractive indices of the light guide and the re-direction layer have been about 1.5. Other refractive indices may be used, preferably in the range of 1.4-1.8. However, as will be recognized by the skilled person, the hitherto discussed dimensions, angles, etc. may need to be adapted accordingly, which the skilled person will be able to do based on the information disclosed herein. 
         [0056]    The pie-shaped sections, or segments, of the rotational symmetric luminaire arrangements that have been discussed in the foregoing may advantageously be used in assembling an optical module according to embodiments of the present invention. In the following, the term “cavity” refers to a through-hole  102  described above, the term “light source” refers to the light source  117  described above, and the term “collimating structure” refers to all of the elements of the luminaire arrangements shown in  FIGS. 2A-2B  and  3 A- 3 B which are outside of the cavity (i.e., the light guide  101 , re-direction layer  121 , etc.). 
         [0057]      FIG. 4  sets forth a flow diagram of method steps for designing an optical module using segments of either the luminaire arrangement illustrated in  FIGS. 2A-2B  or the luminaire arrangement illustrated in  FIGS. 3A-3B , according to various embodiments of the present invention. While the method steps are described in conjunction with  FIGS. 2A-2B  and  5 A- 5 D, persons skilled in the art will recognize that any system configured to perform the method steps, in any order, is within the scope of the present invention. Thus, while, in the following, segments of the luminaire arrangement  100  are discussed, similar teachings may be applied to other luminaire arrangements having a light source positioned within a cavity in a collimating structure, such as e.g. the luminaire arrangement  169 . 
         [0058]      FIGS. 5A-5D  provide schematic illustrations of the design steps set forth in  FIG. 4 , showing a top view of a luminaire arrangement, segments, and an optical module (similar to  FIGS. 2B and 3B ). In  FIGS. 5A-5D , elements with the same numbers and names as shown in  FIGS. 2A-2B  illustrate the same elements as those in  FIGS. 2A-2B  (such as e.g. the surface  105  of the cavity, radius of the cavity R 1 , etc.). Further, dashed lines  191 - 195  illustrate planes perpendicular to the x-y plane, where an intersection of planes  191  and  193  forms an axis of symmetry of the optical module and an intersection of planes  192  and  193  forms the axis of symmetry at the center of the cavity within a segment (i.e. equivalent to the control axis CA). 
         [0059]    As shown in  FIG. 4 , the method begins with step  180 , where a “segment” of the luminaire arrangement  100  to be used in the future optical module is defined.  FIG. 5A  illustrates how a segment is defined. As shown in  FIG. 5A , a segment  197  is a portion of the luminaire arrangement  100  between planes  194  and  195  selected so that the segment  197  is mirror-symmetric with respect to the plane  193 . While the cavity within the segment  197  is shown to be circular, in other embodiments, the cavity may have other shapes as long as the segment  197  maintains mirror-symmetry with respect to the plane  193 . For example, the cavity may be an elliptical cavity, with one of the two main axis of the ellipse coinciding with the line  193  (parallel to the x-axis in  2 D). 
         [0060]    The corner axis of the segment  197  where the planes  194  and  195  intersect, shown in  FIG. 5A  as a corner  198 , is at a distance “d” from the center of the cavity. Planes  194  and  195  form an angle [gamma]. The angle [gamma] and the distance d are selected as follows. 
         [0061]    First, the number of segments to be present in the future optical module should be selected. As previously described herein, the number of segments define the number of light sources that will be present in the optical module. Since the total light output of the optical module is the combination of the light outputs of each light source, the greater the number of light sources, the greater the lumen output of the optical module. Since, as will be described in greater detail below, the segments will be arranged in a “daisy” pattern around an axis of symmetry of an optical module, when N segments are selected to be included in the optical module, each segment spans an angle of 360/N degrees: 
         [0000]      =360°/N
 
         [0062]    In  FIGS. 5A-5D , segments are illustrated for an exemplary embodiment where the optical module includes a total of 6 segments. Of course, in other embodiments, any other number of segments may be used, as long as N is greater than or equal to 2. 
         [0063]    The distance d is selected to be such that the segment  197  includes the whole cavity. Therefore, for N segments, the minimal distance d can be determined as: 
         [0000]        d   min   =R 1/sin(180°/ N )
 
         [0064]    Any distance d greater than d min  can be selected. The greater the distance d, the greater the diameter of the optical module. In one embodiment, it may be preferable to select the distance d to be as small as possible in order to e.g. keep the overall luminaire footprint as small as possible. In other embodiments, it maybe be preferable to select a larger distance d because an additional through-luminaire center hole would allow for the placement of extra optical equipment, such as e.g. a central camera in medical lighting equipment. 
         [0065]    In step  182 , N segments identical to the segment  197  defined in the previous step are produced (one such segment is shown in  FIG. 5B ). Such segments may be fabricated by cutting each segment out of one luminaire arrangement  100 . Alternatively, the segments may be fabricated on their own by keeping the optical design of a single optical segment the same as described for that portion of the luminaire arrangement  100 . 
         [0066]    In step  184 , a first segment is arranged so that the axis of symmetry of the cavity of that segment (i.e., the intersection of planes  192  and  193 , equivalent to the central axis CA of  FIGS. 2A and 3A ) is at a distance d from the axis of symmetry of the future optical module (i.e., the intersection of planes  191  and  193 ). This is shown in  FIG. 5C . 
         [0067]    The method ends in step  186 , where other (N−1) segments are arranged around the axis of symmetry of the optical module so that, for each segment, the axis of symmetry of the cavity of that segment is at a distance d from the axis of symmetry of the optical module. A complete optical module  200  arranged in this manner is shown in  FIG. 5D . The optical module  200  is rotationally symmetric with respect to rotations of an integer multiple of 360/N degrees around the axis of symmetry of the module. Arranging an optical module as described above allows, for each of the segments, maintaining the cavity to be centered according to the rotationally symmetric prism structures on the re-direction layer  121 . In that way, light rays escaping from the optical wedge waveguide  101  only have an inclination angle with respect to the following re-direction layer  121 . Their azimuthal angles (the angle in the plane of the flat re-direction layer  121 ) are substantially zero. Therefore, the width of the output light beam is largely dictated by the [prism] collimation action on the ray inclination angles which results in a decreased beam width of the output light beam. 
         [0068]    In case the azimuthal portion of the light ray angle differs from zero, it will be directly translated into a similar output light beam angle, since the redirection layer  121  does not provide collimation action for the azimuthal part of the light rays. Hence, in one embodiment, the azimuthal angle portion should be lower and preferably significantly lower than the intended final output light beam angle. 
         [0069]    Optionally, the optical module may further include at least partially reflective structure (a mirror) configured, for each of the segments, to direct [at least some of] the light produced by the light source towards the collimating structure of that segment. (i.e., so that the light of each light source is guided only in the optics of its own segment).  FIGS. 6A-6D  illustrate various ways for arranging mirrors in the optical modules  200 A- 200 D, respectively, for directing the light towards the collimating structures. Each of the optical modules  200 A- 200 D may be the optical module  200 , described above. 
         [0070]    In one embodiment, a mirror is used to close the top of all cavities, which could be done e.g. with a flat circular (diffusively reflecting) mirror  202  illustrated in  FIGS. 6A-6D . In other embodiments (not shown in  FIGS. 6A-6D ), each cavity could be closed on the top with it&#39;s own mirror (similar to the mirror  115  shown in  FIGS. 2A and 3A ). Additionally or alternatively to the mirror used to close the top of all cavities, the optical module may further include sidewall mirror(s) configured to reflect the light towards the outer portion of the segments. In various embodiments, this may be implemented e.g. with a central gear shaped sidewall mirror  204 A shown in  FIG. 6A , a central cylinder shaped sidewall mirror  204 B shown in  FIG. 6B , or a central regular polygon shaped sidewall mirror  204 C shown in  FIG. 6C . In yet another embodiment, each segment may be provided with it&#39;s own sidewall minor, such as e.g. illustrated in  FIG. 6D  with minors  204 D which could be e.g. foils bended in the back of the cavities. Persons skilled in the art will recognize that there are numerous other ways for providing minors for guiding the light produced by the light sources towards the respective collimating structure of each segment. 
         [0071]    While the embodiments described above illustrate cavities having a circular cross-section in the x-y plane, in other embodiments such cross-sections of cavities may have other shapes, such as e.g. a regular polygon, en ellipse or a parabola. 
         [0072]    One advantage of the present invention is that light output beam having high lumen output as well as narrow band width may be provided. Therefore, optical modules that have been discussed in the foregoing may advantageously be used in a downlighting application, particularly in surgical lighting. 
         [0073]    While the forgoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof. Therefore, the scope of the present invention is determined by the claims that follow.