Abstract:
A lighting device ( 1 ) comprises a light source ( 2 ) and a lens ( 3, 23, 33, 43, 53, 63 ) positioned in front of the light source ( 2 ). The lens ( 3, 23, 33, 43, 53, 63 ) is provided with a light entrance surface on a side facing the light source ( 2 ) and a light exit surface ( 14, 38 ) on a side remote from the light source ( 2 ). The lens ( 3, 23, 33, 43, 53, 63 ) comprises a number of strip-shaped interconnected elongated light guiding elements ( 4, 24, 34, 54, 64 ), of which first ends ( 7, 27, 37, 57 ) and spaced apart second ends ( 5, 25, 35, 55, 65 ) comprise the light entrance surface and light exit surface, respectively. Light beams emitted by the light source ( 2 ) are transmitted by total internal reflection in the elongated light guiding elements ( 4, 24, 34, 54, 64 ) from the first ends ( 7, 27, 37, 57 ) to the spaced apart seconds ends ( 5, 25, 35, 55, 65 ).

Description:
FIELD OF THE INVENTION 
     The invention relates to a lighting device comprising a light source and a lens positioned in front of the light source, which lens is provided with a light entrance surface on a side facing the light source and a light exit surface on a side remote from the light source. 
     The invention also relates to a lens. 
     BACKGROUND OF THE INVENTION 
     Such a lighting device, which is known from EP 2009345 A2, comprises a lens having a first optical refractive element arranged around a peripheral edge and a second optical refractive element centrally located on the lens. Between the light source, such as a light emitting diode (LED), and the lens a reflector is located. A forward emitted portion of the light of the LED goes directly to the lens whilst a sideward emitted portion is reflected by the reflector before it goes to the lens. 
     A disadvantage of this known lighting device is that the perceived luminance of the lens is of the same order as the luminance of the light source. In the case of a high power LED an intense and blinding light will be emitted by the lighting device. 
     The use of such a lighting device with high power LEDs for general indoor lighting is difficult because of the extreme high luminance of the LED. In order to avoid direct exposure of the observer to the high luminance of the LED, luminance transformers may be added. However, such luminance transformers have the disadvantage that they lead to a decrease of the optical efficiency and an increase of the costs of the lighting device. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a lighting device with a simple structure, a relatively high optical efficiency and luminance transformation to avoid glare. 
     This object is accomplished with the lighting device according to the invention in that the lens comprises a number of strip-shaped interconnected elongated light guiding elements, of which first ends and spaced apart second ends comprise the light entrance surface and light exit surface, respectively, and light beams emitted by the light source are transmitted by total internal reflection in the elongated light guiding elements from the first ends to the spaced apart second ends. 
     Due to the strip-shaped elongated light guiding elements and the spaced apart second ends, a lens with an open structure is obtained. Owing to the strip-shaped elongated light guiding elements, the length in a first direction from the first end to the second end is in the same order or much smaller as the length of the elongated light guiding elements in a second direction perpendicular to the first direction. The open structure provides a relatively large light exit surface compared to the light entrance surface. The brightness of the light source is distributed over a relatively large light exit surface, whereby the observed brightness is strongly reduced. Light beams from the light source are transmitted by total internal reflection, due to which a high optical efficiency of the lens is obtained. Lighting devices with such a lens are applicable for a wide range of indoor and outdoor applications. 
     An embodiment of the lighting device according to the invention is characterized in that at least the strip-shaped second ends of the elongated light guiding elements extend parallel to each other. 
     In this manner, the light exit surface will have a rectangular shape due to which the light emitted by the lighting device may look similar to the light emitted by elongated fluorescent tubes. 
     Preferably, the elongated light guiding elements extend parallel to each other from the first ends to the second ends, which makes it possible to manufacture the lens by means of extrusion, so that a relatively large length in the second direction perpendicular to the first direction from the first end to the second end can be achieved. 
     A device with such a lens is suitable as a light line for shop lighting or a light line for a bus, train or airplane, or tunnel lighting. Such a lens is also suitable for a waterproof luminaire, since only the small entrance area where the LED or the LEDs are located, must be waterproof, together with both side ends. 
     Another embodiment of the lighting device according to the invention is characterized in that at least the second ends of the elongated light guiding elements are ring-shaped strips located concentrically with respect to each other. 
     In this manner, the light exit surface will have a cylindrical shape due to which the light emitted by the lighting device may look similar to the light emitted by incandescent light bulbs. 
     Preferably, the elongated light guiding elements are ring-shaped from the first ends to the second ends, such that each elongated light guiding element is cup-shaped. 
     A device with such a lens is suitable for home lighting, or as a downlighter for office lighting or shop lighting. 
     Yet another embodiment of the lighting device according to the invention is characterized in that at least a part of the elongated light guiding elements are at least connected to each other near the first and/or second ends. 
     A continuous light entrance surface and/or exit surface can thus be obtained, providing the lens with a smooth appearance. The areas where the elongated light guiding elements are interconnected are preferably as small as possible to prevent disturbance of the total internal reflection in the elongated light guiding elements. 
     Another embodiment of the lighting device according to the invention is characterized in that the area of the light exit surface of the lens is at least 100 times and preferably at least 10,000 times larger than the light emitting area of the light source. 
     Due to the enlargement of the light exit surface with respect to the light emitting surface of the light source, a decrease in perceived luminance is obtained. Depending on the desired luminance, the ratio between light entrance surface and light emitting surface is chosen as well as the number and shape of the elongated light guiding elements. 
     Another embodiment of the lighting device according to the invention is characterized in that a surface of the first end of the elongated light guiding element extends substantially perpendicularly to the light beams of the light source directed towards said elongated light guiding element. 
     All light emitted by the light source towards an elongated light guiding element will enter the elongated light guiding element so that the optical efficiency will be optimal. 
     Yet another embodiment of the lighting device according to the invention is characterized in that the light exit surface has an oblique, convex or concave shape. 
     The shape of the light exit surface further improves the light emitted by the lighting device, the visual appearance thereof, and determines to a large extent the beam pattern emerging from the lighting device. This is of particular interest for applications in which beam control is important, for example in automotive headlight systems, for example for generating a dim light beam. 
     Another embodiment of the lighting device according to the invention is characterized in that the lens is made of acryl, polycarbonate or other transparent material. 
     From such materials relatively cheap lenses can easily be produced. 
     Another embodiment of the lighting device according to the invention is characterized in that the lens is made by injection moulding or extrusion. 
     Such a manufacturing process is relatively easy. In the case of injection moulding, the lens can be assembled out of several parts to avoid draft angle problems. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be explained in more detail with reference to the drawing, in which: 
         FIGS. 1A and 1B  are, respectively, a cross sectional view and a perspective top view of a first embodiment of the lighting device according to the invention, 
         FIG. 2  is a cross sectional view of the lens of the lighting device as shown in  FIG. 1 , with light beams emitted by the light source and internally reflected by elongated light guiding elements of the lens, 
         FIG. 3  is a perspective view of a second embodiment of the lighting device according to the invention, 
         FIG. 4A  is a cross section of a third embodiment of the lighting device according to the invention, 
         FIGS. 4B, 4C and 4D  are different embodiments of second ends of the elongated light guiding elements, 
         FIG. 5  is a perspective bottom view of the third embodiment of the lighting device as shown in  FIG. 4A , 
         FIG. 6  is a cross section of a fourth embodiment of the lighting device according to the invention, 
         FIG. 7  is a cross section of a fifth embodiment of the lighting device according to the invention, 
         FIG. 8  is a cross section of a sixth embodiment of the lighting device according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     In the Figures, like parts are indicated by the same reference numerals. 
       FIGS. 1A, 1B and 2  show a first embodiment of a lighting device  1  according to the invention. The lighting device  1  comprises a high power LED  2  as light source and a lens  3 . The lens  3  comprises a number of elongated light guiding elements  4  which are interconnected near spaced apart second ends  5  by means of a plate  6 . First ends  7  of the elongated light guiding elements are located near the LED  2 . The first ends  7  are spaced apart, such that the distance between the first ends  7  is much smaller than the distance between the second ends  5  at the plate  6 . The elongated light guiding elements  4  are cup-shaped, such that both the first ends  7  and the second ends  5  form ring-shaped strips. The first ends  7  of the elongated light guiding elements  4  are each provided with a light receiving surface  8 . The light receiving surfaces  8  of all the elongated light guiding elements  4  form a light entrance surface of the lens  3 . The light receiving surfaces  8  are located with respect to each other in a manner such that all light beams of the LED  2  will enter one of the light receiving surfaces  8  and no light beam will enter an opening  9  between the elongated light guiding elements  4 . Preferably, each light receiving surface  8  extends substantially perpendicularly to the light beams entering said light receiving surface  8 . The two centrally located elongated light guiding elements  4  are connected to each other with their second ends  7 , however just beyond the first ends  7  the elongated light guiding elements  4  are spaced apart. Between the elongated light guiding elements  4  openings  9  are present which become wider in a direction away from the LED  2 . The strip-shaped elongated light guiding elements  4  are relatively long in a direction from the LED  2  towards the plate  6  and in a circumferential direction. The elongated light guiding elements  4  are curved and dimensioned so that a light beam  10 ,  11  entering the light receiving surface  8  will be totally reflected inside the elongated light guiding element  4  by inner and outer surfaces  12 ,  13  of the elongated light guiding element  4  until the light beam  10 ,  11  reaches the second end  5  of the elongated light guiding element  4  and leaves the second end  5  through the light exit surface  14  of the plate  6 . 
     Due to the elongated light guiding elements  4 , the curved form thereof and the openings  9  between the elongated light guiding elements  4 , the light exit surface  14  of the plate  6  interconnecting the second ends  5  of the elongated light guiding elements  4  is much larger than the light entrance surface as formed by the light receiving surfaces  8  of the elongated light guiding elements  4 . Preferably, the light exit surface  14  is at least 100 times and more preferably at least 10,000 times as large as the light emitting surface  2 ′ of the LED  2 . The light emitting surface  2 ′ of the LED  2  is for example 1×1 mm to 3×3 mm and the luminance is for example 10 7  cd/m 2 . Preferably, the perceived luminance should be in the order of 10 4  cd/m 2  to 5×10 4  cd/m 2 . The light exit surface of the lens should therefore preferably be in de order of 5×10 −4  m 2  to 10 −2  m 2 . The number of elongated light guiding elements  4  is preferably at least 3 and at the most 50. The luminance of the LED  2  is strongly fragmented and a much lower luminance is perceived by the observer. However, due to the total internal reflection the optical efficiency of the lens  3  is high and nearly no light is lost. 
     The lens  3  is made of acryl, polycarbonate or other transparent material and is preferably made by injection moulding. It can be made out of several parts to overcome draft angle problems during the injection moulding process. 
       FIG. 3  shows a second embodiment of a lens  23  of a lighting device according to the invention. The lens  23  has a similar cross section as the lens  3  and as shown in  FIG. 1 . However, instead of a round shape, the lens  23  has a more rectangular shape. The lens  23  comprises a number of strip-shaped elongated light guiding elements  24  extending parallel to each other. The elongated light guiding elements  24  are interconnected near second ends  5  by means of a rectangular plate  26 . First ends  27  of the elongated light guiding elements  24  are located near a passage  31  into which a number of LEDs  2  can be positioned in a row or array. The first ends  27  are spaced apart, the distance between the first ends  27  being much smaller than the distance between the second ends  25 , i.e. at the location where the second ends enter the plate  26 . The first ends  27  of the elongated light guiding elements  24  are each provided with a light receiving surface  28 , the light receiving surfaces  28  of all the elongated light guiding elements  24  thus forming a light entrance surface of the lens  23 . The light receiving surfaces  28  are located with respect to each other in a manner such that all light beams of the LEDs  2  will enter one of the light receiving surfaces  28  and that no light beam will enter an opening  29  between the elongated light guiding elements  24 . 
     Light beams of the row of LEDs in the passage  31  will be guided by total internal reflection through the elongated light guiding elements  24  in a manner as shown in  FIG. 2 . The lens  23  can be made by means of extrusion so that a relatively long lens of for example 1 meter in the extrusion direction can be obtained. Such a lens can be used for a lighting device for a bus, train, airplane or parking garage, for example. 
     The total light emitting surface of the row or array of LEDs  2  is the sum of the light emitting surfaces  2 ′ of all the LEDs  2 . Preferably, the light exit surface of the lens  23  at the plate  26  is at least 100 times and more preferably at least 10,000 times as large as the total light emitting surface of the LEDs  2 . 
     Other shapes of lenses can be made by milling the outer surface of the lens  23 . 
       FIGS. 4A and 5  show a third embodiment of a lens  33  of a lighting device according to the invention. The lens  33  comprises cup-shaped curved elongated light guiding elements  34 . First ends  37  of the elongated light guiding elements  34  are located against each other and form a light entrance surface  36 . Second ends  35  of the elongated light guiding elements  34  are spaced apart, such that the distance between the second ends  35  is much larger than the distance between the first ends  37 . Between the elongated light guiding elements  34  openings  39  are located. Near the second ends  35  the elongated light guiding elements  34  are provided with light exit surfaces  38 , such that the light exit surface of the lens  33  is formed by the area in which the light exit surfaces  38  are located. The area of the light entrance surface  36  is much smaller than the area of the light exit surface  38  of the lens  33  due to which the perceived luminance of the lighting device is much lower than the luminance of the LED  2  positioned opposite the light entrance surface  36 . The second ends  35  of the elongated light guiding elements  34  can be provided with a convex light exit surface  40 , a concave light exit surface  41  or an oblique light exit surface  42  as shown in  FIGS. 4B, 4C, 4D , respectively, to amend the emitted light as desired. 
       FIG. 6  shows a fourth embodiment of a lens  43  of a lighting device according to the invention. The lens  43  differs from the lens  33  in that the second ends  35  are located in a convex plane rather than in a common flat plane. 
       FIG. 7  shows a fifth embodiment of a lens  53  of a lighting device according to the invention. The lens  53  differs from the lens  33  in that the thickness of the elongated light guiding elements  54  near the first ends  57  is larger near the outside of the lens  54  than near the inside, whilst the thickness near the second ends  55  of all elongated light guiding elements  54  is equal. 
       FIG. 8  shows a sixth embodiment of a lens  63  of a lighting device according to the invention. The lens  63  differs from the lens  53  in that the second end  65  of the outer cup-shaped elongated light guiding element  64  is flared so that the light exit surface  68  thereof faces away from the light exit surfaces  58  of the other elongated light guiding elements  54 . 
     It is also possible to manufacture a more rectangular lens, such as shown in  FIG. 3  with a cross section as shown in  FIG. 6, 7 or 8 . 
     It is also possible to extend the outer elongated light guiding elements  34  so that the second ends are located in a convex plane, a rippled plane or any differently shaped plane. 
     It is also possible to provide the light exit surface of the elongated light guiding element with a micro structure, such as a frosted structure or diffusion structure, to further improve the light distribution. 
     It is also possible to connect the elongated light guiding elements to each other somewhere between the first and second ends either by a light guiding material, an opaque material or a holder. 
     Preferably, the elongated light guiding elements are rigid. However, it is also possible to manufacture flexible elongated light guiding elements so that the position of the light exit surface of each elongated light guiding element can be amended as desired.