Abstract:
A light tube for producing light, which may be used in the light fittings, desk lamps, vehicle lights etc. The light tube comprises a plurality of segments arranged end to end along the end of the light tube. Each segment has a transparent or translucent rod  706  having two ends, a head spreader  703  by at least one of said ends and an LED  705  mounted to each heat spreader  703  for directing light laterally into the rod  706 . The heat spreaders  703  are mounted to a metal jacket  702  for dissipating heat. The light tube further comprises one or more reflectors  708, 711  for reflecting light out of the light tube.

Description:
RELATED APPLICATIONS 
     The present invention claims priority from U.S. Provisional Patent Application No. 61/213,304, filed 28 May 2009. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a light tube for producing light. Such light tubes may, for example, be used in light fittings, desk lamps, vehicle lights etc. It is to be understood that in this specification the term ‘light tube’ is not limited to tubes of circular cross section, but encompasses all types and shapes of light tube. 
     BACKGROUND OF THE INVENTION 
     LEDs have many advantages such as long life, low power consumption, and high efficiency. In recent times LED-based luminous products have become more and more popular. LED tubes are especially attractive for general illumination due to the above-mentioned advantages. Indeed, LED tubes are a promising potential replacement for traditional fluorescent light tubes. However, there are two major problems which need to be solved. One is their optical performance. Currently, LED arrays are mounted in a tube for generating light. Due to the concentrated light emission of LED chips, the light distribution of an LED tube is not as uniform and comfortable as that of conventional fluorescent tubes. The light from a conventional LED tube can have a “spotty effect” due to the uneven distribution of light throughout the tube. Another problem is thermal management. The heat generated by LED chips needs to be dissipated effectively in order to guarantee the optical performance and the operation life of LED tubes. 
       FIG. 1  is a schematic drawing showing a prior art commercial LED light tube. The light tube  1  comprises a PCB strip  10  on which LEDs  20  are mounted and which is inserted into a transparent plastic cylinder  30 . The cylinder or “shell”  30  is then sealed with end caps  40 . Electric power and a ground line are distributed through the electrode pins  45  on the end caps  40 . In this typical structure, the LEDs  20  are facing towards the front side  30   a  of the tube  39 , emitting light directly to outside the tube. Since the light emitted from LEDs  20  has a relatively small angle, the distribution of the light is rather non-uniform. This gives rise to a “spotty effect”. The surface of the shell  30  may be roughened or a diffuser may be added to scatter the light. However, such arrangements will substantially reduce the optical output. Furthermore, there is limited thermal management that can be done in the existing commercial LED light tubes due to the simplicity of mechanical structure. 
     SUMMARY OF THE INVENTION 
     In general terms the present invention proposes a light tube comprising a plurality of point light sources (e.g. LEDs) in which the point light sources are placed in thermal communication with a metal member for dissipating heat; and/or in which the point light sources are arranged to direct light laterally into a plurality of light guides and the light guides are arranged to reflect the light out of the light tube. In this way heat may be effectively dispersed and a smooth non-spotty distribution of light can be achieved. 
     A first aspect of the present invention provides a light tube comprising a plurality of segments arranged end to end along the length of the light tube; each segment comprising transparent or translucent rod having two ends, a heat spreader at each end of the rod and one or more LEDs mounted to each heat spreader for directing light into the rod; the heat spreaders being mounted to a metal member for dissipating heat; the light tube further comprising one or more reflectors for reflecting light out of the light tube. Preferably each rod has an LED at each of its ends. Each rod may have more than one LED at each end. 
     As the segments are arranged end to end and each segment has at least one LED at either end, the LEDs are distributed throughout the tube. Further, as the light is directed laterally into the rods and then reflected outward, the output light is relatively uniform. This is in contrast to the situation shown in  FIG. 1  where the LEDs face the front side of the tube and project light directly out of the tube. 
     The rods act as light guides and may be made of any suitable material, for example plastic, glass or acrylic. The reflectors are preferably reflective coatings of the rods. Alternatively, or in addition, the light tube may have reflectors external to the rods. 
     Preferably the metal member is a metal jacket and the heat spreaders in the different segments are all mounted to the same metal jacket. The metal jacket may provide structural support for the light tube. 
     Preferably the segments are arranged periodically along the length of the light tube and preferably are of the same length. This further enhances the uniform nature of the output light. 
     A second aspect of the present invention provides a light tube comprising a plurality of light guides and a plurality of point light sources (e.g. LEDs), each said point light source being associated with a respective light guide and arranged for directing light into said respective light guide. The point light sources are in thermal communication with a metal member for dissipating heat. As the point light sources are in thermal communication with the metal member, heat generated by the point light sources may be communicated to (e.g. through conduction) to the metal member and then dissipated (e.g. into the atmosphere or a heat sink from the metal member surface). Preferably at least two of the point light sources are mounted (directly or indirectly) to the same metal member; more preferably all of the point light sources are mounted to the same metal member. 
     Preferably each light guide as two ends and at least one point light source is mounted to each end of the light guide (for injecting light into the light guide). The light guides may be any suitable length. Lengths of 40 mm to 100 mm were tested, but other lengths could be used instead. The light guides may have substantially the same length in order to further smooth the distribution of light. 
     Preferably each point light source is mounted to a heat spreader and each heat spreader is mounted to a metal member for dissipating heat. Preferably the heat spreader has a higher thermal conductivity than the thermal conductivity of the metal member. For example, the heat spreader may be made copper, while the metal member may be made of aluminum. This facilitates rapid transfer of heat from the point light source to the metal member. Generally the metal member has a much larger surface area than the heat spreader; the rate of heat transfer is the same in both materials, but the density of heat transfer higher in the heat spreader. This facilitates transfer of heat from the small surface area of the discrete point light source to the larger surface area on the metal member from where it can be dissipated to the air or otherwise. 
     Preferably all of the heat spreaders are mounted to the same metal member. The metal member may be a metal jacket. Preferably the metal jacket is approximately semi-circular in cross section. It may provide structural support for the whole light tube. Preferably each light guide has a heat spreader and associated LED at each end. 
     Each heat spreader has two opposite sides and preferably at least one point light source (e.g. LED) is mounted to each side. The point light source may be mounted to the heat spreader by via a substrate. 
     Preferably there is a plurality of light guides along the length of the light tube and a heat spreader is provided between each light guide. The light guides are preferably mounted in a transparent or translucent casing (e.g. glass or plastic). The casing may have one or more end caps and electrical connections for conveying electricity to the discrete point light sources. 
     Each light guide preferably has a reflector for reflecting light out of the light tube. The reflector is preferably a reflective coating of the light guide. For example, one side of the light guide may have a reflective coating such that light is reflected out of the opposite (non-coated) side. Additionally, or alternatively, a reflector external to the light guide may be provided. The external reflector may, for example, be an inner surface of the metal jacket. 
     The light guides may, for example, be translucent or transparent rods; preferably plastic, acrylic or glass rods. The light guides may have any cross sectional shape; for example: circular, rectangular, square, diamond, crescent or semi-circle. 
     The light tube may contain two or more parallel rows of light guides. At least one, and preferably both, end(s) of each light guide has a recess for accommodating a point light source such as an LED. 
     The point light sources may be arranged at one or both ends of each light guide. The light guides are preferably arranged periodically along the length of the light tube so as to form a plurality of segments of the light tube. They preferably have substantially the same length so as make the output light even more uniform by uniform distribution of the light sources along the length of the light tube. 
     A third aspect of the present invention provides a light tube comprising at least first and second light guides arranged end to end along a length of the light tube; the first light guide having a first end and a second end; the second light guide having a first end and a second end; the second end of the first light guide facing the first end of the second light guide; a first point light source at a first end of the first light guide; a second point light source between the second end of the first light guide and the first end of the second light guide and a third point light source at the second end of the second light guide. 
     The first point light source directs light into the first light guide; the second point light source directs light into the first and/or second light guides and the third point light source directs light into the second light guide. In this way light is distributed evenly as there is a light source in between the first and second light guides and at their remote ends. 
     Preferably the light tube comprises three or more light guides arranged end to end, each light guide having two ends and point light sources being provided at each end of each said light guide. There may be four or more light guides arranged end to end. Preferably the light guides have the same length as each other. 
     Preferably the light guides have a recess at their ends for accommodating a point light sources, such as an LED. 
     A fourth aspect of the present invention provides a lamp or other illumination apparatus comprising the light tube of any one of the first to third aspects of the present invention. 
     Features of any of the above aspects of the invention may be combined together. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred embodiments of the present invention will now be described, by way of example only, with reference to the appended drawings, in which:— 
         FIG. 1  is a schematic drawing showing a conventional LED light tube; 
         FIG. 2(   a ) is a schematic drawing showing a longitudinal cross-section of a light tube according to a first embodiment of the present invention; 
         FIG. 2(   b ) is a schematic drawing showing a desk lamp having a light tube according to an embodiment of the present invention; 
         FIG. 2(   c ) is a more detailed longitudinal cross-section of a light tube according to an embodiment of the present invention similar. 
         FIG. 2(   d ) is a close-up top view of Region A denoted in  FIG. 2(   c ); 
         FIG. 2(   e ) is a close-up side view of Region A denoted in  FIG. 2(   c ); 
         FIG. 2(   f ) is the cross-sectional view of the light tube at the location B-B denoted in  FIG. 2(   c ); 
         FIG. 2(   g ) is a schematic side view of the backside of the shell of a light tube; 
         FIG. 3(   a ) is a schematic view of the cross-section of the light tube, according to the first embodiment of the present invention; 
         FIG. 3(   b ) is a schematic view of the cross-section of the light tube, according to a second embodiment of the present invention; 
         FIG. 3(   c ) is a schematic view of the cross-section of the light tube, according to the third embodiment of the present invention; 
         FIG. 3(   d ) is a schematic view of the cross-section of the light tube, according to the fourth embodiment of the present invention; 
         FIG. 3(   e ) is a schematic view of the cross-section of the LED light tube, according to the fifth embodiment of the present invention; 
         FIG. 3(   f ) is a schematic view of the cross-section of the LED light tube, according to the sixth embodiment of the present invention; and 
         FIG. 3(   g ) is a schematic view of the cross-section of the LED light tube, according to the seventh embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     References are now made to the drawings, in which the same reference numerals are used throughout the different drawings to designate the same or similar components. 
       FIG. 2  is a schematic drawing of a light tube  700  according to a first embodiment of the present invention. The light tube comprises two casing tubes or “shells”  701   a  and  701   b , which are fixed together side by side. The casings are transparent or translucent and may, for example, be made of glass or plastic. The light tube  700  further comprises a plurality of light guides in the form of transparent or translucent rods  706  which are fitted inside the casing  701 . The rods may, for example, be made of plastic, acrylic or glass. 
     The light guides  706  are arranged end to end along the length of the light tube and form segments of the light tube. Each light guide  706  has two opposite ends and a discrete point light source  705  (e.g. an LED) mounted to or accommodated at each end. An end cap  707  at one end of the light tube provides electrical connections for supplying power to the point light sources. 
     As can be seen in  FIG. 2  ( a ) and  FIG. 2  ( c ) the LEDs  705  are installed facing along the center line of the tube, rather than facing outward directly as in  FIG. 1 . The LEDs  705  are thus arranged to direct light laterally into their associated light guides  706  with the injected light having a main component parallel to the length of the light tube. One or more reflectors are arranged to reflect light out of the sides of the light guide in a direction with a component perpendicular to the length of the light tube. 
     This is best shown in  FIG. 2(   f ), which illustrates how light from the LEDs is reflected out of the light tube by reflectors  708  and  711 . Further, the light guide is mounted in a metal jacket  702  which has a reflector  711  on its inner surface. The light from the LED  705  is reflected outwards of the opposite side of the light tube by reflectors  708  and  711  as shown by the arrows in  FIG. 2(   f ). The reflector  708  may be a reflective coating, such as a spray on coating, on the exterior surface of the light guide facing the metal jacket. The coverage area of the reflective coating  708  may be adjusted in order to achieve different light emitting profiles. The surface of the light guides  706  may be roughened to adjust the optical distribution. The reflector  711  may be a coating on the inner surface of the metal jacket  702 . 
     The light guides  706  and reflectors  708 ,  711  improve the optical performance of the light tube. The design may provide a uniform light distribution and eliminate the “spotty effect” which is a major drawback of traditional LED light tubes of the type shown in  FIG. 1 . 
       FIG. 2  ( b ) shows a desk lamp  800  in which the light tube  700  is used. The light tube faces a reflecting portion  810  of the lamp whereby light is reflected outward. 
       FIG. 2  ( c ) shows the structure of the light tube  700  in more detail. There are first  701   a  and second  701   b  casings made of transparent or translucent material such as plastic or glass. Each casing contains a row of light guides  706  extending end to end along its longitudinal length. Each casing further contains a respective metal jacket  702 , of semi-circular or crescent cross section, which supports the light guides. 
     A plurality of heat spreaders  703  are mounted to the metal jacket  702 . The light guides  706  are positioned between the heat spreaders  703 , so that each end of a light guide is adjacent a heat spreader. Thus in  FIG. 2(   c ) there is a first light guide  706   a  between first and second heat spreaders  703   a ,  703   b  and a second light guide  706   b  between the second  703   b  and third  703   c  heat spreaders. The heat spreaders  703  are in thermal communication with the metal jacket  702 . The heat spreaders  703  have two opposite sides and an LED  705  mounted on one or both sides. 
     As shown in  FIG. 2(   c ) and  FIG. 2(   d ), surface-mount type LEDs  705  are mounted via substrates  710  to the heat spreaders  703 . The heat spreaders  703  are mounted to the metal jacket  702  by one or more screws  704 . The heat spreaders  703  serve to the support the LEDs and conduct heat from the LEDs to the metal jacket  702 . 
     The metal jacket  702  plays an important role in the design. It serves not only as a mechanical support for the light guide tubes and LEDs, but also for dispersing the heat generated by LEDs quickly. The LEDs are mounted to the metal jacket via a heat spreader which has a higher thermal conductivity than the metal jacket. For example, the heat spreader may be made of copper and the metal jacket of aluminum. This facilitates rapid conduction of the heat from the LED through the heat spreader to the metal jacket, from where it is dissipated into the surrounding atmosphere. This is especially a great advantage when high power LEDs are applied, which is an obvious trend in the LED illumination industry. The metal jacket is contained in the transparent or translucent shell casing  701 . As shown in  FIG. 2(   f ), openings  750  may be provided in the shell casing  701 , at the side to which the metal jacket  702  is attached, in order to improve heat dissipation. 
     The LEDs are mounted to a heat spreader and accommodated in a recess or cavity  712  in the end of the light guide  706 , as shown in  FIG. 2  ( d ) and  FIG. 2  ( e ). Each light guide  706  preferably has a lathed cavity  712  at each end for accommodating an LED. Electrical connectors, such as wires  709 , are used to connect the LEDs  705  in series and to the end cap  707 . The wires extend from the end cap to the nearest LED and then along the light guide to the LED on the opposite side of the light guide. Wires also connect LEDs in adjacent light guides and may pass through a hole in the heat spreader  703 , as shown in  FIG. 2  ( d ). 
     The design of the light tube is flexible. The light guides  706  form divisions or segments which can be combined in different ways to form the LED light tube. Generally the light guides will be of the same length, so that the divisions are periodic. The light guides can be combined in double-lines to form a twin tube structure, as shown in  FIG. 2(   a ) and  FIG. 3  ( a ). In this case there are two casings  701  which may be fixed together. 
     Alternatively the light guides  706  could be lined up in a single line to form a single line tube similar to a traditional fluorescent light tube. The light guides  706  are not limited to a cylindrical shape, but may have a variety of cross-sectional shapes. For example, in  FIG. 3(   a ) the cross sectional shape is circular, while in  FIG. 3(   b ) it is rectangular and in  FIG. 3  ( c ) it is diamond shaped. 
     The number of LEDs may be varied as well.  FIG. 3(   d ) shows a crescent shaped light guide having two LEDs accommodated at each end (total of four LEDs). In  FIG. 3  ( d ) the light guide is accommodated in a single large casing  701 . 
     While in  FIGS. 3(   a ) to  3 ( b ) the light guides are housed in two parallel casings  701   a  and  701   b , they may alternatively be housed as parallel multiple rows in one larger casing  701  as shown in  FIG. 3(   e ),  FIG. 3(   f ), and  FIG. 3(   g ). The light-distribution can be adjusted by applying light guides of different shapes. This brings a great ease and flexibility to the design of the light tube. 
     Preferred embodiments of the present invention have been described above with reference to the drawings. However, a person skilled in the art may be able to make variations and modifications to the above described examples, while still remaining within the scope of protection which is defined by the claims.