Abstract:
A light guide panel assembly ( 10 ) comprises a translucent panel ( 12 ), and at least one light-emitting device ( 40 ) (such as an LED) which projects a divergent beam of light into the panel from a light-receiving edge ( 52 ) of the panel. A heatsink ( 22 ) may be provided, with a compressible, thermally-conductive element ( 20 ) disposed in compression against the heatsink and so as to urge the light-emitting device(s) against the light-receiving edge of the panel and to provide a thermal pathway between the light-emitting device(s) and the heatsink. The compressible element therefore serves the dual purposes of holding the light-emitting device in place and also assisting in conducting heat away from it. The rear face ( 28 ) of the panel specularly reflects and scatteringly reflects light incident thereon from within the panel with a ratio of specular to scattering reflection which is preferably non-uniform across the rear face of the panel so as to cause substantially uniform illumination of the front face ( 58 ) of the panel.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This invention relates to light guide panel assemblies. 
         [0002]    Such panel assemblies can be used to provide illuminated signs, such as road signs, which may convey a message by virtue of their shape or by masking provided on the panel assembly. They may also be used for back-lighting of displays such as liquid crystal computer screens or television screens. 
         [0003]    A conventional panel assembly of this type comprises a translucent panel (for example of acrylic) and at least one light-emitting device (such as an light emitting diode, or LED) arranged for producing a divergent beam of light and projecting the light into the panel from a light-receiving edge of the panel. As the light passes through the panel, it is internally reflected by the front and rear faces of the panel and at least some of the light is transmitted by at least one light-emitting surface portion of at least one of the faces so as to illuminate that surface portion. At least one other surface portion of that face may be opaque so that an illuminated pattern is formed on that face. Alternatively, substantially all of one or each face may be light-emitting, so that the whole face (which may have a particular shape such as a direction arrow) is illuminated. 
         [0004]    Although high-power LEDs are reasonably efficient at converting electrical energy into light energy, they do produce heat and can become hot. The hotter the temperature at which an LED is run, the shorter its life. It is therefore desirable to conduct the heat away from the LED. 
         [0005]    It is desirable that the light emitting device(s) and any other electrical components contained in the panel assembly are protected against the ingress of moisture. 
         [0006]    It is also desirable that the light-emitting face of the panel assembly provides uniform illumination (other than in areas where the face is intended to be masked). However, generally speaking, the further the light travels through the panel from the light-emitting device, the less intense it becomes. Also, if the light-emitting device(s) are not properly positioned, for example being canted over, they can produce aberrations in the illumination. 
       BRIEF SUMMARY OF THE INVENTION 
       [0007]    An aim of the present invention, or at least of specific embodiments of it, is to provide a light guide panel assembly which can: conduct heat effectively away from the light-emitting device(s); establish and maintain proper positioning of the light emitting device(s); provide a seal against ingress of moisture to the light-emitting devices; provide substantially uniform illumination of the light-emitting face of the panel assembly (other than in areas where the face is intended to be masked); and accomplish all of the aforesaid in a simple, convenient and inexpensive manner. 
         [0008]    In accordance with a first aspect of the present invention, there is provided a light guide panel assembly comprising: a translucent panel; a light-emitting device (such as an LED) arranged for producing a divergent beam of light and projecting the light into the panel from a light-receiving edge of the panel; a heatsink; and a compressible, thermally-conductive element disposed in compression against the heatsink and so as to urge the light-emitting device against the light-receiving edge of the panel and to provide a thermal pathway between the light-emitting device and the heatsink. The compressible element therefore serves the dual purposes of holding the light-emitting device in place and also assisting in conducting heat away from it. In some embodiments, it can also serve as a seal to prevent ingress of moisture to the light-emitting device. 
         [0009]    The light emitting device is preferably one of a plurality of such light-emitting devices mounted on one face of a common, elongate circuit board, and the compressible element is preferably elongate and is compressed against the heatsink and an opposite face of the circuit board. 
         [0010]    The light-receiving edge of the panel may include at least a portion which is not straight, in which case the circuit board is preferably flexible and conforms to the shape of the light-receiving edge of the panel. This conveniently enables complex shapes of panel assembly to be provided. 
         [0011]    The circuit board may have connection pads at each end of the circuit board, so that a plurality of the circuit boards can be daisy-chained together in a single panel assembly and/or in separate panel assemblies. 
         [0012]    The light-receiving edge of the panel is preferably formed by a side wall of a groove formed in one face of the panel, the light-emitting device(s) and compressible element being disposed in the groove, therefore enabling the opposite face of the panel to be uninterrupted. In this case, the compressible element is preferably disposed in compression against a bottom wall of the groove and against an opposite side wall of the groove, with the heatsink closing off the groove at the rear face of the panel. 
         [0013]    A front face of the panel preferably has at least one light-emitting surface portion which specularly reflects and transmits light incident thereon from within the panel, and a rear face of the panel preferably specularly reflects and scatteringly reflects light incident thereon from within the panel. This enables the panel to act as a light guide and to illuminate the front face. 
         [0014]    The rear face of the panel preferably specularly reflects and scatteringly reflects light incident thereon from within the panel with a ratio of specular to scattering reflection which is non-uniform across the rear face of the panel, preferably so as to cause substantially uniform illumination of the front face. 
         [0015]    This latter feature may be provided independently of other features of the first aspect of the invention. Therefore, in accordance with a second aspect of the invention, there is provided a light guide panel assembly comprising a translucent panel and a light-emitting device arranged for producing a divergent beam of light and projecting the light into the panel from a light-receiving edge of the panel, wherein: a front face of the panel has at least one light-emitting surface portion which specularly reflects and transmits light incident thereon from within the panel; and a rear face of the panel specularly reflects and scatteringly reflects light incident thereon from within the panel with a ratio of specular to scattering reflection which is non-uniform across the rear face of the panel. 
         [0016]    The non-uniform reflection ratio may be provided by the rear face of the panel having a smooth surface interrupted by surface irregularities having a density which is non-uniform across the rear face of the panel. In particular, the surface irregularities may have depths which are non-uniform across the rear face of the panel and/or pitches relative to adjacent irregularities which are non-uniform across the rear face of panel. The surface irregularities may be formed by grooves in the rear surface of the panel. The grooves may be substantially straight and parallel, especially in the case of a rectangular panel illuminated along one edge, and/or they may be curved and/or non-parallel and/or non-concentric in the case of more complex shapes of panel. 
         [0017]    The panel assembly preferably further includes a scatteringly reflective surface disposed against the rear face of the panel so that any light that escapes through the rear face is reflected back to the rear face. In the case where a heatsink is provided, the scatteringly reflective surface may be provided by the heatsink. 
         [0018]    The or each light-emitting surface portion of the front face of the panel preferably (a) specularly reflects substantially all light incident thereon from within the panel at an angle of incidence greater than the critical angle; and (b) transmits substantially all light incident thereon from within the panel at an angle of incidence less than the critical angle. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         [0019]      FIG. 1  is an exploded isometric view of a first embodiment of light guide panel assembly with its centre portion cut away; 
           [0020]      FIG. 2A-C  are sectioned side views of the light guide panel assembly of  FIG. 1 , exploded in  FIG. 2A , partly assembled in  FIG. 2B , and fully assembled in  FIG. 2C ; 
           [0021]      FIGS. 3A-D  are side views of the panel assembly of  FIGS. 1 and 2  showing sample light rays resulting in emission of light from four different regions on a front face of the panel assembly; 
           [0022]      FIGS. 4A  &amp; B are side views on a larger scale showing two different arrangements of surface irregularities on two portions of the rear face of the panel assembly; 
           [0023]      FIGS. 5A  &amp; B are a sectioned side view and a sectioned underplan view, respectively, of a second embodiment of light guide panel assembly; 
           [0024]      FIGS. 6A  &amp; B are a sectioned side view and a sectioned underplan view, respectively, of a third embodiment of light guide panel assembly; and 
           [0025]      FIG. 7  is an isometric view of a modified printed circuit board assembly for use in the embodiments of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0026]    Referring to  FIGS. 1 to 2C , the first embodiment of light guide panel assembly  10  comprises a panel  12 , a printed circuit board (PCB) assembly  14 , a two-core connection lead  16 , a blanking plug  18 , a compression element  20 , a heatsink  22  and a backing sheet  24  (not shown in  FIG. 1 ). 
         [0027]    The panel  12  is generally rectangular and is made of acrylic. A rabbet  26  is formed in the rear face  28  and one edge  30  of the panel  12 . A deep groove  32  is formed in the rabbeted portion of the panel  12  and extends almost to the ends  34  of the panel  12 . The innermost edge  52  of the deep groove  32  is preferably polished Smaller grooves  36  are formed between the ends of the deep groove  32  and the ends  34  of the panel  12 . 
         [0028]    The PCB assembly  14  comprises a flexible PCB  38  made from Kapton® or similar material. The PCB  38  is in the form of a strip having a length slightly less than the length of the groove  32  in the panel  12  and a width slightly less than the depth of the groove  32  in the panel  12 . A plurality of flat-faced surface-mount LEDs  40  are mounted on the PCB  38 . The PCB  38  has a pair of connecting pads  42  at each of its ends. The PCB  38  may also carry other components  44  such as voltage-dropping resistors and/or current regulators. The LEDs  40 , connecting pads  42  and other components  44  are connected by tracks (not shown) of the PCB  38  so that when the connecting pads  42  at one end of the PCB  38  are connected to a electrical supply of a particular voltage and polarity, an appropriate electrical current passes through each LED  40  to illuminate it, and also the supply voltage is fed to the connecting pads  42  at the opposite end of the PCB  38 . 
         [0029]    The connection lead  16  has a moulded resilient grommet  46  adjacent one end which is a tight fit in the smaller groove  36  at one end  34  of the panel  12  and which naturally protrudes slightly above the groove  36 . During assembly of the panel assembly  10 , the cores  48  of the lead  16  are soldered to the connecting pads  42  at the adjacent end of the PCB  38 . The other end of the lead  16  is connected to an electrical supply, such as a battery pack or a mains-to-DC adapter. The blanking plug  18  is a tight fit in the smaller groove  36  at the opposite end  34  of the panel  12  and naturally protrudes slightly above the groove  36 . As an alternative to using the blanking plug  18 , the smaller groove  36  at that end  34  of the panel  12  may be omitted. Alternatively, a further connection lead may be fitted similarly to the connection lead  16  and soldered to the connecting pads  42  at the adjacent end of the PCB  38  so that electricity can be fed to another light guide panel assembly. 
         [0030]    The compression element  20  is a tubular O-rope formed from doped silicone rubber which is resilient, has a high thermal conductivity and is waterproof. For example, the compression element may be made of Primasil PR910/1388 available from Primasil Silicones, HR4 8QU, United Kingdom, and having a thermal conductivity of about 1.5 W·m −1 ·K −1 . The length of the compression element  20  is slightly longer than the length of the deep grove  32 . 
         [0031]    The compression element  20  has an outer diameter which is slightly greater than the depth of the deep groove  32  and slightly greater than the width of the deep groove  32  less the height of the PCB assembly  14 . 
         [0032]    The heatsink  22  is formed from a plate of aluminium alloy having a width equal to the width of the rabbet  26  and a length equal to the length of the panel  12  between its ends  34 . The heatsink  22  may be plain or (as shown) it may be formed with fins  50  on one face. 
         [0033]    The backing sheet  24  is formed from plastics film having a matt-white self-adhesive surface. The backing sheet  24  is the same size as the rear non-rabbeted portion of the panel  12 . 
         [0034]    Referring in particular to  FIGS. 2A  &amp; B, during assembly of the panel assembly  10 , the PCB assembly  14  is placed in the deep groove  32  with the light-emitting faces of the LEDs  40  abutting the polished innermost edge  52  of the deep groove  32 , and the grommet  46  of the connection lead  16  and the blanking plug  18  are squeezed into the smaller grooves  36 . The compression element  20  is then squeezed into the deep groove  32  so that it abuts the rear of the PCB assembly  14 , the outermost edge  54  of the groove  32  and the bottom  56  of the groove  32 . As can be seen in  FIG. 2B , the compression element  20  naturally protrudes from the deep groove  32 . Then, adhesive is applied to the plain face of the heatsink  22  and/or to the rabbet  26 , and the heatsink  22  is pressed into the rabbet  26  so as to compress the compression element  20  into the deep groove  32 , as shown in  FIG. 2C , and also compress the grommet  46  and blanking plug  18  into the smaller grooves  36 . External pressure is maintained until the adhesive has cured. Also, as shown in  FIG. 2C , the backing sheet  24  is adhered to the rear non-rabbeted portion of the panel  12 . 
         [0035]    It will therefore be appreciated that the thermally-conductive compression element  20  urges itself into good contact with the rear of the PCB assembly  14  and the heatsink  22  so as to provide a good thermal path between the two. The compression element  20  also urges the light-emitting faces of the LEDs  40  into contact with the edge  52  of the groove  32 . Moreover, the compression element  20  serves to prevent any moisture which enters between the panel  12  and the heatsink  22  at the edge  30  of the panel  12  from reaching the PCB assembly  38 . 
         [0036]    In operation, the LEDs  40  each produce a divergent beam having a viewing half-angle of, for example, 30 to 40 degrees and possibly greater. The acrylic material of the panel  12  has a refractive index of about 1.49, and therefore the critical angle at the acrylic-air interface is about 42 degrees. The front face  58  of the panel  12  is polished so that:
       substantially all light incident on the front face  58  from within the panel  12  is specularly reflected if the angle of incidence is greater than the critical angle;   substantially all light incident on the front face  58  from within the panel  12  is transmitted out of the panel if the angle of incidence is less than the critical angle; and   no substantial amount of light incident on the front face  58  from within the panel  12  is scatteringly reflected.
 
By contrast, the rear face  28  of the panel  12  has a degree of mattness which is non-uniform over the rear face so that, depending on the position on the rear face  28 :
   a proportion of light incident on the rear face  28  from within the panel  12  is specularly reflected; and   a proportion of light incident on the rear face  28  from within the panel  12  is specularly reflected either directly or by being transmitted through the rear face  28  and then being specularly reflected back to the panel  12  by the matt white surface of the backing sheet  24 .       
 
         [0042]    The outer edge  60  of the panel  12  remote from the LEDs  40  may be substantially totally absorbent to light or may be slightly reflective. The sides  34  of the panel are preferably reflective. 
         [0043]    Referring to  FIGS. 3A-D  and considering only light rays travelling in the plane of the paper, it will be seen that at a point  62  ( FIG. 3A ) on the rear face  28  of the panel  12  adjacent the LED  40 , substantially the only incident light is a high intensity ray  64  received directly from the LED  40 . A proportion of this light (i.e. of lower intensity than the ray  64 ) is specularly reflected, as shown by ray  66 , and a proportion is scatteringly reflected, as shown by the rays  68 , and illuminates the panel  12  in the region  69 . By contrast, at a point  70  ( FIG. 3D ) on the rear face  28  of the panel  12  adjacent the far edge  60  of the panel  12 , the incident light comprises:
       a ray  72  received directly from the LED  40 ;   a ray  74  received after one specular reflection from the front face  58 ;   a ray  76  received after one reflection from the rear face  28  and one specular reflection from the front face  58 ; and   a number of other rays  78  received after multiple reflections from the front and rear faces  58 , 28  of the panel  12 .       
 
         [0048]    Each of these rays  72 - 78  is of lower intensity than the ray  64  received at point  62   
         [0049]    ( FIG. 3A ) due to the greater distance from the LED  40 . Furthermore, the intensity of the rays  76 , 78  is further reduced due to the scattering reflections which occurred at the rear face  28  of the panel  12 . Preferably, at the point  70  adjacent the far edge  60 , the rear face  28  of the panel  12  is totally matt, so that none of the rays  72 - 78  is specularly reflected, in which case all of the rays  72 - 78  are scatteringly reflected as shown by the rays  80 , and illuminate the panel in the region  82 . 
         [0050]    To a first order approximation, the intensity of the ray  64  incident on the point  62  adjacent the LED  40  is substantially greater than the total intensity of the rays  72 - 78  incident on point  70  adjacent the far edge  60  of the panel  12 . Therefore, the rear face  28  of the panel  12  needs to be substantially more glossy (i.e. specularly reflect a greater proportion of light) at point  62  than at point  70  in order that the illumination provided by the rays  68  in region  69  is substantially equal to the illumination provided by the rays  80  in region  82 . Indeed, to a first order approximation, the required glossiness of the rear face  28  of the panel  12  is related to the inverse of the distance from the LED  40 . However, other factors affect uniform illumination of the front face  58  of the panel  12 , notably the radiation pattern of the LEDs  40  (i.e. the variation of intensity with viewing angle) and edge effects. Therefore, the non-uniformity of glossiness of the rear face  28  of the panel  12  that is required in order to achieve uniform illumination of the front face  58  of the panel  12  is best determined by trial and error. 
         [0051]    The non-uniform glossiness of the rear face  28  of the panel  12  may be achieved in a number of ways. For example, discrete formations may be provided in the rear face  28 , such as grooves cut or laser-engraved into the rear face  28 .  FIG. 4A  shows an example where parallel grooves  84  of a particular depth are formed in the rear face  28 , and the pitch between adjacent grooves progressively decreases from the point  62  near the LED  40  to the point  70  near the far edge  60  of the panel  12  parallel grooves  84  of the particular depth but a smaller pitch are formed in the rear face  28 .  FIG. 4B  shows an example where parallel grooves  84  of a particular pitch are formed in the rear face  28 , and the depth of the grooves  84  progressively increases from the point  62  near the LED  40  to the point  70  near the far edge  60  of the panel  12 . It will be appreciated that the arrangements of  FIGS. 4A and 4B  may be combined to provide for surface irregularities with both a non-uniform pitch and a non-uniform depth. 
         [0052]    Instead of providing discrete regular formations, the glossiness of the rear face  28  may also be rendered non-uniform by other treatments, such as rubbing the face  28  with a rotating wire wheel with varying pressure being applied, rubbing the face  28  with varying grades of abrasive paper, or sand-blasting the rear face  28  with varying exposure times. 
         [0053]    In the first embodiment described above, the panel  12  is rectangular. However, the panel may have any regular or irregular shape. 
         [0054]    For example,  FIGS. 5A  &amp; B show a second embodiment of the invention with a circular panel  12  having a circular groove  32  for the PCB assembly  14  and compression element  20 . Because the PCB  38  and compression element  20  are flexible, they can both readily be formed into circles. The heatsink  22  of the second embodiment is a plain circular disc which is secured to the rear of the panel  12  by screws  86 . It will be noted that because the compression element  20  engages both the plate of the heatsink  22  and the outer edge of the groove  32 , the compression element  20  prevents the ingress of any moisture into the region occupied by the PCB assembly  14 . At least the face of the plate of the heatsink  22  facing the panel  12  is finished as matt white so as to serve the same function as the backing sheet  24  in the first embodiment of the invention. The connection lead (not shown) in the second embodiment may enter the panel through a grommet in the plate of the heatsink  22  or in the outer wall of the groove  32 . Because of the circular shape of the panel  12 , the required non-uniformity of the glossiness of the rear face  28  of the panel  12  in order to achieve uniform illumination may be less pronounced than in the case of the first embodiment. The non-uniformity will be rotationally symmetrical through any angle about the centre of the panel  12 . 
         [0055]      FIGS. 6A  &amp; B show a third embodiment of the invention which is similar to the second embodiment except that the panel  12  is elliptical. In this case, the required non-unifomity of the glossiness of the rear face  28  of the panel  12  will be more complex than in the first and second embodiments but can be ascertained by trial and error. 
         [0056]      FIG. 7  shows more detail of a PCB assembly  14  which may be used in the embodiments of the invention. As described above, the PCB assembly  14  comprises a flexible PCB  38  made from Kapton®. The PCB  38  is in the form of a strip many metres long and is divided up into identical sections  88  (three of which are shown in  FIG. 7 ) separated by perforated break lines  90 . Each section  88  comprises two connecting pads  42 A,B at one end and two connecting pads  42 C,D at the opposite end. In each section  88 , the tracks of the PCB  38  connect: pad  42 A to pad  42 C; pad  42 B to pad  42 D; and pad  42 B to pad  42 C via a plurality of LEDs  40  and a current regulator  44  in series. Also, across each break line  90 , the tracks of the PCB  38  connect: pads  42 A and  42 C of the adjacent sections  88 ; and pads  42 B and  42 D of adjacent sections. The PCB assembly  14  can therefore be cut or snapped apart along a break line  90  to form any desired length of assembly  14  which is an integer multiple of the length of each section  88 . The electricity supply can then be connected to the pads  42 A,B at one end of the length of PCB assembly  14 , and the pads  42 C,D at the opposite end may be left unconnected, connected to another PCB assembly  14  or connected back to the pads  42 A,B at the first end in a ring. Each current regulator  44  serves to regulate the current through the LEDs  40  in its section  88  to a desired value provided that the voltage supplied to the section  88  is sufficiently high for the regulator  44  not to drop out and provided the supply voltage is not too high to overload the regulator  44 . Uniform illumination of the LEDs  40  can therefore be achieved despite variation in the supply voltage, voltage drops along the length of the PCB assembly  14 , and the number of sections  88  in the PCB assembly  14 . 
         [0057]    In a modification to the PCB assembly  14  of  FIG. 7 , the current regulators  44  are adjustable, and further interconnected connecting pads are provided at each end of each section  88  for receiving a control voltage and supplying the control voltage to each current regulator  44 . 
         [0058]    In all of the embodiments described above, the panel  12  may be clear or tinted. Also, the front face  58  of the panel  12  may be unmarked so that substantially the whole of the panel  12  is illuminated. Alternatively, the front face  58  of the panel may be masked with an opaque or contrastingly-coloured graphic in a conventional manner The colours of the LEDs  40  may be chosen to suit the colours of the graphic. The LEDs  40  may be driven to provide a flashing sign, and a dimming arrangement may be provided, which may be responsive to ambient light. The panel assembly  10  may be incorporated into another structure. 
         [0059]    It should be noted that the embodiments of the invention has been described above purely by way of example and that many other modifications and developments may be made thereto within the scope of the present invention.