Abstract:
An apparatus and method for assembling a display device to which is coupled a light guide that has smooth surfaces for light-transmission and at least one surface that is for light scattering and internal reflection. The method includes depositing a reflective coating on that surface, and installing the display device with coupled light guide in an interior of a meter housing or casing. A portion of the casing interior is filled with a component, such as epoxy, for sealing the case interior to protect the display device and light guide against water and dust penetration. All surfaces of the light guide are protected from direct contact with the epoxy. This approach greatly simplifies the assembly process and yields a robust display that also prevents light leakage from the light guide.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to a backlit display device such as a liquid crystal display (LCD), and in particular to light guide assembly that provides the back light for the display. 
       BACKGROUND AND SUMMARY OF THE INVENTION 
       [0002]    Meters used on motorcycles include digital displays of running time (hours), engine temperature and other information. Since the motorcycles may be of the trail-type for traversing rough terrain, the meters must be robust to withstand vibration and shock. The meters must also be well sealed to prevent penetration of fine dust and liquid into the meter. 
         [0003]    The meters of interest here include, among other components, a digital LCD that is backlit by light that is provided by a light guide. In this case, the light guide is a transparent glass-like solid through which light propagates from one end of the guide to another. The light guide is coupled to the display and transmits light from a source, such as one or more light emitting diodes (LEDs) carried on a nearby printed circuit board, to the back side of the display for illuminating or “back lighting” the display for viewing through its transparent front or display side. The back lighting is especially helpful for viewing under low ambient light conditions. 
         [0004]    The meter components are housed in a casing that is mounted to the motorcycle. A cable is often included to deliver information from a remote sensor to the meter. An opening in the casing permits viewing of the front of the display. To enhance the robustness of the meter, the assembly process may include a liquid epoxy resin that is injected into the casing interior after the display, light guide, and associated circuit board are in place. The epoxy cures to encapsulate the components against penetration by damaging liquid or dust. The epoxy also serves a dual purpose of anchoring the components in the casing interior for securing the components in place, and for damping vibrations that are received by the meter casing to thus protect the interior components. 
         [0005]    One problem with the epoxy encapsulation technique just mentioned is ensuring that the epoxy does not come into direct contact with the light guide, which may be, for example, formed of optically-clear acrylic. Such contact with the exterior of the light guide will change the refraction characteristics of the outer surface of the guide so that light inside the guide (and propagating from the source to back light the display) will be absorbed by the epoxy. This absorption reduces the illumination of the display to unacceptably low levels. The absorbed, lost light is sometimes referred to as light that has “leaked” from the light guide. 
         [0006]    One prior solution to the forgoing problem is to place a reflective tape around the exterior of the surfaces that are intended to reflect light along the light guide. This approach, however, is time consuming and requires a labor-intensive assembly process. Moreover, given the complex shape of some light guides it is difficult to effectively apply the tape to all of the surface portions that may be exposed to contact with the later applied epoxy. 
         [0007]    The present invention is directed to a solution to the forgoing problems and provides, among other things, an innovative way of assembling a display device to which is coupled a light guide that has smooth surfaces for light-transmission and at least one surface that is configured for light scattering and internal reflection. The method includes depositing on that surface a reflective coating, and installing the display device with coupled light guide in an interior of a meter housing or casing. At least a portion of the casing interior is filled with an epoxy for sealing the display device with coupled light guide against water and dust penetration. All surfaces of the light guide are protected against direct contact with the epoxy or with any contaminant. This approach greatly simplifies the assembly process and yields a robust display without light leaking from the light guide. 
         [0008]    Other advantages and features of the present invention will become clear upon study of the following portion of this specification and drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a perspective view of an exemplary motorcycle meter that includes a display with which the light guide assembly of the present invention may be employed. 
           [0010]      FIG. 2  is an enlarged cross section of a meter that includes a display with coupled light guide assembly of the present invention. 
           [0011]      FIG. 3  is a perspective view of the back of a light guide component of the present invention. 
           [0012]      FIG. 4  is an enlarged detail view of the cross section of  FIG. 2  showing protuberances in the light guide for scattering and reflecting light toward an outlet surface of the light guide. 
           [0013]      FIG. 5  is a perspective view of the front of the light guide component of the present invention. 
           [0014]      FIG. 6  is a bottom plan view of the light guide component. 
           [0015]      FIG. 7  is a perspective view like  FIG. 5  but rotated 180 degrees about a vertical axis. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    Shown in  FIG. 1  is an exemplary motorcycle meter  20  that includes a display with which the light guide assembly of the present invention may be employed. The meter  20  houses internal components inside of a casing  22 , which can be two casing parts attached together. The casing includes an opening  23  on the side of the meter facing the viewer. The meter can include any of a variety of manual control buttons and knobs  26 . 
         [0017]    With reference to  FIG. 2 , a display device such as a liquid crystal display (LCD)  28  is mounted in the interior space  30  of the casing with the edges of the LCD abutting an opening  23  in the casing. A rectangular gasket  24  is present between the edges of the LCD  28  and the opening  23 . Other components of the meter  20  are also carried in this space  30 , including the meter&#39;s printed circuit board (PCB)  32  with circuit components mounted thereto, as well as a power supply, which in this instance can be a coin cell  34 . 
         [0018]    One of the components mounted to the PCB  32  is an array of light emitting diodes LEDs  36 , which may comprise three separate diodes, emitting blue-, red- and green-wavelength light. In this embodiment, there are two such LED arrays  36  (hereafter referred to as simply “LEDs”) carried on the PCB. 
         [0019]    The LCD  28  is backlit with light that is directed through a backlit face  38  on the inward-facing surface of the LCD, so that the display face  40  on the opposite surface of the LCD is sufficiently illuminated for viewing through the opening  23 . 
         [0020]    With reference to  FIGS. 2-7 , a light guide  42  formed in accordance with the present invention is coupled to the LCD  28  and transmits light from the LEDs  36  on the PCB to the backlit face  38  of the LCD  28 . Preferably, the light guide  42  is a transparent thermoplastic formed of PMMA (Poly(methyl methacrylate), the material is hereafter referred to as acrylic. 
         [0021]    The light guide  42  includes a downwardly extending pair of inlets  44  that each include a recess  45  ( FIG. 3 ) for receiving and enclosing therein an LED  36  that protrudes from the surface of the PCB  32 . As a result, light from the LEDs is directed into the core of the light guide for propagating along its length. 
         [0022]    The general path of the light propagating through the light guide  42  is depicted by arrows  46  in  FIGS. 2 and 4 . Generally, the light moves from the inlets  44  until encountering an angled surface  48  ( FIGS. 2 and 5 ) whence the light is reflected to travel through a generally plate-shaped part  50  of the light guide  42  that, when mounted to the casing  22  ( FIG. 2 ) extends across the entire area of the backlit face  38  of the LCD  28 . 
         [0023]    As indicated by the path arrow  46  the light propagating through the light guide  42  emanates from an outlet surface  52  of the light guide  42 . That surface  52  is adjacent to but slightly spaced from the backlit face  38  of the LCD owing to a peripheral rim  54  integrally formed in the light guide  42  to extend upwardly ( FIG. 2 ) and surround the outlet surface  52  like a four-sided frame. 
         [0024]    An adhesive is applied to the flat, contact surface  56  of the rim  54  to bond that surface to the edges of the backlit face  38  of the LCD  28 . This configuration provides a peripheral seal surrounding the spaced-apart junction of the light guide outlet surface  52  and the backlit face  38  of the LCD, thereby preventing fluid from moving between the backlit face and the outlet surface, which would degrade the quality of the display. 
         [0025]    With particular reference to  FIGS. 3 and 4 , the light guide  42  includes a rear surface  58  that is opposite and parallel to the light guide outlet surface  52 . The rear surface  58  includes an array of integrally formed protuberances  60  dispersed across the rear surface to protrude outwardly therefrom. In the present embodiment, the protuberances  60  are cylindrically shaped and act as light traps to capture some of the light that traverses the area of the plate-shaped part  50  of the light guide. The light captured or impinging on the traps (protuberances  60 ) is scattered and reflected generally toward and through the outlet surface  52  of the light guide and into the backlit face  38  of the LCD to thus serve as back light for the display as described above. 
         [0026]    It is noteworthy here that as the distance from the source of light, LEDs  36 , increases, (that is, right to left in  FIGS. 2 and 4 ) the diameters of the cylindrical protuberances  60  increases to for a progressively larger set of light traps to reflect correspondingly increasing amounts of the light toward the LCD  28 . This has the effect of compensating for inevitable losses of light intensity as the light propagates through the guide, and the result is a more uniformly illuminated display than would be the case if the protuberances were all same diameter. 
         [0027]    While the cylindrical-shaped, outwardly protruding protuberances  60  are described as preferred as light traps mentioned above, it is contemplated that other configurations of the light guide rear surface  58  may be employed for accomplishing the light scattering as discussed above. For instance, the rear surface need not be planar, and may be configured with inwardly protruding recesses or pockets of various regular and irregular shapes. Thus, as used here, the term protuberances can be considered as extending inwardly or outwardly, and have various shapes. 
         [0028]    As noted earlier, with all of the meter components assembled, including the light guide assembly and coupled display as just described. The remaining interior space  30  of the casing ( FIG. 2 ) is filled, as by injection, with a seal component  62 , which in this embodiment comprises a liquid, curable epoxy. A portion of the seal component  62  is depicted in  FIG. 2  as cross hatching, with the understanding that the seal component  62  flows into and fills all of the voids in the interior space  30  around the meter components inside the casing. The epoxy cures to encapsulate the components against penetration by potentially damaging liquid or dust. The epoxy also serves a dual purpose of anchoring the components in the casing interior for securing the components in place, and damping vibrations that are received by the meter casing to thus protect the interior components excessive vibration. 
         [0029]    As noted earlier, any light propagating through the acrylic light guide  42  can be lost (leaked) from the guide before reaching the guide&#39;s outlet surface  52  if the seal component  62  described above comes in direct contact with any surface of the guide. Accordingly, prior to applying the seal component  62 , and for that matter prior to assembling the light guide and related components, many of the exterior surfaces of the light guide  42  are coated with reflective material so that the reflective material is applied directly to the light guide, between the guide and the later-applied seal component. 
         [0030]    In a preferred embodiment the reflective material is a thin film of aluminum applied, for example, by vacuum deposition.  FIG. 5  depicts the light guide  42  with the outlet surface  52  facing upwardly. It is contemplated that this surface is protectively masked during the application of the reflective coating since that surface is to remain transmissive to light. With reference to  FIGS. 3 and 5 , the exterior surfaces  64  of the part of the guide that defines the inlets  44  receive the reflective coating, although the recesses  45  are masked to prevent coating the region where the light enters the guide from the source LEDs as discussed above. 
         [0031]    The contact surfaces  56  ( FIGS. 2 and 5 ) of the rim  54  of the light guide are also coated with the reflective material before being bonded to the display as describe above. It is noteworthy here that of the four sides of the rim  48 , the reflective coating need not be applied to the contact surfaces  56  corresponding to the sides that are generally parallel to the path of the light across the guide (that is, the short sides in  FIG. 5 ) since loss of light at those edges would not noticeably degrade the level of illumination of the display. Nonetheless, these sides need not be masked and can in fact be coated with reflective material without deleterious effect on the display. 
         [0032]    The reflective coating is also applied to the angled surface  48  ( FIG. 5 ), so that the reflective aspect of that surface, as discussed above, is maintained after the seal component  62  is added. Moreover, the exterior of the end surface  64  ( FIGS. 2 and 3 ) opposite the angled surface  48  has applied to it the reflective coating. 
         [0033]    With particular reference to  FIGS. 3, 4 and 6 , the entire rear surface  58  of the light guide has applied to it the reflective coating noted above.  FIG. 4  shows in cross section some of these protuberances  60  that that make up the array of integrally formed protuberances  60  dispersed across the rear surface to extend therefrom. As noted earlier, the preferred protuberances  60  are cylindrically shaped and thus define associated outer surfaces  66  ( FIG. 4 ) that protrude for the otherwise planar portions  68  of rear surface  58 . These surfaces  66 ,  68  provide the primary function of the light traps as discussed above. The application of the thin film of reflective material, as by deposition of aluminum, ensures that all of these surfaces are coated to thus protect against direct contact of the surfaces with the later-applied seal component  62  and attendant light leakage as discussed above. 
         [0034]    By coating the entirety of the rear surface  58  (that is, all of surfaces  66 ,  68 ) that surface is completely protected against potentially light leaking contact with the seal component  62  as well as any other contaminants (fine dust, etc.) that may over time make its way into the interior of the meter casing. 
         [0035]    Put another way, any surface of the light guide  42  that may be exposed to direct contact with the seal component when mounted in the casing  22  may be covered with the reflective coating, but the surfaces specifically mentioned as coated above should be so coated for best results. 
         [0036]    While the foregoing description was made in the context of preferred embodiments, it is contemplated that modifications to those embodiments may be made without departure from the invention as claimed. For instance, backlit displays other than LCD type may be employed, and seal components other than epoxy are contemplated.