Abstract:
A system and method is described for distributing light within a light guide which is used for illuminating the user interface of an electronic device. The light guide is formed as a thin film and impressed with a pattern of input and output diffraction gratings to control the transmission of light into and out of the light guide to efficiently illuminate the user interface.

Description:
BACKGROUND OF THE INVENTION 
     Mobile telephones and similar communication devices are rapidly expanding in use and function. Such devices will soon provide Internet access, personal information management, facsimile, and messaging, in addition to telephone communication. This will require a user interface which is more complex, crowded and generally more difficult to use. In addition, electronic devices, such as mobile phones, pagers and the like, are being used in ever expanding situations and environments. Inevitably the devices will be used where only limited light is available, thereby making it even more difficult to operate the user interface. Accordingly effective internal lighting will be an important feature of these devices. 
     Providing a bright and efficient light source at a reasonable price has become more and more difficult as the devices have been reduced in size and packed with an ever increasing number of features. To accommodate the packaging and cost demands, it is desirable to use a printed circuit board that has its components, including light emitting diodes (LEDs) for illumination, mounted on only one side of the board, referred to as the component side. In many instances the buttons, display and other components of the user interface, which require illumination, are located facing the opposite side of the board. The board therefore will impede the illumination of these components. To resolve this problem, an optical light guide is used to receive light through an opening in the board and bend it to illuminate the desired components. A so called through-the-board light source is constructed to direct the light of an LED through the opening in the printed circuit board. 
     The relative positioning of the light guide and light sources requires optimized coupling of the components to maximize the distribution of light within the light guide. In the systems of the prior art, as shown in FIGS. 1 a  and  1   b , edge coupling and surface coupling is often used. Each has its limitations, edge coupling works reasonably well when the light guide is thick enough to receive a majority of the light generated by the LED. Since it is desirable to make mobile communications devices thinner, edge coupling is a limitation on design advance. Surface coupling is inherently less efficient because of the need to bend the light which results in the light escaping out of the light guide, as shown in FIG. 1 b . Modification of the surface geometry of the light guide to retain more light and reduce losses is attempted, but with only limited success. 
     A purpose of this invention is, therefore, to provide a lighting system for the user interface components of an electronic device, such as a mobile communication device. More particularly, it is a purpose of this invention to distribute the light from an LED into a light guide with improved efficiency while allowing the thickness of the light guide to be reduced significantly. Another object of this invention is to construct an input diffraction optical element (DOE), such as a grating structure, operatively associated with a light guide to distribute the light from an LED throughout the light guide. It is a further purpose of this invention to use an output diffraction optical element in association with a light guide to transmit light from the light guide to the components of the user interface. 
     SUMMARY OF THE INVENTION 
     A system for distributing light within a thin light guide is provided using diffraction gratings as a means to optically couple the light from a source, such as a light emitting diode (LED), to the light guide. Planar style light guides have been used to supply light to the user interface of a mobile telephone or other communications device in the past, but the reduction of the thickness of the light guide was limited in order to maintain a reasonable level of coupling efficiency. By the use of diffraction gratings and the like as a coupling mechanism, the light guide can be reduced considerably in thickness while increasing the coupling efficiency. 
     In the system of this invention the input grating coupling is optimized for each application and the system can include an array of LEDs each having its optimized input grating. By also using diffraction gratings also to out couple the light from the light guide an extremely uniform source of illumination is provided to sensitive user interface components such as a liquid crystal display (LCD) 
     The pattern of user interface illumination is established for a particular application. This determines the configuration of the light guide, its associated diffraction gratings and the array of LEDs required. A master grating pattern is constructed by means, for example: electron beam lithography and assembled with the light guide in the molding or pressing process of the light guide. In this manner an extremely thin light guide is constructed having increased coupling efficiency. 
    
    
     DESCRIPTION OF THE DRAWING 
     The invention is described in more detail below with reference to the attached drawing in which: 
     FIG. 1 a  is a schematic illustration of the use of edge coupling of an LED to a light guide; 
     FIG. 1 b  is a schematic illustration of the use of surface coupling of an LED to a light guide; 
     FIG. 2 a  is a bottom view of the light guide of this invention; 
     FIG. 2 b  is a side view of the lighting system of this invention; 
     FIG. 3 a  is an enlarged side view of the lighting system of this invention; 
     FIG. 3 b  is an enlarged view of a grating structure for use in this invention; 
     FIG. 4 is a perspective view of a typical communications device in which the invention may be used; 
     FIG. 5 a  is an alternate embodiment of a grating structure; and 
     FIG. 5 b  is an alternate embodiment of an output diffraction element. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The light distribution system of this invention is described below with reference to a mobile communications device such as a cellular telephone, but it should be noted that the system is equally adaptable to other types of electronic devices such as personal information managers, computers, pagers, game controllers and the like. 
     A mobile device  1  is illustrated in FIG.  4  and it is constructed with a front cover  2 , a printed circuit board  3 , and a back cover or base  4 . These elements are assembled to form an operational unit in a conventional manner. Printed circuit board  3  has a component side  5  to which all of its components are soldered and an inactive side  7  to which no components can be soldered. Front cover  2  contains the user interface  6  which consists of a liquid crystal display  11  and a series of buttons forming a keyboard  12 . In order to use the user interface  6  in situations of limited ambient light, the display and buttons need to be back lit internally. 
     An optical light guide  8  is mounted on the inactive side  7  of the circuit board  3  to receive light from an array of LEDs  9  connected on the component side of the circuit board  3  through openings  15 , as shown in FIGS. 2 b  and  3   a . The guide  8  distributes the light emitted from the LEDs  9  towards, for example, the liquid crystal display  11  and keyboard  12  of the user interface  6 . 
     In the prior art, as shown in FIG. 1 b , an LED  50  is mounted on a printed circuit board  51  with the diode chip  52  emitting light through an opening  53  in circuit board  51 . An optical light guide  54  is mounted opposite to the diode chip  52  over the opening  53 . light guide  54  is constructed with the optimizing shaped surface  55  extending substantially across the opening  53  to bend the light, depicted in FIG. 1 b  as arrows  58 , passing through the opening  53  at approximately right angles. As shown by arrows  56  and  57 , light at the side extremities of the beam will be propagated through the light guide  54  and be wasted. This can be effective, providing there is sufficient thickness in the light guide. Otherwise, the inefficiencies of this configuration result in the use of higher power or a more efficient, i.e. more expensive, LED than is necessary and results in undesirable power dissipation or expense. In FIG. 1 a  an edge illuminated light guide is shown where the light guide  54  is too thin to receive the full light intensity generated by the LED chip  52 . The efficiency of this latter system is limited when the light guide is thin. 
     To provide a more efficient delivery of light to the components of user interface  6  a system is provided which utilizes through the board lighting from an array of LEDs  9 . The light is coupled to light guide  8  by the use of input diffraction optical elements (DOE), such as diffraction gratings  13  associated with the light guide  8 . Utilizing appropriate optical relationships, an input grating pattern is designed which takes into consideration the angular spectrum and dimensions of the LED, the dimensions and composition of the light guide, and the amount of light required. Through these calculations the grating configuration is optimized for each application. In the preferred embodiment, an output grating pattern  14  is also designed to extract the light from the light guide in the appropriate area to illuminate user interface components, such as keyboard  12  and LCD  11 . 
     The basic components of the light delivery system of this invention are shown in FIG. 3 a . Printed circuit board  3  supports and connects the operating components of the electronic device, i.e. mobile communications device  1  on its component side  5 . A light guide  8  is mounted to circuit board  3  at its inactive side  7 . To provide an optical path for the transmission of light to the light guide  8 , an opening  15  is constructed in the circuit board  3 . LED  9  is connected to the component side  5  with its light emitting chip  10  aligned with the opening  15 . A diffraction grating  13  is constructed on the underside of the light guide  8  to receive the light emitted from diode  9 . Grating  13  diffracts the light in accordance with the characteristics of the light guide  8  to cause an efficient distribution of the light within the light guide  8 . As shown in FIG. 3 b , the diffraction surface is varied in order to accommodate the spectrum of incident angles of the typical LED. 
     As shown in FIG. 2 b , to enhance the delivery of light to the user interface  6 , a series of output gratings  14  are constructed in the light guide  8  to extract the transmitted light out of the light guide at predetermined locations coincident with the locations of the interface components, i.e. LCD  11  and keyboard array  12 . Other forms of extracting surfaces are potentially usable, for example the opening  15  having conical surface  16 , as shown in FIG. 5 b.    
     To accomplish the purpose of this invention, the input grating  13  is designed to diffract the light from LED  9  into an angle greater than the total internal reflections γ of the light guide  8 , where, assuming a refractive index of n=1.5, γ≈42°. Using this as a guide, the grating dimensions and pattern may be optimized by using known formulas (see, Diffractive  Optics for Waveguide Display , chapter 3, Pasi Laakonen, Jun. 16, 2000, Doctoral Thesis, University of Joensuu, Joensuu, Finland the substance of which is incorporated into this application by reference. A pattern of gratings which is optimized for each angle of incidence is developed using the Nelder Mead simplex search algorithm. In addition the placement of the LED relative to the grating and the length of the grating are also optimized. The grating comprises an array of minute grooves which are varied in depth, width and length to accommodate the spectrum of incident angles, as shown in FIG. 3 b . Instead of straight gratings a circular grating configuration, as shown in FIG. 5 a , can be generated and used for either out-coupling or in-coupling. 
     Once the input and output grating configurations are established the overall pattern can be generated on a thin film by electron beam lithography or other means. This can be used as a master to impress the grating pattern on the light guide as the light guide is molded or pressed. This will allow the light guide to be manufactured with integral in-coupling and out-coupling diffractive gratings. In this manner the light guide distribution system may be made as thin as possible to accommodate overall design goals for an electronic device. Light guides presently being used have a thickness in the order of from 1.2 to 1.5 mm. Through the use of this invention, such light guides can be executed in thin films having a thickness in the range of 0.2 to 0.4 mm.