Abstract:
A method and device for improving lighting of a display having a top surface for viewing. The device includes a light guiding panel located substantially on the top surface of the display and at least one linear light source located adjacent to the light guiding panel for providing light thereto, wherein the light guiding panel has a first surface adjacent to the top surface of the display, a second surface opposite to the first surface, and at least one side edge for admitting a portion of the light provided by the linear light source through the side edge into the light guiding panel between the first and second surfaces; and the second surface includes a plurality of elongated grooves having dense-rare boundaries for reflecting part of the admitted light in a total-internal reflection fashion towards the display through the first surface of the light guiding panel, and wherein the dense-rare boundaries have a distribution density which varies depending on a distance between the dense-rare boundaries and the side edge.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to a lighting device and, in particular, to a lighting device to be placed in front of a panel to illuminate the panel. 
     BACKGROUND OF THE INVENTION 
     In order to improve the viewing of the non-emissive display, especially reflective type B/W and color display, one may place a light guiding panel on top of the display and adjacent to a light source in order to direct the light produced by the light source onto the viewing surface of the display. Typically, light is channeled into one or more edges of the light guiding panel. In U.S. Pat. No. 5,835,661, Tai et al. discloses a light guiding panel wherein a plurality of reflecting grooves are provided on the top surface of the light guiding panel in order to reflect the light introduced into the edges towards the display. As disclosed in U.S. Pat. No. 5,835,661, the reflecting grooves are uniformly distributed over the top surface of the light guiding panel. Accordingly, the illumination on the display is uneven such that the display section near the light source appears much brighter than the display section further away from the light source. This uneven illumination is more pronounced when the thickness of the light guide is reduced. The uniformly distributed grooves may also produce Moire patterns if the spacing of the pixels in the display is slightly different from the groove spacing. Using the light guiding panel, as disclosed U.S. Pat. No. 5,835,661, specular reflection of the illuminating light and ambient light from the lower surface of the light guiding panel further reduces the viewing quality of the display. In order to reduce this specular reflection, one may provide an anti-reflection coating, such as a thin film, on the lower surface. The anti-reflection coating is, in general, expensive because one or more additional steps must be carried out in order to apply the coating on the light guiding panel during the manufacturing process. 
     There are a number of known ways to provide light to a light guiding panel. For example, a cold cathodic fluorescent tube (CCFT) can be used as a light source and can be placed near the edge of light guiding panel to provide light into the light guiding panel. A CCFT typically consists of a glass tube, coated with phosphor on the inside of the glass tube, which is hermetically sealed and evacuated. When a high voltage is applied across the tube ends, plasma is formed to excite the phosphor for light production. The CCFT is relatively large as compared to the thickness of the light guide. It is especially bulky when it is used in a portable communication device such as a mobile phone. In U.S. Pat. No. 5,835,661, Tai et al. also discloses a linear light pipe which can be made sufficiently thin to be used as a light source to provide illumination to a display in a portable communication device. However, the linear light pipe, as disclosed U.S. Pat. No. 5,835,661, is not very efficient in that the light rays exiting the linear light pipe surface towards the edge of the light guiding panel are scattered in a random fashion. Thus, only a small portion of the light available from the linear light pipe is actually directed towards the viewing surface of the display. 
     Thus, it is desirable and advantageous to provide a method and a device for improving the front lighting of a non-emissive panel or display. 
     SUMMARY OF THE INVENTION 
     The first aspect of the present invention is to provide a method for improving lighting of a display having a top surface for viewing. The method comprises the steps of: 
     providing a light guiding panel substantially on the top surface of the display; and 
     providing at least one light source adjacent to the light guiding panel for providing light thereto, wherein the light guiding panel has a lower surface facing the top surface of the display, an upper surface, and at least one side edge for admitting a portion of the light provided by the light source through the side edge into the light guiding panel between the upper and lower surfaces, wherein the upper surface includes a plurality of grooves having dense-rare boundaries substantially facing the light source for reflecting part of the admitted light in a total-internal reflection fashion towards the display through the lower surface of the light guiding panel, and wherein the dense-rare boundaries have a distribution density which varies according to a distance between the dense-rare boundaries and the side edge. 
     Preferably, the distribution density increases with the distance. 
     Preferably, the lower surface includes a reflection reduction structure to reduce unwanted reflection of light from the lower surface towards the upper surface. 
     Preferably, the reflection reduction structure comprises a plurality of periodic grooves, wherein the periodic grooves have a pitch, which is smaller than half the dominant wavelength range of the light provided by the light source. 
     The second aspect of the present invention is a lighting apparatus for a display having a top surface for viewing. The apparatus comprises: 
     a light guiding panel located substantially on the top surface of the display; and 
     at least one linear light source located adjacent to the light guiding panel for providing light thereto, wherein the light guiding panel has a lower surface facing the top surface of the display, an upper surface, and at least one side edge for admitting a portion of the light provided by the linear light source through the side edge into the light guiding panel between the upper and lower surfaces, and the upper surface includes a plurality of grooves having dense-rare boundaries substantially facing the light source for reflecting part of the admitted light in a total-internal reflection fashion towards the display through the lower surface of the light guiding panel, and wherein the dense-rare boundaries have a distribution density which varies according to the distance between the dense-rare boundaries and the side edge. 
     Preferably, the linear light source includes: 
     a linear light pipe having a peripheral surface and at least one light-input end; and 
     at least one light emitter adjacent the light-input end for providing light thereto, and wherein the peripheral surface allows the light provided by the light emitter to transmit therethrough towards the side edge of the light guiding panel. 
     Preferably, the linear light pipe has a longitudinal axis substantially parallel to the side edge, and the peripheral surface has a cross section substantially perpendicular to the longitudinal axis, wherein the cross section includes a curved section adjoining a reflecting surface which is located away from the side edge of the light guiding panel in order to reflect light towards the side edge through the curved section. 
     Preferably, the curved section has a spherical-shaped section adjacent the side edge for focusing the light reflected by the reflecting surface towards the side edge. 
     Preferably, a plurality of scatterers are provided on the reflecting surface to scatter light towards the curved section. 
     Alternatively, the curved section joins the side edge such that the linear light pipe is an integral part of the light guiding panel. 
     The third aspect of the present invention is to provide a light guiding panel which comprises: a first surface, an opposing second surface and at least one side edge, wherein the first surface has a plurality of grooves having dense-rare boundaries substantially facing the side edge so as to reflect light admitted through the side edge into the light guiding panel between the first and second surfaces towards the second surface, and wherein the dense-rare boundaries have a distribution density which varies according to the distance between the dense-rare boundaries and the side edge. 
     Preferably, the light guiding panel further comprises a reflection reduction structure provided on the second surface. 
     Preferably, the reflection reduction structure includes a plurality of periodic grooves. 
     The present invention will become apparent upon reading the description taken in conjunction with FIGS. 1 to  8 . 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagrammatic representation illustrating a front-lighting device provided on top of a display. 
     FIG. 2 a  is a cross sectional view illustrating the principle of a front-lighting device when it is provided on a display having a transparent top surface. 
     FIG. 2 b  is a cross sectional view illustrating the principle of a front-lighting device when it is provided on a panel. 
     FIG. 2 c  is a diagrammatic representation illustrating the reflection of a light beam encountering a reflecting groove on a light guiding panel. 
     FIG. 3 a  is a cross sectional view illustrating the distributing density of the reflecting grooves on the upper surface of the light guiding panel, according to the preferred embodiment of the present invention. 
     FIG. 3 b  is a cross sectional view illustrating the distributing density of the reflecting grooves on the upper surface of the light guiding panel, according to another embodiment of the present invention. 
     FIG. 4 is a cross sectional view illustrating the structure of the reflection reduction structure, according to the present invention. 
     FIG. 5 is diagrammatic representation of a linear light source located near one edge of the light guiding panel. 
     FIG. 6 is a cross sectional view illustrating the shape of the light pipe, according to the present invention. 
     FIG. 7 a  is a diagrammatic representation illustrating another embodiment of the light guiding panel wherein the light pipe is an integral part of the light guiding panel. 
     FIG. 7 b  is a sectional view illustrating the integration of the light pipe to the light guiding panel. 
     FIG. 8 is a diagrammatic representation illustrating a light guiding panel being used for back lighting. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 illustrates a front-lighting device  10  provided on top of a display  100 . As shown in FIG. 1, the front-lighting device  10  includes a light-guiding panel  20  and a linear light source  50 . The light-guiding panel  20  has an upper surface  22  and a lower surface  24  which is substantially located on top of the upper surface  110  of the display  100 . The linear light-source  50 , located adjacent to a side edge  26  of the light guiding panel  20 , is used to provide light to illuminate the display  100  through the light guiding panel  20  so that the display  100  can be viewed more clearly through the upper surface  110  and the light guiding panel  20 . 
     As shown in FIG. 2 a , the upper surface  22  of the light guiding panel  20  has a plurality of grooves X 0 , X 1 , . . . , X n  to reflect light from the light source  50  towards the lower surface  24  of the light guiding panel  20  through the upper surface  110  to reach a lower layer  120  of the display  100 . As shown in FIG. 2 c , each groove X n  has a dense-rare boundary  150  which is substantially facing the side edge  26  of the light guiding panel  20  so that when a light ray  200  coming from the direction of the side edge  26  encounters the dense-rare boundary  150  at a reasonably large angle of incidence, it undergoes total internal reflection to be directed towards the lower surface  24  of the light guiding panel  20 . It should be noted that, when the light ray  200  encounters the dense-rare boundary at the lower surface  24 , part of the light energy, as indicated by reference numeral  204 , will be reflected back into the light guiding panel  20 . In order to reduce this type of Fresnel reflection at the lower surface  24  of the light guiding panel  20 , it is advantageous to provide a reflection reduction structure  30 . It should be noted that the reflection reduction structure  30  is used to reduce only the unwanted reflection of light rays having an angle of incidence smaller than a certain angle. For example, the reflection reduction structure  30  is used to reduce the Fresnel reflection of light rays with an angle of incidence smaller than 30 degrees, while allowing light rays with an angle of incidence larger than 30 degrees to reflect back to the light guiding panel  20 . As shown in FIG. 2 c , light ray  202  is reflected by the structure  30  back to the light guiding panel  20 . Typically, the display  100 , as shown in FIG. 2 a , is a non-emissive type display, such as an LCD panel. In that case, the light reflected by the grooves X 0 , X 1 , . . . , X n  transmits through a transparent, top layer  130  to reach a liquid-crystal layer as indicated by the lower layer  120  so that the information displayed on the lower layer  120  can be viewed or read more clearly. It should be noted that the front-lighting device  10  of the present invention can also be used on a panel  102  which does not have a transparent layer on top of the displayed text or graphics on the upper layer  122 , as shown in FIG. 2 b.    
     FIG. 3 a  is a cross sectional view illustrating the distributing density of the reflecting grooves on the upper surface  22  of the light guiding panel  20 , according to the preferred embodiment of the present invention. The reflecting grooves, as shown in FIG. 3 a , are denoted by X 0 , X 1 , X 2 , . . . , X n−1 , X n , X n+1 . The spacing between two adjacent grooves X 0 , X 1 , X 2 , . . . , X n−1 , X n , X n+1  is denoted by P 0 , P 1 , P 2 , . . . , P n−1 , P n . Furthermore, the distance from the side edge  26  to the grooves X 0 , X 1 , X 2 , . . . , X n−1 , X n , X n+1  of the light guiding panel  20  is denoted by D 0 , D 1 , D 2 , . . . , D n−1 , D n , D n+1 . As light is transmitted through the side edge  26  of the light guiding panel  20 , a larger part of the light beam encounters the reflecting grooves near the side edge  26  than the part of the light beam encountering the reflecting grooves further away from the side edge  26 . Thus, if the reflecting grooves X 0 , X 1 , X 2 , . . ., X n−1 , X n , X n+1  are distributed evenly throughout the upper surface  22  of the light guiding panel  20 , more light will be reflected towards the lower surface  24  and the display  100  near the light source  50 . The illumination to the display  100  appears to be much brighter on the light source side than the rest of the display  100 . This uneven illumination effect will become more pronounced when the thickness, or the distance between the upper surface  22  and the lower surface  24 , of the light guiding device  20  is reduced. In order to reduce this uneven illumination effect, it is preferred that, as the distance D from the grooves X increases, the spacing P decreases so that the distribution density of the grooves X increases with the distance D. For example, because D 0 &lt;D 1 , we have P 0 &gt;P 1 . Such a distribution density of the grooves X can improve the uniformity of illumination of the display  100 . 
     Alternatively, the distribution density of the reflecting grooves X can be made to vary in another fashion, as shown in FIG. 3 b . As shown in FIG. 3 b , the reflecting grooves are grouped into surface sections S 1 , S 2 , . . . , S m  with each surface section having an equal number of reflecting grooves. For example, the surface section S m  has three reflecting grooves X′ 0 , X′ 1 , X′ 2 , and the surface section S m  has three reflecting grooves X′ n−1 , X′ n , X′ n+1 . The grooves X′ within a surface section S may be evenly spaced. For example, within the surface section S 1 , we have P′ 0 =P′ 1 . Similarly, within the surface section S m , we have P′ n−1 =P′ n . However, because the distance D c  from the side edge  26  to the surface section S 1  is smaller than the distance D f  from the side edge  26  to the surface section S m , it is preferred that P′ 0 &gt;P′ n . Thus, the distribution density of the grooves X′ effectively increases with the distance from the side edge  26  to the reflecting grooves X′. Such a distribution of the grooves X′ can also improve the illumination of the display  100 . 
     It is also possible to provide light sources to two side edges  26 ,  26 ′, as shown in FIG. 3 c.  In that case, one half of the upper surface  22  is provided with reflecting grooves X 0 , X 1 , X 2  and the other half is provided with reflecting grooves X″ 0 , X″ 1 , X″ 2  with P″ 0 &gt;P″ 2 . 
     In order to reduce the Fresnel reflection at the lower surface  24  of the light guiding panel  20 , it is desirable to have the reflection reduction structure  30  provided on the lower surface  24 . It is preferred that the reflection reduction structure  30  includes a plurality of periodic grooves, as shown in FIG.  4 . Such periodic grooves can be produced as an integral part of the light guiding panel  20 . For example, the periodic grooves can be produced on the lower surface  24  by a molding process or a pressing process concurrently with or separately from the production of the reflecting grooves X on the upper surface  22  to reduce the production cost of the reflection reduction structure  30 . The spacing between adjacent grooves is denoted by letter Q, and the depth of the grooves is denoted by letter h. If the light introduced into the light guiding panel has a dominant wavelength λ, it is preferred that the spacing Q is smaller than half the dominant wavelength λ. The antireflection properties of the periodic grooves are well known in the art (see, for example, Diffractive Optics for Industrial and Commercial Application, ed. J. Turunen and F. Wyrowski, Akademie Verlag, 1997, pp. 308-311). 
     The reflecting grooves X and X″ on the upper surface  22  of the light guiding panel  20  can be made by many different ways. For example, laser lithography can be used to make the grooves on a master plate, and a replica of the master plate can be made by an electrolysis process. This replica is known as a nickel shim-plate. The reflection reduction structure  30  can also be made on a master plate by interferometric lithography or electron-beam lithography, and a nickel shim-plate is made from the master plate. These shim-plates are then used as molds for hot embossing or injection molding. Using shim-plates for making a fine surface structure is well known in the art. 
     FIG. 5 shows the structure of the linear light source  50  which is located near the side edge  26  of the light guiding panel  20  to provide light to the light guiding panel  20 . As shown in FIG. 5, the linear light source  50  includes a linear light pipe  52  having two ends  54  and a reflection enhancing surface  56  to help directing light rays  206  towards the side edge  26  of the light guiding panel  20 . One or more light emitters  60  are placed near the ends  54  of the light pipe  52  to provide light to the light pipe  52 . The light emitter  60  can be a light-emitting diode or any other light-emitting device. The light pipe  52  has a longitudinal axis  70  and a peripheral surface  58  surrounding the longitudinal axis  70 . The cross section of light pipe  52 , for illustrating the peripheral surface  58  and part of the light guiding panel  20 , is shown in FIG.  6 . 
     As shown, it is preferred that the peripheral surface  58  includes a front section  62  adjoining a back section  64 , wherein the back section  64  includes a reflecting surface  56  having a plurality of scatterers  57  to direct light towards the front section  62 . It is also preferred that the front section, which faces the side edge  26  of the light guiding panel  20 , is curved so that when light rays  210  encounter the front section  62  as they are exiting the light pipe  52 , the divergence of the light rays  210  is reduced. Thus, the shape of the front section  62  helps focus the light rays exiting the light pipe  52 . For example, the front section  62  can be spherical or aspherical. The reflecting surface  56  can be made of a diffraction grating and/or a silvered surface. 
     FIGS. 7 a  and  7   b  illustrate another embodiment of the lighting device. As shown, the light device  10 ′ includes a linear light source  50 ′, which is an integral part of the light guiding panel  20 . As shown in FIG. 7 a , the two ends  54 ′ of the light pipe  52 ′ are connected to the light guiding panels  20 , and the light pipe  52 ′ remains detached from the light guiding panels  20 . FIG. 7 b  shows a sectional view of the lighting device, as shown in FIG. 7 a.    
     It should be noted that the lighting device  10  has been disclosed as a front-lighting device, as shown in FIGS. 1 to  7   b . The same device can be used as a back lighting device, as shown in FIG.  8 . As shown, the lighting device  10  is placed behind a transmissive display  104 , with the “upper” surface  24  of the light guiding panel  20  being further away from the bottom of the display  104  than the “lower” surface  22 . It should be noted that, for back lighting purposes, it is not necessary to have a reflection reduction structure between the transmissive display  104  and the lighting device  10 . 
     Thus, although the invention has been described with respect to a preferred embodiment thereof, it will be understood by those skilled in the art that the foregoing and various other changes, omissions and deviations in the form and detail thereof may be made without departing from the spirit and scope of this invention.