Patent Publication Number: US-7585094-B2

Title: Optical plate with light diffusion layer and backlight module using the same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is one of four co-pending U.S. patent applications, which is; application Ser. No. 11/550,379, filed on Oct. 17, 2006, and entitled “OPTICAL PLATE AND BACKLIGHT MODULE USING THE SAME”; application Ser. No. 11/557,914, filed on Nov. 8, 2006, and entitled “OPTICAL PLATE AND BACKLIGHT MODULE USING THE SAME”; application Ser. No. 11/565,575, filed on Nov. 30, 2006, and entitled “OPTICAL PLATE WITH DIFFUSION LAYER AND BACKLIGHT MODULE USING THE SAME”; application Ser. No. 11/566,836, filed on Dec. 5, 2006, and entitled “OPTICAL PLATE WITH DIFFUSION LAYER AND BACKLIGHT MODULE USING THE SAME”. In the co-pending applications, the inventors are Shao-Han Chang. The co-pending applications have the same assignee as the present application. The disclosure of the above identified application is incorporated herein by reference. 
   This application is related to a U.S. patent application Ser. No. 11/565,575 entitled “OPTICAL PLATE WITH LIGHT DIFFUSION LAYER AND BACKLIGHT MODULE USING THE SAME”, recently filed with the same assignee as the instant application. The disclosure of the above identified application is incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to optical plates, and more particularly to an optical plate typically used in a backlight module, the backlight module being used in apparatuses such as a liquid crystal display (LCD). 
   2. Discussion of the Related Art 
   In a liquid crystal display device, liquid crystal is a substance that does not itself radiate light. Instead, the liquid crystal relies on light received from a light source, in order that the liquid crystal can provide displaying of images and data. In the case of a typical liquid crystal display device, a backlight module powered by electricity supplies the needed light. 
     FIG. 7  represents a typical direct type backlight module  10 . The backlight module  10  includes a housing  11 , a plurality of lamp tubes  12 , a light diffusion plate  13 , a light diffusion sheet  14 , and a prism sheet  15 . The housing  11  includes a base  111 , and a plurality of sidewalls  113  extending from a periphery of the base  111 . Top ends of the sidewalls  113  cooperatively define an opening  112  therebetween. The light diffusion plate  13 , the light diffusion sheet  14 , and the prism sheet  15  are stacked in that order on the housing  11  above the opening  112 . The lamp tubes  12  are positioned in the housing  11  under the light diffusion plate  13 . Light rays emitted from the lamp tubes  12  are substantially diffused in the light diffusion plate  13 , and finally surface light rays are output from the prism sheet  15 . 
   However, to enhance the uniformity of light rays output by the backlight module  10 , there must be a certain space between the light diffusion plate  13  and the lamp tubes  12 . This space eliminates potential dark strips that may otherwise occur due to the reduced intensity of light between adjacent lamp tubes  12 . Therefore the backlight module  10  may be unduly thick, and the overall intensity of the output light rays (luminance) may be reduced. Alternatively, the light diffusion plate  13  can be constructed to have sufficient thickness to be able to thoroughly diffuse light rays passing therethrough. In such case, the thickness is typically required to be in the range of about 2 to 3 centimeters. 
   In addition, the light diffusion plate  13  is typically manufactured by uniformly dispersing a plurality of light diffusion particles  132  into a transparent resin matrix material  131 . Since numerous light rays are diffused by the light diffusion particles  132  a number of times in the light diffusion plate  13 , an amount of light energy is lost, and a brightness of light output by the backlight module  10  is reduced. 
   Furthermore, the light diffusion plate  13 , the light diffusion sheet  14 , and the prism sheet  15  are in contact with each other, but with a plurality of air pockets existing at the boundaries therebetween. When the backlight module  10  is in use, light rays pass through the air pockets, and some of the light rays undergo total reflection at one or another of the corresponding boundaries. Thus the light energy utilization ratio of the backlight module  10  is reduced. 
   What is needed, therefore, is a new optical plate and a backlight module using the optical plate that can overcome the above-mentioned shortcomings. 
   SUMMARY 
   An optical plate according to a preferred embodiment includes a transparent plate and a light diffusion layer. The transparent plate includes a light output surface, a light input surface opposite to the light output surface, and a plurality of elongated recessed portions formed at the light input surface. The light diffusion layer is coated in the light input surface and the elongated recessed portions such that the light diffusion layer and the light input surface cooperatively define a flat surface. The light diffusion layer includes transparent resin matrix material, and first light diffusion particles and second light diffusion particles each dispersed in the transparent resin matrix material uniformly. A refractive index of the second light diffusion articles is greater than that of the first light diffusion particles. 
   A backlight module according to a preferred embodiment includes a housing, a plurality of linear light sources, and an optical plate. The same optical plate as described in the previous paragraph is employed in this embodiment. The housing includes a base and a plurality of sidewalls extending from the base, the base and the sidewalls cooperatively forming an opening. The optical plate is positioned on the housing. The linear light sources in the receiving cavity underneath the optical plate are positioned in one-to-one correspondence with the elongated recessed portions. 
   Other advantages and novel features will become more apparent from the following detailed description of various embodiments, when taken in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The components in the drawings are not necessarily drawn to scale, the emphasis instead being placed upon clearly illustrating the principles of the present optical plate and backlight module. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views, and all the views are schematic. 
       FIG. 1  is an isometric view of an optical plate according to a first preferred embodiment of the present invention, the optical plate having a transparent plate and a light diffusion layer. 
       FIG. 2  is an enlarged, cross-sectional view of the optical plate of  FIG. 1  taken along line II-II thereof. 
       FIG. 3  is an isometric, inverted view of the transparent plate of the optical plate of  FIG. 1 . 
       FIG. 4  is an enlarged view of one part of the light diffusion layer of the optical plate of  FIG. 2 , showing a light ray being diffused at first light diffusion particles within the light diffusion layer. 
       FIG. 5  is similar to  FIG. 4 , but showing a different light ray being reflected and diffracted at second light diffusion particles within the light diffusion layer. 
       FIG. 6  is an exploded, cross-sectional view of a backlight module according to a second preferred embodiment of the present invention. 
       FIG. 7  is an exploded, cross-sectional view of a conventional backlight module. 
       FIGS. 8(   a ) to  8 ( e ) are partially, side cross-sectional views of five optical plates in accordance with third to seventh embodiments of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Reference will now be made to the drawings to describe preferred embodiments of the present optical plate and backlight module, in detail. 
   Referring to  FIGS. 1 through 3 , an optical plate  20  in accordance with a first preferred embodiment of the present invention is shown. The optical plate  20  includes a transparent plate  21  and a light diffusion layer  22 . The transparent plate  21  includes a light input surface  211 , a light output surface  212  opposite to the light input surface  211 , and a plurality of elongated recessed portions  213  formed at the light input surface  211 . The light diffusion layer  22  is coated in the elongated recessed portions  213 . The light diffusion layer  22  is discontinuous, and is constituted by a group of separate coplanar portions. The light diffusion layer  22  and the light input surface  211  cooperatively define a single flat surface at a bottom of the transparent plate  21 . The light output surface  212  is a flat surface. A thickness H of the transparent plate  21  is configured to be in the range from 1.5 millimeters to 3.0 millimeters. The transparent plate  21  can be made of material selected from the group consisting of polycarbonate (PC), polymethyl methacrylate (PMMA), polystyrene (PS), copolymer of methylmethacrylate and styrene (MS), and any suitable combination thereof. 
   Each elongated recessed portion  213  extends along a direction parallel to a side surface  214  of the optical plate  20 . Each elongated recessed portion  213  has one of the following cross-sections taken along a direction perpendicular to the side surface  214  and the light output surface  212 : triangular, trapezoidal, arcuate, or arc-shaped. In this embodiment, each elongated recessed portion  213  has an isosceles trapezoidal cross-section. In consideration of light diffusing effects, the ratio of a depth h of each elongated recessed portion  213  to the thickness H of the transparent plate  21  is preferably less than 0.3. Thus, the depth h is configured to be preferably in the range from about 0.45 to about 0.9 millimeters. 
   The light diffusion layer  22  includes transparent resin matrix material  224  in an amount of 5 to 90 percent by weight, and first and second light diffusion particles  221  and  222  in a combined amount of 10 to 95 percent by weight. The first light diffusion particles  221  and second light diffusion particles  222  are respectively dispersed into the transparent resin matrix material  224  uniformly. A ratio by weight of the first light diffusion particles  221  to the second light diffusion particles  222  is in the range from 5 to 100. The light diffusion layer  22  is manufactured by solidifying a varnish into which the first and second light diffusion particles  221 ,  222  have been dispersed. The varnish can be selected from the group consisting of acrylic varnish, acrylic amine varnish, epoxy resin varnish, and any suitable combination thereof. Accordingly, the transparent resin matrix material  224  may be one of acrylic, acrylic amine, epoxy resin, and any suitable combination thereof. 
   A refractive index of the second light diffusion particles  222  is larger than that of the first light diffusion particles  221 . The refractive index of the first light diffusion particles  221  is in the range from about 1.4 to about 1.7. The refractive index of the second light diffusion particles  222  is larger than about 2.0, and is preferably selected to be in the range from about 2.0 to about 2.8. A diameter of each of the first light diffusion particles  221  is larger than that of each of the second light diffusion particles  222 . The diameter of the first light diffusion particles  221  is in the range from about 1 micron to 500 microns, and the diameter of the second light diffusion particles  222  is in the range from about 0.01 microns to about 1 micron. 
   The first light diffusion particles  221  may be selected from the group consisting of polystyrene (PS) particles, polycarbonate (PC) particles, styrene acrylonitrile copolymer particles, polypropylene particles, polymethyl methacrylate (PMMA) particles, glass beads, silicon dioxide (SiO 2 ) particles, quartz particles, and any combination thereof. The second light diffusion particles  222  may be selected from the group consisting of titanium dioxide (TiO 2 ) particles, barium sulfate (BaSO 4 ) particles, zinc sulfide (ZnS) particles, zinc oxide (ZnO) particles, antimony oxide (Sb 2 O 3  or Sb 2 O 5 ) particles, calcium carbonate (CaCO 3 ) particles, and any combination thereof. 
   Referring to  FIG. 4 , the first light diffusion particles  221  can substantially diffuse light rays passing through the light diffusion layer  22 . Referring to  FIG. 5 , the second light diffusion particles  222  can further diffract and reflect the light rays due to their smaller sizes and larger refractive index compared to the first light diffusion particles  221 . Thus the light diffusion layer  22  has a good light diffusion capability with the cooperatively effects of the first and second light diffusion particles  221  and  222 . Accordingly, the light diffusion layer  22  of the optical plate  20  may be configured to be very thin, with the optical plate  20  still being able to achieve uniform light diffusion. 
   In this embodiment, the light diffusion layer  22  further includes a plurality of fluorescent particles  223  uniformly dispersed into the transparent resin matrix material  224  amongst the first and second light diffusion particles  221 ,  222 . A ratio by weight of the fluorescent particles  223  to the first light diffusion particles  221  is preferably less than 1 percent. When ultraviolet rays from one or more external light sources irradiate the fluorescent particles  223 , a significant amount of the ultraviolet rays is converted into visible light and infrared light. Therefore, the rate of utilization of light energy of a backlight module using the optical plate  20  is increased. 
   Referring to  FIGS. 1 and 2 , the optical plate  20  further includes a plurality of hemispherical protrusions  23  formed on the light output surface  212  of the transparent plate  21 . The hemispherical protrusions  23  are discrete from each other, and are arranged in a matrix. A diameter of each hemispherical protrusion  23  is configured to be in the range from about 10 microns to about 500 microns. The transparent plate  21  and the hemispherical protrusions  23  can be integrally manufactured by injection molding. When the optical plate  20  is utilized in a backlight module, light rays from lamp tubes (not shown) enter the optical plate  20 . The light rays are substantially diffused in the light diffusion layer  22  of the optical plate  20 . Many or most of the light rays are condensed by the hemispherical protrusions  23  of the optical plate  20  before they exit the light output surface  212 . Thereby, a brightness of the backlight module is increased. 
   In an alternative embodiment, prism lens structures may be foamed on the light output surface  212  of the transparent plate  21  of the optical plate  20  instead of the hemispherical protrusions  23 . The prism lens structures can be configured for increasing the brightness of a corresponding backlight module. Further, the optical plate  20  is not limited to the above-described embodiments. For example, referring to  FIGS. 8(   a )- 8 ( e ), the optical plate  20  may further or alternatively include other suitable brightness enhancement structures formed at the light output surface  212  thereof, such as hemispherical grooves  24 , V-shaped protrusions  25 , V-shaped grooves  26 , arc-shaped protrusions  27 , arc-shaped grooves  28 , and the like. 
   When the optical plate  20  is used in a backlight module, the optical plate  20  may replace a light diffusion plate and prism sheet combination that would ordinarily be used. Therefore, air pockets that would ordinarily exist in the backlight module are eliminated, and loss of light energy in the backlight module is reduced. In addition, because the single optical plate  20  can be used in place of both a light diffusion plate and a prism sheet, the cost of the backlight module is reduced. 
   Referring to  FIG. 6 , a backlight module  30  in accordance with a second preferred embodiment of the present invention is shown. The backlight module  30  includes a housing  31 , a plurality of cold cathode fluorescent lamps  32 , and an optical plate  33 . The housing  31  includes a base  312 , and a plurality of sidewalls  314  extending from a periphery of the base  312 . Top ends of the sidewalls  314  cooperatively define an opening  316  therebetween. The optical plate  33  is positioned on the housing  31  above the opening  316 . 
   The optical plate  33  is substantially the same as the optical plate  20  of the first preferred embodiment, and includes a transparent plate  41  and a light diffusion layer  42 . The transparent plate  41  includes a light output surface  412 , a light input surface  411  opposite to the light output surface  412 , and a plurality of elongated recessed portions  413  formed at the light input surface  411 . The light diffusion layer  42  is coated in the elongated recessed portions  413 . The light diffusion layer  42  and the light input surface  411  cooperatively define a single flat surface at a bottom of the transparent plate  41 . The cold cathode fluorescent lamps  32  are regularly arranged on the base  312  in one-to-one correspondence with the elongated recessed portions  413  of the optical plate  33 . A largest width d of each elongated recessed portion  413  is configured to be equal to or greater than that of the corresponding cold cathode fluorescent lamp  32 . Further, said width d is less than a pitch D between each two adjacent cold cathode fluorescent lamps  32 . Light rays emitted from the cold cathode fluorescent lamps  32  are substantially diffused in the optical plate  33 , and finally uniform surface light rays are output from the optical plate  33 . 
   Because the cold cathode fluorescent lamps  32  are positioned in one-to-one correspondence with the elongated recessed portions  413 , many or most light rays that reach portions of the optical plate  33  directly above the cold cathode fluorescent lamps  32  first pass through the corresponding portions of the light diffusion layer  42 . Thus, the optical plate  33  above the cold cathode fluorescent lamps  32  has relatively low illumination. Accordingly, a distance from the cold cathode fluorescent lamps  32  to the optical plate  33  may be configured to be very short, with little or no risk of dark strips occurring due to reduced intensity of input light corresponding to spaces between adjacent cold cathode fluorescent lamps  32 . Accordingly, the backlight module  30  can have a thin configuration while still providing good, uniform optical performance. It is to be understood that the cold cathode fluorescent lamps  32  can be replaced by other suitable linear light sources. 
   Finally, while particular embodiments have been described above, the descriptions are illustrative of principles of the invention and are not to be construed as limiting the invention. Various modifications can be made to the embodiments by those skilled in the art without departing from the true spirit and scope of the invention as defined by the appended claims.