Abstract:
A light guide plate ( 20 ) has a light incidence surface ( 221 ) for receiving light, a light emitting surface ( 223 ) for emitting light, and a bottom surface ( 222 ). The light emitting surface has a plurality of diffraction grating units. Each diffraction grating unit has a strong diffractive portion and a weak diffractive portion. In each diffraction grating unit, grating directions of the strong and weak diffractive portions are orthogonal to each other. Area ratios of the strong diffractive portions in the diffraction grating units progressively increase with increasing distance away from the light incidence surface. The grating directions of the strong diffractive portions may vary according to the locations of the diffraction grating units relative to a light source. These features improve the overall efficiency of utilization of light, and enable the light emitting surface to output highly uniform light.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a light guide plate utilizing diffraction gratings for controlling of light emissions, and a backlight module for a liquid crystal display using such a light guide plate. 
   2. Description of Prior Art 
   A typical liquid crystal display requires a backlight module in order to be able to provide uniform illumination. The performance of the backlight module greatly depends on a light guide plate employed therein. Means for enhancing the uniformity of light that is output from a light guide plate can be classified into two categories. The first category uses geometrical optic means, such as prisms or micro dots. The second category uses wave optic means, such as diffraction gratings. Light guide plates with multifarious configurations of micro dots and prisms have been developed, and some of these light guide plates can generate quite uniform light beams. However, the uniformity provided by dots is relatively low compared with light guide plates having gratings. This is because the gratings of the latter kind of light guide plate can be precisely configured to correspond to the wavelength band of visible light beams, thereby accurately controlling the uniformity of transmission of the light beams. Nevertheless, there are two main problems associated with gratings. Firstly, a grating is subject to becoming worn over time. Secondly, a grating generates spectral phenomenon. 
   Referring to  FIG. 5 , U.S. Pat. No. 5,703,667, issued on Dec. 30, 1997, discloses a backlight module. The backlight module  1  comprises a light guide plate  2  having a light incidence surface  21 , a bottom surface  22  and a light emitting surface  23 , a fluorescent tube  4  disposed adjacent the light incidence surface  21 , a reflection plate  5  disposed under the bottom surface  22 , and a diffusing plate  6  and a prism plate  7  disposed on the light emitting surface  23  in that order from bottom to top. 
   A plurality of reflective diffraction grating units  3  is provided on the bottom surface  22 . All the diffraction grating units  3  are parallel with the fluorescent tube  4 . Each diffraction grating unit  3  comprises a grating part parallel with the fluorescent tube  4 , and a non-grating part. Because all the grating parts of the diffraction grating units  3  are aligned in parallel as described, the diffraction grating units  3  provide strong diffraction of light beams orthogonally emitting from the fluorescent tube  4 . 
   The ratio of a grating part width to a non-grating part width in the diffraction grating units  3  becomes progressively greater with increasing distance away from the light incidence surface  21 . Therefore, light beams that are available in large quantities at places nearer to the light incidence surface  21  undergo weaker diffraction, and light beams that are available only in small quantities at places more remote from the light incidence surface  21  undergo stronger diffraction. As a result, the light emitting surface  23  provides uniform outgoing light beams. 
   However, in numerous actual applications, the linear fluorescent tube  4  cannot accurately emit light rays in a same direction, or point light sources are used instead of the linear fluorescent tube  4 . In such cases, the light beams arriving at the bottom surface  22  do not have a same direction. If the light beams are mainly incident on a diffraction grating unit  3  at an angle that is other than orthogonal, the diffraction effect of the diffraction grating unit  3  is weak. Furthermore, light beams arriving at the non-grating parts of the diffraction grating units  3  are wasted. As a result, the light guide plate  2  has limited uniformity of outgoing light beams and limited efficiency of utilization of light. 
   It is desired to provide a backlight module having a light guide plate which overcomes the above-described problems. 
   SUMMARY OF THE INVENTION 
   An object of the present invention is to provide a light guide plate for a backlight module which yields high uniformity of outgoing light and which has a high light utilization efficiency. 
   A light guide plate of the present invention comprises a light incidence surface for receiving light, a light emitting surface for emitting light, and a bottom surface. The light emitting surface has a plurality of diffraction grating units. Each diffraction grating unit comprises a strong diffractive portion and a weak diffractive portion. Grating directions of the strong and weak diffractive portions are orthogonal to each other. The grating direction of the strong diffractive portion is substantially perpendicular to a direction of light beams received by the diffraction grating unit. 
   Because the grating directions of the strong diffractive portions in the diffraction grating units vary according to the varying incoming light beams received by the diffraction grating units, the strong diffractive portions have improved light utilization efficiency. In addition, the weak diffraction portions can diffract incoming light beams that are received in oblique directions, which further improves the overall light utilization efficiency. 
   Moreover, area ratios of the strong diffractive portions in the diffraction grating units progressively increase with increasing distance away from the light incidence surface. This enables the light emitting surface to output highly uniform light. 
   Other objects, advantages, and novel features of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings, in which: 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic, side sectional view of a first embodiment of a backlight module having a light guide plate according to the present invention; 
       FIG. 2  is a top elevation of the light guide plate of  FIG. 1 , showing a distribution and structure of a plurality of diffraction grating units on a light emitting surface thereof; 
       FIG. 3  is a schematic, side sectional view of a second embodiment of a backlight module having a light guide plate according to the present invention; 
       FIG. 4  is a top elevation of the light guide plate of  FIG. 3 , showing a distribution and structure of a plurality of diffraction grating units on a light emitting surface thereof; and 
       FIG. 5  is a schematic, side sectional view of a conventional backlight module. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to  FIG. 1 , a backlight module  10  according to the first embodiment of the present invention is shown. The backlight module  10  comprises a linear light source  11 , a plate-like transparent light guide member  12  having a rectangular cross-section, and a reflection plate  13 . The light guide plate  12  comprises a light incidence surface  121 , a light emitting surface  123 , and a bottom surface  122  opposite to the light emitting surface  123 . The light source  11  is a CCFL (cold cathode fluorescent lamp) disposed adjacent the light incidence surface  121 . The reflection plate  13  is disposed under the bottom surface  122 . A plurality of micro dots  124  is formed on the bottom surface  122  for diffusing light beams. 
   Referring to  FIG. 2 , a plurality of diffraction grating units  125  is provided continuously on the light emitting surface  123 . Each unit  125  has a strong diffractive portion  1251  and a weak diffractive portion  1252 . The grating constant in both diffractive portions  1251 ,  1252  is in the range from 2–10 μm, and preferably 3 μm. Since the CCFL light source  11  mainly emits light beams in a direction perpendicular to the light incidence surface  121 , a grating direction of the strong diffractive portion  1251  is arranged to be parallel with the light incidence surface  121  for diffracting the light beams with maximum efficiency. A grating direction of the weak diffractive portion  1252  is perpendicular to that of the strong diffractive portion  1251 . 
   Referring to  FIG. 2 , a ratio of an area of the strong diffractive portion  1251  to an area of the weak diffractive portion  1252  in each unit  125  becomes progressively greater with increasing distance away from the light incidence surface  121 . As shown in  FIG. 2 , units  125  in four regions I, II, III, IV sequenced along a direction away from the light incidence surface  121  have different configurations and different area ratios. Because the diffraction grating units  125  further from the light incidence surface  121  have larger strong diffractive portions  1251 , they have stronger diffracting capability. Since the intensity of light beams decreases with increasing distance of propagation, the quantity of light beams received by the units  125  decreases with increasing distance away from the light incidence surface  121 . The configuration of the diffraction grating units  125  on the light emitting surface  123  compensates the location of each unit  125  with a corresponding diffraction capability. This assures uniformity of the light beams emitting from the light emitting surface  123 . 
   Each unit  125  has the weak diffraction portion  1252  in addition to the strong diffraction portion  1251 . The weak diffraction portion  1252  can diffract incoming light beams that do not travel in directions perpendicular to the light incidence surface  123 . That is, a small quantity of light beams is incident on the unit  125  in oblique directions. The weak diffractive portion  1252  diffracts these light beams to some extent, thereby improving utilization of all light beams. 
   The uniformity of light emitting from the light guide plate  10  can be controlled by configuring the area ratios of the strong diffractive portions  1251  in the units  125  accordingly. Unlike in the prior art, there is no wastage of light beams at non-grating parts. Therefore the light utilization efficiency of the light guide plate  10  is higher, at least fractionally. In high-end products, the change in intensity of light beams emitted from the units  125  having the weak diffractive portions  1252  needs to be considered and is desirable. This is because of the exacting requirements for uniformity of illumination of such products. 
   Further, most light sources including the light source  11  irradiate light beams with various wavelengths, and the UV (ultra violet) light component of the light beams often comprises a large share of the total light energy. In the backlight module  10 , a fluorescent layer (not shown) is provided on the bottom surface  122  to utilize the UV light energy. When UV light impinges on the fluorescent layer, the fluorescent layer emits visible light. This reduces or even eliminates wastage of the UV energy of the light beams, and enhances the brightness of the light beams emitted from the backlight module  10 . 
   Referring to  FIG. 3 , a backlight module  20  according to the second embodiment of the present invention is shown. The backlight module  20  comprises two point light sources  21 , a transparent light guide plate  22  having a wedgy cross-section, and a reflection plate  23 . The light guide plate  22  comprises a light incidence surface  221 , a light emitting surface  223 , and a bottom surface  222  opposite to the light emitting surface  223 . The light sources  21  are LEDs (light emitting diodes), and are disposed adjacent the light incidence surface  221 . The reflection plate  23  is disposed under the bottom surface  222 . A plurality of micro dots  224  is formed on the bottom surface  222 , for diffusing light beams. 
   Referring to  FIG. 4 , a plurality of diffraction grating units  225  is provided continuously on the light emitting surface  223 . Each unit  225  has a strong diffractive portion  2251  and a weak diffractive portion  2252 . The grating constant in both diffraction portions is in the range from 2–10 μm, and preferably 3 μm. Because the light sources  21  are two LEDs, light beams emitted by them are quite different from the light beams emitted by the CCFL  11  of the backlight module  10  of the first embodiment. Propagation of the light beams emitted by the light sources  21  is approximately over a range of angles covering three directions; that is, a 2 o&#39;clock direction, a 3 o&#39;clock direction, and a 4 o&#39;clock direction. The light emitting surface  223  is divided into four regions I, II, III and IV sequenced along a direction away from the light incidence surface  221 . Each region has three units  225  disposed along a direction parallel with the incidence surface  221 . That is, the units  225  are in an upper, a middle, and a lower position. The units  225  in the four regions I, II, III, IV have progressively larger strong diffractive portions  2251 , similar to the diffraction grating units  125  of the backlight module  10 . However, the grating directions of the strong diffractive portions  2251  of the units  225  differ. The units  225  at the upper, middle and lower positions have three different grating directions respectively, each being perpendicular to a main direction of incoming light beams. This enables the light guide plate  20  to emit light beams uniformly even though the light sources  21  do not irradiate light beams in a single uniform direction. In each unit  225 , the grating direction of the weak diffractive portion  2252  is perpendicular to that of the strong diffractive portion  2251 . Thus light beams incident on the weak diffractive portion  2252  in oblique directions are diffracted. 
   The backlight module  20  has a fluorescent layer (not shown) provided on the bottom surface  222 , similar to the backlight module  10 . When UV light impinges on the fluorescent layer, the fluorescent layer emits visible light. This enhances the brightness of the light beams emitted from the backlight module  20 . 
   The above-described embodiments employ progressively increasing area ratios of the strong diffractive portions  1251 ,  2251  in the units  125 ,  225 . Further or alternatively, the grating constants of the units  125 ,  225  can be varied in order to obtain the desired diffraction capabilities. That is, the grating constants of the strong diffractive portions  1251 ,  2251  of the diffraction grating units  125 ,  225  can progressively decrease with increasing distance away from the light incidence surface  121 ,  221 . Whatever embodiment is adopted, the units at various locations have various diffraction capabilities in order to compensate for the differences in intensities of the light beams received at the units. 
   The micro dots  124 ,  224  of the bottom surfaces  122 ,  222  can be replaced by prisms or reflective gratings that similarly diffuse light beams. The fluorescent layer can be made of SrAl 2 O 4 . The units  125 ,  225  can be formed on the light guide plate  12 ,  22  by injection molding. For precision, the mold used for such process is itself preferably manufactured by way of laser beam etching or electron beam etching, or another kind of precision process used in the semiconductor field. 
   It is to be understood, however, that even though numerous characteristics and advantages of the present invention have been set forth in the foregoing description, together with details of the structure and function of the invention, the disclosure is illustrative only, and changes may be made in detail, especially in matters of shape, size, and arrangement of parts within the principles of the invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.