At present, the LED side light type lighting is adopted on a small-sized thin LCD back light module, wherein, an LGP is an essential element.  In the existing structural design of the LGP, reasonable microstructures are usually distributed on the bottom surface of the LGP, and the microstructures are utilized to obtain the light of LED incident to the LGP.  The microstructures can scatter and reflect the light rays, and finally exit from the LGP, thereby reaching the even brightness of the panel surface of the whole LGP. 
Please refer to FIG.  1 which is the planar structure diagram of an existing back light module.  The back light module comprises an LGP 1 and an LED light bar 4.  The LGP 1 comprises a light input surface 2 opposite to the LED light bar 4, a bottom surface 5, a light output surface (not labeled) opposite to the bottom surface 5, and two side surfaces (not labeled) perpendicular to the light input surface 2.  In the Figure, x direction is the direction parallel to the light input surface 2 of the LGP 1; y direction is the direction parallel to the two side surfaces of the LGP 1.  The bottom surface 5 of the LGP 1 is provided with a plurality of microstructures 3 parallel to x direction. 
Generally speaking, the light ray is concentrated in the central area of the LGP 1, which is a relatively ideal light output situation of the LGP 1 and conforms to the human eyes' vision.  Through the experiment, the light output situation of the LGP 1 in y direction is high brightness in the center and low brightness on edges; the light output of the LGP 1 in x direction is that the light is not well concentrated in the center.  Thereby causing the light energy loss, and reducing the light utilization efficiency.