1. A symmetric prism array, comprising: a first refracting portion, comprising a plurality of first right triangle prisms, wherein longer legs of said first right triangle prisms are arranged along an identical line on an incident plane, and a second refracting portion, comprising a plurality of second right triangle prisms which are corresponding to the first right triangle prisms, wherein longer legs of said second right triangle prisms are arranged along the identical line on the incident plane, and wherein said first right triangle prisms and said second right triangle prisms are left-right symmetric with respect to a central symmetry line, and wherein normals of hypotenuses of said first right triangle prisms diverge from said central symmetry line and normals of hypotenuses of said second right hand triangle prisms diverge from said central symmetry line.