A diffuser provided with plural shapes obtained by translation on an xy plane of at least one of z=g(x, y) and z=−g(x, y), z=g(x, y) being a smooth function within a rectangle having sides of length of s in x direction and sides of length of t in y direction, the origin being the center of the rectangle, wherein on the sides of the rectangle, g ⁡ ( x , y ) = 0 , ⁢ ∂ g ⁡ ( x , y ) ∂ x = 0 , ⁢ ∂ g ⁡ ( x , y ) ∂ y = 0 , ⁢ ∂ 2 ⁢ g ⁡ ( x , y ) ∂ x 2 = 0 , and ∂ 2 ⁢ g ⁡ ( x , y ) ∂ y 2 = 0 , and wherein z=g(x, y) has a single vertex at (xv,yv) g(x,y)=h1(x)·h2(y), first derivative of z=h1(x) is continuous in ( - s 2 , s 2 ) , second derivative of z=h1(x) has a single point of discontinuity in ( - s 2 , x v ) and ⁢ ( x v , s 2 ) , first derivative of z=h2(y) is continuous in ( - t 2 , t 2 ) , and second derivative of z=h2(y) has a single point of discontinuity in ( - t 2 , y v ) and ⁢ ( y v , t 2 ) .