A lens includes a light incident surface and a light output surface. In a coordinate system, the light incident surface satisfies a formula (1): Z r 1 = c 1 ⁢ r 1 2 1 + 1 - r 1 2 ⁢ c 1 2 + u 1 2 ⁡ ( 1 - u 1 2 ) 1 - r 1 2 ⁢ c 1 2 ⁢ ∑ m = 0 M ⁢ a m ⁢ Q m ⁡ ( u 1 2 ) . c1 is a curvature of the light incident surface. r1 is a distance from the center of the lens to a joint point between the light input surface and a radius which is substantially perpendicular to the central axis of the lens. u1 is a normalized radius of the light incident surface. am represents the aspheric coefficient. Qm is a polynomial of order m and represents that the sum of am is equal to the sum of the root mean square of the slope of the light incident surface. The light output surface satisfies a formula (2): Z r ⁢ ⁢ 2 = c 2 ⁢ r 2 2 1 + 1 - r 2 2 ⁢ c 2 2 + u 2 2 ⁡ ( 1 - u 2 2 ) 1 - r 2 2 ⁢ c 2 2 ⁢ ∑ m = 0 M ⁢ a m ⁢ Q m ⁡ ( u 2 2 ) .