The present invention provides a prism. The prism includes a lower surface, an upper surface, a first side surface and a second side surface. The first side surface and the second side surface are disposed between the upper surface and the lower surface. The first side surface and the second side surface of the prism are one-dimensional parabolic surfaces. The lower surface is used to receive light. The first side surface is used to reflect the light from the lower surface to the upper surface. The second side surface is used to reflect the light from the upper surface to the lower surface for further analysis in the process unit afterwards.