Illuminating device and its lens used therein

The invention discloses a luminescent device and a lens thereof. The lens comprises a incident surface and a refraction exit surface; the lens is in a slice shape; the subface of the lens is an incident surface; multi-circle ring-shape saw teeth are arranged on the top surface of the lens; these saw teeth are arranged along the direction from the center of the lens toward the inner and outer brims, and the inner lateral-faces and outer lateral-faces of the saw teeth form a refraction exit surface. A total reflection plane can also be arranged on the lateral face of the lens. The luminescent device comprises a light-source and the lens arranged in front of the light source. The invention can reduce the thickness of the lens to the largest extent, as a result, the light loss inside the lens is reduced, and the light emitting efficiency is improved. The very small thickness can realize a relatively small light-beam angle. Besides meeting the requirements of optically designed light-distribution curve, the utilit