Nonlinear optical crystals, such as KTP (ferroelectric potassium phosphate) or BBO(3- boric acid paving). Lenses are optical elements made of transparent substances (such as glass and crystal). Convex lens: thick in the middle and thin in the edge, including biconvex, plano-convex and concave-convex; Concave lens: the center is thin and the side is thick, and there are three types: double concave, flat concave and convex concave. Lens plays an important role in astronomy, military, transportation, medicine, art and other fields.