Lenses are classified by the curvature of the two optical surfaces. Also called a "converging lens" because it converges parallel rays toward a focus point. A convex lens bulges outward from the centre of the lens. When this curve occurs on both sides of a lens it is called "biconvex" and when it occurs on a single side it is called "plano-convex.". The PC Spot is so named because of its use of the plano-convex lens.