These lenses have an aspheric surface to correct for spherical aberration. Aspheric lenses have much lower f-numbers, allowing them to perform better than spherical lenses in light collection, projection, illumination, detection, and condensing applications. Since the focal lengths are often very short, compact designs are often possible.These lenses are fabricated using B270 glass for excellent transmission in the visible to near-infrared regions. The aspheric surface is precision molded, while the plano or spherical surface is ground and polished. Lenses with a plano second surface have minimum aberration. Lenses with a convex back have lower f-numbers. Only single-layer MgF2 coatings are available on these lenses since the steep aspheric curvatures present a performance problem near the edges for multilayer coatings.