Aspheric lenses (or more simply, aspheres) are used in a wide variety of applications as a way to optimize optical performance while ensuring an efficient design. They are characterized by a radius of curvature that varies from the center to the edge of the lens, affording them greater optical functionality. Because of this, they can reduce the number of elements needed in an optical assembly (better performance meaning the use of fewer lenses) leading to both weight and size reduction in systems.
They are particularly effective at reducing the effects of spherical aberration, and additionally help manage other higher order aberrations and can facilitate improved light focusing and collection efficiency. Their use is being stimulated by improvements in optical manufacturing processes and metrology techniques---and the demand for highly functional small optical devices.