section_path: Non spherical types
section_name: Non spherical types
paragraph_text: Non spherical types
  
  Cylindrical lenses have curvature along only one axis. They are used to focus light into a line, or to convert the elliptical light from a laser diode into a round beam. They are also used in motion picture anamorphic lenses.
  
  Aspheric lenses have at least one surface that is neither spherical nor cylindrical. The more complicated shapes allow such lenses to form images with less aberration than standard simple lenses, but they are more difficult and expensive to produce. These were formerly complex to make and often extremely expensive, but advances in technology have greatly reduced the manufacturing cost for such lenses.
  
  A Fresnel lens has its optical surface broken up into narrow rings, allowing the lens to be much thinner and lighter than conventional lenses. Durable Fresnel lenses can be molded from plastic and are inexpensive.
  
  Lenticular lenses are arrays of microlenses that are used in lenticular printing to make images that have an illusion of depth or that change when viewed from different angles.
  
  Bifocal lens has two or more, or a graduated, focal lengths ground into the lens.
  
  A gradient index lens has flat optical surfaces, but has a radial or axial variation in index of refraction that causes light passing through the lens to be focused.
  
  An axicon has a conical optical surface. It images a point source into a line along the optic axis, or transforms a laser beam into a ring.
  
  Diffractive optical elements can function as lenses.
  
  Superlenses are made from negative index metamaterials and claim to produce images at spatial resolutions exceeding the diffraction limit. The first superlenses were made in 2004 using such a metamaterial for microwaves. Improved versions have been made by other researchers. As of 2014 the superlens has not yet been demonstrated at visible or near- infrared wavelengths.
  
  A prototype flat ultrathin lens, with no curvature has been developed.