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The design of optical systems gains profit from the application of aspheric elements: to correct for image aberrations, to reduce weight or system length or to cope with other functional needs like the number of reflections or the field size. Manufacturing, however, is suffering in practice from problems resulting from the reduced symmetry of the components as compared to spherical optics. Almost all large optical companies in the world are working on solving the problem of fabrication of accurate aspherical elements at reasonable cost.In this paper fabrication and metrology aspects of aspherical elements (mirrors and lenses) in the IR, visible, UV and EUV at Carl Zeiss and others companies will be discussed. The specifications achieved presently are ranging between some im to nm for figure accuracy and lOOnm to 0mmfor microroughness.

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High quality lenses have reached a standard where wavefront tolerances of a few nanometers for transmission and surface figure of individual components are necessary. The standard measuring tool in production is digital interferometry. To satisfy the demands of current and future production quality interferometric equipment has to provide an overall accuracy below one nanometer under fabrication environment. This is reasonable especially if ,,work man's ruJe is applied demanding a tolerance/accuracy factor for metrology of 10:1. In this paper the limitations on interferometry for material and surface inspection as well as possible improvements are discussed. 2 Principle test setups Typical setups for interferometric tests are of the Fi_-eau type or the Twvman-Green type /1/. In a Fizeau interferometer test and reference beams travel across the same optical components except for the cavity, which is the optical path between reference and test piece (fig. 2). A Fizeau interferometer is thus called a common-path inter ferometer. In an arrangement of the Twyman-Green type reference and test beams are separated, the beams cross different optical components (fig. I). The latter is more sensitive towards mechanical shocks, vibrations, air turbulences and changes in environmental influences. If a Fizeau interferometer has a small cavity only consisting of reference and the test piece very stable test conditions can be established. Either Twyman-Green or Fizeau interferometer are suitable to test . relative deviations of plane, spherical and aspherical surface shapes in reflection, SPIE Vol. 3482 • 0277-786X198/$1O.OO laser Test surface Reference n,Irrar J Camera Fig. 2: Fi:eau interferometer jbr testing spherical surfaces (transmission sphere with rejerence surface) Transmismon sphere Camera Fig. 1: Twyman-Green interferometerjbr testing spherical surfaces ('transmission sphere without reference surface) Downloaded From: http://proceedings.spiedigitallibrary.org/ on 05/15/2015 Terms of Use: http://spiedl.org/terms

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