Interferometric Testing Of Aspheric Surfaces

Abstract

Methods for testing aspheric surfaces fall into two categories: null tests and non-null tests. In a null test, accessory optics are included in the test setup to cancel the asphericity of the aspheric optics under test. Thus, if the aspheric optics under test are perfect, an interferometric test will give straight equally-spaced fringes. In a non-null test, non-straight, non-equally-spaced fringes will be obtained even if the optics under test are perfect. As long as computer analysis is available and the fringe spacing is sufficiently large that the fringes can be detected, non-null tests are as good as null tests, and they are generally easier to perform. Non-null tests can measure the wavefront directly, or a shearing interferometer test can be performed which measures the slope of the wavefront, rather than the wavefront itself. A shearing interferometer test has the advantage that for a given wavefront aberration, fewer interference fringes are obtained; however it has the disadvantage that the measurement accuracy is less than if the wavefront is measured directly. This paper discusses and compares various aspheric optics testing schemes. Null tests that are discussed include the use of real and computer-generated holograms and the use of refractive and reflective null optics. Non-null tests that are discussed include lateral and radial shearing interferometry, direct phase measurement using large linear and two-dimensional solid-state detector arrays, sub-Nyquist interferometry, long-wavelength interferometry, and two-wavelength holography (or interferometry). The importance of combining lens analysis (design) software with interferogram analysis software is discussed.