Testing of Aspheric Wavefronts and Surfaces

Abstract

Aspheric wavefronts with spherical aberration are produced by optical systems using spherical as well as aspherical surfaces. Aspheric surfaces are used in optical systems in order to improve aberration correction and, frequently, to decrease the number of optical elements needed to make this correction satisfactorily. However, if these surfaces are tested while being isolated from the rest of the optical system to which they belong, they frequently produce aspherical wavefronts. The interferometric testing and measurement of aspherical wavefronts are not as simple as in the case of spherical or flat wavefronts. To test aspherics, often a null test is issued. The usual definition of a null test is that which produces a fringe-free field when the desired wavefront is obtained. Then, if a tilt between the wavefront under test and the reference wavefront is added and the paraxial curvature of them are equal, straight and parallel fringes are obtained. Under these conditions, any deviation from straightness of the fringes is a graphical representation of the wavefront deformation. This is the ideal testing procedure because the desired wavefront is easily identified and measured with high accuracy. There are several methods to obtain this null test, but sometimes this is not simple and may even be a source of possible errors. Typically, if a quantitative retrieval of the wavefront is desired, the interferogram is imaged onto a CCD detector. Then, the straightness of the fringes for a perfect wavefront is useful but not absolutely necessary. However, the minimum fringe spacing should be larger than twice the pixel size in the detector. This is the wellknown Nyquist condition, which may be impossible to satisfy if the wavefront has a strong asphericity. In a Fizeau or Twyman–Green interferogram, a strong rotationally symmetric aspheric wavefront has many fringes when taken at the paraxial focus setting as shown in Figure 12.1(a). By adding a small curvature to the wavefront, that is, by adding defocusing, the minimum fringe spacing can be slightly reduced. For