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

Null Methods in Optical Testing

“…This optical system was proposed after several previous devices designed by different authors using reflective and refractive components [8]. …”

Two computer-generated holograms for testing convex aspheric surface

“…For special convex conical surfaces such as paraboloids and hyperboloids, the most common test for these surfaces has been implemented by using a Hindle sphere. The problem with this method is that a very large concave spherical surface, much larger than the surface under test, is required [1] . For those general aspherics, convex aspheres made of solid glass may be measured using a null lens looking through the back of the blank, but refractive index inhomogeneity limits the application of this test [2~3] .…”

Measuring aspheres quickly: tilted wave interferometry

“…20,21 To avoid both of the above-mentioned problems (retrace errors and vignetting), so-called compensation optics comes into play to compensate these departures and ensure normal or close-to-normal incidence of the testing light onto the SUT. State of the art is the use of computer generated holograms (CGH), [20][21][22][23] where the perfect shape of the asphere under test and typically also alignment aids are encoded in the CGH. The full field measurement then reveals the deviations from this perfect shape.…”