Fizeau interferometry for large convex surfaces

Measurements of large convex surfaces are notoriously difficult because they require auxiliary optics that are larger than the surface being tested. Fizeau interferometry is well suited for these surfaces because the only surface required to be made to high accuracy is the concave reference surface, which is only slightly larger than the surface being measured. Convex surfaces which are spherical or aspheric can be measured using spherical, aspherical, or holographic test plates. The reference surfaces for these tests must be of good quality and measured to high accuracy. The optical systems that provide illumination and create an image of the interference pattern do not have to be made to high quality. The illumination systems can typically have errors several orders of magnitude larger than the allowable surface measurement errors, so these systems can be made at low cost. Several such systems using low cost aspheric mirrors and lenses for measuring convex spherical and aspherical surfaces are presented.