Michael Kuchel

Interferometric measurement of rotationally symmetric aspheric surfaces

The measurement of aspheric surfaces in a Fizeau interferometer implies a sometimes dramatic increase in dynamic range, in terms of acceptable slope and departure, which can run the risk of introducing substantial measurement errors. Common approaches to relaxing the dynamic range requirement include reducing the area of the surface measured in a single measurement and stitching together the partial results, or using compensation techniques with the help of additional components like null-lenses or computer generated holograms. This paper reviews these methods, with special attention to the questions of degrees of freedom for misalignment. These considerations lead to a proposed method that uses the inherent symmetry of the problem to scan along the optical axis, gathering measurements at zones of normal incidence. These measurements are independent from each other; their ensemble represents directly the surface-deviation in normal direction to the surface and the result is in the object coordinates of the design surface. Using an absolutely calibrated spherical reference surface, the result is absolute. It is shown that this is very different from the technique of stitching of zones, even when Intrinsic Coma is preserved through partially overlapping measurement regions.The measurement of aspheric surfaces in a Fizeau interferometer implies a sometimes dramatic increase in dynamic range, in terms of acceptable slope and departure, which can run the risk of introducing substantial measurement errors. Common approaches to relaxing the dynamic range requirement include reducing the area of the surface measured in a single measurement and stitching together the partial results, or using compensation techniques with the help of additional components like null-lenses or computer generated holograms. This paper reviews these methods, with special attention to the questions of degrees of freedom for misalignment. These considerations lead to a proposed method that uses the inherent symmetry of the problem to scan along the optical axis, gathering measurements at zones of normal incidence. These measurements are independent from each other; their ensemble represents directly the surface-deviation in normal direction to the surface and the result is in the object coordinates of the design surface. Using an absolutely calibrated spherical reference surface, the result is absolute. It is shown that this is very different from the technique of stitching of zones, even when Intrinsic Coma is preserved through partially overlapping measurement regions.