Misalignment of spheres, aspheres and freeforms in optical measurement systems

When measuring surfaces it is always a challenge to differentiate whether differences to the expected form originate from positioning errors or from surface errors. In interferometry it is common to subtract tilt and power terms from the measurement result to remove misalignment contributions. This is a suitable approximation for spherical surfaces with small NA. For high NAs and increasing deviations from a spherical shape, which applies to aspheres and freeforms, additional terms show increasing magnitudes. A residual error remains after subtraction of tilt and power. Its form depends on the surface's nominal shape and oftentimes has a non-negligible magnitude, therefore imposing the risk of being misinterpreted as topography error.