Tilted Wave Interferometry (TWI) is a measurement technique for fast and flexible interferometric testing of aspheres and freeform surfaces. The first version of the tilted wave principle was implemented in a Twyman-Green type setup with separate reference arm, which is intrinsically susceptible to environmentally induced phase disturbances. In this contribution we present the TWI in a new robust common-path (Fizeau) configuration. The implementation of the Tilted Wave Fizeau Interferometer requires a new approach in illumination, calibration and evaluation. Measurements of two aspheres and a freeform surface show the flexibility and also the increased stability in both phase raw data and surface measurements, which leads to a reduced repeatability up to a factor of three. The novel configuration significantly relaxes the tolerances of the imaging optics used in the interferometer. We demonstrate this using simulations on calibration measurements, where we see an improvement of one order of magnitude compared to the classical Twyman-Green TWI approach and the capability to compensate higher order error contributions on the used optics.

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.