Calibration of interferometric non-null test setups

The calibration of an interferometric test setup is the essential prerequisite for quantitative measurements. Measuring in null condition provides the optimal results in terms of measurement accuracy since the measurements do not deviate substantially from the calibration measurement. However, actual production needs require a high degree of flexibility in terms of specimen to be tested. A good example is form measurements on optical surfaces, such as aspheres and free form surfaces. In this contribution we present an approach to calibrate wave front based setups not only in the null test condition but for a measuring volume. This approach was first applied to TWI (tilted wave interferometry). We show how it can be applied to standard interferometry setups that need to measure far away from fluffed out fringes.