A workpiece is placed on a stage in an interferometer. Measurements of the workpiece are taken using the interferometer. A random two-dimensional tilt is applied to the workpiece before each of the measurements using at least one tilt motor. A stage is configured to hold a workpiece in a path of the beam of light from a beam splitter. The tilt motor is connected with the stage and randomly move the stage in two dimensions before a measurement.

A workpiece is placed on a stage in an interferometer. Measurements of the workpiece are taken using the interferometer. A random two-dimensional tilt is applied to the workpiece before each of the measurements using at least one tilt motor. A stage is configured to hold a workpiece in a path of the beam of light from a beam splitter. The tilt motor is connected with the stage and randomly move the stage in two dimensions before a measurement.

An interferometer system for measuring the displacement of a location of a test surface includes a reflective beamsplitter having a through-hole through which light enters into and exits from a reference arm and having a second through-hole through which a portion of light from the measurement arm of the interferometer passes through the beamsplitter and is incident on a position sensing device (PSD). The output of the PSD is then used as an indicator of the amount and direction of tilt of the surface under test so that systemic errors of the interferometer induced by the tilt of the test surface can be determined and removed from the displacement measurement.

An interferometer system for measuring the displacement of a location of a test surface includes a reflective beamsplitter having a through-hole through which light enters into and exits from a reference arm and having a second through-hole through which a portion of light from the measurement arm of the interferometer passes through the beamsplitter and is incident on a position sensing device (PSD). The output of the PSD is then used as an indicator of the amount and direction of tilt of the surface under test so that systemic errors of the interferometer induced by the tilt of the test surface can be determined and removed from the displacement measurement.