A relatively inexpensive interferometer the residual errors of which can be calibrated inexpensively is desirable.
The uncalibrated interferometer errors should be somewhat less than a wave ptv to ensure accurate measurement.
The random ball test can be used to calibrate any fast spherical wave interferometer with an accessible real test beam focus and sufficient working distance so that the center of the ball can be made to coincide with the test beam focus.
Suitable Grade 5 Silicon nitride balls are readily available.
A Twyman-Green interferometer using a "microscope" objective to produce a suitably located external focus is one possibility.
Another is to use a PDI plate located at the long conjugate of a finite conjugate (or equivalent) microscope objective together with a beamsplitter located in the slow beam to allow coaxial illumination of the test surface from a near diffraction limited point source.
The PDI version is a common path interferometer allowing short coherence length sources to be used.
One option for the "microscope" objective is an aspheric singlet typically used as a fiber collimator plus a long focal length PCX singlet lens (or equivalent).
The "microscope" objective forms a real image of the test surface which in turn is imaged onto the image sensor by the camera optics.
The distortion in the imaging of the test surface onto the image sensor has to be corrected when calculating the surface error.
However achieving a sufficient working distance to permit calibraion via the random ball test is an issue when an NA of 0.2 is required unless a 12mm diameter ball is used with an 11mm efl lens.
Edited by BGRE, 07 June 2019 - 11:07 PM.