The Fourier method of interferogram analysis requires the introduction of a constant tilt into the interferogram to serve as a 'carrier signal' for information on the figure of the surface under test. This tilt is usually removed in the first steps of analysis and ignored thereafter. However, in the problem of aligning optical components and systems, knowledge of part orientation is crucial to proper instrument performance. This paper outlines an algorithm which uses the normally ignored carrier signal in Fourier analysis to compute an absolute tilt (orientation) of the test surface. We also provide a brief outline of how this technique, incorporated in a rotating Twyman-Green interferometer, can be used in alignment and metrology of optical systems.
Ronald W. Toland, Ronald W. Toland,
"Absolute measurement of tilts via Fourier analysis of interferograms", Proc. SPIE 5531, Interferometry XII: Techniques and Analysis, (2 August 2004); doi: 10.1117/12.559810; https://doi.org/10.1117/12.559810