This is a demonstration of fourier transform frequency scanning interferomtery.
You can use a swept frequency laser source to measure suface figure and homogeneity of transmisive optics like windows and lenses. This is a different technique than phase shifting interferometry which requires you to physically perturb the cavity length to create swept fringe images.
This is based on work by Dr. Leslie L. Deck of the Zygo Corporation:
Fourier-transform phase-shifting interferometry
This technique is used in some of Zygo's optical surface measurement tools, such as this:
Zygo Verifire™ MST
This was written in 2008 or so. This is not an example of excellent programming (I write much better software every day...) but is a demo implementation of the tricky physical and mathematical technique desribed in Deck's paper.
This has been updated to a Visual Studio 2017 C++ solution, you can clone and compile it easily.