TERMITES is typically implemented on collimated beams. It uses a Michelson interferometer with a flat mirror in one arm, and a convex spherical mirror in the other. In this second arm, the central part of the beam is converted into a diverging beam, which eventually fully overlaps with the beam to becharacterized and is used as a reference, as in point diffraction interferometry. The spatio-spectral comparison of these two beams is achieved by measuring, on each pixel of the detector, the signal resulting from their superposition as a function of delay. Their cross-spectral density function is then obtained by a Fourier transformation of this first order cross-correlation function.
“Space-time characterization of ultra-intense femtosecond laser beams”, G. Pariente, V. Gallet, A.Borot, O. Gobert and F. Quéré, Nat. Photonics (2016) 10 p547