The challenging constraints imposed on the Euclid telescope imaging performances have driven the design, manufacturing and characterisation of the multi-layers coatings of the dichroic. Indeed it was found that the coatings layers thickness inhomogeneity will introduce a wavelength dependent phase-shift resulting in degradation of the image quality of the telescope. Such changes must be characterized and/or simulated since they could be non-negligible contributors to the scientific performance accuracy. Several papers on this topic can be found in litterature, however the results can not be applied directly to Euclid's dichroic coatings. In particular an applicable model of the phase-shift variation with the wavelength could not be found and was developped. The results achieved with the mathematical model are compared to experimental results of tests performed on a development prototype of the Euclid's dichroic.
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