An algorithm to characterise multilayer optical coatings with large numbers of layers by spectroscopic ellipsometry, using a discrete spectral-zone fitting approach, has been demonstrated by characterising a 27-layer TiO₂/SiO₂ multilayer beam combiner 22 in the wavelength range of 280–1200 nm. The ellipsometric spectra are fitted first in the wavelength regime of 700–1200 nm and the sample structure was determined. TiO₂ and SiO₂ layers have been assumed to be transparent in this wavelength regime 22 with dispersion-less refractive indices. The data were then fitted in the wavelength range of 280–340 nm to find the dispersion relation for the optical constants of TiO₂. Finally, the fitting has been done in the wavelength range of 340–700 nm and the true dispersion of refractive index of TiO₂ along with the best fit sample structure have been obtained.