1. A method of determining at least one property of a substrate, comprising: measuring a plurality of scatterometry spectra from one or more targets on said substrate; and performing reconstructions of each of said spectra to derive one or more values for said property by comparing representations of each of said measured spectra with one or more modeled representations of spectra calculated using variable parameter values, wherein at least one parameter in the reconstruction for each spectrum is linked to the value of the parameter used in the reconstruction for a different spectrum.