An apparatus and a method are disclosed for measuring at least one desired parameter of a patterned structure having a plurality of features defined by a certain process of its manufacturing, wherein the structure represents a grid having at least one cycle formed of at least two metal-containing regions spaced by substantially transparent regions with respect to incident light defining a waveguide. The method utilizes an optical model on at least some of the features of the structure defined by a certain process of its manufacturing, and is capable of determining theoretical data representative of photometric intensities of light components of different wavelengths specularly reflected from the structure and of calculating said at least one desired parameter of the structure.