1. A method of model-based optical metrology to measure a critical dimension of a grating layer, the method comprising: illuminating an area of a geometrical structure of dispersive materials with incident electromagnetic radiation from an illuminator of a scatterometer apparatus, wherein the incident electromagnetic radiation is polarized; measuring spectral components of the incident electromagnetic radiation reflected from the area using a detector of the scatterometer apparatus; and using a computer for the scatterometer apparatus to determine parameter values for the semiconductor structure that minimize an objective function J which depends on differences between the measured spectral components and computed spectral components based on a parameterized geometrical model of the geometrical structure, wherein steps for determining the parameter values that minimize the objective function include: computing ∇ p J where p is a parameterized vector of the parameter values, wherein the parameter values comprise ƒ and h, where h is a height of the grating layer, and ƒ is a critical dimension fraction comprising the critical dimension divided by a pitch of the grating layer.