1. A method of monitoring critical dimensions in a semiconductor fabrication process comprising: a. capturing at least one image of a first structure having a geometry which has an effect on the polarization state of light reflected therefrom; b. calculating, for each of the at least one images of the first structure a value that is indicative of an intensity of the light reflected from the first structure; c. obtaining a critical dimension of the first structure; d. correlating a critical dimension of the first structure with the calculated value; e. capturing at least one image of a subsequent structure; and, f. determining, based at least in part on the calculated value, a critical dimension of the subsequent structure.