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. generating a differential image for each captured image; c. averaging pixel intensity differences of the respective differential images over the entire differential image to obtain an average image intensity value for each differential image; d. obtaining a critical dimension of the first structure; e. correlating a critical dimension of the first structure with the average image intensity value; f. capturing at least one image of a subsequent structure; and, g. determining, based at least in part on the average image intensity value, a critical dimension of the subsequent structure.