1. A method of controlling a manufacturing process, the method comprising the steps of: a) providing a testing area with a periodic structure, wherein the periodic structure includes a series of sets of patterned features; b) illuminating the periodic structure with a light, thereby producing a non-zero order diffraction signal; c) collecting the diffraction signal to produce a test signature; d) matching the test signature with a reference signature, wherein the reference signature was previously produced by performing steps a), b), and c) with respect to a reference structure that is at least similar to the periodic structure; and e) controlling a manufacturing process using a control setting set associated with the matching reference signature.