1. A production line having a plurality of successive stages for construction of a product comprising at least one layer on a substrate, and routers for transferring partly constructed products between the stages such that each stage receives a respective predefined partly constructed product as its input, the production line comprising: a predetermined reflected light intensity spectrum for at least one stage representing the respective predefined part construction for the stage, a reflected light intensity spectrum deriver located at said at least one stage operable to obtain reflected light intensity spectra of incoming partly constructed product, and a comparator, for comparing said obtained reflected light intensity spectra with said predetermined reflected light intensity spectrum, to determine whether said incoming partly constructed products correspond with said respective predefined part construction for the stage.