The present invention is directed at least in part to methods and apparatus for optically monitoring selected optical characteristics of coatings formed on substrates during the deposition process and controlling the deposition process responsive thereto. In one aspect, the system includes a retroreflector for reflecting an electromagnetic beam transmitted by the coating and substrate back through the substrate and coating before selected properties of the retroreflected beam are measured. The system and method improve the signal to noise properties of the measured beam. The present invention may be used in systems for coating one or an array of substrates, and is particularly suitable for deposition processes where the substrates are translated past the sources of material to be deposited, and wherein the angle of incidence of a monitor beam on the substrate changes as the substrate translates past the beam source.