Virtual synchronization: uncoupling synchronization annotations from synchronization code

In apparatus for sensing and controlling the deposition of a thin film on to a substrate (12) from a gas or vapor phase, the optical reflectance or transmittance is sensed, the resonant frequency of a crystal (14) also exposed to the deposition is sensed, and the quotient of the change in each signal over a predetermined time interval is determined. The deposition can be terminated at a suitable thickness, for example a reflectance or transmittance maximum or minimum. Instead of an optical property, the electrical resistivity of the film may be sensed and used in the same way.