1. An apparatus to fabricate a thin-film layer, the apparatus comprising:a material source;
a support structure to support a substrate;
a conveyance system to provide relative transport between the material source and the substrate along a conveyance path, wherein during the relative transport, the material source is to deposit a material onto the substrate, in order to form the thin-film layer;
an optical source to form an optical beam;
an optical detector fixed in position relative to at least one of the material source or the substrate, the optical detector to detect divergence of the position of the optical detector from the optical beam; and
at least one transducer to displace at least one of the material source or the detector relative to the conveyance path, the at least one transducer to be driven in dependence on an output of the optical detector so as to cause the position of the optical detector to remain coincident with the beam during the relative transport.