A sheet of material is received at a sensor assembly, which includes a sensor operable to measure a property of the sheet. An air flow is generated that is substantially tangential to the sheet in order to at least partially control a position of the sheet relative to the sensor assembly. For example, the sheet may be associated with an upstream boundary layer of air and a downstream boundary layer of air. At least part of the air from the upstream boundary layer could be removed and used to provide an air flow forming at least part of the downstream boundary layer. Also, the air flow could be provided between a surface of the sensor assembly and the sheet to at least partially control a distance of the sheet from the surface of the sensor assembly and/or an angle at which the sheet passes the surface of the sensor assembly.