A method of assessing the surface roughness of an object includes directing gas supplied at a constant pressure through a control orifice and subsequently to a measurement nozzle adjacent to and spaced from a surface of the object, with subsequent escape of the gas to the atmosphere. The object is moved past the measurement nozzle and the resultant back pressure of the gas upstream of the measurement nozzle and downstream of the control nozzle is measured to provide a back pressure signal. The frequency content of the back pressure signal is examined to thereby obtain an assessment of the surface roughness of the object.