Surface quality is a measure of surface defects on optical surfaces such as lenses or mirrors. They can be caused during the manufacturing of the part or handling. These imperfections are part of the surface and cannot be removed by cleaning. Surface quality is commonly characterized either by the American military standard notation, eg "60-40", or by specifying required RMS (root mean square) roughness, eg "0.3 nm RMS". The former notation characterizes how visible defects on the surface are and can be described as a "cosmetic" specification, while the latter notation specifies an objectively measurable property of the surface. In either case, a tighter specification increases the cost of fabricating the optical element while a looser one may affect performance.