The amount by which an optical surface deviates from its ideal design value, expressed in wavelengths of light. Optical manufacturers use this value to determine two common characteristics of an optic: 1.) surface flatness – the difference between the actual surface shape and the difference between the peaks and valleys of the optic under test and the surface it would have if it was defect-free. 2.) scratch-dig- often referred to as surface quality, this value is related to the apparent size and number of visible surface defects such as scratches and pits ( called digs) on surface of the part.