Surface Accuracy

An indication of the peak-to-valley flatness of a surface given in terms of waves (a multiple or part of a reference wavelength). For example, a specification of 1/4 wave means that the flatness of the surface deviates by one quarter of the wavelength used to test the surface. If the wavelength was 550nm, the flatness would be held to within 140nm in order to produce a surface accuracy of 1/4 wave.  Also given in RMS (root mean squared)  where the peak-to-valley values are squared and then the square root is taken of their mean.