Irregularity

A type of surface accuracy specification that describes how the shape of a surface deviates from the shape of a reference surface. Regularity refers to the sphericity of the circular fringes that are formed from the comparison of the test surface to the reference surface.  When the power of a surface is more than 5 fringes, it is difficult to detect small irregularities of less than 1 fringe. So surfaces are specified with a ratio of power to irregularity of approximately 5:1.