When attempting to specify how closely an optical surface conforms to its intended shape, a measure of surface accuracy is needed. Surface accuracy can be determined by interferometric techniques. Traditional techniques involve comparing the actual surface to a the test plate gage. In this approach, surface accuracy is measured by counting the number of rings or fringes and examining the regularity of the fringe. The accuracy of the fit between the lens and the test gage (as shown below) is described by the number of fringes seen when the gage is in contact with the lens. Test plates are made flat or spherical to within small fractions of a fringe. Modern techniques for measuring surface accuracy utilize phase measuring interferometry with advanced computer data analysis software. During manufacture, a precision component is frequently compared with a test plate that has an accurate polished surface that is the inverse of the surface under test. When the two surfaces are brought together and viewed in nearly monochromatic light, Newton’s rings (interference fringes caused by the near-surface). The number of rings indicates the difference in radius between the surfaces. This is known as power or sometimes as figure. It is measured in rings that are equivalent to half wavelengths. Beyond their number, the rings may exhibit distortion that indicates non-uniform shape differences. The distortion may be local to one small area, or it may be in the form of noncircular fringes over the whole aperture. All such non-uniformities are known collectively as irregularity.