A German national research institute active in metrology has developed a measurement process, which enables the efficient measurements of strong curved spherical segments which are often used in optical measurement equipment. Measurement uncertainties due to thermal expansion of the measurement equipment are reduces by at least factor 2. The method uses an interferometer signal coming from the front onto the spherical surface to be measured. In the center (on the optical axis) a phase value is determined, which is determined down to half a wavelength (typically the wavelength is 632 nm).