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Form and Wavefront Metrology

Working Group 4.21

Interferometric flatness and sphericity measurement

The flatness of optical surfaces with diameters of up to 300 mm is normally measured by means of a calibrated Fizeau interferometer. The associated measurement uncertainty amounts to 5.5 nm (or, as expanded uncertainty according to GUM, to 11 nm)

Fig.: Interferometer set-up for flatness measurement

The calibration of the interferometer and also special calibrations with lowest measurement uncertainty are performed with the high-accuracy ESAD measuring facility developed at PTB. This facility allows uncertainties in the subnanometer range to be achieved and test specimens up to an extension of 500 mm to be measured.

A Twyman-Green extension is available for the measurement of wave aberrations. It allows, for example, the wave aberration of lenses (for a wavelength of 632 nm or 543 nm and infinite object distance) or the sphericity of reflection spheres to be measured.