Flatness Interferometer – Optical Flatness Measurement

The flatness interferometer is used at METAS for the high-precision calibration of flatness standards such as optical flats and optical mirrors. The measurement system operates according to the Fizeau interferometer principle, using a stabilised helium-neon laser light source, and enables the non-contact determination of minute flatness deviations in the nanometre range.

For the measurement, interference fringes are generated between a high-precision reference optical flat and the object under test. A piezo-controlled phase shift and analysis of the interference images using a CCD camera enable the surface topography of the object under test to be determined with high accuracy. The interferometric measurement method provides traceable, areal characterisation of precision optical surfaces.

The flatness interferometer is suitable for optical flats and mirrors with diameters of up to 95 mm. Measurement uncertainties of down to 10 nm can be achieved for flatness measurements, enabling the calibration of optical reference surfaces at the highest level of metrological accuracy.