National Physical Laboratory

Optical Flatness

Fizeau interferometerThe NPL 305 mm Fizeau interferometer Vertically mounted flatCalibrating a vertically mounted flat 

Measurement of optical flats by interferometry

The measurement of the shape or form of precision engineered components is an essential part of production quality control. Many techniques for such measurements, including interferometry, require a reference surface of known form with which to compare the surface being measured. Optical flats are often used for this purpose. They are also employed as mirrors in optical systems where it is important to preserve the shape of the wavefront upon reflection. NPL offers a service to calibrate the form of optical flats up to 300 mm in diameter. A key feature of this calibration is its traceability to the form of a liquid surface.

Key features

  • Calibration of flats up to a diameter of 300 mm
  • Best measurement capability of ± 0.020 µm P-V by comparison of reference surfaces of known quality
  • Master reference flats standardised by liquid surface interferometry
  • NPL certificate of calibration issued
  • Option to include three dimensional plots, profiles and a table of optical path differences


Customer Service tel: +44 20 8943 6245

Last Updated: 18 Aug 2015
Created: 1 Feb 2011


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