National Physical Laboratory

Flatness, Radius and Sphericity Calibration Services

Radius and Sphericity

Radius gauges are widely used to check the form of spherical surfaces used in optical imaging systems. These gauges are made in pairs to a specified radius of curvature, one disc having a concave surface and the other a matching convex surface. Before they can be adopted as test gauges for checking the form of manufactured optical surfaces, their radii of curvature must be accurately measured. NPL provides a calibration service for radius gauges of 5 mm to 1000 mm radius of curvature, with measurements traceable to national standards. Measurements of sphericity can also be carried out using NPL calibrated optics.

Key features

• Measurement of the radius of curvature of radius gauges
• Measurement range 5 mm to 1000 mm radius of curvature
• Radius measurements traceable to national standards
• Sphericity measurements using NPL calibrated reference optics
• NPL certificate of calibration issued

Measurement uncertainty

• Uncertainty of radius of curvature measurements in range ± 0.002 mm to ± 0.010 mm
• Uncertainty of sphericity measurements ± 0.05 λ (p-v departure from best fitting sphere) where λ is the illuminating wavelength
• Uncertainties quoted at the 95% confidence level

Optical Flatness

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

Associated services

• Measurement of radius of curvature and sphericity of other concave or convex surfaces in glass metal or plastic
• Calibration of customer supplied spherometers and associated equipment
• Technical consultancy and training

For further information or to discuss your particular calibration requirements
Contact: Length Services
Direct Line: +44 208 943 6245
Fax: +44 208 943 6184

Last Updated: 25 Mar 2010
Created: 5 Sep 2007


Please note that the information will not be divulged to third parties, or used without your permission