How does NPL calibrate its own Reference Flats?
NPL gets the traceability of its 300 mm diameter Master Flat from the surface of a pool of silicone oil, which conforms to the local radius of curvature of the earth and from which the departure from flatness can easily be calculated. This oil surface provides the reference surface in an NPL designed Fizeau type interferometer and enables the errors in the Master Flat to be established. Transfer Standards can then be measured to enable the flatness of the Reference Flats used in other interferometers to be assessed. As noted above, reference flats should be calibrated in the attitude in which they are to be used. (See details of flatness measurement service.
