National Physical Laboratory

How often should I get my reference flats, spherical surfaces and test lenses recalibrated? (FAQ - Length)

We would recommend that reference flats in interferometers be recalibrated every three to five years depending upon the precision of measurement required. If the flats are used as 'test plates', i.e. they are used in contact with a test surface, then the calibration frequency will also depend on the amount of wear they are subjected to (indicated by scratches on the surfaces). The fringe analysis systems of modern interferometers can be affected by surface damage and contamination. The use of software masks to exclude such artefacts is desirable if errors are to be avoided. It is also important that reference flats are calibrated in the attitude in which they are to be used to account for gravitational sag. The recalibration of spherical surfaces will also depend on their use, with the significance of wear being more significant on long radii than on short. Radius-scopes become more difficult to use since surface damage can cause the contrast of the image to deteriorate. Accurate calibration of test lenses with scratched or badly worn surfaces can be difficult because the damage leads to poor measurement repeatability. The use of damaged test lenses in customers' test equipment can also be problematic, some modern laser scanning focimeters misbehave when scratches are encountered. It is therefore advisable to monitor the surface damage and replace test lenses as soon as trouble is experienced.

Last Updated: 25 Mar 2010
Created: 18 Jul 2007

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