Phase correction measurements in gauge block metrology

Project Description

The aim of this project is to establish harmonisation in the methods of measuring a phase correction by quantifying the variability of the measurements. Participants are asked to measure a phase correction for a number of NPL-supplied steel gauges on steel and quartz platens. Any method for accomplishing this can be used. NPL will then analyse the results.

Final Report 1999-10-05

A set of eight gauge blocks, two steel platens and two quartz platens have been circulated to ten European laboratories, plus laboratories in the USA and Australia, in order to compare methods for measuring a phase correction when obtaining the interferometric length of a gauge block. Measurements were made of the central length and a phase correction for each gauge block. The participants were free to use any method to measure a phase correction. The four methods used to measure a phase correction were: the phase stack technique, optical scattering methods, stylus profilometry and a mechanical contacting technique. The results for the central length measurements showed a linear secular decrease in length as the project progressed - the total change in length being about 30 to 40 nm. Within a 95% confidence level, the central length measurements all agree, but the phase correction results vary by as much as 30 nm in the worst case.
Significant changes to the surface were recorded as the project progressed - the scratch content increased and there was evidence of a reaction of the gauge block surfaces with the box in which they were housed. The largest difference between phase corrections determined by the participants was approximately 30 nm.
This is a relatively large figure compared to the interferometric length measurement uncertainties which are usually quoted. There is no obvious difference in the variability of the values for a phase correction for the quartz and steel platens.In general the results obtained with scattering instruments are in good agreement with the results obtained using the phase stack technique. The exceptions to this are the final NPL results where surface changes invalidate this comparison and all NPL measurements on platen PLBI7. The reason for this latter disagreement is not fully understood at this stage, but may be due to the reliance of the NPL TIS method on the type of steel (or finishing technique) that is used during its calibration.

A report on the comparison has been written and submitted to Metrologia for publication.