Virdee, M (1993) A new approach for establishing a reference plane for absolute measurement of shape and flatness with nanometre precision. NPL Report. MOM 114

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Abstract

An alternative optical method to interferometry has been developed to measure form of flat and of shallow curvature optical surfaces to nanometre accuracy without reference to a master surface. The method consists of measuring the surface slopes at points along a line with a resolution of 1/100 of a second of arc. Slope values are integrated with respect to distance to obtain profile height. The surface is scanned with a laser beam whose direction and straightness can be maintained to a high degree of accuracy by using enclosures that insulate the acoustic noise and improve the thermal stability to better than 0.1 °C. The line profiles measured in X and Y directions are combined using a least-squares solution to obtain a complete surface map and reference plane. The calculated reference surface is further corrected for twist by measuring the diagonal profiles. Recent measurements using a new two-dimensional profilometer system have shown that it can repeatably measure shape over an area of 120 x 120 mm to within ­± 2 nanometres with angular deviations from flatness of up to 5 minutes of arc. This profilometer has a capability of readily examining surfaces with specular reflectivities varying from 4% to 100% up to 200 rnrn in diameter. Recently an assessment of systematic uncertainties of the two-dimensional instrument has been made by comparing flatness results with those obtained by Fizeau and Zygo interferometry, and the two sets of measurements agree to within ± 5 nanometres. The slope integration method has the potential to be developed further to characterise non-specular diffusing and specular surfaces over an area of at least 400 x 400 mm.

Item Type:	Report/Guide (NPL Report)
NPL Report No.:	MOM 114
Subjects:	Engineering Measurements Engineering Measurements > Dimensional
Last Modified:	02 Feb 2018 13:19
URI:	http://eprintspublications.npl.co.uk/id/eprint/342

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