National Physical Laboratory

Do surface coatings affect the measurements we perform? (FAQ - Length)

To some degree yes. Ideally, the reflectivity of both the reference and test surface should be equal to give maximum contrast fringes. Highly reflecting coatings may reduce the fringe contrast to such a low level that fringes are almost invisible and fringe analysis systems cease to work. In the case of radius of curvature measurements, anti-reflection coatings can sometimes make surfaces difficult to locate. The importance with non-metallic films is that they are parallel. Any out of parallelism in a dielectric film is seen as an error in the surface shape and, more importantly, the observed error will vary according to the test wavelength used. At NPL Reference Flats up to 150 mm diameter having high or low reflectance's can normally be calibrated since these artefacts are measured using a commercial interferometer having a reference surface with a special, high reflectivity coating. Flats of 150 mm diameter and over, which have to be measured on the large NPL Fizeau, are more problematic if the fringe contrast is low.

Last Updated: 25 Mar 2010
Created: 18 Jul 2007


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