BS EN ISO 3868:1995 Metallic and other non-organic coatings. Measurement of coating thicknesses. Fizeau multiple-beam interferometry method
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This International Standard specifies a method for the measurement of the thickness of thin, highly reflective coatings (up to 2 μm) by the use of Fizeau multiple-beam interferometry.
The method described cannot be applied to vitreous enamel coatings.
PRINCIPLE
By completely dissolving a small area of the coating without attack of its substrate (or by masking before plating), a step is formed from the surface of the coating to that of its substrate. The height of this step is measured with a multiple-beam interferometer.
A monochromatic light beam is reflected back and forth between the specimen surface and a superimposed trans- parent mirror serving as a planar reference plate, so as to produce a pattern of interference fringes observed through a low-power microscope. The reference plate is tilted slightly with respect to the surface being inspected, so that the fringe pattern is a series of parallel lines. A step in the specimen surface causes a shift in the fringe pattern. A shift of one full fringe spacing is equivalent to a vertical displacement of 1/2 of the wavelength of the monochromatic light. The whole and fractional number of fringe spacings occupied by the fringe shift is determined with an eyepiece micrometer.
