XXXI New contributions to interferometry. Part V.—New multiple beam white light interference fringes and their applications

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Abstract
A description is given of some new thin film multiple beam white light fringes. Both the theory and properties of these multicoloured fringes are discussed in detail and some of their many possible applications to precision interferometry are considered. It is proposed, on theoretical grounds, to call them “fringes of equal chromatic order.” They afford an extremely powerful new method for examining thin film interference phenomena. For example, surface defects can be measured if even only of the order of λ/200. New topographical features are revealed on the cleavage faces of mica crystals, with the same degree of precision. New properties of thin mica films can be demonstrated, with a precision some 60 per cent, better than in the case of air films. The fringes have wider application and are much more powerful analytically than even highly sharpened multiple beam monochromatic Fizeau fringes. Extremely small surface angles can be measured over very small areas, the resolution available being 100 times better than that of the Rayleigh limit.