Photoelectric Fringe Signal Information and Range in Interferometers with Moving Mirrors*

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Abstract
Photoelectric receptors and electronics have greatly increased the accuracy and simplicity of interferometric measurements. Theoretical and experimental investigations of instrumental and light source conditions required to ensure adequate information content and “visibility” of fringe signals have been made. Mirror parallelism requirements needed to maintain maximum signal modulation and to reduce apparent local displacement errors, for use with equal inclination interferometers, are expressed by the fractional number of equal thickness fringes within the mirror aperture, the sensitivity to varying rotation along the mirror traverse being reducible by decreasing this aperture and ensuring best local parallelism. The “effective” length per fringe, determined by photoelectric flux integration over an angular range within the source angular radius αs is approximately (λ/n)/2(1−αs2/4). Theoretical expressions obtained for the fringe “visibility” with simple single isotope lines of Gaussian shape are in good accord with experiments, showing that the fringe amplitude with perfectly adjusted continuously moving interferometers is governed by an expression of the form

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