Phase dispersion of three-layer zinc sulphide-cryolite coatings and interferometric measurement of wavelength in the visible spectrum

Abstract

The phase dispersion over the range 4046 to 6438 Å has been determined for a pair of three layer zinc sulphide-cryolite dielectric coatings from measurements of the change with wavelength in the retardation of an etalon made with five lines of known wavelength emitted by a mercury isotope lamp and the red line emitted by a cadmium lamp. Over the range 4350 to 5800 Å the phase dispersion (expressed as the change in retardation of an etalon—the most convenient method for interferometric measurements of wavelength) has been found in this research to be linear to an accuracy within the limit of accuracy of the measurements. It is therefore possible to use a Fabry-Perot etalon with these coatings satisfactorily for measurements of wavelengths in this range, the retardation being determined by linear interpolation between the retardation found for the four lines radiated by a standard mercury isotope lamp. The advantages of this method when low intensity or limited life of the light source presents difficulties are shown to be significant. Measurements of wavelength of some lines in the spectrum xenon ɪ are compared with earlier measurements by Humphreys & Meggers (1933), and show that this method of measuring wavelengths is satisfactory.