Interference rings as length standards. These Fabry-Perot interferometer observations (40-centimetre path difference) are for the green line of mercury-198 (left, wavelength: 5462 angstroms) and the orange line of krypton-86 (right, wavelength: 6058 angstroms). These results were used as international length standards. However, the krypton display allowed more accurate measurements. From 1960, the wavelength of light from a krypton-86 lamp replaced the International Prototype Metre as the primary length standard. These sets of interference rings were recorded at the National Physical Laboratory, Teddington, UK.