Frequency and wavelength standards, present and in prospect

Abstract

Techniques have now been developed that allow direct measurement of frequencies through the infrared and into the visible. They have already yielded spectroscopic standards more precisely known through their frequencies than their wavelengths and have made it attractive to redefine the international metre in terms of the speed of light and the primary time standard. The implications for spectroscopy of these techniques and of improved methods for wavelength comparison are discussed.