Standards of Length, Wavelength and Optical Frequency

Abstract

Measurements are made in terms of given units by comparing the quantity to be measured (e.g. a length) to a standard (e.g. a metre stick) that is preferably known in terms of other standards accepted by a wide community. The accuracy of the measurement can be limited by the reproducibility of the standard, the measurement process and the nature of the quantity being measured. One usually tries to arrange things so that the last named is the major limitation but, as was the case during the history of the measurement of the speed of light, the measurement process can, by orders of magnitude impose the major limitation. It also happens occasionally that the development of techniques leads to a situation where, for certain measurements, the primary standard is the limit, as happened in the 1950’s and again at present in the case of the international Metre; in this case international agreement is sought to define a new standard. This lecture is concerned with the development of primary standards for length and with some of the important secondary or workings standards used for transfer to practical measurements. We shall start with a brief look at history, not simply for entertainment but to help put into perspective the factors to be considered in making and using standards. For a more complete historical review see references 1–3.