ABSTRACT
The provision of length standards and the ability to measure length to a required accuracy are of fundamental importance to any technologically developed society. Throughout history, there have been many standards for the length beginning with simple defi nitions based on the human body, e.g., cubit and feet. The continuing refi nement of standards led to more specifi c defi nitions and more accurate methods of realizing them. As a milestone, in 1887 Michelson proposed the use of optical interferometers for the measurement of length. However, it needed many years until the meter was defi ned in terms of the wavelength of light from a krypton lamp in vacuum. In 1960, this defi nition replaced the International Prototypes deposited in 1889 at the Bureau International des Poids et Mesures (BIPM), where they remain today. Since 1983 the meter, one of the seven base units of the SI, is defi ned as the length of the path tra veled by light in vacuum during a time interval of 1/299,792,458 of a second.
