The definition of the metre, SI base unit of length, is as follows:
The metre, symbol m, is the SI unit of length. It is defined by taking the fixed numerical value of the speed of light in vacuum c to be 299 792 458 when expressed in the unit m s–1, where the second is defined in terms of the caesium frequency ΔνCs.
The practical realization of the metre is based on the measurement of the frequency (or the wavelength) of special laser sources stabilized by means of consolidated and agreed techniques (Mise en Pratique for the definition of the meter). The INRIM’s standards are based on lasers locked to molecular transitions (He-Ne laser locked to I2 @ 633nm, Nd:YAG laser locked to I2 @ 532 nm and 1064 nm) and on a laser frequency comb locked to the second for wavelengths from 500 nm to 1600 nm.
In practical precision length measurements, the traceability to the metre comes through interferometers based on lasers which wavelength is measured accurately. Hence, the dissemination of the metre is implemented by measuring the wavelength of the lasers by direct comparison with INRIM’s standards (chosen according to the wavelength and the accuracy required).