**2.1 Definition and realization of meter**

The definition of the meter—whether in terms of a prototype meter bar, a wavelength of light, or the propagation of an electromagnetic wave in an interval of time—has provided the basis for the lowest-uncertainty realization of the unit.

In 1983, the meter was re-defined again to the one in effect today, namely: "The meter is the length of path traveled by light in vacuum during the interval of 1/299 792 458 of a second". At that time, the International Committee on Weights and Measures (CIPM) gave three basic methods for the practical realization of the meter: time-of-flight, using time intervals, and interferometry, using wavelengths or frequencies. CIPM gave five recommended radiations with assigned frequencies, wavelengths, and uncertainties (Quinn, 2003).

Of the recommended radiations, that of the iodine stabilized helium-neon (He-Ne) laser is the most widely used for practical realization of the meter. It has a wavelength of He-Ne = 632.991 398 22 nm, with a relative standard uncertainty *ur* of 2.5×10–11.

The effect of the re-definitions and advances in measurement of the frequencies of recommended radiations was to decrease the relative uncertainty attainable in realization of the meter by five orders of magnitude (Swyt, 2001).

Measurements of dimensions of material goods are most often referenced to the SI unit of length through material artifacts calibrated as dimensional standards. The meter, the basic unit for length, is usually transferred to measurement standards in the form of line scales or photoelectrical incremental encoders by length measuring machines that typically use a laser interferometer in air as reference measuring system. The measurement results are traceable to the meter due to the use of the wavelength of the laser interferometer.
