Realization

In metrology, the realization (or embodiment) of a measurement unit or a measurement scale is the practical method by which the unit can be measured (or the scale put into practice). The realization of a unit or of a scale creates one or more measurement standards against which an unknown physical quantity of the same kind can be compared.

There are three general methods of realizing a unit or a scale. The first and most obvious is simply to follow the definition of the unit or scale, for example by measuring the distance travelled by light in vacuum in ¹⁄_299,792,458 of a second to realize the metre. The second method is to use a physical phenomenon which is accepted to be equivalent to the definition, for example realizing the metre by laser interferometry with a measured frequency of light. The third method is to have a physical standard, for example the now-obsolete International Prototype Metre.

As a realization is a measurement, it is associated with a measurement uncertainty, which is called the standard measurement uncertainty. The standard measurement uncertainty is a component of the uncertainty in any measurement result which relies on the measurement standard, although it is often (indeed usually) negligeable compared to other components of the uncertainty.