Angle metrology

Metrological traceability of length measurements is based on the use of special laser sources whose wavelength is known. It is then possible to demonstrate that measurements can be traced back to the unit of length, in strict compliance with the definition of the metre.

In the case of angle measurements, the reference is the turn, a purely geometric definition, as is the definition of the unit of plane angle: "The radian (rad) is the plane angle between two rays which intercept, on the circumference of a circle, an arc of length equal to that of the ray".

The abstract nature of the angle unit, based solely on geometric considerations, gives it an important property for all users: accessibility. It might therefore be considered unnecessary to institute a calibration chain linking every angle measurement to the definition of the quantity or to a materialization of this definition, as is the case for the metre.

But the ability to perform angular measurements with a certain level of accuracy requires instrumentation and implementation that are not easily accessible to calibration laboratories, a fortiori in an industrial context, hence the production of standards materializing angles, either in discrete form or through measuring instruments.

Consequently, angle metrology relies above all on instrumentation and on the ability of means or methods to divide the circle precisely, and not on the possession of a primary reference standard.

An exception is made in the field of small angles, where techniques based on length measurements are sometimes more appropriate.