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The length references have been implemented with the aim of providing to the users traceability to the metre. In order to achieve this, it is required to develop references in two distinct yet complementary sub-fields :

- The wavelength (which includes all the works dedicated to the application of the metre definition) ;
- The dimensional quantities (that covers more specifically the material standards).

Since 1983, the metre is directly connected to the second: “the metre is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second”. This definition sets the numerical value of the speed of light in vacuum c_{0} which is equal to 299 792 458 m/s.

**This definition can be put into practice :**

- By measuring the time interval required by one luminous impulse to travel the path between two points to deduce the distance separating these two points ;
- By using the relationship that characterises the propagation of electromagnetic waves in vacuum : l = c
_{0}/ n , which links the wavelength to the luminous radiation I and its frequency n (i.e. to measure the frequency) ; - By using a well-defined (transition used for control, experimental conditions...) laser with a known wavelength.

These three methods translate how the metre definition is put into practice. For the measurements to the metre scale, the last method is generally used. The CIPM keeps an updated list of the wavelengths that are matching the types of lasers most commonly used. Said list specifies :

- The radiations recommended and the experimental conditions (type of laser, method of stabilisation,...) ;
- The wavelengths as well as the corresponding frequencies ;
- The standard uncertainties relating to each radiation.

The LNE-INM has implemented lasers, primarily in-house designed lasers, for materialising four wavelengths which put into practice will be matching the main needs of dimensional metrology :

- Two 633 nm iodine-stabilised He-Ne lasers ;
- Two 532 nm iodine-stabilised frequency-doubled Nd:YAG lasers (figure 1) ;
- One 543 nm iodine-stabilised dye laser ;
- One 612 nm iodine-stabilised He-Ne laser.

These reference lasers are used more particularly for calibration at the best level of uncertainties applying the frequency beat comparison method of the LNE’s lasers used for measuring the material standards with interferometric installations.

Fig. 1. - 532 nm iodine-stabilised frequency-doubled YAG laser

Traceability to the length unit is primarily carried out in the industry through two large families of measuring instruments :

- The fixed reading instruments (end standards, lines standards, gauges, ...) ;
- The variable reading instruments (callipers, micrometers, comparators,...).

At the best level of uncertainty, the dissemination of the length unit, via the material standards, is primarily carried out by using the fixed reading instruments.

Regarding traceability within the industry, the gauge blocks play a preponderant role. At the best level of uncertainty they are calibrated at the LNE with one Michelson (TESA-NPL) interferometric bench through the method of fractional excess using three wavelengths (543 nm, 612 nm and 633 nm). This bench allows to calibrate the gauge blocks between 0,1 mm and 300 mm. The means implemented also enable to calibrate via interferometric comparison the gauge blocks up to 3 m.

The LNE is also implementing a suite of testing benches that allow to calibrate different material standards used in the industry (lines standards, different plug gauges, rings, spheres, step gauges,..) using interferometric comparison or mechanical comparison.

Knowing the geometry of one artefact requires, in addition to length measurements, measurements of forms. It is often about making a comparison between one form and a geometric artefact (circle, cylinder, straight line,...). The two specific testing benches implemented at the LNE aim at :

- measuring the circularity gaps on internal and external cylinders ;
- straightness measurement.

The angle is included in the dimensional quantities. The unit of this quantity is a SI derived unit: the radian (symbol: rad). It is defined as the angle subtended at the centre of a circle by an arc of equal length to the radius: its expression in base SI units is m·m^{-1}. In practice, angles are materialized by cutting the circle the more steadily possible.

The LNE has developed jointly with the Ensam of Lille one plate with permanent positioning (figure 2) which oscillating uncertainty is in the magnitude of ± 0,03 seconds of angle (± 1,5·10^{-7} rad). This plate is the first realization of the laboratory that is based on the concept of dissociated metrology structure (the structure linked to the measurement is dissociated from the supporting structure). It is based on two commercial coders that are assembled so as to free themselves from the accuracy errors peculiar to each of the coders.