Fundamental length metrology

This chapter describes the use of lasers for length measurement traceable to the definition of the metre, which is defined today via the speed of light. High-precision and traceable length measurements are especially important for production engineering with a measurement range of up to some ten metres. Today for traceable measurements the use of laser interferometers relying on frequency-stabilized lasers is state-of-the-art. Laser interferometers and grating scales with interferometric readout are also widely used in industrial applications. For machine tools and coordinate measurement machines, normally encoders are used for length measurements, because interferometers are sensitive to refractive index variations, mainly caused by temperature and pressure fluctuations, and more expensive than encoders. Frequency-stabilized lasers of both categories can be found in a list of recommended radiations for the realization of laser wavelength or frequency standards for the realization of the metre.