**1. Introduction**

Accurate length measurement plays a vital role in meeting the needs of industry and commerce for traceability to common national and international standards, especially in view of the common market and world trade. Such measurement needs arise across a wide applications base, from large-scale engineering projects such as dam construction, aerospace and shipbuilding, through automotive engineering and components manufacture, to precision engineering and nanotechnology (DTI/NMDS, 2002b).

The lowest uncertainty attained in dimensional measurements of a material object occurs in semiconductor industry and integrated circuit (IC) production. The dimensional feature of interest in a line scale is the critical dimension (CD). The CD corresponds to the width of the smallest line that can be produced on a wafer with an acceptable yield of manufactured devices; presently this parameter is less than 0.1 µm. Requirements in other areas, such as manufacture of precision instruments, large machines (e.g. planes), and others also rise fast. In all these areas the principle "to stop means to fall behind" is in force. Development of measurement systems is impelled by the augmentation of customer needs as well as by steadily evolving state-of-the-art measurement technologies (Bosse & Flügge, 2001).

Length metrology has a fundamental role to maintain the primary standard of length, the metre, and to provide the infrastructure to enable a wide range of dimensional and positional measurements to be made traceable to the metre. National metrology institutes (NMIs) in a number of countries and companies that produce precision high-tech products pay much attention to accuracy-related research with the aim to improve properties of length calibration systems and to specify their uncertainty budget. Metrological programmes in the area of length measurement are consistently carried out in the USA, Japan, UK, Germany (Bosse & Flügge, 2001; Beers & Penzes, 1999; Israel et al., 2003), and other countries. The programmes impel the creation of metrological infrastructure that increases industry competitiveness, supports industrial innovations, and improves control of manufacturing processes and quality. For example, systematic research of accuracy of vacuum nano-comparator, performed in German National Metrology Institute (PTB) in 2000 – 2006, resulted in reducing the measurement repeatability error from 14 nm down to 0.2 nm. NIST, the National Metrology Institute of the USA, is carrying out research on nm-accuracy one dimensional (1D) metrology with the development of components of next generation length scale interferometer. In conceptual design, the system would have a range for 1D measurements from 100 nm to 1 m with a target expanded uncertainty of from 1 nm to 10 nm.

One of the most sophisticated challenges for science and the high technologies engineering is the growing need to address real industrial problems rather than the ideal measurement situation and embed the traceable length metrology directly into technological processes by performing precise dynamic measurements in more demanding environments than those of calibration laboratories.

This chapter will present a synopsis and analysis of literature and existing scientific and technical solutions of precision length calibration. It covers analysis of laser interferometers, line detection systems, measurement signals and algorithms, as well as measurement capabilities of state-of-the art calibration systems worldwide. The contribution also addresses a thorny issue of achieving reliable measurements and meeting contradictory requirements of accuracy and productivity of line scale calibration in non-ideal environmental conditions, under the influence of many external influencing factors. The problems will be also upon the anvil of the development of an interferometer-controlled comparator that is operated in dynamic mode and enables to trace the calibration of line scale of up to L ≤ 3.5 m long to the wavelength standard.
