Light is not only a subject of measurement but can be a very useful tool for measuring. It plays a dominant role in the measurement of dimensional quantities. Coherent light from a highly stable laser source is represented by a precise wavelength which can be seen as an elementary etalon of length. Metrology of length is in fact metrology of optical frequencies and a broad family of interferometric techniques converting a precise wavelength into the measurement of real objects. The precision (stability) of the best optical oscillators are now close to overcoming the stability of an atomic clock. This, with help of femtosecond optical frequency synthesis, will most likely lead to unification of standards of time and length into one precise laser, called an optical clock. and Josef Lazar a kolektiv oddělení koherenční optiky.