| Zugriffsnummer | 21886 |
| Dokumenttyp | Buchartikel |
| Sprache | Englisch |
| Titel | Length and size |
| Autor(in); Institution |
Schödel, René; 5.4, Interferometrie an Maßverkörperungen, PTB-Braunschweig
|
| Quelle/Jahr | Handbook of optical metrology : principles and applications:(2009), 365 - 390 |
| Herausgeber(in) |
Yoshizawa, Toru
|
| ISBN | 0-8493-3760-7 ; 978-0-8493-3760-4 |
| Verlag | Boca Raton, Fla. [u.a.]: CRC Press, Taylor & Francis |
| Freie Schlagworte | interferometry ; length ; metrology ; interferometer |
| Zusammenfassung | The provision of length standards and the ability to measure length to a required accuracy are of fundamental importance to any technologically developed society. Throughout history there have been many standards for the length beginning with simple definitions based on the human body, e.g. cubit and feet. The continuing refinement of standards led to more specific definitions and more accurate methods of realising them. As a milestone 1887 Michelson proposed the use of optical interferometers for the measurement of length. However, it needed many years until the meter was defined in terms of the wavelength of light from a krypton lamp in vacuum. In 1960, this definition replaced the International Prototypes deposited 1889 at the BIPM, where they remain today. Since 1983 the meter, one of the seven base units of the SI, is defined as the length of the path travelled by light in vacuum during a time interval of 1/299,792,458 of a second. This definition is based on the availability of primary frequency standards (atomic clocks) defining a second accurately. It opens two alternative ways for the realisation of length measurements i) propagation delay: the length L is the path travelled in vacuum by a plane electromagnetic wave in a time t which is obtained using the relation L=c0t and the value of the speed of light in vacuum c0 = 299,792,458 m s-1, ii) interferometry: by means of the vacuum wavelength λ0 of a plane electromagnetic wave of frequency f which is obtained from the relation λ0 = c0/f. The frequency of atomic clocks serving as primary standards can be transferred to frequencies of laser radiation used in length measurements by optical interferometry. This transfer is realised by frequency chains or more modern techniques utilising femtosecond laser frequency combs. There are two ways for providing laser sources whose frequency is traceable to the SI. The common way is the use of recommended radiations generated by lasers whose frequencies are stabilised to selected hyperfine absorption lines. Currently, twelve reference frequencies covering the visible and infrared regions of the electromagnetic spectrum are recommended by the Comité International des Poids et Mesures (CIPM). As an alternative to the usage of recommended radiations, the laser light source may be directly synchronized to the primary frequency standard by femtosecond laser frequency combs. The practical realisation of length measurements depends upon the application. The propagation delay method is basically useful for long distances, e.g. in space, while for the calibration of secondary length standards, e.g. gauge blocks, interferometry by means of known wavelengths is preferable. The length of these primary calibrated material artefacts basically transfers the SI unit of the length to the industry and society to be used subsequently in mechanical calibrations based on the comparison length measurements. Almost any relevant usage of length standards is applied under air, i.e. not under vacuum conditions. Accordingly, the primary calibration of the material artefacts has to be performed under air, otherwise the length is effected by the air pressure due to the material’s compressibility. This means that the actual relations L=ct and λ=c/f have to be considered in the primary calibration measurements. Here the speed of light is reduced by the refractive index n of air according to: c=c0/n. Accordingly, the accurate evaluation of the air refractive index is one of the key points for the transition from the SI definition of the metre to the actually provided length standards. Other key points regard the use of adjustment methods to make sure that the SI definition is realised properly, e.g. that plane waves are used. Besides these points, the laser frequency itself is no longer the limiting factor regarding measurement uncertainty, provided that stabilised lasers are used as mentioned above. This conclusion even holds for measurements under vacuum conditions. Therefore, the often found designation of stabilised lasers serving as secondary frequency standards as "primary length standards" is misleading. |
