The Kelvin and temperature measurements

The International Temperature Scale of 1990 (ITS-90) is defined from 0.65 K upwards to the highest temperature measurable by spectral radiation thermo metry, the radiation thermometry being based on the Planck radiation law. W hen it was developed, the ITS-90 represented thermodynamic temperatures as closely as possible. Part I of this paper describes the realization of cont act thermometry up to 1234.93 K, the temperature range in which the ITS-90 is defined in terms of calibration of thermometers at 15 fixed points and v apor pressure/temperature relations which are phase equilibrium states of p ure substances. The realization is accomplished by using fixed-point device s, containing samples of the highest available purity, and suitable tempera ture-controlled environments. All components are constructed to achieve the defining equilibrium states of the samples for the calibration of thermome ters. The high quality of the temperature realization and measurements is w ell documented. Various research efforts are described, including research to improve the uncertainty in thermodynamic temperatures by measuring the v elocity of sound in gas up to 800 K, research in applying noise thermometry techniques, and research on thermocouples. Thermometer calibration service s and high-purity samples and devices suitable for "on- site" thermometer c alibration that are available to the thermometry community are described. P art II of the paper describes the realization of temperature above 1234.93 K for which the ITS-90 is defined in terms of the calibration of spectrorad iometers using reference blackbody sources that are at the temperature of t he equilibrium liquid-solid phase transition of pure silver, gold, or coppe r. The realization of temperature from absolute spectral or total radiometr y over the temperature range from about 60 K to 3000 K is also described. T he dissemination of the temperature scale using radiation thermometry from NIST to the customer is achieved by calibration of blackbody sources, tungs ten-strip lamps, and pyrometers. As an example of the research efforts in a bsolute radiometry, which impacts the NIST spectral irradiance and radiance scales, results with filter radiometers and a high-temperature blackbody a re summarized.