International Temperature Scale of 1990 International Temperature Scale of 1990 The International Temperature Scale of 1990 (ITS–90) published by the Consultative Committee for Thermometry (CCT) of the International Committee for Weights and Measures (CIPM) is an equipment calibration standard for making measurements on the Kelvin and Celsius temperature scales. ITS–90 is an approximation of the thermodynamic temperature scale that facilitates the comparability and compatibility of temperature measurements internationally. It specifies fourteen calibration points ranging from 0.65±0 K to ±0 K (−272.50±0 °C to ±0 °C) and is subdivided into multiple temperature ranges which overlap in some instances. ITS–90 is the latest (as of 2014) of
Hounsfield scale Hounsfield scale The Hounsfield scale , named after Sir Godfrey Hounsfield, is a quantitative scale for describing radiodensity. It is frequently used in CT scans, where its value is also termed CT number. The Hounsfield unit (HU) scale is a linear transformation of the original linear attenuation coefficient measurement into one in which the radiodensity of distilled water at standard pressure and temperature (STP) is defined as zero Hounsfield units (HU), while the radiodensity of air at STP is defined as -1000 HU. In a voxel with average linear attenuation coefficient formula_1, the corresponding HU value is therefore given by: