This part of IEC 61757 defines detail specifications for distributed temperature measurement by a fibre optic sensor, also known as fibre optic distributed temperature sensing (DTS). DTS includes the use of Raman scattering, Brillouin scattering and Rayleigh scattering effects. In addition, Raman scattering and Rayleigh scattering based measurements are performed with a single-ended fibre configuration only. Brillouin scattering based measurements are performed with a single-ended fibre or fibre loop configuration. The technique accessible from both sides at same time (e. g. Brillouin optical time domain analysis, BOTDA) is referred to here as a loop configuration. Generic specifications for fibre optic sensors are defined in IEC 61757-1. This part of IEC 61757 specifies the most important DTS performance parameters and defines the procedures for their determination. In addition to the group of performance parameters, a list of additional parameters has been defined to support the definition of the measurement specifications and their associated test procedures. The definitions of these additional parameters are provided for informational purposes and shall be included with the sets of performance parameters. A general test setup is defined in which all parameters can be gathered through a set of tests. The specific tests are described within the clause for each measurement parameter. This general test setup is depicted and described in Clause 4 along with a list of general information that has be documented based upon the specific DTS instrument and test setup used to measure these parameters per this standard. Annex A provides a blank performance parameter table which should be used to record the performance parameter values for a given DTS instrument and chosen optical test setup con-figuration. Annex B provides a guideline to optional determination of point defect effects.