Machin, G (2025) Using photons to measure temperature. Precision (37). pp. 6-9.

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Abstract

The practice of thermometry has evolved over the years to incorporate the findings of research and advances in technology [1]. For example, liquid-in-glass thermometers, whose use was ubiquitous in the 19th Century, progressively gave way to electrically based sensors such as resistance thermometers and thermocouples in the 20th. In addition, the introduction and use of temperature scales has successfully ensured reliable thermometry has been performed on a global basis for around 100 years. The first temperature scale was introduced in 1927, this was followed by revised and improved temperature scales in 1948 and 1968 with the most recent one in widespread use, incorporating further improvements, is the International Temperature Scale of 1990 (ITS-90) [2]. The ITS-90 has been tremendously successful at facilitating low uncertainty, consistent temperature measurement around the world for more than 30 years.

However, these defined scales are only ever approximations to “true” temperature, known as thermodynamic temperature, which is the temperature one finds in physical equations, and which was first elaborated by Lord Kelvin (then William Thomson) in the mid-1850s. After the kelvin redefinition in 2019 [3] there has been significant research activity, on a global basis, to develop approaches to determine thermodynamic temperature at the point-of-measurement. There are a number of approaches to doing this. Firstly, one can calibrate practical sensors against thermodynamic temperatures instead of ITS-90 and then use those sensors to measure thermodynamic temperature directly [4]. However, this approach has all the drawbacks of classical thermometry in that sensors still drift in use and would still require periodic recalibration to deliver reliable temperature values. Secondly, and more radically, one can utilise the physics associated with the sensor, which relates temperature to another measurable quantity, to determine thermodynamic temperature directly without recourse to sensor calibration. This approach is loosely known as in-situ traceability [5] because it delivers and maintains traceability direct to thermodynamic temperature (that is to the kelvin definition) in-process. Significant advances have been made in this approach both electron-based [6] and photon-based and it is the latter I want to focus on today in the rest of this article.

Item Type:	Article
Keywords:	Photonic thermometry, quantum thermometry, practical primary thermometry, in-situ traceability
Subjects:	Engineering Measurements > Thermal
Divisions:	Thermal & Radiometric Metrology
Last Modified:	03 Jul 2026 14:32
URI:	https://eprintspublications.npl.co.uk/id/eprint/10477