Calibration and validation of thermal imagers

Abstract

This paper will consider how to improve confidence in the use of thermal imagers quantitatively, that is for actual temperature measurement. The proposed route will be through the implementation of best international measurement practice via calibration, traceability and accreditation. Reference blackbody standards that have been rigorously qualified will be described, with emphasis on developments in clinical thermography. IntroductionThe practice of thermal imaging is at a crossroads. In the past, for the vast majority of applications, qualitative analysis was sufficient. However there is a growing need to have properly quantified thermal imaging. For example, if cross-centre image exchange is required to study a problem, if clinical or other decisions are made based on thermal imager temperature measurement (e.g. preventing or delaying transit through an airport) or for the construction of a database of images taken by different thermal imagers, then questions concerning normalisation and calibration need to be addressed. This brings with it a requirement of measurement rigour that has not generally been practised or appreciated.The purpose of this paper is to outline how, through the use of appropriate and available metrology "tools", the accurate calibration of thermal imagers can be obtained. The "tools" namely calibration, [and associated uncertainty analysis] traceability and accreditation will be described in some detail. Emphasis will be given to the requirements of the international standard ISO17025 and how its implementation in the thermal imaging community, and in particular in the laboratories and manufacturers testing and supplying thermal imagers, would significantly improve the practice of accurate thermal imaging. The paper will then describe, for illustrative purposes, the facilities at the NPL for performing the traceable calibration of thermal imagers. Particular attention will be paid to developments in lower temperature medical applications, both of a variable temperature blackbody source and novel in-field fixed point blackbodies, that are used to provide in-situ image assurance for critical applications.Finally a review of the recent proposed standard 62D_616_CDV for implementing thermal imaging in an airport/port-of-exit/port-of-entry environment will be given. This will highlight the requirement for key metrological input at the early stage of standards formulation. Assuring temperature measurementsProper temperature (in fact any) measurement requires the establishment of a rigorous process that enables all measurements to be traced back to a reliable national standard, which itself has been qualified internationally through top-level comparisons with standards in other national measurement institutes. From a temperature perspective the current standard is the International Temperature Scale of 1990 (ITS-90) [1]. All temperature measurements throughout the world should ultimately be traceable back to this scale. The process of ensuring that this is the case has three steps eac...

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