Calibration of infrared thermometers
One symptom COVID- 19 is an elevated body temperature. Body temperature measurement is therefore a crucial element in combating the pandemic. In many countries devices for temperature measurement are being used on a large scale — not only at airports, but also in shopping centres, etc. In public spaces, non-contact infrared forehead thermometers and thermographic cameras are widely in use and hand-held ear infrared thermometers are being used to a lesser degree. Ear or tympanic thermometers are considered medical devices and are more often used in hospitals.
Due to the increased use of infrared thermometers (IRTs), the demand for their calibration at metrology institutes has also risen. Governments expect metrology institutes to ensure that instruments measure accurately and that their measurements are traceable to the SI Units. PTB was approached by some of its international partners on this matter. That is why PTB is providing some general information and links to guidelines and other websites. PTB has procured infrared thermometer calibration equipment as a short-term assistance measure for some countries.
Application and calibration of non-contact skin thermometers and hand-held ear thermometers
In the case of a health emergency like COVID-19, medical non-contact skin thermometers (e.g. forehead and ear thermometers) are considered safer than contact skin thermometers. This is because there is no direct contact between the person holding the thermometer and the person whose temperature is being taken. However, especially forehead thermometers could be less accurate due to several factors, such as the body’s capacity to emit thermal radiation and environmental conditions as well as the thermometer’s capacity to detect the influence of the emitted radiation on the measurement. Of course, there are also great variations in quality that go hand in hand with the price of the devices.
The difficulties in the application of forehead thermometers yielding to a possible higher uncertainty are even more critical for industrial hand-held thermometers and thermographic cameras.
For the calibration and testing of instruments that measure body temperature, one must also differentiate between the two distinct types of instruments, namely clinical or medical thermometers and industrial thermometers. For the performance verification and calibration of clinical thermometers, e.g. ear or tympanic and forehead thermometers, an international standard is available that describes the maximum permissible error of these devices and how they can be tested and calibrated with standard uncertainties below 100 mK under laboratory conditions.
For industrial thermometers and thermal imagers, it is recommended to use an extended area blackbody source (a surface emitter, for example) because of its universal usage. In any case, it must be ensured that the measurement spot of the instrument to calibrate is smaller than the emitting area of the blackbody.
If possible, the use of ear thermometers is recommended as they have two distinct advantages: 1) Regarding application, the tympanic temperature is quite close to the internal body temperature due to its proximity to the hypothalamus. 2) These thermometers have been widely studied by European National Metrology Institutes. An international comparison was also carried out and confirmed their accuracy. To provide SI traceability, a special water bath blackbody and an appropriate chiller is required. Chapter 2.2 of the “Guidelines for metrological verifications of medical devices with a measuring function – LMKM“ describes the calibration of ear thermometers (https://www.ptb.de/cms/fileadmin/internet/publikationen/wissensch_tech_publikationen/LMKM-V3-Part1-Englisch_2020.pdf).
Such a set-up could also be used to verify calibrators for ear thermometers which are provided by ear thermometer manufacturers. The calibration of water bath blackbodies is described in this document:
https://www.ptb.de/cms/fileadmin/internet/publikationen/wissensch_tech_publikationen/Testing_the_suitability_of_cavity_radiators_used_for_the_metrological_verification_of_infrared_thermometers__Supplement_to_LMKM_.pdfIf you need further information or consultation, please contact PTB. Where possible, short-term solutions are being developed during the COVID-19 pandemic.
The following is a collection of guidelines that give detailed information on the calibration of infrared thermometers. PTB is in negotiation regarding whether these guidelines can be offered free of charge.
For clinical instruments, e.g. forehead and ear or tympanic thermometers, an international standard is available that describes the necessary conditions, the required instrumentation and the achievable uncertainties:
ISO 80601-2-56:2017(E) Medical electrical equipment – Part 2-56: Particular requirements for basic safety and essential performance of clinical thermometers for body temperature measurement
A useful source for additional information on the testing of infrared thermometers is the ASTM E1965 - 98(2016) Standard Specification for Infrared Thermometers for Intermittent Determination of Patient Temperature
For the calibration of radiation thermometers in general, the VDI/VDE has published a series of guidelines:
VDI/VDE 3511 Part 4 Temperature measurement in industry - Radiation thermometry
VDI/VDE 3511 Part 4.2 Temperature measurement in industry - Radiation thermometry - Maintenance and intended operation of radiation thermometers
3511 Part 4.4 Temperature measurement in industry; Radiation thermometry; Calibration of radiation thermometers:
VDI/VDE 3511 Part 4.5 Temperature measurement in industry - Radiation thermometry - Practical application of radiation thermometers
VDI/VDE 3511 Part 4.6 Temperature measurement in industry - Radiation thermometry - Set-up and use of calibration radiators
Available free of charge:
You might also find these freely available technical guides which have been published by our colleagues in New Zealand useful:
TG 22 https://measurement.govt.nz/download/28 and https://measurement.govt.nz/download/45
TG 26 https://measurement.govt.nz/download/31
The calculation of uncertainty budgets for the calibration of radiation thermometers is discussed in this paper:
Several free Fluke Calibration on-demand webinars are dealing with the calibration of infrared thermometers:
Our partner institute in Beijing, the National Metrology Institute of China, kindly provided a webinar on infrared thermometers on August 8, 2020.
The presentations are available online:
The National Metrology Institute of China (NIM) has compiled an article about the calibration of radiation thermometers. Please see pages 6-9 of their monthly newsletter:
The National Metrology Institute of Spain (CEM) published a guide in which they compared different types of non-contact radiation thermometers (the English version starts on page 12):
The U.S. Food and Drug Administration (FDA) has also compiled some information on this topic:
The Journal of Medical Engineering & Technology includes an article on the “Comparison of non-contact infrared skin thermometers”:
This article from INEN (NMI Ecuador) is in Spanish:
This article from INMETRO (NMI Brazil) is in Portuguese: