The challenge of measuring body temperature with thermography and how to solve it
24. 11. 2020
On the global scale, the Covid-19 pandemic has laid bare, among other things, the challenge of measuring body temperature as one of the measures for preventing uncontrolled spreading of the disease. Lately, there has been a lot of controversy, mainly regarding mass control at entry points such as airports, large companies, medical and educational institutions, over the application of thermal imaging resp. thermal imaging cameras. For the specifics of the method for measuring temperature in different spots on the face, and application of mathematical models to thus calculate the temperature of the a body` core have yet not been finalized from the aspect of precision and traceability. This is what the experts will have to work on in the future.
Though fever is no 100% evidence that a person has been suffering from Covid-19, increased body temperature is one of the earliest and most common symptoms of the viral respiratory infections, and, as a consequence, the basis for many screening tests for detecting potential patients. Notwithstanding the apparent simplicity, measuring of body temperature is far from a trivial concept, moreover so in aiming for precise and traceable results.
Measuring of body temperature is far from a trivial concept, moreover so in aiming for precise and traceable results in combating a pandemic.
Reference measuring spots
Body temperature is a concept depending on whether it is measured inside a body or on it, deemed the temperature of a body`s core represented by the heart of the brain. Since temperature cannot be measured on these two organs directly, certain reference spots have been defined as their best proxy - the outer surface of the eardrum or the tympanic membrane, distal esophagus, lung artery and urinary bladder. As a rule, body temperature is measured in the armpit, in mouth, in ears and in rectum. These measuring spots are clinically approved relating to the correlation of temperature as compared to the reference measuring spots. The correlation of, for example, forehead and face, where temperature is measured by means of contact-less thermometers or by means of thermal imaging cameras, and of reference measuring spots, however, has not been established yet.
Thermometers without the legislative regulations
Considering variety of types of thermometers available on the market and ever growing number of companies and public institutions opting for measuring of body temperature of employees and visitors by means of thermal imaging cameras, there is additional challenge to overcome when buying equipment - thermometers as medical instruments are not deemed to fall within the regulated metrology. Meaning that the state does neither defines nor monitors calibration of these.
Factors impacting the measured temperature
Another fact to be pointed out when measuring of temperature by means of thermal imaging cameras is that the relation between skin temperature and temperature of the body`s core varies in correlation to the spot of measuring. Further element impacting the measuring result is the distance of measuring in combination with other conditions such as thermal regulation of an individual resp. his/her activity level, time of the year, and subjective factors such as smoker/non-smoker, food intake, menstrual cycle, age, gender. Challenges of the contact-less measuring of body temperature are very complex. The parameter of emissivity, depending on temperature, wave length and observation angle, is of vital importance here. Same goes for the traceability, which is a prerequisite for precision of measurements with established measurement uncertainty. Traceability must be ensured for the measuring instrument as well as for the measuring method resp. procedure.
Absolutely precise measuring only possible in lab environment
Nowadays, temperature can be measured to a very high precision level, for the most precise thermometers measure even one millionth of a degree Celsius, however this is only possible under lab conditions. Precision level of contact-less measurements within the IR specter (between minus 30 dC and plus 600 dC) only amount to the size of some tenths of a degree Celsius, at best. In praxis, measuring results are influenced by the above mentioned factors, therefore the results are, as a rule, less precise than stated in the specifications of producers, however, they are surely less precise than those obtained under strictly controlled laboratory conditions.
What can various measuring instrument do?
In September this year, the Laboratory of Metrology and Quality (LMQ) operating under the Faculty of Electrical Engineering of the University of Ljubljana issued an explanation regarding the problems of technologies for measuring of body temperature, for this is an issue very topical in this time of pandemic. At the same time, the topic requires quite some explanation regarding precision, appropriate interpretation and consequent appropriate decision-making in the process of purchasing appropriate measuring equipment. Some vital features of different thermometers for measuring of body temperature are given in the below table.
Quality results are more than a statement of the precision level of measuring equipment
Given body temperature is deemed a basis for decision-making with relevant consequences, data in the table is important for the very display of capabilities of different measuring instruments, and an indicator of required elements for a correct and coherent interpretation of the measuring results. Quality of measuring results is much more than just a statement of precision of a measuring device, for specifications of producers in general fail to encompass the conditions under which measurements are carried out in real situations. Thermometers tend to measure different objects with different level of precision, therefore a good understanding of measuring instruments resp. mastering of the entire measuring procedure is a prerequisite in ensuring precise measuring results. To sum up: a precise instrument in itself does not ensure precise measuring, as a person highly experienced in precise measuring cannot acquire precise results with an imprecise instrument.
Validation by means of contact measuring
In order to ensure highest level of precision in measuring of body temperature in our fight with the pandemic, in July this year the LMQ took on a leading role in an international group for preparation of Guidelines for measuring of body temperature within a technical committee for temperature with the International Bureau of Weights and Measures (BIPM). Said Guidelines will be applied by the WHO as a support tool in the process of ensuring correct and precise measuring of body temperature. In face of the fact that measuring of body temperature by means of contact-less measuring is not sufficiently reliable; I, as head of this work team, believe that the international protocol we are about to pass will still require additional validation of increased body temperature by means of a classic contact measuring resp. by means of a contact-less ear thermometer, although we would like to avoid a contact with the person being measured.
Pay attention to calibration certificates
Though state-of-art technologies of contact-less measuring are applied at mass entering points today, these are still considered low-precision tools which may cause inappropriate further steps. It is therefore our advice to all users that they use an additional tool for a reliable identification of potentially sick - e.g. contact measuring of temperature. When selecting measuring devices please pay attention to the quality resp. calibration certificates regarding precision resp. measuring imprecision.
Some providers pave a solid path
I am often asked for my opinion on the compliance of the solutions from the field of thermal imaging currently available on the market. The available standard for clinical thermometers and chapters on laboratory precision state precision of the level of ±0,2 degrees Celsius for contact thermometers and ±0,3 dC for ear thermometers. There is an appendix on clinical evaluation, for we know that measurements in praxis undergo various impacts from the environment. To return to my opinion on suitability of measuring tools, I dare stress that there is no such thing as a perfect final solution available yet, though some of the providers are on the right way with their development. One of these providers is Kolektor with its development of Cordea, with a somewhat non-typical process of validation. The device is firstly checked in a lab to reach all regulated lab precision levels, and after this the device is validated in praxis.
About the author and about the Laboratory of Metrology and Quality
Igor Pušnik, PhD, Head of Quality at the LMQ at the Faculty of Electrical Engineering of the University of Ljubljana is the person responsible for contact-less temperature measurements. The LMQ as the carrier of the national master sample for temperature and humidity represents highest expert level in Slovenia, and Mr.Pušnik, beside his pedagogic work, is actively engaged in numerous domestic and international research-oriented and specialized projects, mainly from the field of calibration of thermometers and hygrometers with particular focus on contact-less thermometry. Since decision-making on basis of a measured value of body temperature is a topical issue in several interest groups, a more detailed explanation is provided at www.lmk.si
Our lab also organizes training courses from the field of measuring body temperature thus solves the dilemmas possibly arising thereunder.
Available recording of the webinar
In November 2020, LMQ and Kolektor Digital organized a webinar called Challenges of measuring body temperature - how to overcome them and standpoint of experts A link to the recording https://bit.ly/posnetek_webinarja.
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