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A Study on the Agreement of Body Temperatures Measured by Infrared Cameras and Oral Thermometry

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Abstract and Figures

Background: The COVID-19 pandemic has led to the rapid adoption and rollout of thermal camera-based Infrared Thermography (IRT) systems for fever detection. These systems use facial infrared emissions to detect individuals exhibiting an elevated core-body temperature, which is present in many symptomatic presentations of Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2). Despite the rollout of these systems, there is little independent research supporting their efficacy. The primary objective of this study was to assess the precision and accuracy of IRT screening solutions in a real-world scenario. Methods and Findings: A single-centre, observational study investigated the agreement of three IRT systems compared to digital oral thermometer measurements of body temperature. Over five days, 107 measurements were taken from individuals wearing facial masks. During each entry, two measurements of the subject’s body temperature were made from each system to allow for the evaluation of the measurement precision, followed by an oral thermometer measurement. Each participant also answered a short demographic survey. This study found that the precision of the IRT systems was wider than 0.3 °C claimed accuracy of two of the systems. This study also found that the IRT measurements were only weakly correlated to those of the oral temperature. Additionally, it was found that demographic characteristics (age, gender, skin colour, mask-type) impacted the measurement error. Conclusions: This study indicates that using IRT systems in front-line scenarios poses a potential risk, where a lack of measurement accuracy could possibly allow febrile individuals to pass through undetected. Further research is required into methods which could increase accuracy and improve the techniques viability.
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A Study on the Agreement of Body Temperatures Measured by Infrared Cameras and
Oral Thermometry
Scott Adams1*, Tracey Bucknall2,3,4, ¶, Abbas Kouzani1, ¶
1School of Engineering, Deakin University, Geelong, Australia
2School of Nursing & Midwifery, Deakin University, Geelong, Australia
3Nursing Services, Alfred Health, Melbourne, Australia
4Centre for Quality and Patient Safety - Alfred Health Partnership, Institute for Health Transformation,
5School of Information Technology and Electrical Engineering, University of Queensla nd, Brisbane, Australia
*Corresponding Author
Email: scott.adams@deakin.edu.au
These authors contributed equally to this work as Joint Senior Authors
The Authors have no potential competing interests to declare.
Abstract
Background:
The COVID-19 pandemic has led to the rapid adoption and rollout of thermal camera-based Infrared
Thermography (IRT) systems for fever detection. These systems use facial infrared emissions to detect
individuals exhibiting an elevated core-body temperature, which is present in many symptomatic
presentations of Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2). Despite the rollout
of these systems, there is little independent research supporting their efficacy. The primary objective of
this study was to assess the precision and accuracy of IRT screening solutions in a real-world scenario.
Methods and Findings
A single-centre, observational study investigated the agreement of three IRT systems compared to
digital oral thermometer measurements of body temperature. Over five days, 107 measurements were
taken from individuals wearing facial masks. During each entry, two measurements of the subject’s
body temperature were made from each system to allow for the evaluation of the measurement
precision, followed by an oral thermometer measurement. Each participant also answered a short
demographic survey. This study found that the precision of the IRT systems was wider than 0.3 °C
claimed accuracy of two of the systems. This study also found that the IRT measurements were only
weakly correlated to those of the oral temperature. Additionally, it was found that demographic
characteristics (age, gender, skin colour, mask-type) impacted the measurement error.
Conclusions:
This study indicates that using IRT systems in front-line scenarios poses a potential risk, where a lack
of measurement accuracy could possibly allow febrile individuals to pass through undetected. Further
research is required into methods which could increase accuracy and improve the techniques viability.
Introduction
Core body temperature is one of the four key vital signs, which is regularly assessed by healthcare
settings, alongside respiration rate, blood pressure and heart rate 1. In an in-patient setting, core body
temperature can be assessed from different body locations using oral, rectal, tympanic or temporal artery
thermometers, or even through urinary or pulmonary artery catheters with in-built temperature sensors
2. The accuracy, precision, advantages and disadvantages of these temperature measurement devices in
clinical settings has been well established 2–4.
In addition, in modern medical practice, every device must be assessed against national and
international regulations. To ensure that a device meets appropriate levels of quality, accuracy and
safety, strict medical equipment certification standards must be met by the device prior to its use in a
clinical setting. The bodies administering these standards for medical devices include the Therapeutic
Goods Authority in Australia, the Food and Drug Administration in the USA and the Medicines and
Healthcare products Regulatory Agency in the UK 5. The use of evidence-based assessment to evaluate
technologies for use in a hospital setting is a common and critical part of modern healthcare, ensuring
patient safety and assisting in the delivery of high-quality care 6.
The COVID-19 pandemic has led to an unprecedented adoption and rollout of new fever detection
technologies 7. As fever is present in a significant proportion of symptomatic SARS-CoV-2 cases, the
goal of the screening is to identify individuals exhibiting an elevated temperature, isolate them, and
refer them for a more comprehensive assessment to a health practitioner 8. Currently, fever screening
technologies are typically installed in high-traffic areas, such as train-stations or airports, and also at
the entrance of high-risk sites, such as hospitals, where the consequences of an outbreak could be
catastrophic. Many of the deployed fever screening solutions have not yet been assessed by regulatory
agencies.
The ideal screening technology must be accurate, rapid, widely available, and operate in a way that
keeps both the test administrator and subject safe from viral transmission. In addition, an ideal solution
would operate without consumables to deal with global supply chain shortages of critical resources,
such as consumables, personal protective equipment (PPE) and medical devices, which have been
experienced during the COVID-19 pandemic 9. When compared to this ideal screening technology, it
is clear that traditional measurement techniques have a variety of limitations which restrict them from
being highly suited for use as mass-screening tools. Traditional hospital-grade, contact-based
measurement techniques all require close proximity between the test administrator and the subject, and
some methods are too invasive, too slow, or too expensive to be widely used. This has led to the
increased adoption of infrared thermal detection systems for fever screening applications.
Infrared thermal detection systems operate through the measurement of thermal radiation emitted in the
infra-red wavelengths, of the electromagnetic spectrum 10. The thermal radiation is converted through
a transducer to an electrical signal which can be interrogated and measured on-board of the device. In
fever screening applications, detection systems fall into two main categories: handheld Non-Contact
Infrared Thermometry (NCIT) devices, and Infrared Thermography (IRT) systems. These systems meet
many of the criteria for a successful mass screening system, they are non-contact, require no
consumables, are rapid, and in the case of IRT systems the operator can be physically distanced from
the subject. While the use of NCIT devices has been explored in a hospital setting through a number of
clinical trials and research articles, and that many of NCIT devices have achieved appropriate medical
device approvals, these are not yet established for most IRT systems 11–13.
Despite the wide-spread use of IRT systems, there remains limited independent evidence demonstrating
their efficacy and accuracy when measuring body temperature for fever screening. Some clinical trials
have been conducted, but the results have been mixed, and there is a lack of consensus in the literature
on the effectiveness of IRT systems. A 2015 study in Singapore found that one system was able to
achieve a high level of sensitivity and specificity (89.7% and 92%, respectively) 14, and a similar result
found from a study conducted in the USA in 2010, reporting a sensitivity of 91.0% and specificity of
86.0%. However, these results were not found to be broadly repeatable, as revealed by three experiments
performed in Hong-Kong and NZ between 2011-2013 15–17. The most recent of these studies only
reported a maximum sensitivity and specificity of 64% and 86%, respectively, when measurements
were taken in a comparative manner 15. International standards such as the ISO/TR 13154:2017 18 which
are used to explain and outline current best-practice approaches suggest a number of considerations to
be taken into account (e.g. measurement location, number of subjects who can be measured
simultaneously, and recommended distance to subject) when performing fever screening. The existing
studies did not report that any of the systems which were installed according to these standards. This is
likely due to the fact that the finalised version of the ISO/TR 13154:2017 standard was released after
the studies were conducted.
As such, this study, conducted in Australia during the COVID-19 pandemic, evaluates the precision
and accuracy of three different types of IRT system for human temperature measurements when
installed in a real-world scenario with a mask-wearing population. This was performed to determine
their efficacy as screening systems, when compared with a certified benchmark temperature
measurement device commonly used in hospitals.
Method
Study Design
The investigation was designed as a single-centre observational study comparing the accuracy of three
IRT systems to core-temperature measurements taken using certified oral thermometers. Additionally,
the precision of each IRT system was determined through repeat measurements.
Setting
The study was conducted at a University during a five-day period in August 2020 from 9AM to 5PM.
Sampling and Eligibility Criteria
The study was performed on a community sample and used a convenience method for participant
recruitment. The study aims, participation requirements, and methods of consent and requirements of
participation were provided in an email to staff and students in the building. Verbal consent was
gathered on the day. These measures were performed to ensure that social distancing guidelines were
able to be maintained during the study. Every employee and student who attended the building during
the study period was invited to participate. Due to the nature of the facility, the participants were all
over 18 years of age.
IRT Systems
Three IRT systems were selected for use which represented three of the main types of systems that are
being sold in the Australian market. These were as follows:
System 1 A dual-camera system with a 40 °C external reference temperature device
(blackbody), advertised to be able to measure up to 30 subjects simultaneously while
performing facial recognition tasks.
System 2 A single camera system with laser-assisted autofocus which operates without a
blackbody.
System 3 – A single camera system with a 35 °C blackbody which is deployed in-line with the
guidance provided in the ISO/TR 13154:2017 technical standard (apart from the guidance on
masks) 18.
The specifications for each system are included in Table 1:
Table 1. Specifications of the IRT systems used in the study.
System 1
System 2
System 3
Camera Pixels
400 × 300
464 × 348
640 × 480
Thermal
Sensitivity
≤40 mK
<40 mK
<50 mK
Stated Accuracy
±0.3 °C
±0.3 °C
Not stated
Dual Cameras
Yes
No
No
Blackbody Used
(temperature)
Yes (40 °C ±0.1
°C)
No (N/A)
Yes (35 °C ± 0.1
°C)
Measurement
Location
Whole Face
Inner Canthus
Inner Canthus
Data Reporting
Corrected
Raw
Raw
Number of
Subjects in
Simultaneous
Measurement
30
1
1
Required a
Sightboard
No
Yes
No
Experimental Setup
Each of the three IRT systems were loaned to the researchers by the manufacturing companies for the
purpose of conducting this experiment. To ensure the independence of this trial, each company provided
the equipment free of charge and signed a research services waiver giving the researchers the right to
publish the results without commercial input or oversight. The IRT systems were all setup according to
the manufacturer’s directions. The researchers were provided training on the assembly, deployment and
operation of the IRT systems to ensure correct operation. Each manufacturer confirmed the
configurations of the systems, the setup environment and the experimental methodology prior to the
beginning of the trial through video conference but were otherwise uninvolved in the experiment.
The setup conditions of the three IRT systems are visualised in Fig 1.
Fig 1. Experimental setup of the three IRT systems. In each case, the location of the subject being
measured is indicated by a cross.
The conditions of the experimental environment were as follows:
The area was not directly under any active Heating, Ventilating, and Air Conditioning (HVAC)
system.
The room in which the trial was located was temperature and humidity controlled.
No lights or thermal radiation sources were directly in view of any of the cameras, and the
cameras were not pointed at any reflective surfaces.
Each IRT system was allowed 30 minutes of temperature stabilization each day prior to the first
measurement being taken.
The room had no direct entry from the outside, each doorway had an airgap which had to be
traversed prior to entering the experimental area.
Systems 2 and 3 were focussed at the beginning of the trial, and focus was checked four times
per day (the systems remained in focus throughout the trial). The manufacturer of System 1
indicated that manual focussing was not required.
Systems 2 and 3 were directly in-line with the subjects face. The manufacturer of System 1
indicated that this was not necessary.
TV Screens were setup for Systems 2 and 3 so that subjects could orient themselves within the
camera targeting area. System 1 used a web-based application and a laptop to display the data.
Experimental Procedure
Every step of this experiment was strictly conducted according to socially distancing guidelines
between the researcher and the subject.
As a potential participant entered the building, they were approached by the researcher who confirmed
their knowledge of the study (information which had been provided through email) and asked for their
verbal consent. If the answer was in the affirmative, the participant then verbally completed a
questionnaire of demographic data (age, gender, skin tone and mask-type), and also was queried as to
their current health status, in particular:
“Have you experienced any fever symptoms in the last 24 hours?”
“If yes, did you take any medications to treat your fever in the last 4 hours?”
“Have you experienced any symptoms in the last 24 hours of sore throat, cough, runny nose,
loss of taste or smell?”
Have you had a hot or cold drink in the last 30 mins?
This data was entered into a case report form for each individual in a secure REDCap (Research
Electronic Data Capture) database.
The participant was then questioned to if they had been outside the building in the past 5 minutes, if the
answer was in the affirmative, they were asked to wait for 5 minutes to acclimatise. The purpose of this
was to avoid readings from being impacted by the exterior environment. The participants were also
asked to remove hats or glasses to ensure accurate readings.
All participants were wearing masks during the trial, apart from when using the oral thermometers, as
during the SARS-CoV-2 pandemic in Victoria, Australia, there was a state-wide mandate that all
individuals must wear masks when outside their own homes, this means that any screening technology
used during this time would be required to operate with masked participants 19.
Each participant was then asked to spend 5 seconds standing in front of each IRT system facing directly
into each camera at a distance of 1.5m (indicated by a mark on the floor). This was then repeated. At
the conclusion of these measurements (6 in total), the participant was provided with a DT-01B oral
thermometer (Measurement accuracy: ±0.1°C [35.5°C-42.0°C]) and requested to stand in an isolated
location (indicated by a mark on the floor), where they proceeded to take an oral thermometer reading.
The two readings from each IRT system and the single oral thermometer reading were then entered into
the secure REDCap database case report form.
Analysis
The RStudio integrated development environment for R (version 1.3.1056, R version 4.0.2) was used
for the statistical analysis, and for all tests, a 0.05 level of significance was used. The frequency
distribution (reported as mean and standard deviation), was calculated for the participants age, the
number and percentage distribution of the participant’s gender, skin colour and mask-type were also
calculated.
An investigation into the measurement precision was then performed to analyse the difference between
the first and second measurements of each IRT system and to determine if the claimed 0.3 °C of
accuracy was observed in a multi-measurement precision test. This precision error was also reported as
a boxplot to observe the distribution of the quartiles as well as to identify outliers. The frequency
distribution of the precision error was then calculated and reported as mean and standard deviation.
The measurements of the IRT systems were then compared against the reading of the oral thermometer,
and the mean and standard deviation were calculated. The Pearson's correlation coefficient (ρ) was also
calculated to determine the correlation between the measurements. The thermal camera measurements,
in comparison to the oral thermometer readings were then fitted to a linear model, and the coefficient
of determination for each system was calculated. The error of each measurement for the three systems
was then calculated. Finally, the accuracy was assessed in relation to each of the demographic attributes
(age, gender, skin-color, mask type), the mean and standard deviation of the error was calculated, and
tests of statistical significance between the systems were calculated using Welch’s t-test (as the sample
sizes were unequal 20).
Ethical Considerations
This study was approved by the Deakin Human Ethics Advisory Group (Health) (approval number:
HEAG-H 154-2020). All data was anonymised before being stored in a secure double identifier
password protected REDCap (Research Electronic Data Capture) database administered by Deakin
University with access limited to the study investigators.
Results
Participant Characteristics
Over the five-day period of the study, a total of 107 measurements were taken from the participating
subjects within the building. As individuals were able to take part in the study on subsequent days, the
number of unique participants was seventy-one. Table 2 details the demographic attributes of the study
participants. All participants wore face masks as mandated by the Victorian Department of Health and
Humans Services 19.
Table 2. Demographic attributes of participants.
Mean
SD
Age (year)
35.66
10.09
Self-Reported Fever
0
0
Other Symptoms of
COVID-19
0
0
Gender
n
%
- Male
77
71.96
- Female
30
28.04
Skin Colour
n
%
- Light
69
64.49
- Medium & Dark
38
35.51
Mask Type
n
%
- Thin
56
52.34
- Thick
51
47.66
In this study, no participants reported having a fever in the past 24 hours, or experiencing other COVID-
19 related symptoms (such as sore throat, cough, runny nose etc.) which is due to a level of selection-
bias, as these symptoms would have precluded their entrance to the campus.
IRT System Precision Test
The first test performed on the IRT systems was to determine if the systems were able to achieve a
precision of within ±0.3 °C through a repeated measurement precision test, the results are shown in Fig
2 and Table 3. As each participant in the study was measured twice by each system within 30 seconds,
the ideal system would have had a temperature difference of 0 °C between the two measurements. From
our results it was clear that none of the systems achieved a precision within ±0.3 °C. The mean and
standard deviation of precision error for each system were as follows: System 1: 0.024 ±0.183 °C,
System 2: 0.019 ±0.194 °C, and System 3: 0.044 ±0.214°C. The mean errors were low in this case as
all systems experienced both positive and negative precision errors which caused error cancellation.
Using the 2-standard deviation confidence interval, the systems were found to have precisions of: ±0.34
°C for System 1, ±0.37 °C for System 2 and ±0.38 °C for System 3. From the box-plot in Fig 2B, it can
be clearly observed that System 2 was the system with the most measurements within ±0.3 °C
measurement precision, however it still recorded a number of outliers.
Fig 2. Repeated measruements precision test results. (A). A bubble-plot which displays the
first measurement against the second. The dotted lines is the range in which a system with 0.1
°C of precision would fit, the continuous lines are the range in which a system with 0.3 °C of
precision would fit. (B). A boxplot of the difference between the first and second
measurement clearly displaying the quartile ranges and the outliers from each system.
IRT System Accuracy and Correlation Test
The second test compared the IRT system readings against the oral thermometer to determine how
correlated the readings from each system were to core body temperature. While not required to be exact,
an accurate system for detection of elevated body temperature should have a strong degree of correlation
with the oral thermometry readings, with an increase in core temperature resulting in an increased IRT
measurement. The results of this test were tabulated and are displayed in Table 3. The oral thermometer
measurements were similar to those found in previous studies 3,4. Firstly, there were significant
differences found between the IRT systems and the oral thermometer measurements with the mean
difference in System 1 being 0.26 °C, System 2 being -1.90 °C, and System 3 being -1.31 °C. In
addition, the correlation coefficients calculated were weak (<0.5), with System 3 being the most
correlated to the oral thermometer results (ρ = 0.49) and System 1 being the least (ρ = 0.33)
Table 3. The data from each IRT system, and the oral thermometer. The correlation
coefficient is for each IRT system against the oral thermometer.
System
Mean (SD) °C
Mean Difference
°C
Correlation
Coefficient (ρ)
Oral
Thermometer
36.37 (0.452)
-
-
System 1
36.62 (0.338)
0.255
0.333
System 2
34.47 (0.685)
-1.902
0.407
System 3
35.06 (0.671)
-1.311
0.491
A linear model was developed for each of the systems relative to the oral thermometer results and is
displayed as a bubble-plot in Fig 3. Each of the systems had a generally increasing trend in their reported
temperature readings as the oral thermometer readings increased. However, the results from this
analysis agree with the results in Table 3 with the three IRT systems only presenting weak coefficient
of determination’s (r2), with the strongest being System 3 (r2 = 0.24) and the weakest being System 1
(r2 = 0.11).
Fig 3. Bubble-plot of the IRT measurements vs the Oral thermometer measurements.
Participant Characteristics and IRT System Error
In addition to examining the participants in aggregate, a set of analyses was also performed to determine
if any of the recorded demographic attributes had an impact on the measurement error experienced by
the IRT systems compared to the oral thermometer. For each of the recorded characteristics (age,
gender, skin colour and mask type), the mean and standard deviation of the error and p-values were
calculated, using Welch’s t-test (as the sample sizes are unequal 20). These results were tabulated and
are displayed in Table 4. From the data, it can be seen that the IRT systems errors were impacted by the
demographic factors, however, each of the systems was not impacted by the same demographic factors.
System 1 was impacted by the age of the subject as well as by the mask type. System 2 and 3 were
impacted by the participants gender and skin colour.
Table 4. Results of examining the error of each IRT system in relation to each of the
recorded characteristics. Factors which have a p-value of <0.05 are shaded.
n
System 1
System 2
System 3
Mean
(SD) °C
p-
value
Mean
(SD) °C
p-
value
Mean
(SD) °C
p-
value
Subjects
Age*
- <40 150
0.192
(0.415)
<0.05
-1.874
(0.662)
0.86
-1.277
(0.594)
0.31
- >=40 44
0.395
(0.525)
-1.855
(0.627)
-1.389
(0.643)
Gender
- Male 154
0.273
(0.480)
0.35
-1.762
(0.608)
<0.05
-1.211
(0.569)
<0.05
- Female 60
0.210
(0.424)
-2.260
(0.615)
-1.570
(0.595)
Skin Colour
- Light 138
0.220
(0.478)
0.13
-1.817
(0.664)
<0.05
-1.222
(0.592)
<0.05
- Medium /
Dark
76
0.318
(0.436)
-2.055
(0.594)
-1.475
(0.573)
Mask Type
- Surgical
Mask
112
0.339
(0.456)
<0.05
-1.884
(0.693)
0.67
-1.271
(0.609)
0.30
- Thick
Mask
102
0.163
(0.459)
-1.922
(0.599)
-1.356
(0.584)
*Subjects were given the option to not give an age, 10 subjects indicated that they did not want
their age recorded.
Validation of System 2 against External Temperature Reference
Devices
System 2 was checked against two separate blackbody reference devices (35 ±0.1 °C and 40 ±0.1 °C)
and was found to report the correct temperature within 0.1 °C, demonstrating its high degree of
measurement accuracy.
Discussion
IRT systems are being installed in a wide variety of locations worldwide in response to the COVID-19
pandemic. Yet, there is limited evidence available in the literature reporting on their accuracy or efficacy
in these application spaces, in particular on how they correlate to core-body temperature measurements.
In order to address this research gap, the current study investigated the use of three different IRT
systems from different manufacturers in a real-world setting, with a community mask-wearing
population.
Firstly, the precision of each of the three systems was determined through repeated measurements taken
a short time apart (<30 s). Each of the systems were found to have a precision wider than the 0.3 °C
accuracy claimed by two of the systems. Secondly, in regard to measurement accuracy, our results
suggest that the IRT systems each experience a deviation from the core temperature measurements.
Thirdly, this study has shown that in our sample, the IRT systems measurements only have a weak
correlation to the oral temperature measurements, with System 3 having the highest ρ and r2 with values
of 0.49 and 0.24, respectively. Fourthly, the participant characteristics were associated with changes in
the error between oral measurements and IRT systems measurements, however, these were not
consistent across all three IRT systems.
Some existing studies have found IRT systems to be sensitive and specific in assessing the febrile status
of subjects 14,21. These studies were also conducted with participants >18 years of age and with multiple
IRT systems. However, neither of these studies were conducted with a mask-wearing population, and
nor with IRT systems installed at the first stage of entry to a building as in our study, which is a common
use-case for these systems in hospitals and workplaces in 2020. In addition, the study by Nguyen et al.
21 which found the IRT systems to be sensitive and specific, was performed on individuals after they
had been registered in the emergency department, which may have allowed for an extended period of
acclimatisation time when compared to a regular building entry.
Our study is the only recent study reporting on repeated measurement precision of the IRT systems, this
is a significant result as it describes the repeatability of the system when measuring the same person
multiple times. From our results it seems unlikely that a 0.3 °C level of accuracy would be achievable
with such a wide measurement precision in each of the systems. Indeed, all 3 of the systems reported
measurements with differences of > 0.5 °C on the same individuals only 30 seconds apart.
The research conducted by Ghassemi et al. 22, and the ISO/TR 13154:2017 standard suggest that using
an external reference device (blackbody) increases the accuracy of the measurements. Our study results
show that this is a good recommendation; System 3, which used a blackbody reference, had a mean
difference 0.59 °C closer to the core temperature measurement than System 2, which used a similar
camera without a blackbody. In addition, System 3 was found to have a greater correlation to core
temperature than System 2, with their respective correlation coefficients being 0.491 and 0.407.
The current ISO standards also recommend that measurements should be taken from the inner canthus
of the eye rather than the forehead or general facial measurements 18,22. Our study found that System 1
which was not specifically taking measurements from the inner canthus in fact reported measurements
which were closest to the core body temperature measurements (mean difference increase of 0.26 °C).
This was an unexpected result, as the literature suggests that the facial skin temperature is generally
expected to be lower than the core-body temperature 16,23. This suggests that System 1 is employing a
correction algorithm on the measurement results, which may shift the measurements into a more
“acceptable” range. Additionally, the measurements from System 1, which were not taking
measurements from the inner canthus were found to have the smallest coefficient of determination of
any of the systems to the core body temperature (r2 = 0.11). This suggests that the use of the algorithm
does not significantly improve the efficacy of whole-face temperature measurement, and that the current
guidance is correct in recommending measurements be taken from the inner-canthus region over a
general facial measurement.
The correlation found in our study (maximum being ρ = 0.49) is generally in agreement with the existing
literature, which reported values between 0.3 – 0.5 for well-functioning systems 15,21,24. However, the
study by Chan et al. 15 found that the correlation is higher among febrile populations with core-
temperatures 38 °C, which we were not able to include in our study, so the correlation may have been
improved with a population including a large cohort of febrile individuals.
The experiment involving the validation of System 2 against external temperature reference devices
(blackbodies) demonstrated that this system is capable of measuring emitted thermal radiation with a
high degree of accuracy in the experimental environment. When measuring these near-ideal sources,
System 2 reported a measurement value within 0.1 °C of the expected temperature, which is within the
margin of error of the reference device. This suggests that the measurement error observed in
experiments with human subjects is likely due to the physiological link between core-temperature and
facial temperature, rather than inherent technological error.
Our study also found that demographic characteristics had a significant impact on the measurement
error of the systems, however, this was not consistent across the three IRT systems. System 1 which
measured the whole face temperature exhibited an increased error on subjects ≥40 years of age, and
those who were wearing thinner masks. Systems 2 and 3 which measured the inner canthus of the eye
had an increase in error on subjects who identified as female, or those with darker skin. Other studies
have found gender and age to be factors which impact measurement accuracy 15,21. To the authors
knowledge, this is the first IRT study which has reported skin tone and face-mask type as demographic
factors and investigated their impact on measurement error. Our earlier study using NCIT devices in a
hospital setting found that these demographic factors (age, skin tone and gender) also impacted
measurement results, so it appears that there is scope for future research to determine their precise
impact on the performance of infrared measurement systems in fever-screening scenarios 12.
Strengths and Limitations
To the authors’ knowledge, this the first study of this type which has been conducted on a mask-wearing
population, which was a mandated intervention in Victoria, Australia during parts of the COVID-19
pandemic. The limitations of the study were as follows: this was a convenience selected community
sample, with no febrile subjects, there were no subjects >65, the duration of the camera loan agreements
and facility agreements dictated a one week timeframe which restricted the sample size. Additionally,
the variations in height of individuals impacted the measurement process as some subjects had to bend,
lean, or use chairs in order to be in focus in the measurement. The inclusion of an NCIT device could
have allowed for the exploration of the source of the measurement error, and finally the lack of febrile
individuals made it impossible to assess the sensitivity and specificity for individual febrile detection.
Future Work
There is a clear need to expand this study into a setting with more febrile individuals to allow for the
assessment of sensitivity and specificity of fever detection, however, the low correlation values found
between each of the measurement sources raises doubt to the efficacy of these systems at detecting
individuals with low-grade fevers (37.5-38 °C). In addition, there is clear scope to perform further
investigations into the impact of age, gender and skin-tone on the measurement results. Finally, there is
clearly an interesting avenue of investigation in relation to improving the precision of these
measurement techniques for mass screening.
Conclusion
This paper presented the first study on assessing the capabilities of IRT systems in a face-masked
population in a real-world mass screening scenario. This system was tested outside of a hospital setting,
at the entrance to a research facility within a building, mimicking the installation scenario of many
currently operating IRT systems. Our results show that using the systems as a front-line intervention
for fever-screening poses a potential risk, where the lack of measurement repeatability could negatively
impact sensitivity and specificity, possibly allowing febrile individuals to pass through undetected.
Although these systems are currently seeing widespread use due to the COVID-19 pandemic, our results
show that there is still further research required to improve their precision and accuracy so that users
can be confident in their operation. There remains an opportunity for new technology to meet this gap.
Acknowledgements
We sincerely thank Bridey Saultry (Deakin University, Australia) and Dr Andrew Valentine (University
of Queensland, Australia) for their contributions towards the conduct of the study and for critical
commentary at various stages of this work.
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