In light of the on-going COVID-19, pandemic agencies and businesses are starting to consider how to reopen and keep people safe as they begin the process of returning to a relatively normal life. Currently, many companies and health care facilities are turning to thermal imaging as a means of screening people while keeping their employees at a safe distance. Utilizing thermal imagers, along with best practices, can be very useful in mitigating the risks of getting back to businesses as usual. This article will discuss recommendations from the FDA and IEC, best practices to effectively utilizing an elevated body temperature (EBT) monitoring system, and things to be aware of when researching potential solutions for yourself.
This method of screening, though more well known now, has been in use since it’s more extensive use during the SARS epidemic of 2002, particularly in Asian countries that were more dramatically affected by the virus. Large scale testing is costly and complicated, so thermal imagers were very useful in identifying individuals that were more likely to have an infection. As with SARS, a common symptom of an infection of COVID-19 is fever. In a screening application, thermal imagers are used to accurately measure the surface temperature of a person’s skin to identify individuals that could have a fever from infection. Some thermal imager manufacturers use the term Elevated Skin Temperature (EST). Scientific literature has shown that the use of thermal imagers in a screening capacity can be very effective in estimating a person’s internal temperature when used correctly.
When implementing an EBT (Elevated Body Temperature) monitoring system, it is essential to meet some minimal requirements to allow for accurate measurements according to standards defined by the International Electrotechnical Commission (IEC) and the Food and Drug Administration (FDA) (Table 1). These requirements included minimum specifications of resolution, accuracy, as well as subject placement. In operation, the system should measure one person at a time, measuring the temperature of the medial canthus of the eye near your tear ducts (Fig. 1). Studies have shown this to be the best and most consistent target for accurate screening because the skin in other locations is much more dramatically affected by external factors. For example, if a person is outside in cold weather, the temperature of their forehead will drop significantly compared to their internal temperature (Fig. 2). This significant change does not occur at the inner canthus. To measure the inner canthus, the person should stand still and not have hair or glasses obstructing the imager’s view of the eye. The person screened should be positioned so that their face fills the majority of the field of view. If using the minimum required resolution of 320x240 pixels, the face should fill 240x180 pixels. Some EBT monitoring systems will include a reference temperature black body emitter. These emitters allow the imager to compensate for ambient temperatures that cause drift in the measurements. These temperatures include the body of the imager, lens, and sensor temperatures. Imagers that have an internal sensor such as the FLIR A400 can self adjust for these factors without the use of a reference source. This adjustment on the camera is called Ambient Temperature Compensation.
|Minimum Temperature Range||30°C - 40°C (86°F-104°F)|
|Accuracy||≤± 0.5°C (± 9°F|
|Stability and Drift||< 0.2°C (0.36°F) within 14 days or the manufacturer specified time frame whichever is longer|
|Minimum resolvable temperature difference (MRTD)||≤ 0.1°C|
The temperature of the tear duct is the most accurate area to measure on a body
Note the difference in temperature between the forehead and tear duct
The best EBT monitoring system that is best for your application will depend on several factors. One factor might include pre-existing security. Depending on your industry, you might have security at entrances to your facility or a reception desk. You could create a screening station that could be easily operated by the security officer or receptionist that is checking people into the building through the lobby or security desk. A handheld imager, like the FLIR E95, could be used in this scenario. A fixed imager on a tripod, such as the Optris PI400i, could also be used. Pre-existing security camera systems are also something to consider when choosing an EBT monitoring system. Some fixed imagers like the FLIR A400 or A700 can be integrated easily into existing security, allowing a more automated system. The layout of your facility could also play a role in what system would work best for you. When using limited space as an area to screen individuals, it can become challenging to use a more complex system that requires an external reference temperature emitter. When utilizing an emitter, it must be in the same plane as the person being screened, which can present some difficulties with installation and space. Also, you will need a computer with software to process the data from the emitter, so the set-up is more extensive. Finally, The minimum resolution required for EBT screening is 320x240 pixels. However, there is a benefit with imagers with higher resolution. One advantage is improved accuracy and further distances. With any imager, you want the person as close as possible to the unit. Allowing for more pixels on the desired target and, as a result, better measurements. As the unit moves further away from the person, the pixels get larger in comparison to the target. Once far enough away from the imager, the measurements will start to “average,” causing inaccuracy as the temperature drops. For example, the FDA recommends pixel sizes less than 1mm. This pixel small enough pixel to get an accurate measurement of the inner canthus. As you move away from the target, the pixel size will increase and will become larger than the inner canthus. At this point, you will not measure the small hot spot at the inner canthus, but averaging a larger area. With a higher resolution imager, the pixels are smaller, allowing you to be farther away from the person while still achieving accurate results.
Pixel size gets larger as the target moves away from the imager
First, It is essential to know that the thermal imager cannot diagnose individuals or to identify the presence of a fever. The imager identifies people with elevated body temperatures that likely indicate the presence of fever. Secondary methods, such as a sublingual thermometer should be used to confirm the presence of a fever. The imager also cannot identify the cause of EBT. The cause could be COVID-19 or the flu. It’s strongly recommended to use the systems and best practices described by the IEC and FDA for the best results. Given the current demand for EBT monitoring systems, some are being advertised with the minimum specification that shows individuals walking quickly past the imager with glasses on. You should be careful when purchasing a system like this for a few reasons. The first is that, especially with glasses on, the imager will be measuring the forehead or other areas of the face (Fig. 3). As discussed previously, these areas are dramatically affected by the environment, hair, makeup, and sweat, leading to false negatives. Secondly, uncooled imagers used for screening are not good at precisely measuring targets that are moving quickly, again leading to false negatives.
Glasses appear opaque and don’t allow measurement of the medial canthus of the eye
In light of the COVID-19 pandemic agencies and organizations will need to implement protective measures for their employees as we work towards normalcy. Systems like thermal imaging EBT monitoring are growing in popularity as a way to protect people. It is important to be aware of recommendations and to be wary of systems that promise more than they are likely to be able to deliver on. If you have any questions about EBT or systems to monitor EBT, please reach out to us.
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- Ring, Francis J., and E. Y. K. Ng. “Infrared thermal imaging standards for human fever detection.” Medical Infrared Imaging: Principles and Practices. CRC Press, 2007
- IEC 80601-2-59:2017