When COVID-19 hit, one of our employees jokingly said, “We should start making COVID-19 sensors instead of biometric sensors”. That triggered the development team to investigate whether we could use our diverse background to make a contactless fever sensor that would be more optimized for live entertainment. So we started learning about the world of thermal infrared temperature measurement to see what we could build. Although we were experienced with a lot of related technology, temperature measurement was something new. We’ve learned a lot and want to share our findings in order to help others make informed decisions.
First, a few disclaimers. Fever scanning of any type can only detect an elevated temperature; it cannot detect any particular pathogen or disease. Furthermore, many people (as many as 50+%) who have COVID-19 do not exhibit fever symptoms at all. For the rest of this post, we will use the technical term Elevated Body Temperature (EBT) scanning, as that is more accurate than calling these scanners “fever scanners.” The most important thing to understand is this:
EBT (fever) scanning cannot detect illness and should only be used as a
screening tool, followed by an actual medical diagnosis.
Let’s Talk About Thermal Energy
Feel free to skip this background section if you just want the recommendations.
Contactless temperature measurement is accomplished by measuring infrared radiation emitted by an object. When you hold your hand next to a hot stove, your are sensing infrared energy. This can sometime be confusing to people because of another part of the infrared spectrum called near infrared (NIR) which works differently.
On the electromagnetic spectrum, NIR is just above the color red. Human eyes can only detect a small part of NIR, and then only when it is pretty dark. That’s why NIR is used to allow security cameras to “see in the dark.” Because of how they are manufactured, cameras are more sensitive to NIR that humans, so security cameras can shine a bright NIR light to illuminate dark areas without bothering people. Just like with regular cameras, a NIR camera is only as good as the source of light used.
Confusingly, infrared is also used to describe thermal infrared (LWIR) in the chart. The is energy emitted by an object as opposed to energy reflected from a light source. It is a property of the object itself. When you put your hand close to a hot stove, you are sensing thermal infrared.
The reason this is important is that unlike conventional visible or NIR sensors, the performance of a thermal infrared sensor cannot be improved with external lighting. It can only be improved with better measurements.
Measuring Thermal Energy
Using contactless methods to measure temperature will almost always be less accurate than measuring temperatures with a sensor attached to an object. With this method, the sensor and the measured object will reach an equilibrium and can achieve a high degree of accuracy. Using a contactless method for temperature measurement requires the sensor to be pointed accurately at the object from a distance with a particular field of view for the sensor. This potentially introduces a large (sometimes massive) source of error, but contactless temperature measure is convenient for cases where we don’t want to get close to the object we are measuring.
There are two methods for contactless temperature measurement – thermal cameras or individual thermal sensors. A thermal camera is similar to a regular digital camera, but the pixels are sensitive to thermal energy, not visible light. A thermal sensor can be (roughly) thought of as a single pixel from a thermal camera, but more accurate and less expensive.
Thermal cameras allow a human to see a representation of thermal energy. Outside of EBT scanning, thermal cameras are typically used to measure temperatures of equipment that may be hard to reach – circuit boards, machines, motors, heating ducts, etc. For most thermal camera use cases, high accuracy (+/- 1 °F) is not needed.
Thermal cameras are typically much lower resolution than regular digital cameras – 640×480 or even 320×200 pixels. However, because they are essentially video cameras, companies are using them to measure the temperature of people in motion, as shown below:
This approach is problematic for a number of reasons. First, effective EBT screening requires an absolute minimum accuracy of +/- 0.5 °C to be effective, and the vast majority of thermal cameras are only accurate to +/- 2.0 °C. The cheapest camera FLIR offers with the required accuracy is the E75 at $6,999 and that doesn’t include other required software and hardware. Second, both the FDA and FLIR state clearly that subjects who are being screened need to stop and remove their glasses. Because of the low resolution of thermal cameras, a system cannot reliably pick up enough pixels on the correct location on the face to measure fever in moving people.
A thermal sensor is a discrete component that is sensitive to thermal energy, but you can think of it as essentially a thermal camera with one pixel. However, since it is a single sensor as opposed to an array, it is easier to make it more accurate and to compensate for ambient temperature influences. These sensors are also a tiny fraction of the cost of a thermal camera – around $4-$40 for a single sensor.
If you have used an infrared thermal “gun” you have used a device with a thermal sensor. These thermal sensor guns are used in home improvement, HVAC, as well as fever screening.
These devices are very similar, but are tuned and designed differently. Home improvement guns are designed to cover a large range of temperatures (0-250 °F) and are correspondingly inaccurate – typically +/- 2 °C or +/- 5 °C, so are not suitable for fever scanning. Medical temperature guns measure a much smaller range of temperatures, but because of how they are designed they typically require a measurement distance of less than an inch (this example photo is too far away). These devices have the required accuracy, but they are difficult to use without touching the subject and impossible to use while maintaining social distancing.
We believe thermal sensors are objectively superior to thermal cameras for the following key reasons:
- The FDA recommends people stop for an accurate measurement, eliminating the advantage of motion/video
- An accurate thermal camera solution will require an expensive camera ($7,000) and an expensive black-body radiator ($4,000) as minimum hardware, plus associated software
- Because they are dramatically cheaper, thermal sensors can be deployed at the perimeter of a location, in “self service” mode
- Thermal sensors are available that are specifically calibrated for medical accuracy
The FDA has stated that fever scanners are considered medical devices, but in the interest of fostering innovation, the FDA will not regulate fever scanners during this crisis. That means it is the Wild West in terms of vendor claims and some vendors are taking full advantage of it. The main grievances are claims of exaggerated accuracy and exaggerated range of detection. These particular exaggerated claims are especially troubling because they hit at the heart of what the device is supposed to do.
When a device with only +/- 2 °C accuracy is used for tever scanning, it can easily report a normal person as having a fever (annoying) or report a fever person as normal (dangerous). Home improvement style thermal guns, FLIR cameras that attach to phones, and other “inexpensive” (< $7,000 !!) thermal cameras all fall into this category and cannot be made accurate enough to be used for fever scanning.
Exaggerating a device’s range is equally problematic. A sensor or camera has a specified field of view (FOV) and needs to have the measured object fill up the FOV to take an accurate temperature reading. Fever guns have a 90 degree FOV, meaning they need to be held about one inch from the forehead. As they are moved farther away, they will start to read the forehead and surrounding ambient air together, which will lower the measured temperature. At about two feet, these devices will read about 2 °C lower than actual.
Normally, the FDA regulates devices like fever scanners under a lightweight regulatory mechanism called 510K. This is meant for devices that are functionally similar to existing devices, meaning they need less scrutiny. A vendor would not need to show contactless temperature measurement works, just that the device they are making is essentially similar to existing devices. This 130 day review process is normally required before marketing any medical device, and it ensures the claims made by the vendor are not fraudulent, misleading or dangerous. But since the FDA has waived this requirement due to Covid-19, so there is no government agency checking claims and in the case of Chinese imports that oversight was never there in the first place.
If you are buying an EBT sensor you can’t assume their published specifications are accurate, especially if the company is new. You should ask any prospective vendor for test data or independently calibrated test data tied to their system showing an accuracy of at least +/- 0.5 °C. Otherwise, your EBT scanning solution may give the illusion of safety, making it worse than doing nothing at all.
If you just skipped to the end, here is what you need to know:
- Fever screening cannot detect pathogens, nor can it detect everyone who is sick
- In-motion fever scanning is not recommended as a viable solution by the FDA or FLIR
- The minimum FLIR system for accurate fever scanning will require ~$11,000 worth of hardware
- Most thermal cameras are only accurate to +/- 2.0 C
- Off-the-shelf thermal guns for home use are not accurate enough for fever scanning
- Off-the-shelf fever guns are accurate, but require less than one inch measurement distance
- Solutions in this area are not currently regulated by the FDA