Human body temperature and how to measure it correctly with a non-contact thermometer and thermal camera
13. 3. 2020
The importance of infrared imaging system during two phases of epidemiological situation
27. 3. 2020Why Workswell Medicas?
Measuring human body temperature is very metrologically demanding. Precision thermal imaging for healthcare MEDICAS in healthcare, a minor temperature measurement error (for instance +2 °C) can lead to critical false alarms and, even worse, overlooked risks. After all, even a small difference in temperature, such as that between 37° C and 39° C, indicates a very different situation – it marks the difference between a health and illness.
The standard uncertainty of an infrared camera for thermodiagnostic purposes is ± 2 °C @ 35 °C. What’s more, as temperature increases, uncertainty tends to increase as well.
This measurement uncertainty is common across virtually all thermal cameras, including Workswell’s WIRIS Pro. As a result, most thermal cameras are not suitable for healthcare applications. From a technical standpoint, it is not possible to achieve better accuracy in terms of long-term measurement without continuous black body calibration. That’s why we developed the Workswell MEDICAS infrared camera for body temperature measurement during the coronavirus pandemic.
Continuous re-calibration using a black body
A black body is permanently placed in the Workswell MEDICAS’ field of view. This black body has very low measurement uncertainty, and it is used as a so-called local working standard. This ensures the highest possible measurement accuracy, which cannot be achieved by any other measurement procedure.
The use of a black body is therefore essential.
That’s why we always include a calibrated black body with our system – and, since system calibration is achieved through the re-calibration of a black body, the Workswell MEDICAS offers significant cost savings.
Resolution, measuring distance and errors
The resolution of the infrared camera is very important for accurate temperature measurement. The Workswell MEDICAS offers 640 x 512 pixel resolution with a FOV (Field of View) of 45° x 37° (f = 13 mm).
If the temperature of the surface to be measured is greater than the ambient temperature, the measurement error generally increases with increasing distance. That is why the resolution of the thermal camera is crucial for accurate measurement.
Using Workswell MEDICAS, the optimum distance of the measured object is 5 – 12 meters, with a screened area from 4.1 x 3.3 m to 9.9 m x 8 m.
The Workswell MEDICAS helps medical workers maintain a safe distance from infected people.
640×512 pixels, 45° x 37°
| Distance | image area (Horizontal/Vertical) | spot size 1×1/3×3 |
| 5 m | 4.1 x 3.3 m | 6.5 mm / 2 cm |
| 6 m | 5 m x 4 m | 7.8 mm / 2.4 cm |
| 7 m | 5.8 m x 4.7 m | 9.2 mm / 2.7 cm |
| 8 m | 6.6 m x 5.4 m | 1 cm / 3.1 cm |
| 9 m | 7.5 m x 6 m | 1.2 cm / 3.5 cm |
| 10 m | 8.3 m x 6.7 m | 1.3 cm / 3.9 cm |
| 11 m | 9.1 m x 7.4 m | 1.4 cm / 4.3 cm |
| 12 m | 9.9 m x 8 m | 1.6 cm / 4.7 cm |
| 13 m | 10.8m x 8.7 m | 1.7 cm / 5.1 cm |
| 14 m | 11.6 m x 9.4 m | 1.8 cm / 5.5 cm |
| 15 m | 12.4 m x 10 m | 2 cm / 5.9 cm |
| 17 m | 14.1 m x 11.4 m | 2.2 cm / 6.7 cm |
| 20 m | 16.6 m x 13.4 m | 2.6 cm / 7.8 cm |
Now, let’s try to calculate the same table for a thermal camera with lower resolution and the same FOV: 336 x 256 pixels, 45° x 35°.
336 x 256 pixels, 45° x 35°
| Distance | image area (Horizontal/Vertical) | spot size 1×1/3×3 |
| 5 m | 4.1 m / 3.2 m | 1.1 cm / 3.4 cm |
| 6 m | 5 m / 3.8 m | 1.4 cm / 4.1 cm |
| 7 m | 5.8 m / 4.4 m | 1.6 cm / 4.8 cm |
| 8 m | 6.6 m / 5 m | 1.8 cm / 5.4 cm |
| 9 m | 7.5 m / 5.7 m | 2 cm / 6.1 cm |
| 10 m | 8.3 m / 6.3 m | 2.3 cm / 6.8 cm |
| 11 m | 9.1 m / 6.9 m | 2.5 cm / 7.5 cm |
| 12 m | 9.9 m / 7.6 m | 2.7 cm / 8.2 cm |
| 13 m | 10.8 m / 8.2 m | 2.9 cm / 8.8 cm |
| 14 m | 11.6 m / 8.8 m | 3.2 cm / 9.5 cm |
| 15 m | 12.4 m / 9.5 m | 3.4 cm / 10.2 cm |
| 17 m | 14.1 m / 10.7 m | 3.9 cm / 11.6 cm |
| 20 m | 16.6 m / 12.6 m | 4.5 cm / 13.6 cm |
Results: As you can see from the above tables, a thermal camera with a resolution of 640×512 pixels offers a geometric resolution (edge length of spot size 1×1/3×3 pixels) of 1.3 cm / 3.9 cm for a distance of 10m. A 336×256 pixel thermal camera could only offer the same geometrical resolution at a distance of 5.5 m! Of course, the measurement distance for a thermal camera with even lower resolution is only a few meters, and it cannot capture more than one person at once. As a result, it is difficult for healthcare professionals to reach a safe distance while executing these measurements.
You can also use our FOV calculator to verify this calculations.
Frequently asked questions (FAQ)
It is possible to maintain a safe distance using the Workswell MEDICAS?
Yes. The Workswell MEDICAS offers a safe social distance to both screened persons and camera operators. The optimum measurement distance of the Workswell MEDICAS is 6m to 12m. According to current information, a safe social distance for COVID-19 may range from six feet to 4.5 meters, so the Workswell MEDICAS well exceeds both requirements. In addition, the image from the camera can be transmitted over long distances, using both HDMI and TPC / IP interfaces.
Some information about “safe distance:”
- ny.gov – Try to keep at least 6 feet of distance between you and others at the event.
- com/news – bus passenger infected fellow travellers sitting 4.5 metres away
Can we measure people outdoors?
We do not recommend measuring people outdoors. Definitely do not measure people who are currently exposed to sunlight or have recently been exposed to sunlight, as this will increase the temperature reading. Also, do not measure people who have recently been exposed to low temperatures or wind. The surface of the body undergoes cooling in these conditions, and any temperature measured by the camera will be lower than actual temperatures.
How should we set the thermal camera parameters?
The emissivity value of human skin is approximately 0.96. When measuring skin with a thermal camera, we recommend deflecting it by approximately 5° from the surface of the forehead – to be more exact, from the normal incline of the measured surface. The reflected apparent temperature should be set either to the value equal to the surrounding air temperature, or it can be defined exactly as per the ISO 18434-1 standard. However, due to the high emissivity of the human skin, the exact reflected apparent temperature is not of major importance. That being said, this parameter should not be neglected.
Is a standard industrial thermal camera suitable for body temperature measurement?
No, standard industrial thermal cameras are not suitable for body temperature measurement. The standard uncertainty of an infrared camera for thermodiagnostic purposes is ± 2 °C @ 35 °C, and uncertainty usually increases with increased temperatures. That is too much uncertainty for a healthcare application. For instance, the difference between a body temperature of 37°C and 39°C is highly significant – it is simply the difference between a healthy person, and a sick person. The Workswell MEDICAS offers an uncertainty of ± 0.3 °C, which no other thermal camera can offer.
It is possible to measure human body temperature using a drone?
No, drones are not suitable for this application, for two reasons: It is impossible to maintain a minimum distance with a drone, and drones do not allow for continuous re-calibration using a black body.
How should we interpret human body temperature measurements?
The following table shows how to understand body temperature measurement results.
|
Temperature status |
Ear / Forehead |
Armpit / Mouth |
|
Low body temperature |
35.7 and less |
35.8 and less |
|
Normal body temperature |
35.8 – 36.9 |
35.9 – 37.0 |
|
Increased body temperature |
37.0 – 37.5 |
37.1 – 37.5 |
|
Low-grade fever |
37.6 – 38.0 |
37.6 – 38.0 |
|
Moderate fever |
38.1 – 38.5 |
38.1 – 38.5 |
|
High fever |
38.6 – 39.4 |
38.6 – 39.4 |
|
Very high fever |
39.5 – 42.0 |
39.5 – 42.0 |
How many people can we scan at once?
The distance at which one can precisely measure temperature is between 2-8 meters. This distance varies depending on the situation where the system is installed, and the skills of the operator are critical. When examining the thermal image, the operator will see all the pixels above the critical temperature levels highlighted in colors. This means that the operator will have a visual alarm, and can react when a problem arises. We offer a table of the distances of camera installations and black body sources.
Do we need to install anything on our PC?
The Workswell MEDICAS system works autonomously using firmware inside of the camera. If you want to analyze the radiometric images, you will have the install Workswell CorePlayer software, which will be delivered as part of the package.
Are any special accessories needed for system installation?
Not really. You can use standard tripods or any other anchor system using standard camera tripod screws. Installation is very simple.
