Accurate measurement of body temperature

Why Workswell Medicas

How to measure body temperature accurately and avoid false alarms

Measuring human body temperature is a metrologically demanding application. In the industry, usually a minor temperature measurement error (for instance +2 °C) can not lead to critical erroneous alarms and, what is worse, risk overlooks (infected person). But that is not true when measuring human body temperature. For instance temperature 37 °C and 39 °C in terms of body temperature is very different situation (yes, it is simply the difference between a healthy and a sick person).

The standard uncertainty of the infrared camera for thermodiagnostic purposes is ± 2 °C @ 35 °C (ie, for a temperature 35 °C; with increasing temperature the uncertainty usually increases).

This is a measurement uncertainty that is common for instance for WIRIS Pro thermal camera (but you will find this value with virtually all manufacturers) and it is definitely not suitable value. Technically, it is not possible to achieve better accuracy in terms of long-term measurement, without continuous black body calibration! And that’s why we developed infrared camera the Workswell MEDICAS for body temperature measurement at a time of risk of a pandemic.

Continuous re-calibration using a black body

A black body is permanently placed in the field of view of the thermal camera Workswell MEDICAS. This black body has very low measurement uncertainty and is used as a so-called local working standard. This ensures the highest possible measurement accuracy, which cannot be achieved by another measurement procedure.

The use of a black body is therefore irreplaceable.

A calibrated black body is therefore always included with our system. And the big advantage is that the system calibration is done by re-calibrating a black body, which is much cheaper than a infrared camera.

Workswell MEDICAS re-calibration using black body

Resolution, measuring distance and errors

The resolution of the infrared is very important for accurate temperature measurement. This is due Workswell MEDICAS is offering 640×512 pixels resolution with FOV (Field of View) 45°×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 screened area from 4.1×3.3 m to  9.9×8 m.

Due high resolution Workswell MEDICAS helps keep a safe distance from infected people.

640×512 pixels, 45°×37°

Distanceimage area (Horizontal/Vertical)spot size 1×1/3×3
5 m4.1×3.3 m6.5 mm/2 cm
6 m5×4 m7.8 mm/2.4 cm
7 m5.8×4.7 m9.2 mm/2.7 cm
8 m6.6×5.4 m1 cm /3.1 cm
9 m7.5×6 m1.2 cm  / 3.5 cm
10 m8.3×6.7 m1.3 cm / 3.9 cm
11 m9.1×7.4 m1.4 cm / 4.3 cm
12 m9.9×8 m1.6 cm / 4.7 cm
13 m10.8×8.7 m1.7 cm / 5.1 cm
14 m11.6×9.4 m1.8 cm / 5.5 cm
15 m12.4×10 m2 cm / 5.9 cm
17 m14.1×11.4 m2.2 cm / 6.7 cm
20 m16.6×13.4 m2.6 cm / 7.8 cm

Now lets try to calculate the same table for thermal camera with lower resolution and the same FOV: 336×256 pixels, 45°×35°.

336×256 pixels, 45°×35°

Distanceimage area (Horizontal/Vertical)spot size 1×1/3×3
5 m4.1 m / 3.2 m1.1 cm / 3.4 cm
6 m5 m / 3.8 m1.4 cm / 4.1 cm
7 m5.8 m / 4.4 m1.6 cm / 4.8 cm
8 m6.6 m / 5 m1.8 cm / 5.4 cm
9 m7.5 m / 5.7 m2 cm / 6.1 cm
10 m8.3 m / 6.3 m2.3 cm / 6.8 cm
11 m9.1 m / 6.9 m2.5 cm / 7.5 cm
12 m9.9 m / 7.6 m2.7 cm / 8.2 cm
13 m10.8 m / 8.2 m2.9 cm / 8.8 cm
14 m11.6 m / 8.8 m3.2 cm / 9.5 cm
15 m12.4 m / 9.5 m3.4 cm / 10.2 cm
17 m14.1 m / 10.7 m3.9 cm / 11.6 cm
20 m16.6 m / 12.6 m4.5 cm / 13.6 cm

Results: As you can see form tables, thermal camera with a resolution 640×512 pixels could for instance offer a geometric resolution (edge length of spot size 1×1/3×3 pixels) 1.3 cm/3.9 cm for distance 10 m. Same geometrical resolution could offer a 336×256 pixels thermal camera only for distance 5.5 m!  Of course, measure distance for a thermal camera with even lower resolution is only a few meters and than is not able to capture more people at once and it is then difficult for people to reach a safe distance.

You can also use our FOV calculator to verify this calculations.

Frequently asked questions (FAQ) about Workswell MEDICAS and human body temperature measurement

Yes. Workswell MEDICAS could offer a safe distance for both – screened persons and camera operator. Optimum measurement distance using Workswell MEDICAS is 6 m to 12 m, that is enough to safe distance for COVID-19. That is more than  6 feets or even 4.5 meters. And for sure, however, the image from the thermal imager can of course be transmitted over long distances, using both HDMI and TPC / IP interfaces.

Some informations about “safe distance”

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. Also, do not measure people who have recently been exposed to low temperatures or even weaker winds. Their body surfaces are cooled by these effects and the measured temperature is lower than the actual temperature.

Human skin emissivity value is approximately 0.96. When measuring 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 such very high emissivity of the human skin, the exact reflected apparent temperature is not of big importance (but this parameter really cannot be neglected).

No, standard industrial thermal camera is not suitable for body temperature measurement. The standard uncertainty of the infrared camera for thermodiagnostic purposes is ± 2 °C @ 35 °C (ie, for a temperature 35 °C; with increasing temperature the uncertainty usually increases). That’s too much. For instance temperature 37 °C and 39 °C in terms of body temperature is very different situation (yes, it is simply the difference between a healthy and a sick person). Workswell MEDICAS is offering ± 0.3 °C which no other thermal camera can offer.

No, using drone is not possible for this purpose. It is impossible to maintain a minimum distance and continuous re-calibration using black body.

The following table shows how to understand body temperature measurement results.

Temperature statusEar / ForeheadArmpit / Mouth
Low body temperature35.7 and less35.8 and less
Normal body temperature35.8 – 36.935.9 – 37.0
Increased body temperature37.0 – 37.537.1 – 37.5
Low-grade fever37.6 – 38.037.6 – 38.0
Moderate fever38.1 – 38.538.1 – 38.5
High fever38.6 – 39.438.6 – 39.4
Very high fever39.5 – 42.039.5 – 42.0