How to achieve effective camera-based body temperature measurements
IEC / DIN EN 80601-2-59:2017 - guidance for thermography measurements for human febrile temperature screening - includes minimum specifications of resolution, accuracy, as well as the subject placement for the effective measurement of body temperature.
The IEC’s guidelines indicate that the Canthus (inner corner of the eye) is known to provide a reasonable indicative body temperature reading, and that all other methods are inaccurate. According to the international standard “the current evidence indicates that the region medially adjacent to the inner Canthi is the preferred site for fever screening due to the stability of measurement. This is because this region is directly over the inner carotid artery.”
Assuming the optics are very good, to be able to get an accurate measurement from this region 4x4 pixels need to cover it. Any objects smaller than this will appear cooler than they actually are. If the canthus is approximately 3mm in diameter, this would mean a resolution of at least 320x240 would be required to be able to measure the temperature in that region accurately - with the subjects face covering most of the field of view.
The camera needs to be pointing directly at the subjects face, with both canthus regions visible, and there must be no hair or glasses obstructing the face.
Long Wave Infrared cameras can’t “see” through glass, because the transmittance of most glass is very low at the LWIR wavelengths (8–14μm).
This is why measuring large groups of people numbers of people in the cameras field of view is a very inaccurate method of fever screening.
The standards also state that a black body reference must be in focus at the same time as the subjects face, to ensure camera stability (compensating for any potential drift).
It’s important to remember that by following this the system will only be measuring elevated body temperatures in accordance with the guidelines, it’s not detecting any diseases.
How thermal cameras (or, more accurately, thermographic cameras) work
What is blackbody calibration and why would you want it?
By Darth Kule - Own work, Public Domain, https://commons.wikimedia.org/w/index.php?curid=10555337