How to achieve effective camera-based body temperature measurements
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).
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