by Mikael Cronholm and Gary Orlove
From a biological standpoint, human beings are so called warm blooded animals. That means that we maintain a fairly constant body temperature, regardless of the surrounding temperature. The term body temperature (that we compare with when we decide whether a person has a fever or not) refers to the inside temperature, or core temperature of the body. The outside of the body is nearly always colder. It must be, because as we convert the energy from our food when we do work, we also produce heat. That heat has to go somewhere and if the outside and inside temperatures were the same, no heat transfer would be occurring.
None of us have a constant metabolism, or energy conversion, over time. It varies with our activity level. That means that the amount of heat we need to lose also changes with time. Our surrounding temperature also changes up and down, which means that sometimes we need to conserve heat and sometimes we need to increase the cooling by increased evaporation of liquid, we start to sweat more. Sweating is something we always do, just more or less depending on the situation.
To be able to use non-contact measurement for fever screening purposes, we need to find a point on the outside of the body that is close to the inside temperature – our “body temperature”. Because the outside is colder, and varies from place to place on the body, it is obviously the highest temperature on the outside of the body that is also the closest to the inside temperature. So we want to look for a warm spot on the outside.
We also want that spot to vary up and down in the same way that the inside body temperature does – we want what is called high correlation between outside and inside temperatures. We have to settle with that, because the two temperatures will not be the same.
So what points are available to us then? Generally speaking, the brain is one of the most stable parts of the body, in terms of temperature. It can only tolerate very small deviations in temperature. One part of the brain is called the hypothalamus, and it is responsible for the temperature regulation in the whole body. To do that, it needs to have a stable temperature. So we should look for points close to the brain. That suits us well, because the head is most seldom covered with clothes and therefore accessible.
Two pairs of points on the body are close to the brain and have a high rate of blood flow, which they share with the brain to a high degree. Those points are the eardrums and the areas in the corners of our eyes, close to the nose.
The eardrums are too hidden for our purposes, so we are left with the corners of the eyes. The inner canthus of the eye has been demonstrated to be a robust measurement site and is supplied by the internal carotid artery. It is for this reason that a
screening thermograph utilizes the inner canthus for determining the body temperature. Use an emissivity setting of 0.98.