Infrared Thermometer: How to achieve non-contact temperature measurement?
Human body temperature will change due to many conditions (diseases), and measuring body temperature can be used to detect and record abnormalities.
Normal body temperature is the average temperature for most people under normal conditions, around 37°C (98.6°F). Each human body temperature can vary by 1°F (0.6°C) above or below this normal value.
Changes in normal body temperature are related to your activity level and time of day, as well as the secretion of hormones in your body. For example, when a woman ovulates or has a period, her body temperature will rise and fall even more.
The temperature inside the human body also varies. The temperature of the rectum or eardrum is 0.3~0.6°C higher than the oral temperature, while the temperature of the armpit is 0.3~0.6°C lower.
The use of ordinary mercury thermometers generally needs to be kept in the mouth and armpits for 3 to 5 minutes. It is relatively quick to measure body temperature by infrared method, and it only takes a few seconds to measure the temperature of forehead and eardrum.
Measuring temperature with an infrared temperature sensor seems straightforward. Align, press the button, and read the temperature value. However, if you do not master the measurement principles and methods, the measured temperature results will be greatly deviated.
Temperature measurement methods can be divided into two categories: contact and non-contact. Contact sensors include thermocouples, thermistors, RTDs, and semiconductor temperature sensors. The signals output by such sensors actually reflect their own temperature changes, and they need to be in full contact with the temperature-measured object to make their temperatures consistent.
In some cases, it will be troublesome to use a contact temperature sensor, such as: the measured object or medium is far away, or in a dangerous environment, which is not easy to reach; the measured object is in motion; the measured object is small and the temperature will be affected by the sensor. These problems can be solved by using non-contact temperature measurement methods.
Infrared thermometers belong to non-contact temperature measurement, which use the relationship between the thermal radiation of objects and the temperature of objects to work.
Heat is usually transferred in three ways: conduction, convection, and radiation. Thermal radiation is essentially electromagnetic waves of a certain wavelength, ranging from 0.7 to 1000 microns. The actual use of infrared thermometers to measure thermal radiation has a wavelength range of 0.7 to 14 microns, and most objects radiate the strongest in this range.
The absorption of energy (including heat) by an object causes its temperature to rise, which also radiates heat. In thermal equilibrium, the heat energy absorbed (Wa) is equal to the heat energy emitted (We). The temperature of an object is reflected in radiant heat energy in two forms.
One way is that the total amount of heat energy is related to the fourth power of the absolute temperature of the object:
We: thermal radiation capability; E: object emissivity; σ: Stefan-Boltzmann constant. T: absolute temperature of the object; A: emission area.
Usually, the E, A, and σ of the measured object are constant, so the temperature of the object can be obtained by measuring We in reverse. This method needs to determine parameters such as E and A through calibration in advance.
