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Infrared thermometer
- 2019-01-18-

We usually go to the hospital for an examination. The nurse sister rarely uses infrared thermometers to measure your temperature. Generally, they are mercury thermometers and electronic thermometers. So why can't such a "tall" infrared thermometer completely replace the traditional thermometer, so popular? It also has to start with how it works ...

As the name suggests, the core of infrared thermometers is infrared, which is invisible to the naked eye, and is not the "red light" emitted by infrared thermometers (that is, lasers, which only help users determine the location of the measured object and do not participate in temperature measurement).


When the temperature of the object is higher than absolute zero [1], due to the molecular vibration of the substance, the energy generated by it is radiated outward in the form of electromagnetic waves, most of which are infrared. According to Stefan-Boltzmann's law, in a unit time, the total energy radiated by the black body [2] is proportional to the fourth power of its surface temperature.

Φ = A × σ × T4

Among them, A is the emissivity note [3], and σ is a constant. The infrared thermometer is based on this principle. By collecting all the infrared energy of the measured object, the thermodynamic temperature of the object can be calculated indirectly.

So how can the insignificant infrared light change into a temperature that can be read by the naked eye? The process is not complicated. First of all, you just need to point the thermometer's filter directly at the object to be measured, and the infrared rays generated by the object will be actively "thought and carried". After passing through the filter, it will be collected by the photoelectric system and transmitted to the photodetector. The signal is converted into an electric signal and sent to a signal processor. After being corrected by the signal processor, it is transmitted to the display output system and finally converted into a temperature value.


Nowadays, infrared thermometer technology has long been able to achieve high-precision temperature measurement, but the ideal is beautiful, the reality is very skinny. In actual life, an object cannot radiate all the infrared energy it generates to the outside world. It is generally believed that the measured object can radiate 95% of infrared rays (emissivity 0.95) to the thermometer. [4] The thermometer adjusts accordingly and can measure Get accurate temperature. Unfortunately, the emissivity of an object is affected by many factors, such as soil sticking to the surface, dusting, and distortion, which can change its emissivity. To ensure accurate results, it is necessary to prevent the influence of the surrounding environment (steam, dust, smoke) and keep the surface of the measured object clean.

As we all know, the readings of mercury thermometers only increase, the readings of the naked eye are prone to deviation, the temperature measurement time is long, and there is the danger of mercury poisoning after breaking. Therefore, China will stop production in 2020. The infrared thermometer is the opposite. It is safe and convenient. It does not need to touch the measured object, the temperature is fast, and the temperature of any part of the surface can be measured. However, it cannot measure internal temperature and objects with smooth surfaces and high transparency [5]. In addition, using an infrared thermometer, in order to prevent the infrared rays radiated by other objects from "disrupting", it is necessary to keep the laser perpendicular to the measured object and measure at a short distance.



It can be seen that due to its harsh conditions, infrared thermometers are often used in areas with low accuracy requirements, such as industrial temperature measurement or rapid screening of patients with abnormal body temperature (contact thermometers will bring the risk of infectious diseases) . However, high-precision body temperature detection is required. Contact mercury thermometers, alcohol thermometers, and electronic thermometers are obviously more reliable than infrared thermometers. After all, 37 ° C and 37.5 ° C are very different!