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Recently, due to the spread of the epidemic, people's body temperature has been tested everywhere, and most of them are using non-contact infrared thermometers. The staff took an infrared thermometer and fired a shot at your forehead, arm and other places, and the display showed your body temperature. So why can the temperature measurement function be achieved without contact? Does infrared temperature measurement emit infrared light for temperature measurement? What happened from pressing the trigger to shoot you to displaying the numbers? Let us be meticulous and unveil its mystery.
How was infrared detected?
In 1665, a great plague occurred in London, England, and more than 100,000 people died. Newton, 22, returned home to escape the plague. During this period, he did a dispersion experiment to study the composition of sunlight. He let a beam of white light pass through a prism, and found that what came out of the prism turned out to be seven light beams-red, orange, yellow, green, blue, indigo, and purple. Where does the white light go? Was it the shadow avatar technique, one divided into seven, or was it caused by some kind of reaction with the prism? He let these colorful lights converge through a convex lens, and then through a prism, the white light returned. (Surprise) So he came to a conclusion that white light is a mixture of seven colors of light.
The world can deceive you not only your mouth, but also your most trusted eyes. If you don’t see it, then does the invisible really not exist? 100 years later, Herschel in the United Kingdom did an experiment. He wanted to see which of the seven kinds of light could make an object heat up the fastest. He first disperses the white light into seven kinds of light, and then measures the temperature rise when a thermometer is placed in each place where the light is irradiated. As a comparison, thermometers are also placed on both sides of the seven kinds of lights. The experimental results startled him, but it was actually red light. The thermometer in the invisible place nearby heats up fastest. Is it hell? He understood that the eyes deceived him, and there is light in the invisible places, and this light irradiation makes the temperature rise the fastest, and he named this light infrared. The naming of ultraviolet rays is the same. (So are there any ghosts? Worth thinking about)
What is black body radiation?
What exists in the world is reasonable, and what exists has its meaning and value. What can infrared rays do? We can use infrared as a remote control, and remote sensing technology just uses the long wavelength of infrared, and of course we want to talk about temperature measurement. So how does an infrared thermometer use infrared to measure temperature? Let's first understand black body radiation. The so-called black body refers to black objects? A black body is actually an idealized object, which can absorb all external electromagnetic radiation without any reflection or transmission. No matter what object, as long as it is higher than absolute zero temperature, it is radiating electromagnetic waves. Electromagnetic waves are oscillating particle waves emitted in space derived from electric and magnetic fields that are in phase and perpendicular to each other. They are electromagnetic fields that propagate in the form of waves. The black body looks black when it is below 700K, but that is only because the radiation energy emitted by the black body below 700K is very small and the radiation wavelength is outside the visible light range. If the temperature of the black body is higher than the above temperature, the black body will no longer be black, it will start to turn red, and as the temperature rises, orange, yellow, white and other colors will appear. Take steel as an example, according to the process of temperature increase, it turns into red, orange, and yellow respectively. When the temperature exceeds 1300 degrees Celsius, it starts to turn white and blue. When the black body turns white, it will also emit a lot of ultraviolet light.
How does an infrared thermometer measure temperature?
The black body radiation law points out that the higher the temperature of the object, the stronger the radiation, as shown in the following formula.
Among them, the radiation power per unit area is called p, T is the absolute temperature, and σ is a constant. The sun can be approximately regarded as a black body. It is measured that the radiant power per unit area of the sun’s surface is 6×10^7 watts per square meter. According to the above formula, the sun’s surface temperature is 5700 Kelvin, which is 5153.7 degrees Celsius. This is the first time humans have calculated the sun’s surface temperature. .
The sun far in the sky can measure its surface temperature. Wouldn't it be easy to measure the surface temperature of the human body in front of you? So just as we saw, the staff took the infrared thermometer in his hand and fired at us. The infrared thermometer can collect the radiant power per unit area on the forehead to get your body temperature. The human body temperature is around 37, and the radiated electromagnetic waves are mainly concentrated in the infrared band, so this method of temperature measurement is called infrared temperature measurement. Seeing this, we understand that the original infrared thermometer does not emit infrared to measure temperature by itself, but our human body is radiating infrared rays, and the infrared thermometer is collecting these infrared rays.
What happened from shooting to showing the numbers?
Now we know that the infrared thermometer collects the infrared radiation power per unit area on the forehead, and we also know that the temperature is obtained through the formula, so how can we get the temperature value displayed on the display? The infrared thermometer is composed of optical system, photodetector, signal amplifier, signal processing, display output and other parts. The optical system condenses the infrared radiation energy of the target in its field of view, and the infrared energy is focused on the photodetector and converted into a corresponding electrical signal. After the electrical signal is amplified and filtered, it undergoes analog/digital conversion and is sent to the single-chip microcomputer (a small and complete microcomputer integrated on a silicon chip) for signal processing. The liquid crystal display unit displays the temperature value of the measured target.
Congratulations, your temperature is normal.
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