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Mar 21 2020

Working Principle and Accuracy of Infrared Thermometers

Catalog

I Introduction

II What is Infrared?

III Theoretical Principle of Infrared Temperature Measurement

IV The Principle of Infrared Thermometer

V Differences in Accuracy of Different Types of Infrared Thermometers

 5.1 Three Categories of Infrared Thermometers

 5.2 Differences Between Mainstream Infrared Thermometers

 5.3 Infrared Temperature Gun

VI Infrared Thermometer Accuracy And Factors Affecting Accuracy

 6.1 Precision of Infrared Thermometer

 6.2 Factors Affecting The Accuracy of The Infrared Thermometer Measurement

VII Factors to Consider When Choosing An Infrared Thermometer

VIII How To Make Infrared Thermometers More Accurate

IX One Question Related to Infrared Thermometers

 9.1 Question

 9.2 Answer

I Introduction

In the past two months, due to the outbreak of Coronavirus Disease 2019 (COVID-19), output of infrared thermometers exceeded the whole year of last year, driving the shipments and demand for chips such as sensors, MCUs, and operational amplifiers.

Infrared thermometer is a non-contact diagnostic technology that can scan and image the thermal radiation of objects and display data. It has the advantages of wide measurement range, fast temperature measurement, high accuracy and high sensitivity. With the widespread use of infrared thermometers, some users have doubts about its working principle and accuracy. This article will introduce how the infrared thermometer works, and explain its accuracy and the factors that affect it.

Figure1. Infrared Thermometer

Figure1. Infrared Thermometer

II What is Infrared?

Infrared is an electromagnetic wave with a wavelength between microwave and visible light. The wavelength is between 1mm and 760 nanometers (nm), which is invisible light longer than red light.

Anything above absolute zero (-273.15°C) can generate infrared rays. Modern physics calls it heat rays. Medical infrared can be divided into 2 categories: near infrared and far infrared. Containing thermal energy, the sun's heat is mainly transmitted to the earth through infrared rays.

Infrared is a part of the many invisible rays of the sun's rays. It was discovered by British scientist Herschel in 1800 and is also called infrared thermal radiation. It has a strong thermal effect. He split the sunlight with a prism, and placed thermometers on the ribbons of various colors in an attempt to measure the heating effect of light of various colors. It was found that the thermometer located outside the red light warmed the fastest. Therefore, it is concluded that in the solar spectrum, there must be invisible light outside the red light, which is infrared. Can also serve as a medium of transmission. The wavelength of infrared light in the solar spectrum is greater than visible light, with a wavelength of 0.75 to 1000 μm. Infrared can be divided into three parts, namely near infrared, with a wavelength between (0.75-1) to (2.5-3) μm; mid-infrared, with a wavelength between (2.5-3) to (25-40) μm; far infrared , The wavelength is between (25-40) ~ l500μm.

Figure2. Infrared

III Theoretical Principle of Infrared Temperature Measurement

In nature, when the temperature of an object is higher than absolute zero, due to the existence of internal thermal movement, it will continuously radiate electromagnetic waves to the surroundings, including infrared rays with a wavelength range of 0.75µm ~ 100µm. Its biggest feature is that at a given temperature and wavelength, the radiant energy emitted by an object has a maximum value. This substance is called a black body, and its reflection coefficient is set to 1; the reflection coefficient of other substances is less than 1, and is called gray body. Because the black body's spectral radiant power P (λT) meets Planck's law between the absolute temperature T, it shows that at the absolute temperature T, the radiant power of the black body per unit area at the wavelength λ is P (λT). According to this relationship, the relationship curve can be obtained as shown in the figure below:

(1) As the temperature increases, the stronger the radiant energy of the object. This is the basis of the theory of infrared radiation and the design basis of a single-band infrared thermometer.

(2) As the temperature rises, the radiation peak shifts to the short-wave direction (to the left) and satisfies the Wien shift theorem. The wavelength at the peak is inversely proportional to the absolute temperature T, and the blue curve is the line connecting the peaks. This formula tells us why the high temperature thermometer works mostly in the short wave and the low temperature thermometer works mostly in the long wave.

(3) The rate of change of radiant energy with temperature is larger at the short wave than at the long wave, that is, the thermometer working at the short wave has a relatively high signal-to-noise ratio (high sensitivity) and strong anti-interference. This is particularly important at wavelengths, especially for small targets at low temperatures.

Figure3. Planck's Law of Blackbody Radiation

IV The Working Principle of Infrared Thermometer

The infrared thermometer consists of the optical system, photodetector, signal amplifier, signal processing and display output. The radiation of the measured object and the feedback source is adjusted according to the modulator and input to the infrared detector. The difference between the two signals is amplified by the inverse amplifier and the temperature of the feedback source is controlled so that the spectral radiance of the feedback source is the same as that of the object. The display indicates the brightness temperature of the object being measured.

How does an Infrared Thermometer work?

V Differences in Accuracy of Different Types of Infrared Thermometers

5.1 Three Categories of Infrared Thermometers

According to different uses and accuracy, infrared thermometers can be roughly divided into medical-grade infrared thermometers, consumer-grade infrared thermometers, and industrial-grade infrared thermometers.

Strictly divided, medical-grade infrared thermometers have the highest accuracy requirements. The accuracy needs to be between 0.1 and 0.2 degrees. High-precision infrared ear thermometers can meet the medical-grade temperature standards. However, to avoid cross-infection, hospitals use ear thermometers. One-time sheath is needed for warm guns; consumer grades are next, and accuracy around 0.5 can meet our daily temperature measurement needs. The accuracy is about 0.3 degrees, which belongs to the consumer-grade infrared thermometer; the industrial grade has the lowest, generally the maximum allowable error is more than ± 1 ° C, and the distance is far.

5.2 Differences Between Mainstream Infrared Thermometers

In fact, whether it is a medical or industrial infrared thermometer, they use the same principle of receiving infrared waves from the human body, but the object distance ratio has been adjusted differently, and the surface temperature is measured. The normal forehead temperature is about 2-3 ° C lower than the temperature of the armpit, and the forehead is directly affected by the environment. It is for preliminary investigation and reference and cannot be used as a basis for medical diagnosis. In addition, the temperature of the ear and neck will be more stable than the temperature of the forehead and barely affected by the environment. This is one of the reasons why the ear thermometer is more accurate than the forehead.

5.3 Infrared Temperature Gun

The medical thermometer has been revised by software or the relevant range has been limited by the software before leaving the factory. The emissivity of a normal human body is 0.98 (the thermometer defaults to 0.95), so the measured result is about 34-35 ° C. All infrared products (infrared cameras) can correct the difference by changing the emissivity to 0.8 to avoid inaccurate body temperature when used by non-professionals; and industrial-grade thermometers provide more realistic feedback on temperature measurement. It shows the actual temperature detected.

Figure4. Infrared Temperature Gun

VI Infrared Thermometer Accuracy And Factors Affecting Accuracy

6.1 Precision of Infrared Thermometer

The accuracy of contact measurement is about 0.1 degrees. Compared with contact temperature measurement, the accuracy of non-contact temperature measurement is lower. The infrared thermometer with higher accuracy is about 0.2 degrees, and the worse temperature error is 1 degree. Even above 1 degree. In general, the accuracy of infrared thermometers is ± 2 ° C.

Today, temperature measurement products such as handheld infrared thermometers on the market are easily affected by measurement distance and ambient temperature, and the measurement error is often around 1 degree.

6.2 Factors Affecting The Accuracy of The Infrared Thermometer Measurement

6.2.1 Emissivity

All objects reflect, transmit, and emit energy, and only the emitted energy can indicate the object's temperature. When the infrared thermometer measures the surface temperature, the instrument can receive all three kinds of energy. Therefore, all infrared thermometers must be adjusted to read only the emitted energy. Measurement errors are usually caused by infrared energy reflected from other light sources. Some infrared thermometers can change the emissivity, and emissivity values for many materials can be found in published emissivity tables. Other instruments have a fixed pre-set emissivity of 0.95. The emissivity value is the surface temperature of most organic materials, paints or oxidized surfaces, which is compensated by applying a tape or flat black paint to the measured surface. When the tape or lacquer reaches the same temperature as the base material, measure the temperature of the surface of the tape or lacquer, which is its true temperature.

Figure5. Emissivity

6.2.2 Ratio of Distance To Light Spot

The optical system of the infrared thermometer collects energy from a circular measurement spot and focuses it on the detector. The optical resolution is defined as the ratio of the distance from the infrared thermometer to the object to the size of the measured spot (D: S). The larger the ratio, the better the resolution of the infrared thermometer and the smaller the spot size to be measured.

6.2.3 Field of View

Make sure the target is larger than the spot size of the infrared thermometer. The smaller the target, the closer it should be. When accuracy is particularly important, make sure the target is at least 2 times the spot size.

Figure6. Field of View

VII Factors to Consider When Choosing An Infrared Thermometer

(1) Temperature range

The temperature measurement range is actually the range of the infrared thermometer, and the range of different thermometers will be different. The temperature measurement range is generally -50 ~ 360 ° C, -30 ~ 380 ° C, -18 ~ 280 ° C, -32 ~ 450 ℃, -32 ~ 650 ℃, -32 ~ 1050 ℃, etc., and the range for measuring body temperature is generally 35 ~ 42.5 ℃. You need to choose the appropriate range according to the temperature range of the measured object.

(2) Measurement accuracy

Measurement accuracy is the only indicator to ensure the accuracy of the measurement, and it is also a key indicator to determine the performance of the infrared thermometer. Accuracy is usually expressed as ± X ℃ or ± X%. The smaller the value, the higher the accuracy.

(3) Display resolution

The display resolution is the last digit of the temperature display, usually 0.1 ° C or 0.1 ° F.

(4) Optical resolution

The optical resolution is the ratio of the distance D from the thermometer to the target to the diameter S of the measurement spot, that is, the ratio of the distance to the spot diameter D; S, D: S, the greater the accurate temperature measurement distance. In order to obtain accurate temperature readings, the distance between the thermometer and the test target must be within a suitable range. If the pyrometer must be measured away from the target due to environmental conditions, and a small target is to be measured, a pyrometer with high optical resolution should be selected.

(5) Emissivity

Emissivity is the ratio of the energy radiated by an object at a specific temperature to the energy radiated by an ideal radiator at the same temperature. Different objects have different emissivities. Some infrared thermometers have a fixed emissivity of 0.95, while others are adjustable. The emissivity of the infrared thermometer can be adjusted according to the material of the measured object to ensure the accuracy of the measurement results.

(6) Response time

The response time is the time it takes for the infrared thermometer to reach 95% of its final reading. It represents the speed at which the infrared thermometer responds to changes in the measured temperature. The response time of the new infrared thermometer can even reach 1ms. If the target moves fast or measures a fast-heated target, a fast-responding infrared thermometer should be selected; otherwise, a sufficient signal response cannot be achieved, which will reduce the measurement accuracy.

Figure7. Infrared Thermometer

VIII How To Make Infrared Thermometers More Accurate

(1) Accurately determine the emissivity of the measured object;

(2) Avoid the influence of high-temperature objects in the surrounding environment;

(3) For transparent materials, the ambient temperature should be lower than the temperature of the measured object;

(4) The thermometer should be vertically aligned with the surface of the measured object. Under no circumstances should the angle exceed 30 ° C.

(5) Can be applied to the temperature measurement of bright or polished metal surfaces, and cannot be measured through the glass;

(6) Correctly follow-off coefficient, the target diameter is full of field of view;

(7) If the infrared thermometer is suddenly in a situation where the ambient temperature difference is 20 ° C or higher, the measurement data will be inaccurate, and then take the measured temperature value after the temperature is balanced.

IX One Question Related to Infrared Thermometers

9.1 Question

What is infrared radiation?

A. It's the transfer of energy by electromagnetic waves

B. The radiation given off by radioactive particles

C. Infrared radiation is a type of gas

D. It is the reaction that occurs by freezing water

9.2 Answer

A

 

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