Shenzhen Jinghailiang Electronics Technology Co., Ltd.
Working principles for infrared temperature measuring and thermometer
Date: 2021-05-24
Abstract: This article analyses the working principles for infrared temperature measuring through the blackbody radiation law, investigates and analyses the reasons for deviations in temperature measurement from the perspective of emissivity, distance factor, environment, etc., briefly introduces the thermometer based on infrared temperature measuring technology, and outlines the classification, properties, selection and application of infrared thermometers.
Introduction: In the nature, when the temperature of an object is higher than the absolute zero, there will be thermal motion in it, as a result, it will keep radiating electro-magnetic waves in all directions, among which is the infrared ray with the band of 0.75～100μm. Infrared thermometers are manufactured by following this principle. Temperature, a physical quantity measuring the degree of coldness and hotness of an object, is a common and important thermal parameter in industrial production, which must be monitored and controlled in many manufacturing processes, especially in the industries of chemical engineering, foods, etc., as measurement and control of temperature will directly affect product quality and properties. Traditional contact thermometers, such as thermocouples, thermal resistors, etc., need exchange heat with the substance to be measured, and require certain period of time before reaching thermal equilibrium, thus there is a time lag in temperature measurement, limiting their application in continuous production and quality inspection. Currently, infrared thermometers are getting more widely adopted day by day, thanks to their advantages such as convenience in use, quick response, high sensitivity, wide temperature measuring range and on-line non-contact continuous measurement. Table 1 lists out common temperature measuring methods and their features, and infrared temperature measuring as a common temperature measuring technology possesses obvious advantages.
1 . Working principles and features of infrared thermometers
1.1 Blackbody radiation and infrared temperature measuring principle
Any object with a temperature higher than the absolute zero is radiating infrared energy to the surrounding space all the time. The magnitude of infrared radiation energy of the object, and its distribution according to wave length -- are closely related to its surface temperature. Therefore, its surface temperature can be accurately measured by measuring the infrared energy radiated by the object itself, and this is the basis of infrared temperature measuring. Blackbody radiation law: Blackbody is an idealized radiator, which can absorb radiation energy of all wave lengths, without energy reflection or penetration. The emissivity is 1 on its surface while those of other substances’ are less than 1, thus they are called gray bodies. It should be pointed out that there is no such a thing as blackbody, it is a proper model which is a must in theoretical research so as to understand and obtain a law for infrared radiation distribution, that is the quantized oscillator model of blackbody chamber radiation expounded by Planck, from which the Planck blackbody radiation law is deduced, i.e., use wave length to indicate the blackbody spectral radiant power, which is called the blackbody radiation law, the source of all infrared radiation theories.
1.2 Features of infrared thermometers
Any object with a temperature higher than the absolute zero is radiating infrared energy to the surrounding space all the time. The magnitude of infrared radiation energy, and its distribution according to wave length, are closely related to its surface temperature. Therefore, its surface temperature can be accurately measured by measuring the infrared energy radiated by the object itself. Infrared thermometers can receive invisible infrared radiation energy emitted from many objects. Infrared ray is a part of the electro-magnetic spectrum, which is between visible lights and radio waves. When the instrument is measuring a temperature, infrared radiation energy emitted from the object goes into the optical system of the thermometer, which will be modulated into an electric signal by the detector, and displayed as the surface temperature of the object on the screen of the infrared thermometer. Features of the infrared thermometers: non-contact measurement, wide temperature measuring range, quick response and high sensitivity. However, due to the influence of emissivity of the object, it’s almost impossible to measure the actual temperature of the object but its surface temperature.
2 . Composition of infrared thermometers
Infrared measuring uses the method of point-by-point analysis, that is, to focus the heat radiation from an area of the object on a single detector, and to transform radiation power into temperature through the known emissivity of the object . Infrared thermometers may vary in appearances and internal structures for different measurement objects, ranges and scenarios but they are generally similar in basic structure, which is mainly consisted of a optic system, a photoelectric detector, a signal amplifier and a processor as well as output display. Please refer to Figure 2 for the basic structure. Infrared rays emitted from the object goes into the optical system, which will be modulated into alternating rays by the modulator, and converted into a corresponding electric signal by the detector. The signal goes through the amplifier and signal processing circuit, and will be converted into a temperature value of the measured object after being rectified according to the algorithm in the instrument and emissivity of the object.
As indicated in Figure 2, based on the infrared radiation feature of an object, an infrared thermometer relies on its built-in optic system to converge infrared radiation energy of the object onto the detector (sensor), converts it into an electric signal, processes it through the amplifier circuit, compensating circuit and linear process, and displays the temperature of the object on the displaying terminal. The system is consisted of optic system, photoelectric detector, signal amplifier, processor, output display, etc., among which infrared detector is at the core, as it converts inbound radiation energy into a measurable electric signal (see Figure 3).
3 .Deviation analysis of infrared measuring
4 .Several methods of infrared measuring
5 .How to select the correct infrared thermometer
Infrared thermometers can be selected based on considerations in three aspects: performance indicators, such as temperature range, size of light spot, working wave length, accuracy of measurement, response time, etc.; environment and working conditions, such as ambient temperature, window, display and output, protective accessories, etc.; and other aspects, such as convenience of use, repair and calibration properties, price, etc. Infrared thermometers can be classified into three major series, namely, portable, on-line and scanner series, with various optional parts and computer softwares. Besides, each of them has a variety of models and specifications. To pick the desired infrared thermometer from different models and specifications, please pay attention to the following conditions:
（1）Be clear about measurement requirements and the problem to be solved. Such as temperature, size, material and environment of the object to be measured, measuring distance, speed of response, accuracy requirement, portable or on-line type, etc;
（2）Compare measurement requirements and the problem to be solved with existing infrared thermometers of different models, pick out the model which can meet the above-mentioned requirements;
（3）Select the best one in performance, function and price from various suitable models.
We have sorted out the following points:
1> determine temperature range
2> determine dimensions of the object
3> determine optic resolution
4> determine wave length range
5> determine response time
6> signal processing function
7> consider environmental conditions
8> calibration of the infrared radiation thermometer
9> operation and usage
6 . Application of infrared thermometers
Infrared thermometers have the advantage of non-contact & quick measurement, and can be used extensively in industry, agriculture, medical treatment and scientific research. Based on applications, infrared thermometers can be divided into two categories, the first one is for measuring surface temperature of an object, and the second one is for indirect measurement, using heat profile of the object to judge other properties of the object related with heat profile. The examples are as follows:
1>The steel industry claims more than half of the total infrared thermometers in use. Measurement and control of temperature play an important role in improving product quality during steel melting, rolling, casting and tempering. Thermal failure monitoring of furnace wall and machinery & equipment can provide data to extend service life and guarantee safety.
2>In mechanical processing, temperature measurement and control for parts under heat treatment is critical for their quality.
3>In chemical industry, the equipment work under high temperature and high pressure, so it’s very helpful to monitor its heat profile, judge its working status, check for failures like heat loss and leakage at the connectors of hot piping .
4>In power and electricity industries, checking for abnormal temperature at power equipment, electricity distribution equipment, cables, electric appliance connectors when the equipment is running and live, can guarantee operation safety of the equipment in certain degree.
5>In construction industry, heat profile inspection on the walls, floors, roofs of buildings can determine the location of potential insulation, crack and leakage risks, as well as defects. Determine management of heat consumption in factories and buildings.
6>In agriculture, there are surface temperature measurement for the soil and plants, temperature measurement during the drying process for grains and seeds, temperature monitoring during processing of agricultural and by-products such as tobacco leaves and tea leaves, and temperature monitoring for drying of Chinese herbal medicine and manufacturing of pharmaceuticals.
7>In agriculture, there are surface temperature measurement for the soil and plants, temperature measurement during the drying process for grains and seeds, temperature monitoring during processing of agricultural and by-products such as tobacco leaves and tea leaves, and temperature monitoring for drying of Chinese herbal medicine and manufacturing of pharmaceuticals.
8>In scientific research, infrared thermometers are widely used, as their prominent advantages enable them to provide means of temperature measurement under special experimental conditions.
To sum up, infrared thermometers are essential for the development of both industry and agriculture, whose application covers so many fields that can not be fully listed here.
7 . Conclusion Infrared temperature measuring technology is getting perfected day by day with the development of modern technologies, whose application is reaching various fields gradually. Developing new technologies for infrared temperature measurement and improving performance of infrared thermometers are the requirements of the times. The infrared temperature measuring technology, based on the blackbody radiation law, is playing a more and more important role in modernization. Its edge over other thermometers is getting bigger and bigger with the following advantages: realizing remote measurement based on its non-contact measurement; ensuring the balance of the temperature field to be measured ; providing quick response with photons as information carriers; ensuring high sensitivity which is 1-4 orders of magnitude higher than traditional sensors; ensuring large dynamic range based on wide band; realizing digital and intelligent operation by connecting to computers easily; facilitating installation with few accessories, simple maintenance and improved productivity; improving service life and reducing production cost significantly with non-contact measurement; and reducing price with compact structure and light-weight.