The Principles of Noncontact Temperature Measurement © Stahl-Zentrum © TamGlass Infrared Theory Content CONTENT ................................................................................. 2 1 INTRODUCTION ........................................................................ 3 2 DISCOVERY OF INFRARED RADIATION ...................................... 3 3 ADVANTAGES OF USING INFRARED THERMOMETERS ............... 3 4 THE INFRARED SYSTEM ............................................................. 4 4.1 The Target ..................................................................................4 4.1.1 Determining Emissivity ............................................................6 4.1.2 Measuring Metals ...................................................................7 4.1.3 Measuring Plastics ..................................................................7 4.1.4 Measuring Glass ......................................................................7 4.2 Ambient Conditions....................................................................8 4.3 Optics and Window .................................................................. 10 4.4 Sighting Devices ....................................................................... 12 4.5 Detectors .................................................................................. 13 4.6 Display and Interfaces .............................................................. 13 4.7 Technical Parameters of IR Thermometers .............................. 14 4.8 Calibration ................................................................................ 15 5 SPECIAL PYROMETERS .............................................................16 5.1 Fiber-optic Pyrometers .............................................................16 5.2 Ratio Pyrometers .......................................................................16 5.3 Imaging Systems .......................................................................18 5.3.1 IR Linescanners ......................................................................18 5.3.2 Matrix Cameras ..................................................................... 20 6 SUMMARY ...............................................................................21 7 BIBLIOGRAPHY .........................................................................22 Introduction 1 Introduction ry opened up new possibilities for measuring tem- perature – without contact and thus without affect- This booklet is written for people who are unfamiliar ing the object being measured and the measure- with noncontact infrared temperature measurement. ment device itself. Compared to early infrared A conscious attempt has been made to present the temperature measurement devices, which were subject matter as briefly and as simply as possible. heavy, awkward and complicated to operate, the Readers who wish to gain more in-depth knowledge image of such devices today has completely can follow the suggestions for further reading in the changed. Modern infrared thermometers are small, bibliography. This manual focuses on the practical ergonomic, easy to operate and can even be in- operations of noncontact temperature measurement stalled into machinery. From versatile handheld de- devices and IR thermometry, and answers important vices to special sensors for integration into existing questions that may arise. If you plan to use a non- process systems, the spectrum of product offerings contact temperature measurement device and re- is vast. A variety of accessories and software for the quire further advice, send us the completed form collection and analysis of measurement data are (see appendix) prior to use. provided with the majority of infrared temperature sensors. 2 Discovery of Infrared Radiation 3 Advantages of Using Infrared Fire and ice, hot and cold – elemental extremes have always fascinated and challenged people. Var- Thermometers ious techniques and devices have been used Temperature is the most frequently measured phys- throughout time in an effort to accurately measure ical parameter, second only to time. Temperature and compare temperature conditions. For example, plays an important role as an indicator of the condi- in the early days of ceramics manufacture, meltable tion of a product or piece of machinery, both in materials were used, which indicated through de- manufacturing and in quality control. Accurate tem- formation that certain higher temperatures were perature monitoring improves product quality and reached. A baker on the other hand, used a piece of increases productivity. Downtimes are decreased paper – the quicker it became brown in the oven, since the manufacturing processes can proceed the hotter the oven was. The disadvantage of both without interruption and under optimal conditions. of these techniques was that they were not reversi- Infrared technology is not a new phenomenon. It has ble – cooling could not be determined. Also, the been utilized successfully in industry and research accuracy of the results was very dependent on the for decades. But new developments have reduced user and his or her experience. It was not until the costs, increased reliability, and resulted in smaller invention of the first thermoscope in the first half of noncontact infrared measurement devices. All of the 17th Century that temperatures could begin to be these factors have led to infrared technology be- measured. An evolution of the thermoscope (which coming an area of interest for new kinds of applica- had no scale) the thermometer had various scales tions and users. proposed. Between 1724 and 1742 Daniel Gabriel Fahrenheit and by Anders Celsius defined what we probably consider as the 2 most common. Fig. 2: Digital Infrared Pyrometer in miniature size (Raytek MI3 Series) Fig. 1: William Herschel (1738 – 1822) discovers IR radiation The discovery of infrared radiation by the physicist Wilhelm Herschel at the beginning of the 19th Centu- 3 Raytek® Principles of Noncontact Temperature Measurement What are the advantages offered by noncontact 4 The Infrared System temperature measurement? An IR thermometer can be compared to the human 1. It is fast (in the ms range) – time is saved, allow- eye. The lens of the eye represents the optics ing for more measurements and accumulation of through which the radiation (flow of photons) from more data (temperature areas can be deter- the object reaches the photosensitive layer (retina) mined). via the atmosphere. This is converted into a signal 2. It facilitates measurement of moving targets that is sent to the brain. Fig. 3 shows how an infra- (conveyor processes). red measuring system works. 3. Measurements can be taken of hazardous or physically inaccessible objects (high-voltage parts, large measurement distances). Target Sensor Display and 4. Measurements of high temperatures (above with Optics Interfaces 1300°C) present no problems. Contact thermom- eters often cannot be used in such conditions, or they have a limited lifetime 5. There is no interference as no energy is lost from the target. For example, in the case of a poor heat conductor such as plastic or wood, meas- Atmosphere urements are extremely accurate with no distor- tion of measured values, as compared to meas- urements with contact thermometers. Fig. 3: Infrared Measuring System 6. Noncontact temperature measurement is wear- free – there is no risk of contamination and no 4.1 The Target mechanical effect on the surface of the object. Lacquered or coated surfaces, for example, are Every form of matter with a temperature (T) above not scratched and soft surfaces can be meas- absolute zero (-273.15°C / -459.8°F) or emits infra- ured. red radiation according to its temperature. This is called characteristic radiation. The cause of this is Having enumerated the advantages, there remains the internal mechanical movement of molecules. The the question of what to keep in mind when using an intensity of this movement depends on the tempera- IR thermometer: ture of the object. Since the molecule movement represents charge displacement, electromagnetic 1. The target must be optically (infrared-optically) radiation (photon particles) is emitted. These pho- visible to the IR thermometer. High levels of dust tons move at the speed of light and behave accord- or smoke make measurement less accurate. Sol- ing to the known optical principles. They can be id obstacles, such as a closed metallic reaction deflected, focused with a lens, or reflected by reflec- vessel, do not allow internal measurements. tive surfaces. The spectrum of this radiation ranges 2. The optics of the sensor must be protected from from 0.7 to 1000 µm wavelength. For this reason, dust and condensing liquids. (Manufacturers this radiation cannot normally be seen with the na- supply the necessary equipment for this.) ked eye. This area lies within the red area of visible 3. Normally, only surface temperatures can be light and has therefore been called "infra"-red after measured, with the differing emissivities of differ- the Latin, see Fig. 4. ent material surfaces taken into account. Fig. 5 shows the typical radiation of a body at differ- ent temperatures. As indicated, bodies at high tem- peratures still emit a small amount of visible radia- Summary tion. This is why everyone can see objects at very The main advantages of noncontact IR ther- high temperatures (above 600°C) glowing some- mometry are speed, lack of interference, and where from red to white. Experienced steelworkers the ability to measure