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Infrared Thermometry Understanding and Using the Infrared Thermometer

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Infrared Thermometry Understanding and Using the Infrared Thermometer Infrared Thermometry Understanding and using the Infrared Thermometer Advances in electronic and detector Electromagnetic Spectrum the curve increases, increasing the area (energy) technology have resulted in a variety of Infrared radiation is part of the electromagnetic beneath it, and (2) the wavelength associated spectrum which includes radio waves, with the peak energy (highest point of the curve) non-contact infrared thermometers (IR) for microwaves, visible light, ultraviolet, gamma- shifts to the shorter wavelength end of the scale. industrial and scientific use. It is important and X-rays (Figure 2). These various forms of to understand their major differences in energy are categorised by frequency or This relationship is described by Wien’s order to select the proper unit for a given wavelength.* Note that visible light extends from Displacement Law: 3 application. .4 to .7 micron, with ultraviolet (UV) shorter than λmax = 2.89 x 10 /T .4 micron, and infrared longer than .7 micron, where λmax = wavelength of peak Infrared Theory extending to several hundred microns. In energy in microns Energy is emitted by all objects having a practice, the .5 to 20 micron band is used for IR T = temperature in degrees Kelvin temperature greater than absolute zero. This temperature measurement. energy increases as the object gets hotter, For example, the wavelength for peak energy permitting measurement of temperature by Planck’s Law emitted from an object at 2617 degrees Celsius measurement of the emitted energy, particularly The amplitude (intensity) of radiated energy can (2890 degrees Kelvin) is: the radiation in the infrared portion of the be plotted as a function of wavelength, based on 3 spectrum of emitted radiation. Figure 1 shows Planck’s law. Figure 3 shows the radiation λmax = 2.89 x 10 /2890K = 1.0 µm a typical infrared radiation thermometer. emission curves for objects at two different Lens or mirror temperatures. By convention, longer wavelengths Another illustration involves heating a steel billet. Dare shownAmp to the right on IR graphs, reverse of At about 600°C (1100°F), a dull, red glow is Lens or mirror electromagnetic spectrum charts, such as Figure emitted from the steel. As the temperature D Amp 2. The area under each curve represents the total increases, the colour changes from red to Output to Signal controller, energy radiated atConditioner the associated temperature. orange and yellow as the peak passes into the Output to Signalrecorder, etc. controller, Conditioner visible light spectrum. Finally, the energy emitted recorder, etc. Note that two changes occur simultaneously as throughout the entire visible spectrum is at such Figure 1: Block diagram, infrared pyrometer temperature is increased: (1) the amplitude of a high level that white light is given off by the steel at about 1650°C. Because the peak of the curve shifts as temperature increases, selection ed owaves oadcast oadcast; ed α owaves elevision Radar Infrar Visible Light Ultraviolet X-Rays -Rays Longwave Radio AM Radio Br Shortwave Radio FM Radio Br T Micr of the optimum portion of the spectrum is oadcast oadcast; elevision α 10 KHz 1 MHz Longwave Radio 100 MHz AM Radio Br 100 µm Shortwave Radio FM Radio 1 Br µm T Micr Radar Infrar Visible Light Ultraviolet X-Rays -Rays important to achieving satisfactory infrared The IR to UV spectrum 10 KHz 1 MHz 100 MHz 100 µm 1 µm thermometer performance. is explained as shown. The IR to UV spectrum Emissivity is explained as shown. een ellow Infrared Red Y Gr Blue Violet UV Emissivity is defined as the ratio of the energy 20 µm .7 µm .4 µm radiated by an object at a given temperature to een the energy emitted by a perfect radiator, or ellow Infrared Red Y Gr Blue Violet UV blackbody, at the same temperature. The 20 µm .7 µm .4 µm emissivity of a blackbody is 1.0. All values of Figure 2: Electromagnetic spectrum and light spectrum *Wavelength (λ) is inversely related to frequency (f): λ = c/f, where c = velocity of the wave; λ is measured in microns (µm); 1 µm = 10-6m. emissivity fall between 0.0 and 1.0. Emissivity (E), a major but not uncontrollable factor in IR temperature measurement, cannot be ignored. Related to emissivity are reflectivity (R), a measure of an object’s ability to reflect 700C infrared energy, and transmissivity (T), a measure of an object’s ability to pass or transmit IR 25C Relative Radiation Intensity energy. Since all radiation must be either transmitted, reflected or absorbed: l 1 µm 10 µm 20 µm A + R + T = 1.0 Figure 3: Radiation intensities derived from Planck’s Law (Blackbody radiation curves) Calex Electronics Limited Tel: +44 (0)1525 373178/853800 Fax: +44 (0)1525 851319 Online: http://www.calex.co.uk R E Hot Source T Pyrometer Consider the example in Figure 4a. Object X is a hot block of material, Y is colder; therefore, 1.0 heat will be radiated from X to Y. Some heat will be absorbed by Y, some reflected, and some transmitted through Y. The three dispositions 5 must equal 100%, represented as 1.0 for ransmission T coefficients of absorption, reflection, and transmission. If A = 1.0, all the heat is Wavelength 0 in microns absorbed; if R = 1.0, then A = T = 0. Usually 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 (µm) some combination exists: Figure 5: IR transmission through atmosphere A = .7 (70% absorbed) window to pass a high percentage of the IR 2. Increase the size of object B until it fills R = .2 (20% reflected) energy at that wavelength. the thermometer’s FOV. T = .1 (10% transmitted) 3. Decrease the emissivity compensator Atmospheric Absorption (described later) to compensate for the Sum = 1.0 (100% energy radiated from One of the first considerations in selecting the loss of energy. X to Y) spectral response (wavelength range at which 4. Get a thermometer with a smaller FOV. an instrument is sensitive to IR) of a device is atmospheric absorption. Certain components Field of view is described either by its angle or of the atmosphere, such as water vapour, CO2 by a distance-to-size ratio (D:S). If D:S = 20:1, A and other materials, absorb IR at certain and if the distance to the object divided by the wavelengths, increasing the amount of energy diameter of the object is exactly 20, then the R X Y absorbed with the distance between the object object exactly fills the instrument’s field of view. T and the instrument. Therefore, if these A D:S ratio of 60:1 equals a field of view of 1°. absorbents are ignored, an instrument may read correctly when near the object, but several Since most IR thermometers have fixed focus Figure 4a: Energy budget for all radiated IR energy degrees lower a few feet away because the optics, the minimum measurement spot occurs displayed temperature represents an average of at the specified focal distance. Typically, if an If an object is in a state of thermal equilibrium, it the object temperature and the atmosphere instrument has fixed-focus optics with a 120: 1 is getting neither hotter nor colder; the amount 700C temperature. The reading may be affected by D:S ratio and a focal length of 1.5 m the of energy it is radiating must equal the amount of changes in humidity or the presence of steam or minimum spot (resolution) the instrument can energy it is absorbing, so A = E25C (emissivity). By Relative Radiation Intensity certain gases. Fortunately, there are “windows” achieve is 1.5 m divided by 120, or 12.5 mm at substitution: in the IR spectrum which allow these absorption a distance of 1.5 m from the instrument. This is l bands to be avoided. Figure 5 illustrates these significant when the size of the object is close 1 µm 10 µmE + R + T = 1.0 20 µm windows. to the minimum spot the instrument can If any two of these values are known, the third is measure. easy to find. Figure 4b illustrates this relationship. Optics Target size and distance are critical to accuracy Most general-purpose IR thermometers use a for most IR thermometers. Every IR instrument focal distance between 50 cm and 150 cm; R has a field of view (FOV), an angle of vision in special close-focus instruments use a 12.5 mm which it will average all the temperatures it sees to 300 mm focal distance, and may be E Hot Source (Figure 6). equipped with a light-spot aiming device to T ensure that the instrument is measuring the Pyrometer exact spot desired. Some long-range instruments for checking insulators and Figure 4b: Total IR radiation reaching pyrometer transformers on pylons use a 15 m focal Object B Wall distance. Sighting scopes are often used at longer distances or for small spot sizes. Some Transmission Object A IR thermometers use variable-focus optics, In some applications, particularly glass and thin- especially high performance fixed-mount types film plastics, transmission becomes an important Figure 6: Field of view of instrument with through-the-lens sighting. factor. If it is desired to measure the temperature of these substances using IR, a wavelength Object A fills the field of view of the sensor; the Fibre optics are alternatively used in special must be chosen where the material appears only temperature seen is that of object A, so the applications where there is not enough space opaque or semi-opaque. Often it is desired to temperature of object A will be accurately to mount a sensing head, or where radio measure temperatures under the surface of an indicated.
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