1. Infrared Thermal Testing Reading III- SGuide-IRT Part 1 of 2 My ASNT Level III Pre-Exam Preparatory Self Study Notes 29th April 2015 Charlie Chong/ Fion Zhang 2. Infrared Thermography Charlie Chong/ Fion Zhang 3. Infrared Thermography Charlie Chong/ Fion Zhang 4. Infrared Thermography Charlie Chong/ Fion Zhang 5. DEADLY French Military Mistral Anti Aircraft Missile System Charlie Chong/ Fion Zhang https://www.youtube.com/embed/_3c0NpYapM0 https://www.youtube.com/watch?v=_3c0NpYapM0 6. See Through & Fun Thermal Camera Experiments Charlie Chong/ Fion Zhang https://www.youtube.com/embed/pXAzZoWLzSo https://www.youtube.com/watch?v=pXAzZoWLzSo 7. LEAKED Body Scan Images From The TSA! Charlie Chong/ Fion Zhang https://www.youtube.com/embed/QRkWmRVs-nk https://www.youtube.com/watch?v=QRkWmRVs-nk 8. How to see through clothing 2 Charlie Chong/ Fion Zhang https://www.youtube.com/embed/0wQlyCNPw8M https://www.youtube.com/watch?v=0wQlyCNPw8M 9. Bf4 little bird ah-6j night vision infrared real combat footage helmet cam montage funker tactical. Charlie Chong/ Fion Zhang https://www.youtube.com/embed/dRra63kOwWE https://www.youtube.com/watch?v=XfXShaTzAhI&list=PL7D451B08CD9A119B 10. Apache IR Thermal Weaponry Charlie Chong/ Fion Zhang https://www.youtube.com/watch?v=XfXShaTzAhI&list=PL7D451B08CD9A119B https://www.youtube.com/embed/XfXShaTzAhI?list=PL7D451B08CD9A119B 11. Infrared Electrical Testing Charlie Chong/ Fion Zhang https://www.youtube.com/embed/DgXsmvv7Q9o https://www.youtube.com/watch?v=DgXsmvv7Q9o 12. Charlie Chong/ Fion Zhang 13. Fion Zhang at Shanghai 29th May 2015 http://meilishouxihu.blog.163.com/ Charlie Chong/ Fion Zhang 14. Charlie Chong/ Fion Zhang 15. Charlie Chong/ Fion Zhang http://greekhouseoffonts.com/ 16. Charlie Chong/ Fion Zhang Greek letter 17. Charlie Chong/ Fion Zhang IVONA TTS Capable. http://www.naturalreaders.com/ 18. Charlie Chong/ Fion Zhang SGuide-IRT Content Part 1 of 2 Chapter 1 - Introduction to Principles & Theory Chapter 2 - Materials and Their Properties Chapter 3 Thermal Instrumentation Part 2 of 2 Chapter 4 Operating Equipment and Understanding Results Chapter 5 Applications Appendices A, B, C 19. Charlie Chong/ Fion Zhang Chapter 1 Principles & Theory 20. Charlie Chong/ Fion Zhang 1.1 Introduction to Principles & Theory Infrared/thermal testing involves the use of (1) temperature and (2) heat flow measurement as a means to predict or diagnose failure. This may involve the use of contacting or noncontacting devices, or a combination of both. A fundamental knowledge of heat flow and the thermal behavior of materials is necessary to understand the significance of temperature and temperature changes on a test sample. Contacting devices include thermometers of various types, thermocouples, thermopiles and thermochromic coatings. Noncontacting devices include convection (heat flux) devices, optical pyrometers, infrared radiation thermometers, infrared Line scanners and infrared thermal imaging (thermographic) equipment. Infrared thermography is the nondestructive, non-intrusive. noncontact mapping of thermal patterns on the surface of objects. It is usually used to diagnose thermal behavior and, thereby, to assess the performance of equipment and the integrity of materials, products and processes. 21. Charlie Chong/ Fion Zhang Keywords: Principles: temperature and heat flow measurement as a means to predict or diagnose failure. Techniques: contacting or noncontacting devices, or a combination of both. Contacting devices include: thermometers of various types, thermocouples, thermopiles and thermochromic coatings. Noncontacting devices include: convection (heat flux) devices, optical pyrometers, infrared radiation thermometers, infrared Line scanners and infrared thermal imaging (thermographic) equipment. 22. Charlie Chong/ Fion Zhang The infrared thermal imaging equipment used in infrared thermography is available in numerous configurations and with varying degrees of complexity. The thermal maps produced by infrared thermal imaging instruments are called thermograms. To understand and interpret thermograms, the thermograpber must be familiar with the fundamentals of temperature and heat transfer, infrared radiative heat flow and the performance of infrared thermal imaging instruments and other thermal instruments. An understanding of the equipment, materials and processes being observed is also important to effectively assess the full significance of infrared/thermal measurements. A more detailed discussion of the performance parameters of infrared thermal imaging instruments is provided in Chapter 3. Keywords: infrared thermography - The thermal maps produced by infrared thermal imaging instruments are called thermograms. 23. Charlie Chong/ Fion Zhang 1.2 Fundamentals of Temperature and Heat Transfer Heat is a transient form of energy in which thermal energy is transient. What is often referred to as a heat source (such as an oil furnace or an electric heater) is really one form or another of energy conversion the energy stored in one object being converted to heat and nowing to another object. Heat flow is thermal energy in transit and heat always flows from warmer objects to cooler objects. (transient Temperature is a measure of the thermal energy contained in an object - the degree of hotness or coldness of an object that is measurable by any of a number of relative scales. Comments: HBNDEv C9 -Transfer of heat energy can be described as either steady-state or transient . In the steady-state condition, heat transfer is constant and in the same direction over time. However, In this PPT, both steady state and transient are both transient form of energy. 24. Charlie Chong/ Fion Zhang The three modes of heat transfer are: conductive, convective and radiative. All heat is transferred by one of these three modes. In most situations, beat is transferred by a combination of two or all three modes. Of these three modes of heat transfer, infrared thermography is most closely associated with the radiative process, but it is essential to study all three to understand the meaning of thermograms and to pursue a successful program of thermography. As a result of heat transfer, objects tend to increase or decrease their temperature until they come to thermal equilibrium with their surroundings. To maintain a steadystate heat flow condition, energy must be continuously supplied by some means of energy conversion so that the temperature differential, and hence the heat flow remains constant. 25. Charlie Chong/ Fion Zhang The three modes of heat transfer are: conductive, convective and radiative. http://www.chem.purdue.edu/gchelp/liquids/character.html 26. Charlie Chong/ Fion Zhang The three modes of heat transfer are: Water in 3 phases. http://dli.taftcollege.edu/streams/Geography/Animations/WaterPhases.html http://dli.taftcollege.edu/streams/Geography/Animations/WaterPhases.swf 27. Charlie Chong/ Fion Zhang Temperature and Temperature Scales Temperature is expressed in either absolute or relative terms. There are two absolute scales called Rankine (English system) and Kelvin (metric system). There are two corresponding relative scales called Fahrenheit (English system) and Celsius or centigrade (metric system). Absolute zero is the temperature at which no molecular action takes place. This is expressed as zero Kelvin or zero degrees Rankin (0 K or 0 R). Relative temperature is expressed as degrees Celsius or degrees Fahrenheit (C or F). The numerical relations among the four scales are as follows: converting C to F, (9/5 x C +32) F converting F to C, (5/9 x F -32) C T Rankine = T Fahrenheit+ 459.7 T Kelvin = T Celsius + 273.16 Exercise: Temperature (not temperature interval) 0 C = 32 F thus -273.16 C = (-273.16 x 9/5 + 32) F = 459.7 F 28. Charlie Chong/ Fion Zhang Temperature and Temperature Scales http://www.mathsisfun.com/temperature-conversion.html 29. Charlie Chong/ Fion Zhang Temperature and Temperature Scales REMEMBER 0C = 32F converting C to F, (9/5 x C +32) F for my ASNT exam 30. Charlie Chong/ Fion Zhang Boston Tea Party New governances not the Old Fahrenheit & . 31. Charlie Chong/ Fion Zhang Boston Tea Party New governances not the Old Fahrenheit & . 32. Charlie Chong/ Fion Zhang The Mighty Fahrenheit & , English System. 33. Charlie Chong/ Fion Zhang The Mighty Fahrenheit & , English System. 34. Charlie Chong/ Fion Zhang Absolute zero is equal to - 273.16 C and also equal to approximately - 459.7 F. To conveIt, a change in temperature or delta T (T) between the English and metric systems, the simple 9/5 (1.8 to 1) relationship is used: T Fahrenheit (or Rankine) = 9/5 x T Celsius (or Kelvin) or simply; T Fahrenheit (or Rankine) = 1.8 x T Celsius (or Kelvin) Table 1.1 (pages 12 to 14) is a conversion table that will assist in the rapid conversion of temperature between fabrenheit and celsius values. Instructions for the use of the table are shown at the top of the table. (not in this PPT) 35. Charlie Chong/ Fion Zhang Conductive Heat Transfer Conductive beat transfer is probably the simplest form to understand. lt is the transfer of beat in stationary media. It is the only mode of heat flow in solids, but it can also take place in liquids and gases. Conductive heat transfer occurs as the result of atomic vibrations (in solids) and molecular collisions (in liquids) whereby energy is moved, one molecule at a time, from higher temperature sites to lower temperature sites. An example of conductive heat transfer is when one end of a section of metal pipe warms up after a flame is applied to the other end. There are physical laws that allow the amount of conductive heat flow to be calculated, and they are presented here to show the factors on which conductive heat flow depends. Keywords: atomic vibrations molecular collisions (atomic collisions in inert gas) 36. Charlie Chong/ Fion Zhang The Fourier conduction Law expresses the conductive heat flow, Q per unit area A, through a slab of solid material of thickness L as illustrated in Figure 1.1. Thermal resistance Rt is defined as: Thermal conductivity is defined as: Heat flow per unit area is defined as: 37. Charlie Chong/ Fion Zhang Where: Q/A = the rate of heat transfer through the slab per unit area (BTU/hft2) or (W/m2) perpendicular to the flow, L = the thickness of the slab (ft or m), T1 = (F) or (C) is the higher temperature (at the left), T2 = the lower temperature (at the right) k = the thermal conductivity of the slab material (BTU/hftF) or (W/mK) Rt = the thermal resistance of the slab material (Fhft2fBTU) or (m2K/W) 38. Charlie Chong/ Fion Zhang The Fourier conduction Law ( One dimension heat flow) The mathematical relationship that describes heat transfer as a function of the material that heat is conducting through is known as Fourier's law and is given below. Fouriers Law: q = k A TH-TC) L-1 Where: q = heat transfer per unit time (W) A = heat transfer area (m2) k = thermal conductivity of material (W/mK) L = material thickness (m) 39. Charlie Chong/ Fion Zhang Thermal conductivity is highest for metals such as aluminum and lower for porous materials such as brick. It is inversely proportional to thermal resistance. K= 1/Rt Comment: k 1/R, R= thermal resistivity and the thermal resistance Rt = LR Thermal conductivity is highest for metals such as aluminum and lower for porous materials such as brick. It is inversely proportional to thermal resistance. In real terms, the Fourier expression means that the rate of heat flow increases with increasing temperature difference. increases with increasing thermal conductivity and decreases with increasing slab thickness. Heat flow may be expressed in English units or metric units. 40. Charlie Chong/ Fion Zhang Convective Heat Transfer Convective heat transfer takes place in a moving medium and is almost always associated with heat transfer between a solid and a moving fluid (such as air). Forced convection takes place when an external driving force, such as a wind or an air pump, moves the fluid. Free convection takes place when there is no external driving force - the temperature differences necessary for heat transfer produce density changes in the fluid. The warmer fluid rises as a result of increased buoyancy. In convective heat flow, heat transfer takes effect by direct conduction through the fluid and the mixing motion of the fluid itself. Figure 1.2 illustrates convective heat transfer between a flat plate and a moving fluid. 41. Charlie Chong/ Fion Zhang Figure 1.2: Convective heat flow 42. Charlie Chong/ Fion Zhang Figure 1.2: Convective heat flow fluid velocity Distancefrom boundarylayer Thermal Boundary layer Tsurface T 43. Charlie Chong/ Fion Zhang The presence of the plate causes the velocity of the fluid to decrease to zero at the surface and influences its velocity throughout the thickness of a boundary layer. The thickness of the boundary layer depends on the free fluid velocity V - the higher the free fluid velocity, the thinner the boundary layer. It is greatest for free convection where V = 0. The rate of heat flow depends, in turn, on the thickness of the boundary layer as well as the temperature difference between Ts and T , Ts being the surface temperature and T being the free field fluid temperature outside the boundary layer. 44. Charlie Chong/ Fion Zhang Newton's cooling law defines the convective heat transfer coefficient as: where: h = (BTU/b-ft2-F) or (W/m2-K) This is rearranged to obtain an expression for convective heat flow per unit area: If Rc= 1/h is the resistance to convective heat flow, then: 45. Charlie Chong/ Fion Zhang Rc is easier to use than h when determining combined conductive and convective heat transfer because then they are additive terms. In real terms, this expression means that the rate of convective heat flow increases with increasing temperature difference, increases with higher convective heat flow coefficient and decreases with increasing convective thermal resistance. Conductive and convective heat transfer are very similar. In both, the heat transfer is directly proportional to the temperature difference and the speed at which th is energy is transferred (rate of heat flow) depends on the transfer coefficient of the media or material through which the heat energy flows. By comparison, radiative heat transfer takes place in accordance with a different set of rules. 46. Charlie Chong/ Fion Zhang Radiative Heat Transfer Radiative heat transfer is unlike the other two modes because: 1. it occurs by electromagnetic emission and absorption in a manner similar to light; 2. it propagates at the speed of light; 3. like light, it requires a direct line of sight; 4. the heat energy transferred is proportional to the fourth power T4 of the temperature of the objects; and 5. it can take place across a vacuum in fact, a vacuum is the most efficient medium for radiative heat transfer. The electromagnetic spectrum is illustrated in Figure 1.3 and shows that X- rays. radio waves. light waves (ultraviolet and visible) and infrared radiation are all related. Radioactive heat transfer takes place in the infrared portion of the spectrum, from 0.75m to about 100m, although most practical measurements can be calculated to about 20m . The symbols m (m is preferred) stand for micrometers or microns. A micron is one-millionth of a meter and the measurement unit for radiant energy wavelength. Wavelength is inversely related to frequency (longer wavelengths have lower frequencies). 47. Charlie Chong/ Fion Zhang Figure 1.3: Infrared in the electromagnetic spectrum Practical Infrared Thermography ;2m to 6m and 8m to 14m 48. Charlie Chong/ Fion Zhang Figure 1.4: Infrared radiation leaving a target surface () 49. Charlie Chong/ Fion Zhang 1.3 Fundamentals of Radiative Heat Flow Radiation Exchange at the Target Surface The measurement of infrared/thermal radiation is the basis for non-contact temperature measurement and infrared thermography. The surface to be evaluated is called the target surface. Thermal infrared radiation leaving a surface is called exitance or radiosity. It can be emitted from the surface, reflected by the surface, or transmitted through the surface. This is illustrated in Figure 1.4. The total radiosity is equal to the sum of the emitted component (We), the reflected component (Wr) and the transmitted component (Wt ). It is important to note that the surface temperature Te is related to the emitted component We only. Keywords: Exitance Radiosity 50. Charlie Chong/ Fion Zhang Thermal infrared radiation impinging on a surface can be absorbed, reflected, or transmitted as illustrated in Figure 1.5. Kirchhoff's law states that the sum of the three components is always equal to the total received radiation, Et The fractional sum of the three components equals unity or 100 percent: Et = E + E + E , (for blackbody E = E ) where: Et = total energy Likewise, the sum of the three material properties, transmissivity, reflectivity and emissivity, also always equals unity: + + =1 51. Charlie Chong/ Fion Zhang Figure 1.5: Infrared radiation impinging on a target surface Kirchhoff's law 52. Charlie Chong/ Fion Zhang Reflections off Specular and Diffuse Surfaces A perfectly smooth surface will reflect incident energy at an angle complementary to the angle of incidence as shown in Figure 1.5. This is called a specular reflector. A completely rough or structured surface will scatter or disperse all of the incident radiation. This is called a diffuse reflector. No perfectly specular or perfectly diffuse surface can exist in nature, and all real surfaces have some diffusivity and some specularity. These surface characteristics will determine the type and direction of the reflected component of incident radiation. When making practical measurements, the specularity or diffusivity of a target surface are taken into account by compensating for the effective emissivity (*) of the surface. The thermographer's use of effective emissivity is reviewed as part of the detailed discussion of equipment operation in Chapter 5. Keywords: Specular reflector Diffuse reflector 53. Charlie Chong/ Fion Zhang Reflections off Specular and Diffuse Surfaces 54. Charlie Chong/ Fion Zhang Reflections off Specular and Diffuse Surfaces 55. Charlie Chong/ Fion Zhang Transient Heat Exchange The previous discussions of the three types of heat transfer deal with steady state heat exchange for reasons of simplicity and comprehension. Heat transfer is assumed to take place between two points, each of which is at a fixed temperature. However, in many applications, temperatures are in transition so that the values shown for energy radiated from a target surface are the instantaneous values at the moment measurements are made. In many instances, existing transient thermal conditions are exploited to use thermography to reveal material or structural characteristics in test articles. In infrared nondestructive testing of materials, thermal injection or active thermography techniques are used to generate controlled thermal transient flow based on the fact that uniform structural continuity results in predictable thermal continuity. These techniques will be discussed in greater detail in Chapter 5. 56. Charlie Chong/ Fion Zhang Radiant Energy Related to Target Surface Temperature All target surfaces warmer than absolute zero radiate energy in the infrared spectrum. Figure 1.6 shows the spectral distribution of energy radiating from various idealized target surfaces as a function of surface temperature (T) and wavelength (A.). Very hot targets radiate in the visible as well, and our eyes can see this because they are sensitive to light. The sun, for example, is at a temperature of about 6000 K and appears to glow white bot. The heating element of an electric stove at 800 K glows a cherry red and, as it cools, it loses its visible glow but continues to radiate. This radiant energy can be felt with a hand placed near the surface even though the glow is invisible. The idealized curves shown in Figure 1.6 are for perfect radiators known as blackbodies. Blackbodies are defined and discussed in greater detail later in this chapter. Figure 1.6 also shows two key physical laws regarding infrared energy emitted from surfaces. 57. Charlie Chong/ Fion Zhang Radiant Energy Related to Target Surface Temperature All target surfaces warmer than absolute zero radiate energy in the infrared spectrum. 58. Charlie Chong/ Fion Zhang The Stefan-Boltzmann law: W= T4 Where: W = radiant flux emitted per unit area (W/m2) = emissivity (unity for a blackbody target) = Stefan-Boltzmann constant= 5.673 x I0-8 W/m-2K-4 T = absolute temperature of target (K) (Comments: for blackbody =1, =.) illustrates that W, the total radiant flux emitted per unit area of surface, (the area under the curve) is proportional to the fourth power of the absolute surface temperature T4. It is also proportional to a numerical constant , and the emissivity of the surface, . 59. Charlie Chong/ Fion Zhang Figure 1.6: Typical blackbody distribution curves and basic radiation laws Stefan-Boltzmann Law Radiant Flux per Unit Area In W/cm2 W= T4 = emissivity (unity for a blackbody target) = Stefan-Boltzmann constant = 5.673 x I0-8 W/m-2K-4 T = absolute temperature of target (K) Wien's Displacement Law max = b/T where: max = peak wavelength (m) b = Wien's displacement constant (2897 or 3000 approximately) 60. CharlieChong/FionZhang Figure1.6:Typicalblackbodydistributioncurvesandbasicradiationlaws 61. Charlie Chong/ Fion Zhang Wien's displacement law: max = b/T Where: max wavelength of maximum radiation (m) b Wien's displacement constant or 2897 (mK) illustrates that the peak wavelength, max (m) at which a surface radiates, is easily determined by dividing a constant b (approximately 3000) by the absolute temperature T (Kelvin) of the surface. 62. Charlie Chong/ Fion Zhang 1.4 Practical Infrared Measurements ln practical measurement applications, the radiant energy leaves a target surface, passes through some transmitting medium. usually an atmospheric path, and reaches a measuring instrument. Therefore, when making measurements or producing a thermogram, three sets of characteristics must be considered: 1. characteristics of the target surface, 2. characteristics of the transmitting medium and 3. characteristics of the measuring instrument. This is illustrated in Figure 1.7. 63. Charlie Chong/ Fion Zhang Figure 1.7: Three sets of characteristics of the infrared measurement problem obj amb assumed = 0 atm atm 64. Charlie Chong/ Fion Zhang Characteristics of the Target Surface Target surfaces are separated into three categories; blackbodies, graybodies and nongraybodies (also called real bodies, selective radiators or spectral bodies). The target surfaces shown in Figure 1.6 are all perfect radiators (or blackbodies). A blackbody radiator is defined as a theoretical surface having unity emissivity at all wavelengths and absorbing all of the radiant energy impinging upon it. Emissivity, in turn, is defined as the ratio of the radiant energy emitted from a surface to the energy emitted from a blackbody surface at the same temperature. Blackbody radiators are theoretical and do not exist in practice. The surface of most solids are graybodies, that is, surfaces with high emissivities that are fairly constant with wavelength. Figure 1.8 shows the comparative spectral distribution of energy emitted by a blackbody, a graybody and a nongraybody, all at the same temperature (300 K). 65. Charlie Chong/ Fion Zhang Figure 1.8: Spectral distribution of a blackbody, graybody and nongraybody 66. Charlie Chong/ Fion Zhang Referring back to Figure 1.5, the total exitance available to the measuring instrument has three components: emitted energy (We), reflected energy (Wr) from the environment and other reflecting sources, and for nonopaque targets, energy transmitted through the target (Wt) from sources behind the target. Because a theoretical blackbody has an emissivity of 1.00, it will reflect and transmit no energy = 0, = 0. Real targets, however, are not blackbodies. and figure 1.9 shows the three components that comprise Wx, the total exitance that an instrument sees when aimed at a real Ufe target surface. Because only the emitted component, We, is related to the temperature of the target surface, it becomes apparent that a significant part of the measurement problem is eliminating or compensating for the other two components. This is discussed in greater detail in Chapter 4. 67. Charlie Chong/ Fion Zhang Figure 1.9: Components of energy reaching the measuring instrument 68. Charlie Chong/ Fion Zhang Characteristics of the Transmitting Medium Because lhe infrared radiation from the target passes through some transmitting medium on its way to the target, the transmission and emission characteristics of the medium in the measurement path must be considered when making non contact thermal measurement. No loss of energy or self emission (atm) is encountered when measuring through a vacuum. However. most measurements are made through air. For short path length (a few meters, for example), most gases (including the atmosphere) absorb and emit very little energy and can be ignored. However. when highly accurate temperature measurements are required, the effects of atmospheric absorption must be taken into account. (atm,atm). 69. Charlie Chong/ Fion Zhang As the path length increases to more than a few meters, or as the air becomes heavy with water vapor, atmospheric absorption may become a significant factor. Therefore, it is necessary to understand the infrared transmission characteristics of the atmosphere. Figure 1.10 illustrates the spectral transmission characteristics of a 10 m (33 ft) path of ground level atmosphere at a temperature of 25 C (77 F) and 50 percent humidity. It is immediately apparent that the atmosphere is not as transparent in the infrared ponion of the spectrum as it is in the visible ponion. Two spectral intervals have very high transmission. These are known as the 3 to 5 m and the 8 to 14m atmospheric windows, and almost all infrared sensing and imaging instruments are designed to operate in one of these two windows. The absorption segments shown in Figure 1.10 were formed by carbon dioxide and water vapor, which are two of the major constituents in air. For measurements through gaseous media other than atmosphere, it is necessary to investigate the transmission spectra of the medium before validating the measurements, which is explained in greater detail in Chapter 2. 70. Charlie Chong/ Fion Zhang Figure 1.10; Transmission of 10m (33ft) of ground level atmosphere at 50 percent humidity and 25 C (77F) PercentageTransmission Wave Length m 71. Charlie Chong/ Fion Zhang When there is a solid material, such as a glass or quartz viewing port, between the target and the instrument, the spectral characteristics of the solid media must be known and considered. Figure 1.11 shows transmission curves for various samples of glass. Most significant is the fact that glass does not transmit infrared energy at 10m where ambient (30 C, 86 F) surfaces radiate their peak energy. In practice, infrared thermal measurements of ambient targets can never be made through glass. One practical approach to this problem is to eliminate the glass, or at least a portion through which the instrument can be aimed at the target. If a window must be present for personal safety, vacuum, or product safety, a material might be substituted that transmits in the longer wavelengths. Figure 1.12 shows the spectral transmission characteristics of several infrared transmitting materials, many of which transmit energy past 10m. In addition to being used as transmitting windows, these materials are often used as lenses and optical elements in infrared sensors and imagers. Of course, as targets become hotter, and the emitted energy shifts to the shorter wavelengths, glass and quartz windows pose less of a problem and are even used as elements and lenses in high temperature sensing instruments. Characteristics of the measuring instrument are addressed in Chapter 4. 72. Charlie Chong/ Fion Zhang Figure 1.11: Transmission, absorption and reflectance characteristics of glass 73. Charlie Chong/ Fion Zhang Figure 1.12: Transmission curves of various infrared transmitting material 74. Charlie Chong/ Fion Zhang Figure 1.12: Transmission curves of various infrared transmitting material 75. Charlie Chong/ Fion Zhang Convective Heat Transfer Convective heat transfer, often referred to simply as convection, is the transfer of heat from one place to another by the movement of fluids. Convection is usually the dominant form of heat transfer in liquids and gases. Although often discussed as a distinct method of heat transfer, convective heat transfer involves the combined processes of conduction (heat diffusion) and advection (heat transfer by bulk fluid flow). The term convection can sometimes refer to transfer of heat with any fluid movement, but advection is the more precise term for the transfer due only to bulk fluid flow. The process of transfer of heat from a solid to a fluid, or the reverse, is not only transfer of heat by bulk motion of the fluid, but diffusion and conduction of heat through the still boundary layer next to the solid. Thus, this process without a moving fluid requires both diffusion and advection of heat, a process that is usually referred to as convection. Convection that occurs in the earth's mantle causes tectonic plates to move. Convection can be "forced" by movement of a fluid by means other than buoyancy forces (for example, a water pump in an automobile engine). Thermal expansion of fluids may also force convection. In other cases, natural buoyancy forces alone are entirely responsible for fluid motion when the fluid is heated, and this process is called "natural convection". An example is the draft in a chimney or around any fire. In natural convection, an increase in temperature produces a reduction in density, which in turn causes fluid motion due to pressures and forces when fluids of different densities are affected by gravity (or any g-force). For example, when water is heated on a stove, hot water from the bottom of the pan rises, displacing the colder denser liquid, which falls. After heating has stopped, mixing and conduction from this natural convection eventually result in a nearly homogeneous density, and even temperature. Without the presence of gravity (or conditions that cause a g-force of any type), natural convection does not occur, and only forced- convection modes operate. The convection heat transfer mode comprises one mechanism. In addition to energy transfer due to specific molecular motion (diffusion), energy is transferred by bulk, or macroscopic, motion of the fluid. This motion is associated with the fact that, at any instant, large numbers of molecules are moving collectively or as aggregates. Such motion, in the presence of a temperature gradient, contributes to heat transfer. Because the molecules in aggregate retain their random motion, the total heat transfer is then due to the superposition of energy transport by random motion of the molecules and by the bulk motion of the fluid. It is customary to use the term convection when referring to this cumulative transport and the term advection when referring to the transport due to bulk fluid motion. http://en.wikipedia.org/wiki/Convective_heat_transfer 76. Charlie Chong/ Fion Zhang Chapter 1 Review Questions 13. d 14. e I5. d 16. e 17. b 18. d 19. a 20. d 21. b 22. e 1. b 2. d 3. c 4. a 5. c 6. d 7. b 8. b 9. d 10. d 11. a 12. a Q&A 77. Charlie Chong/ Fion Zhang Q1. At a temperature of absolute zero: a. hydrogen becomes a liquid. b. all molecular motion ceases. c. salt water is part solid and part liquid. d. fahrenheit and celsius readings are the same. Q2. Conductive heat transfer cannot take place: a. within organic materials such as wood. b. between two solid materials in contact. c. between dissimilar metals. d. across a vacuum. Q3. The only three modes of heat transfer are: a. resistive, capacitive and inductive. b. steady state, transient and reversible. c. conduction, convection and radiation. d. conduction. convection and absorption. 78. Charlie Chong/ Fion Zhang Q4. Heat can only flow in the direction from: a. hotter objects to colder objects. b. colder objects to houer objects. c. more dense objects to less dense objects. d. larger object to smaller objects. Q5. Thermal resistance is: a. analogous to electrical current. b. proportional to the fourth power of emissivity. c. inversely proportional to the rate of heat flow by conduction. d. a measure of material stiffness. Q6. The radiation of thermal infrared energy from a target surface: a. occurs most efficiently in a vacuum. b. is proportional to the fourth power of the absolute surface temperature. c. is directly proportional to surface emissivity. d. is all of the above. 79. Charlie Chong/ Fion Zhang Q7. The mode of heat transfer most closely associated with infrared thermography is: a. induction. b. radiation. c. convection. d. conduction. Q8. To convert a fahrenheit reading to celsius: a. divide by 1.8. b. subtract 32 and divide by 1.8. c. multiply by 1.8 and add 32. d. add 273. Q9. Thermal radiation reaching the surface of an object can be: a. absorbed only in the presence of atmosphere. b. reflection and absorbed only in a vacuum. c. transmitted only if the surface is organic. d. absorbed, reflected and transmitted. 80. Charlie Chong/ Fion Zhang Q10. The follow ing spectral band is included in the infrared spectrum: a. 0.1 to 5.5 m. b. 0.3 to 10.6 m. c. 0.4 to 20.0 m. d. 0.75 to 100 m. Q11. Mosl instruments used in infrared thermography operate somewhere within the; a. 2 to 14 m spectral region. b. 5 to 10 m spectral region. c. 10 to 20 m spectral region. d. 20 to 100 J m spectral region. Q12. As a surface cools, the peak of its radiated infrared energy: a. shifts to longer wavelengths. b. shifts to shorter wavelengths. c. remains constant if emissivity remains constant. d. remains constant even if emissivity varies. 81. Charlie Chong/ Fion Zhang Q13. The peak emitting wavelength of a 300 C (572 F) blackbody is approximately: a. 1.5 m. b. 3 m. 0. 10 m. d. 5 m. Q14. An opaque surface with an emissivity of 0.04 would be: a. transparent to infrared radiation. b. a fairly good emitter. c. almost a perfect reflector. (=0, =0.04, = 0.96) d. almost a perfect emitter. Q15. If a surface has an emissivity of 0.35 and a reflectivity of 0.45. its transmissivity would be: a. impossible to detennine without additional information. b. 0.80. c. 0.10. d. 0.20. [1-(0.35+0.45)] max = b/T( in K) = 2897/573 m 82. Charlie Chong/ Fion Zhang Q16. In forced convection, the boundary layer: a. increases as the fluid velocity increases. b. remains the same as the fluid velocity increases. c. decreases as the fluid velocity increases. d. increases in proportion to the fourth power of the fluid velocity. Q17. When heating one end of a car key to thaw a frozen automobile door lock, heat transfer from the key to the lock is an example of: a. forced convection. b. conductive heat transfer. c. free convection. d. radiative heat transfer. Q18. The infrared atmospheric window that transmits infrared radiation best is the: a. 2.0 to 3.0 m region. b. 3.0 to 6.0 m region. c. 6.0 to 9.0 m region. d. 9.0 to 11.0 m region. 83. Charlie Chong/ Fion Zhang Q19. The spectral band in which glass transmits infrared radiation best is the: a. 2.0 to 3.0 m region. b. 3.0 to 6.0 m region. c. 6.0 to 9.0 m region. d. 9.0 to 11.0 m region. Q20. Reflectance of infrared radiation by a glass surface is greatest in the: a. 2.0 to 3.0 m region. h. 3.0 to 6.0 m region. c. 6.0 to 9.0 m region. d. 9.0 to 11.0 m region. Q21. A diffuse reflecting surface is: a. a polished surface that reflects incoming energy at a complementary angle. b. a surface that scatters reflected energy in many directions. c. also called a specular reflecting surface. d. usually transparent to infrared radiation. 84. Charlie Chong/ Fion Zhang Q22. In the 8 to 14 m spectral region: a. the atmosphere absorbs infrared radiant energy almost completely. b. the atmosphere reflects infrared radiant energy almost completely. c. the atmosphere transmits infrared energy very efficiently. d. infrared instruments do not operate very accurately. 85. Charlie Chong/ Fion Zhang Chapter 2 Materials and Their Properties 86. Charlie Chong/ Fion Zhang 2.1 Materials Characteristics A knowledge of the characteristics of materials is important to the thermographer for numerous reasons, but the two most important arc the need to know how a particular target surface e mits. transmits and refl ects infrared radiant energy. and the need 10 know how heat flows within a particular material. 2.2 Surface Properties of Materials The surface properties of materials include emissivity. reflectivity and transmissivity. 87. Charlie Chong/ Fion Zhang Emissivity When using infrared thermography to measure surface temperature of a target. it is essential to know the effective emissivity (*) of the surface material. This is the value that must be set into the instrument's menu under the specific conditions of measurement for the instrument to display an accurate surface temperature value. When attempting to make temperature measurements on a target of unknown emissivity. an estimate of emissivity may be the only available alternative. There are numerous reference tables available that list generic values of emissivity for common materials and these can be used as guides. Table 2.2 is an example of a reference table. As previously noted. emissivity depends on the material and the surface texture. It may also vary with surface temperature and with the spectral interval over which the measurement is made. These variations, though usually small , cannot always be ignored. 88. Charlie Chong/ Fion Zhang For an emissivity reference table to be useful. conditions of target temperature and spectral interval (wavelength) must also be presented. If the temperature and wavelength listed do not correspond to the actual measurement conditions. the emissivity listed must be considered to be a rough estimate. Even if there is an exact match to the measurement conditions, the lookup method is not the best approach for accurate temperature measurement. Ideally. the way to determine effective emissivity is to measure it with one of the several established protocols. using a sample of the actual target surface material and the actual instrument to be used for the measurement mission. The protocols for measuring effective emissivity of material samples are discussed in Chapter 4. 89. Charlie Chong/ Fion Zhang Reflectivity Reflectivity of a surface generally increases as emissivity decreases. For opaque graybody surfaces =0. the sum of emissivity and reflectivity is unity (1.0). Therefore. an opaque graybody surface with a low effective cmissivity will be highly reflective, which can result in erroneous temperature readings even if the correct emissivity is set into the instrument. These errors can be the result of either point source reflections, background reflections or both entering the instrument . There are two components of reflected energy the diffuse componenl and the specular component. If the surface is relatively specular (smooth). most of the reflected energy is specular, that is. it reflects off the surface at an angle complementary to the angle of incidenct. If the surface is relatively diffuse (textured) most of the renected energy is scattered uniformly (haphazardly?) in all directions regardless of the angle of incidence. Keywords: Therefore. an opaque graybody surface with a low effective cmissivity will be highly reflective 90. Charlie Chong/ Fion Zhang Errors because of point source reflections are usually larger when the target surfaces are specular, and errors because of background reflections are not affected by the specularity or diffusivity of the target surface. Both types of reflective errors are more serious when the target surface is cool compared to the temperature of the point source or the background because the point source makes a greater contribution to the total radiant exitance than the target does. In practice, the thermographer can learn to recognize and avoid errors due to point source reflections. The thermographer also can learn to measure and compensate for errors due to background reflection. This is discussed in Chapter 4. 91. Charlie Chong/ Fion Zhang Transmissivity When the target surface is a non-graybody, the target material may be partly transparent to infrared radiation. This means the target material has a transmissivity greater than 0. Due to this transparency. radiant thermal energy may be transmitted through the target from sources behind the target. This energy may enter the instrument and cause temperature measurement errors even if the correct emissivity is set into the instrument and reflective errors are eliminated. Although errors due to transmission are the least common in practice. errors due to energy transmiued through the target usually require the most sophisticated procedures to correct them. In most cases, spectral filtering is the best solution. Methods for correcting these errors are discussed in Chapters 4 and 5. Keywords: spectral filtering non-graybody (could be any others like black body, selective emitter, could be a body with > 0) 92. Charlie Chong/ Fion Zhang View Angle The angle between the instrument's line of sight and the surface material will have a minimal effect on the material properties described above, providing this angle is kepi as close as possible to normal (perpendicul ar) and no greater than 30 degrees from normal (for many nonmetallic surfaces this may be increased 10 as large as 60 degrees from normal. if unavoidable). If it is not possible to view a target at an angle within this range, the effective emissivity may Change. particularly if it is low to begin with. This will most likely compromise the accuracy of temperature measurements. Note that the emissivities listed in Table 2.2 are normal emissivities and are not valid at acute viewing angles. On curved (nonflat) surfaces. view angle can be even more critical and measurements should be made cautiously. Note: An acute angle is an angle whose degree measure is greater than 0 but less than 90. 93. Charlie Chong/ Fion Zhang 2.3 Heat Conducting Properties of Materials The use of infrared themlography for nondestructive material testing is generally based on the assumption that uniform structural continuity provides uniform thermal continuity. Both unstimulated and stimulated approaches to thermographic material testing depend on this assumption. as will be discussed in greater detail in Chapters 4 and 5. It is necessary. therefore, that the thermographer have a clear basic understanding of the manner in which heat flows within a material and the material properties that affect this flow. Keywords: The use of infrared themlography for nondestructive material testing is generally based on the assumption that uniform structural continuity provides uniform thermal continuity. 94. Charlie Chong/ Fion Zhang Thermal Conductivity Thermal conductivity k is the relative one dimensional capability of a material to transfer heat. It affects the speed (thus time, t) that a given quantity of heat applied to one point in a slab of material will travel a given distance within that material to another point cooler than the first. Thermal conductivity is high for metals and low for porous materials. It is logical. therefore. that heat will be conducted more rapidly in metals than in more porous materials. Although thermal conductivity varies slightly with temperature in solids and liquids and with temperature and pressure in gases, for practical purposes it can be considered a constant for a particular material. Table 2.1 is a list of thermal properties for several conunon materials. 95. Charlie Chong/ Fion Zhang Heat Capacity The heat capacity of a malerial or a structure describes its ability to store heat. It is the product of the specific thermal energy Cp and the density of the material. When thermal energy is stored in a structure and then the structure is placed in a cooler environment, the sections of the structure that have low heat capacity will change temperature more rapidly because less thermal energy is stored in them. Consequently, these sections will reach thermal equilibrium with their surroundings sooner than those sections with higher heat capacity, The term thermal capacitance is used to describe heat capacity in terms of an electrical analog. where loss of heat is analogous to loss of charge on a capacitor. Structures with low thermal capacitance reach equilibrium sooner when placed in a cooler environmcnt than those with high thermal capacitance. This phenomenon is exploited when performing unstimulated nondestructive testing of structures, specifically when locating water saturated sections on flat roofs. This is discussed in greater detail in Chapter 5, 96. Charlie Chong/ Fion Zhang Thermal Diffusivity As in emissivity . the heat conducting properties of materials may vary from sample to sample. depending on variables in the fabrication process and other factors. Thermal diffusivity is the 3D expansion of thermal conductivity in any given material sample. Diffusivily relates more to transient heat flow, whereas conductivity relates to steady state heat flow. It takes into account the thermal conductivity k of the sample, its specific heat Cp, and its density . Its equation is = k/ Cp cm2s-1. Because thermal diffusivity of a sample can be measured directly using infrared thermography, it is used extensively by the materials flaw evaluation community as an assessment of a test sample's ability to carry heat away, in all directions, from a heat injection site. Table 2.1 lists thermal diffusivities for several common materials in increasing order of thermal diffusivity. Several protocols for measuring the thermal diffusivity of a test sample are described by Maldague. 97. Charlie Chong/ Fion Zhang Thermal Diffusivity Diffusivily relates more to transient heat flow, whereas conductivity relates to steady state heat flow. 98. Charlie Chong/ Fion Zhang Partial 2.1 99. Charlie Chong/ Fion Zhang Partial Table 2.1 100. Charlie Chong/ Fion Zhang Partial Table 2.2 101. Charlie Chong/ Fion Zhang Thermal Diffusivity As in emissivity . the heat conducting properties of materials may vary from sample to sample. depending on variables in the fabrication process and other factors. Thermal diffusivity is the 3D expansion of thermal conductivity in any given material sample. Diffusivily relates more to transient heat flow, whereas conductivity relates to steady state heat flow. It takes into account the thermal conductivity k of the sample, its specific heat Cp, and its density . Its equation is = k/ Cp cm2s-1. for my ASNT exam 102. Charlie Chong/ Fion Zhang Chapter 2 Review Questions 1. c 2. b 3. a 4. d 5. a 6. b 7. a 8. b 9. b 10. b Q&A 103. Charlie Chong/ Fion Zhang 1. The best way to determine the effective emissivity of a target surface is: a. to look it up in a table. b. to calcu late it. c. to measure the effective emissivity of the material itself or a similar sample. d. all of the above. 2. For an opaque graybody target surface, emissivity equals: a. 1/refleclivity. b. 1-reflectivity. c. 1.0. d. reflectivity to the fourth power. 3. The effective emissivity of a surface is always affected by: a. the material, its surface texture and the viewing angle. b. the material, its thermal conductivity and humidity. c. the material, its surface texture and its thermal diffusivity. d. the material, its visible color and its thermal conductivity. 104. Charlie Chong/ Fion Zhang 4. When measuring the temperature of a nongraybody target: a, the viewing angle is not critical. b. always assume an emissivity of 1.0. c. reflections off the near surface may be ignored. d. errors may be caused by hot sources behind the target. 5. The effective emissivity of a target surface: a, can vary at different wavelengths. b. is the same for all wavelengths if the viewing angle is kept constant. c. is always higher at longer wavelengths. d. is always lower at longer wavelengths. 6. Unfinished, unoxidized metal surfaces usually have: a. high and uniform emissivities. b. low and uniform emissivities. c. non-graybody characteristics. d. low specular reflectivity. 105. Charlie Chong/ Fion Zhang 7. Thermal diffusivity is: a. high for metals and low for porous materials. b. the same for all metals. c, low for metals and high for porous materials. d. the same for all porous materials. 8. Thermal diffusivity is: a, the same as diffuse reflectivity. b. related more to transient heat flow than to steady Slale heat flow. c. related more 10 steady stale heat flow than to transient heat flow. d. the same as spectral transmittance. 9. Thermal capacitance: a. describes the heating of a condenser. b. expresses the heat capacity of a material in a form analogous to electrical capacitance. c. is zero for a blackbody radiator. d. describes the maximum temperature rating of a capacitor. 106. Charlie Chong/ Fion Zhang 10. A highly textured surface is said to be diffuse. A smooth surface is said to be: a. opaque. b. specular. c. convex. d. transparent. 107. Charlie Chong/ Fion Zhang Chapter 3 Thermal Instrumentation 108. Charlie Chong/ Fion Zhang 3.1 Thermal Instrumentation Overview Equipment for temperature measurement and thermography includes contacting as well as noncontacting devices. Contacting devices for temperature measurement include thermopiles. thermocouples, liquid thermometers, gas expansion devices (bourdon gas thermometers), liquid crystals (cholesterol crystals ?), heat flux indicators and fiber optic sensors. Aside from some specialized instruments, the vast majority of noncontacting temperature measurement devices are infrared sensing instruments and systems. Infrared sensing instruments and systems are divided into (1) point sensors (radiation thermometers), (2) line scanners and (3) thermal imagers. This chapter begins with a review of contacting thermal measurement instruments and a discussion of the basic configurations of infrared sensing and imaging instruments. This is followed by a discussion of performance parameters and, finally, descriptions of commercial thermal sensing and imaging equipment, thermographic image processing software and image hard copy recording accessories. 109. Charlie Chong/ Fion Zhang What is Thermopile A thermopile is an electronic device that converts thermal energy into electrical energy. It is composed of several thermocouples connected usually in series or, less commonly, in parallel. Thermopiles do not respond to absolute temperature, but generate an output voltage proportional to a local temperature difference or temperature gradient. Thermopiles are used to provide an output in response to temperature as part of a temperature measuring device, such as the infrared thermometers widely used by medical professionals to measure body temperature. They are also used widely in heat flux sensors (such as the Moll thermopile and Eppley pyrheliometer) and gas burner safety controls. The output of a thermopile is usually in the range of tens or hundreds of millivolts. As well as increasing the signal level, the device may be used to provide spatial temperature averaging. Thermopiles are also used to generate electrical energy from, for instance, heat from electrical components, solar wind, radioactive materials, or combustion. The process is also an example of the Peltier Effect (electric current transferring heat energy) as the process transfers heat from the hot to the cold junctions. http://en.wikipedia.org/wiki/Thermopile 110. Charlie Chong/ Fion Zhang Thermopile- Thermoelectric Seebeck module http://en.wikipedia.org/wiki/Thermopile 111. Charlie Chong/ Fion Zhang The Working Principle: Thermopile, composed of multiple thermocouples in series. If both the right and left junctions are the same temperature, voltages cancel out to zero. However if one side is heated and other side cooled, resulting total output voltage is equal to the sum of junction voltage differentials. 112. Charlie Chong/ Fion Zhang What is a IR Thermopile? (non-contact) A thermopile is a serially-interconnected array of thermocouples, each of which consists of two dissimilar materials with a large thermoelectric power and opposite polarities. The thermocouples are placed across the hot and cold regions of a structure and the hot junctions are thermally isolated from the cold junctions. The cold junctions are typically placed on the silicon substrate to provide effective heat sink. In the hot regions, there is a black body for absorbing the infrared, which raises the temperature according to the intensity of the incident infrared. These thermopiles employ two different thermoelectric materials which are placed on a thin diaphragm having a low thermal conductance and capacitance. http://www.ge-mcs.com/download/temperature/930-164A-LR.PDF 113. Charlie Chong/ Fion Zhang IR Thermopiles Sensor (non-contact) 114. Charlie Chong/ Fion Zhang IR Thermopile Quad Sensor (non-contact) 115. Charlie Chong/ Fion Zhang Thermocouple General description: Thomas Seebeck discovered in 1821 that when two wires composed of dissimilar metals are joined at both ends and one of the ends is heated, there is a continuous current which flows in the thermoelectric circuit. (Seebeck effect). The junctions can be exposed, grounded or ungrounded. The thermocouple is normally directly connected to a standard temperature controller. Thermocouples are among the easiest temperature sensors used in science and industry and very cost effective. (usually less than $50.00) thermocouple embedded in Dalton cartridge heater http://www.deltat.com/thermocouple.html 116. Charlie Chong/ Fion Zhang Thermocouple A thermocouple is a temperature-measuring device consisting of two dissimilar conductors that contact each other at one or more spots, where a temperature differential is experienced by the different conductors (or semiconductors). It produces a voltage when the temperature of one of the spots differs from the reference temperature at other parts of the circuit. Thermocouples are a widely used type of temperature sensor for measurement and control, and can also convert a temperature gradient into electricity. Commercial thermocouples are inexpensive, interchangeable, are supplied with standard connectors, and can measure a wide range of temperatures. In contrast to most other methods of temperature measurement, thermocouples are self powered and require no external form of excitation. The main limitation with thermocouples is accuracy; system errors of less than one degree Celsius (C) can be difficult to achieve. Any junction of dissimilar metals will produce an electric potential related to temperature. Thermocouples for practical measurement of temperature are junctions of specific alloys which have a predictable and repeatable relationship between temperature and voltage. Different alloys are used for different temperature ranges. Properties such as resistance to corrosion may also be important when choosing a type of thermocouple. Where the measurement point is far from the measuring instrument, the intermediate connection can be made by extension wires which are less costly than the materials used to make the sensor. Thermocouples are usually standardized against a reference temperature of 0 degrees Celsius; practical instruments use electronic methods of cold-junction compensation to adjust for varying temperature at the instrument terminals. Electronic instruments can also compensate for the varying characteristics of the thermocouple, and so improve the precision and accuracy of measurements. Thermocouples are widely used in science and industry; applications include temperature measurement for kilns, gas turbine exhaust, diesel engines, and other industrial processes. Thermocouples are also used in homes, offices and businesses as the temperature sensors in thermostats, and also as flame sensors in safety devices for gas-powered major appliances. http://en.wikipedia.org/wiki/Thermocouple 117. Charlie Chong/ Fion Zhang Liquid or Gas Expansion Devices Many physical properties change with temperature, such as the volume of a liquid, the length of a metal rod, the electrical resistance of a wire, the pressure of a gas kept at constant volume, and the volume of a gas kept at constant pressure. Filled-system thermometers use the phenomenon of thermal expansion of matter to measure temperature change. The filled thermal device consists of a primary element that takes the form of a reservoir or bulb, a flexible capillary tube, and a hollow Bourdon tube that actuates a signal-transmitting device and/or a local indicating temperature dial. A typical filled-system thermometer is shown in Figure 7-1. In this system, the filling fluid, either liquid or gas, expands as temperature increases. This causes the Bourdon tube to uncoil and indicate the temperature on a calibrated dial. 118. Charlie Chong/ Fion Zhang Bourdon Gas Thermometers 119. Charlie Chong/ Fion Zhang Liquid Crystal Thermometer A liquid crystal thermometer or plastic strip thermometer is a type of thermometer that contains heat-sensitive (thermochromic) liquid crystals in a plastic strip that change color to indicate different temperatures. Liquid crystals possess the mechanical properties of a liquid, but have the optical properties of a single crystal. Temperature changes can affect the color of a liquid crystal, which makes them useful for temperature measurement. The resolution of liquid crystal sensors is in the 0.1C range. Disposable liquid crystal thermometers have been developed for home and medical use. For example if the thermometer is black and it is put onto someone's forehead it will change colour depending on the temperature of the person. There are two stages in the liquid crystals: 1. the hot nematic stage is the closest to the liquid phase where the molecules are freely moving around and only partly ordered. 2. the cold smectic stage is closest to a solid phase where the molecules align themselves into tightly wound chiral matrixes. Liquid crystal thermometers portray temperatures as colors and can be used to follow temperature changes caused by heat flow. They can be used to observe that heat flows by conduction, convection, and radiation. In medical applications, liquid crystal thermometers may be used to read body temperature by placing against the forehead. These are safer than a mercury-in-glass thermometer, and may be advantageous in some patients, but do not always give an exact result, except the analytic liquid crystal thermometer which show the exact temperature between 35.5 to 40.5 Celsius. http://en.wikipedia.org/wiki/Liquid_crystal_thermometer 120. Charlie Chong/ Fion Zhang Liquid Crystal Thermometer A liquid crystal thermometer or plastic strip thermometer is a type of thermometer that contains heat-sensitive (thermochromic) liquid crystals in a plastic strip that change color to indicate different temperatures. Liquid crystals possess the mechanical properties of a liquid, but have the optical properties of a single crystal. 121. Charlie Chong/ Fion Zhang Thermocouple http://www.omega.com/temperature/z/pdf/z021-032.pdf Thermocouple grade wires Stainless steel sheath Wire junction Adjustable nut Flexible SS sheath 122. Charlie Chong/ Fion Zhang Bimetallic Thermometers http://www.omega.com/temperature/z/pdf/z021-032.pdf 123. Charlie Chong/ Fion Zhang Resistance Thermometers - Resistance thermometers, also called resistance temperature detectors (RTDs), are sensors used to measure temperature by correlating the resistance of the RTD element with temperature. Most RTD elements consist of a length of fine coiled wire wrapped around a ceramic or glass core. The element is usually quite fragile, so it is often placed inside a sheathed probe to protect it. The RTD element is made from a pure material, typically platinum, nickel or copper. The material has a predictable change in resistance as the temperature changes and it is this predictable change that is used to determine temperature. They are slowly replacing the use of thermocouples in many industrial applications below 600 C, due to higher accuracy and repeatability. http://www.npl.co.uk/content/ConMediaFile/113 http://en.wikipedia.org/wiki/Resistance_thermometer 124. Charlie Chong/ Fion Zhang In RTD devices; Copper, Nickel and Platinum are widely used metals. These three metals are having different resistance variations with respective to the temperature variations. That is called resistance-temperature characteristics. Platinum has the temperature range of 650C, and then the Copper and Nickel have 120C and 300C respectively. The figure-1 shows the resistance-temperature characteristics curve of the three different metals. For Platinum, its resistance changes by approximately 0.4 ohms per degree Celsius of temperature. The purity of the platinum is checked by measuring R100 / R0. Because, whatever the materials actually we are using for making the RTD that should be pure. If it will not pure, it will deviate from the conventional resistance- temperature graph. So, and values will change depending upon the metals. http://en.wikipedia.org/wiki/Resistance_thermometer 125. Charlie Chong/ Fion Zhang Platinum Resistance Thermometer http://www.aoip.com/product/670-standard-platinum-resistance-thermometers/ 126. Charlie Chong/ Fion Zhang Platinum Resistance Thermometer 127. Charlie Chong/ Fion Zhang Resistance Temperature Detector (RTD) - Principle of Operation, Materials, Configuration and Benefits by Innovative Sensor Technology Overview Innovative Sensor Technology is a world-class manufacturer of thin-film RTD temperature sensors, capacitive humidity sensors, and mass flow sensors at the component level. With our state-of-the-art manufacturing technology, Innovative Sensor Technology offers both standard and custom sensors to satisfy unique applications. Additionally, our highly qualified staff is now offering R&D consulting services for industrial applications. Our sensors have applications in the automotive, HVAC, appliance, controls, and test & measurement industries. Resistance Temperature Detector (RTD) - Principle of Operation An RTD (resistance temperature detector) is a temperature sensor that operates on the measurement principle that a materials electrical resistance changes with temperature. The relationship between an RTD resistance and the surrounding temperature is highly predictable, allowing for accurate and consistent temperature measurement. By supplying an RTD with a constant current and measuring the resulting voltage drop across the resistor, the RTD resistance can be calculated, and the temperature can be determined. http://www.azom.com/article.aspx?ArticleID=5573 128. Charlie Chong/ Fion Zhang RTD Materials Different materials used in the construction of RTD offer a different relationship between resistance and temperature. Temperature sensitive materials used in the construction of RTD include platinum, nickel, and copper; platinum being the most commonly used. Important characteristics of an RTD include the temperature coefficient of resistance (TCR), the nominal resistance at 0 degrees Celsius, and the tolerance classes. The TCR determines the relationship between the resistance and the temperature. There are no limits to the TCR that is achievable, but the most common industry standard is the platinum 3850 ppm/K. This means that the resistance of the sensor will increase 0.385 ohms per one degree Celsius increase in temperature. The nominal resistance of the sensor is the resistance that the sensor will have at 0 degrees Celsius. Although almost any value can be achieved for nominal resistance, the most common is the platinum 100 ohm (pt100). Finally, the tolerance class determines the accuracy of the sensor, usually specified at the nominal point of 0 degrees Celsius. There are different industry standards that have been set for accuracy including the ASTM and the European DIN. Using the values of TCR, nominal resistance, and tolerance, the functional characteristics of the sensor are defined. http://www.azom.com/article.aspx?ArticleID=5573 129. Charlie Chong/ Fion Zhang RTD Configurations In addition to different materials, RTD are also offered in two major configurations: wire wound and thin film. Wire wound configurations feature either an inner coil RTD or an outer wound RTD. An inner coil construction consists of a resistive coil running through a hole in a ceramic insulator, whereas the outer wound construction involves the winding of the resistive material around a ceramic or glass cylinder, which is then insulated. The thin film RTD construction features a thin layer of resistive material deposited onto a ceramic substrate through a process called deposition. A resistive meander is then etched onto the sensor, and laser trimming is used to achieve the appropriate nominal values of the sensor. The resistive material is then protected with a thin layer of glass, and lead wires are welded to pads on the sensor and covered with a glass dollop. Thin film RTD have advantages over the wire wound configurations. The main advantages include that they are less expensive, are more rugged and vibration resistant, and have smaller dimensions that lead to better response times and packaging capabilities. For a long time wire wound sensors featured much better accuracy. Thanks to recent developments, however, there is now thin film technology capable of achieving the same level of accuracy. http://www.azom.com/article.aspx?ArticleID=5573 130. Charlie Chong/ Fion Zhang Operations of RTD An RTD takes a measurement when a small DC current is supplied to the sensor. The current experiences the impedance of the resistor, and a voltage drop is experienced over the resistor. Depending on the nominal resistance of the RTD, different supply currents can be used. To reduce self-heating on the sensor the supply current should be kept low. In general, around 1mA or less of current is used. An RTD can be connected in a two, three, or four-wire configuration. The two-wire configuration is the simplest and also the most error prone. In this setup, the RTD is connected by two wires to a Wheatstone bridge circuit and the output voltage is measured. The disadvantage of this circuit is that the two connecting lead wire resistances add directly two the RTD resistance and an error is incurred. http://www.azom.com/article.aspx?ArticleID=5573 2-Wire Configuration 131. Charlie Chong/ Fion Zhang The four-wire configuration consists of two current leads and two potential leads that measure the voltage drop across the RTD. The two potential leads are high resistance to negate the effect of the voltage drop due to current flowing during the measurement. This configuration is ideal for canceling the lead wire resistances in the circuit as well as eliminating the effects of different lead resistances, which was a possible problem with the three-wire configuration. The four-wire configuration is commonly used when a highly accurate measurement is required for the application. 4-Wire Configuration http://www.azom.com/article.aspx?ArticleID=5573 132. Charlie Chong/ Fion Zhang Benefits of Thin Film RTD There are many options when considering contact temperature measurement, including thermocouples, thermistors, and RTD (wire wound and thin film). While thermocouples can handle very high temperatures and thermistors are inexpensive, there are many advantages of RTD. Some of these advantages include their accuracy, precision, long-term stability, and good hysteresis characteristics. Even beyond these, there are advantages of thin film RTD over wire wound, including smaller dimensions, better response times, vibration resistance, and relative inexpensiveness. New advancements has even made thin film technology just as accurate as wire wound at higher temperatures ranges. http://www.azom.com/article.aspx?ArticleID=5573 133. Charlie Chong/ Fion Zhang Thermistor A thermistor is a type of resistor whose resistance varies significantly with temperature, more so than in standard resistors. The word is a portmanteau of thermal and resistor. Thermistors are widely used as inrush current limiter, temperature sensors (NTC type typically), self-resetting overcurrent protectors, and self-regulating heating elements. Thermistors differ from resistance temperature detectors (RTDs) in that the material used in a thermistor is generally a ceramic or polymer, while RTDs use pure metals. The temperature response is also different; RTDs are useful over larger temperature ranges, while thermistors typically achieve a higher precision within a limited temperature range, typically 90 C to 130 C http://en.wikipedia.org/wiki/Thermistor 134. Charlie Chong/ Fion Zhang Thermistor http://swordrock.wordpress.com/category/robotic-2/ 135. Charlie Chong/ Fion Zhang Thermistor http://en.wikipedia.org/wiki/Thermistor 136. Charlie Chong/ Fion Zhang 3.2 Contacting Thermal Measuring Devices The most commonly used contacting devices include bimetallic thermometers, thermochromic liquid crystals, thermocouples, resistance thermometer, thermistors and heat flux indicators. These devices are discussed briefly here. For more detailed information, refer to ASNT Nondestructive Testing Handbook. third edition: Volume 3. Infrared and Thermal Testing. Bimetallic Thermometers Bimetallic thermometers are sensors constructed of dissimilar metallic strips bonded together. Typically. different iron nickel alloys are used. The strips differ in temperature coefficient of expansion such that temperature changes result in predictable bending of the assembly. Arranged in a spiral or helical configuration. one end of the bimetallic element is fixed and the other end is attached to a pointer. Properly calibrated, the angular position of the pointer can be made to indicate temperature on a scale. 137. Charlie Chong/ Fion Zhang Thermochromic Liquid Crystals Thermochromic liquid crystals (also called cholesterol crystals) change color with temperature. Coatings made of liquid crystals are commonly used as temperature threshold indicators. Depending on the mixture. a coating applied to a surface will change color predictably when the surface exceeds a threshold temperature. The color change may be reversible or irreversible. and the sensing range for most mixtures is limited to a narrow temperature span. Typically. a set of liquid crystal markers provides a selection of transition temperatures. This allows the user to select the appropriate marker for the desired temperature. Keywords: Threshold temperature 138. Charlie Chong/ Fion Zhang Thermocouple Thermocouples are contact temperature sensors based on the thermoelectric effect. or Seebeck effect. Thomas Seebeck discovered that, when two dissimilar metals arc joined at both ends and these ends are at different temperatures, a predictable direct current will flow through the circuit. The thermoelectric coefficient determines the relationship between this current and the temperature difference between the two junctions. This coefficient is known for each type of thermocouple. To configure a thermometer. the circuit is broken and the open-circuit voltage is measured by a volt meter. One of the two junctions is then held al a reference temperature. such as an ice bath, and the voltage is calibrated to indicate the temperature of the other junction. which then becomes the temperature sensing junction. Thermopiles arc banks of thermocouples connected in parallel or in series to increase output gradient. The reference temperature is important because of the thermocouples' non linear response. Keywords: thermoelectric coefficient 139. Charlie Chong/ Fion Zhang Resistance Thermometers Resistance temperature detector (RTDs) arc contact sensors thaI measure tcmpcralUrc by a predictable change in resistance as a function of temperature. Platinum is the most popular resistance temperature detector material because of its excellent stability and its linear response to temperature change. Other materials used include nickel. copper. tungsten and iridium. In operation. the resistance temperature detector may be placed in a bridge circuit such that the bridge output voltage is a measure of the resistance and hence the temperature at the resistance temperature detector. A more accurate measurement may be achieved by using a constant current source and a digital volt meter (DVM). such that the digital volt meter reading is proportional to the resistance temperature detector resistance and hence the temperature at the resistance temperature detector. 140. Charlie Chong/ Fion Zhang Thermistors Thermistors arc also sensors that measure temperature by a predictable change in resistance as a fun ction of temperature. Thermistors are made of semiconductor materials. Whereas resistance temperature detectors are low impedance devices. thennistors are high impedance devices. Thermistors typically are more sensitive to temperature changes than resistance temperature detectors but thermistors are not as stable. Keywords: Thermistors typically are more sensitive to temperature changes than resistance temperature detectors 141. Charlie Chong/ Fion Zhang Heat Flux Indicators Heat flux indicators are heat flow meters and are used to measure rates in conduction, convection, radiation and phase change systems such as building walls, boiler tubes and air conditioning ducts. A typical heat flux indicator consists of a sensitive thermopile, composed of many fine gage thermocouples connected in series on opposite sides of a nat core wilh known and stable thermal resistance. The entire assembly is covered with protective material. The voltage generated across the thermopile is calibrated to be a measure of the steady state heat flux through the device. Transient heat flux can be related to the transient thermopile output and the geometry of the device. 142. Charlie Chong/ Fion Zhang 3.3 Optical Pyrometers Optical pyrometers include brightness pyrometers and infrared pyrometers. Infrared pyrometers are also called infrared radiation themlometers. Various types are discussed in the next section. Brightness pyrometers are also called matching pyrometers. They incorporate a calibrated light source (lamp) powered by a calibrated current supply. Looking through a viewer. the operator matches the brightness of the target to be measured with the brightness of the calibrated lamp. The adjustment control is cal ibrated in temperature units. such that when the brightnesses arc matched, the control indicates the temperature of the target to be measured. 143. Charlie Chong/ Fion Zhang Pyrometer A pyrometer is a device that is used for the temperature measurement of an object. The device actually tracks and measures the amount of heat that is radiated from an object. The thermal heat radiates from the object to the optical system present inside the pyrometer. The optical system makes the thermal radiation into a better focus and passes it to the detector. The output of the detector will be related to the input thermal radiation. The biggest advantage of this device is that, unlike a Resistance Temperature Detector (RTD) and Thermocouple, there is no direct contact between the pyrometer and the object whose temperature is to be found out. Optical (brightness) Pyrometer In an optical pyrometer, a brightness comparison is made to measure the temperature. As a measure of the reference temperature, a color change with the growth in temperature is taken. The device compares the brightness produced by the radiation of the object whose temperature is to be measured, with that of a reference temperature. The reference temperature is produced by a lamp whose brightness can be adjusted till its intensity becomes equal to the brightness of the source object. For an object, its light intensity always depends on the temperature of the object, whatever may be its wavelength. After adjusting the temperature, the current passing through it is measured using a multimeter, as its value will be proportional to the temperature of the source when calibrated. The working of an optical pyrometer is shown in the figure below. http://www.instrumentationtoday.com/optical-pyrometer/2011/08/ 144. Charlie Chong/ Fion Zhang Pyrometer A pyrometer is a type of remote sensing thermometer used to measure temperature. Various forms of pyrometers have historically existed. In the modern usage, it is a non-contacting device that intercepts and measures thermal radiation, a process known as pyrometry and sometimes radiometry. The thermal radiation can be used to determine the temperature of an object's surface. The word pyrometer comes from the Greek word for fire, "" (pyro), and meter, meaning to measure. The word pyrometer was originally coined to denote a device capable of measuring the temperature of an object by its incandescence, or the light that is emitted by the body as caused by its high temperature. Modern pyrometers are capable of interpreting temperatures of room temperature objects by measuring radiation flux in the infrared spectrum. A modern pyrometer has an optical system and a detector. The optical system focuses the thermal radiation onto the detector. The output signal of the detector (temperature T) is related to the thermal radiation or irradiance j* of the target object through the StefanBoltzmann law, the constant of proportionality , called the Stefan-Boltzmann constant and the emissivity of the object. J* = T4 This output is used to infer the object's temperature. Thus, there is no need for direct contact between the pyrometer and the object, as there is with thermocouples and resistance temperature detectors (RTDs). http://en.wikipedia.org/wiki/Pyrometer 145. Charlie Chong/ Fion Zhang Brightness Pyrometers http://www.instrumentationtoday.com/optical-pyrometer/2011/08/ 146. Charlie Chong/ Fion Zhang Brightness Pyrometers Wiens Law http://www.instrumentationtoday.com/optical-pyrometer/2011/08/ 147. Charlie Chong/ Fion Zhang 3.4 Basic Configurations of Infrared Radiation Sensing and Imaging Instruments In terms of configuration and operation. most thermal imagers are considered to be extensions of radiation thermometers or radiation thermometers plus scanning optics. The performance parameters of thermal imagers are extensions of the performance parameters of radiation thermometers. To aid comprehension. the basic measurement problem is discussed in this chapter in terms of the measurement of a single point. It is then expanded to cover thermal scanning and imaging. Figure 3.1 illustrates the basic configuration of an infrared sensing instrument (infrared radiation thermometer), showing the components necessary to make measurements. Collecting optics (an infrared lens, for example) arc necessary for gathering the energy emitted by the target spot and focusing this energy onto the sensitive surface of an infrared detector. 148. Charlie Chong/ Fion Zhang The processing electronics unit amplifies and conditions the signal from the infrared detector and introduces corrections for such factors as detector ambient temperature drift and target effective surface emissivity. Generally. a readout. such as a meter. indicates the target temperature and an analog output is provided. The output signal is used to record, display. alarm, control, correct or any combination of these. 149. Charlie Chong/ Fion Zhang Figure 3.1: Basic configuration of an infrared radiation thermometer 150. Charlie Chong/ Fion Zhang Infrared Detector An infrared detector is at the heart of every infrared sensing and imaging instrument. whatever its configuration. Infrared detectors can sense infrared radiant energy and produce useful electrical signals proportional to the temperature of target surfaces. Instruments using infrared detectors and optics to gather and focus energy from the targets onto these detectors are capable of measuring target surface temperatures with sensitivities better than 0.10 C (0.18 F). and with response limes in the microsecond (s) range. An instrument that measures the temperature of a spot on a target in this manner is called an infra red radiation thermometer. An instrument that combines this measurement capability with a means or mechanism for scanning the target surface is called an infrared thermal imager. It can produce thermal maps, or thermograms, where the brightness intensity or color hue of any spot on the map represents the apparent temperature of the surface at that point. 151. Charlie Chong/ Fion Zhang Figure 3.2 illustrates the spectral responses of various infrared radiation detectors. Radiant energy impinging on their sensitive surfaces causes all infrared detectors to respond with some kind of electrical change. This may be an impedance change. a capacitance change, the generation of an electromotive force (emf) known as Voltage, or the release of photons, depending on the type of detector. Infrared detectors are divided into (1) thermal detectors and (2) photon detectors. Thermal detectors have broad uniform spectral responses, somewhat lower sensitivities and slower response times (measured in millisecond): photon detectors (also called photo detectors) have limited spectral responses. higher peak sensitivities and faster response times (measured in microsecond). Thermal detectors usually operate at or near room temperature. whereas photon detectors are usually cooled to optimize performance. Keywords: Thermal Detector- broad uniform spectral responses/ slower Photon Detector- limited spectral responses/ faster 152. Charlie Chong/ Fion Zhang Figure 3.2: Response Curves of Various Infrared Detectors 153. Charlie Chong/ Fion Zhang Discussion Subject: Why (or How) there are 2 MCT; MCT(215K), MCT(77K)? 154. Charlie Chong/ Fion Zhang The mercury cadmium telluride (HgCdTe) detectors shown in Figure 3.2 are photon detectors cooled to 77 K (-321 F) for operation from 8 to 12 m and to 195 K (-109 F) for operation from 3 to 5 m. Because of their fast response, these detectors are used extensively in high speed scanning and imaging applications. In contrast to the mercury cadmium telluride detector, the radiation thermopile shown in Figure 3.2, is a broad band thermal detector operating uncooled. It is used extensively for spot measurements. Because it generates a direct current electromotive force proportional to the radiant energy reaching its surface. it is ideal for use in portable, battery powered instruments. The lead sulfide (PbS) detector is typical of those used in radiation thermometers that measure and control the temperature of very hot targets. Its peak sensitivity at 3m matches the peak energy emitted by a 1000K (727 C = 1340 F) graybody. Because of the atmospheric absorption considerations previously discussed. most infrared thermal imagers operate in either the 3 to 5 m or the 8 to 12 m spectral region. Note: 195K = [(-273+195) x 9/5] + 32 = -108 F 155. Charlie Chong/ Fion Zhang Figure 3.2: Response Curves of Various Infrared Detectors Photon Detectors Indium Antimony 156. Charlie Chong/ Fion Zhang Infrared Optics - Lenses, Mirrors and Filters There are two types of infrared optics; (1) refractive (lenses. filters, windows) and (3) reflective (mirrors). Refractive optics transmit infrared wavelengths of interest. When used for higher temperature applications. their throughput losses can usually be ignored. When used in low temperature measurement instruments and imagers, absorption is often substantial and must be considered when making accurate measurements. Reflective optics. which are more efficient are not spectrally selective and somewhat complicate the optical path. Reflective optics are used more often for low temperature applications. where the energy levels cannot warrant throughput energy losses. When an infrared radiation thermometer is aimed at a target, energy is collected by the optics in the shape of a solid angle determined by the configuration of the optics and the detector. 157. Charlie Chong/ Fion Zhang The cross section of this collecting beam is called the field of view (FOV) of the instrument and it detennines the size of the area (spot size) on the target surface that is measured by the instrument at any given working distance. On scanning and imaging instruments this is called the instantaneous field of view (lFOV) and becomes one picture element on the thermogram. An infrared interference filter is often placed in front of the detector to limit the spectral range of the energy reaching the detector. The reasons for spectral selectivity will be discussed later in this chapter. Processing Electronics The processing electronics unit amplifies and conditions the signal from the infrared detector and introduces corrections for factors such as detector ambient temperature drift and effective target surface emissivity. In radiation thermometers, a meter is usually provided to indicate the targets apparent temperature. An analog or digital output signal is provided to record, display, alarm, control, correct or any combination of these. 158. Charlie Chong/ Fion Zhang Field of View (FOV) A field of view (FOV) is a specification that defines the size of what is seen in the thermal image. The lens has the greatest influence on what the FOV will be, regardless of the size of the array. Large arrays, however, provide greater detail, regardless of the lens used, compared to narrow arrays. For some applications, such as work in outdoor substations or inside a building, a large FOV is useful. While smaller arrays may provide sufficient detail in a building, more detail is important in substation work. See Figure 4-7. 159. Charlie Chong/ Fion Zhang Figure 4-7. The field of view (FOV) is a specification that defines the area that is seen in the thermal image when using a specific lens. 160. Charlie Chong/ Fion Zhang What is IFOV? A measure of the spatial resolution of a remote sensing imaging system. Defined as the angle subtended by a single detector element on the axis of the optical system. IFOV has the following attributes: Solid angle through which a detector is sensitive to radiation. The IFOV and the distance from the target determines the spatial resolution. A low altitude imaging instrument will have a higher spatial resolution than a higher altitude instrument with the same IFOV http://www.ssec.wisc.edu/sose/tutor/ifov/define.html 161. Charlie Chong/ Fion Zhang What is IFOV? IFOV (instantaneous field of view) smallest object detectable The IFOV (instantaneous field of view), also known as IFOVgeo (geometric resolution), is the measure of the ability of the detector to resolve detail in conjunction with the objective. Geometric resolution is represented by mrad and defines the smallest object that can be represented in the image of the display, depending on the measuring distance. The thermography, the size of this object corresponds to a pixel. The value represented by mrad corresponds to the size of the visible point [mm] a pixel at a distance of 1 m. http://www.academiatesto.com.ar/cms/?q=ifov 162. Charlie Chong/ Fion Zhang Instantaneous Field of View (IFOV) An instantaneous field of view (IFOV) is a specification used to describe the capability of a thermal imager to resolve spatial detail (spatial resolution). The IFOV is typically specified as an angle in milliradians (mRad). When projected from the detector through the lens, the IFOV gives the size of an object that can be seen at a given distance. An IFOV measurement is the measurement resolution of a thermal imager that describes the smallest size object that can be measured at a given distance. See Figure 4-8. It is specified as an angle (in mRad) but is typically larger by a factor of three than the IFOV. This is due to the fact that the imager requires more information about the radiation of a target to measure it than it does to detect it. It is vital to understand and work within the spatial and measurement resolution specific to each system. Failure to do so can lead to inaccurate data or overlooked findings. H D in meter IFOV, in milli-radian H in mm = D 163. Charlie Chong/ Fion Zhang Figure 4-8. An IFOV measurement is the measurement resolution of a thermal imager that describes the smallest size object that can be measured at a given distance. IFOV is similar to seeing a sign in the distance while IFOV measurement is similar to reading the sign, either because it is closer or larger. Instantaneous field of view