understanding infrared thermography-note 1

Infrared Thermal TestingHB-NDEv-C9Thermal Infrared TestingMy ASNT Level III Pre-Exam Preparatory Self Study Notes 20th April 2015

Charlie Chong/ Fion Zhang

Infrared Thermography


Petrochemical Applications


IR Thermography


IR Thermography

Charlie Chong/ Fion Zhang http://www.thehumansolution.com/flir-e-series-infrared-cameras.html

IR Thermography Military Application


IR Thermography - Surveillances


IR Thermography - Rescue


IR Thermography - Syrveillance


IR Thermography - Military

Charlie Chong/ Fion Zhang http://www.flir.com/surveillance/display/?id=64958



https://www.youtube.com/embed/BE661hmLjtU

https://www.youtube.com/watch?v=BE661hmLjtU

IR Thermography A FLIR pod on a French Air Force helicopter.


IR Thermography MilitaryThe Thales NAVFLIR / DAMOCLES (forward-looking infrared day / night vision and target designator). Developing advanced electronic vision systems offers employment opportunities for a variety of related disciplines.

Charlie Chong/ Fion Zhang http://en.wikipedia.org/wiki/NAVFLIR

Combat Night Vision Gaggles - Not IR Thermography



Charlie Chong/ Fion Zhang http://www.physics.ohio-state.edu/~wilkins/writing/Samples/shortmed/johnmedium/index.html

Night Vision Non-IR Light Enhancement Camera

Charlie Chong/ Fion Zhang http://148.243.99.22/CCTV/ARECONTVISION/AV%20Images%20&%20Videos/Arecont%20Vision%20Images/AV%20M-series/av3130/

http://www.arecontvision.com/

Fion Zhang at Shanghai20th May 2015

http://meilishouxihu.blog.163.com/


Charlie Chong/ Fion Zhang http://greekhouseoffonts.com/


Greek letter


IVONA TTS Capable.

http://www.naturalreaders.com/


Video on - Introduction to Infrared Thermography


https://www.youtube.com/watch?v=eH0hsh5Nfug


Video on - Infrared Thermography and Building Efficiency - presentation by Matthew Knights of FLIR systems


https://www.youtube.com/embed/4uhz1rIaMj0


Video on - Introduction to Infrared Thermography


https://www.youtube.com/watch?v=9TCK1Sa0_Vc


CHAPTER 9THERMAL INFRAREDTESTING

Charlie Chong/ Fion Zhang Handbook of nondestructive evaluation


1. HISTORY AND DEVELOPMENTHumans have always been able to detect infrared radiation. The nerve endings in human skin can respond to temperature differences of as little as 0.0162F (0.009C). Although extremely sensitive, nerve endings are poorly designed for thermal nondestructive evaluation. Even if humans had the thermal capabilities of a pit viper, which can find its warmblooded prey in the dark, it is most probable that better heat detection tools would still be needed. Thus, as inventive beings, we have turned to mechanical and electronic devices to allow us to become hypersensitive to heat. These devices, some of which can produce thermal images, have proved invaluable for thermal inspection in countless applications. Sir William Herschel, who in 1800 was experimenting with light, is generally credited with the beginnings of thermography. Using a prism to break sunlight into its various colors, he measured the temperature of each color using a very sensitive mercury thermometer. Much to his surprise, the temperature increased when he moved out beyond red light into an area he came to term the dark heat. This is the region of the electromagnetic spectrum referred to as infrared and recognized as the electromagnetic radiation that when absorbed causes a material to increase in temperature.



Twenty years later, Seebeck discovered the thermoelectric effect, which quickly lead to the invention of the thermocouple by Nobili in 1829. Thissimple contact device is based on the premise that there is an emf(electromotive force) or voltage that occurs when two dissimilar metals comein contact, and that this response changes in a predictable manner with achange in temperature. Melloni soon refined the thermocouple into athermopile (a series arrangement of thermocouples) and focused thermalradiation on it in such a way that he could detect a person 30 feet away. Asimilar device, called a bolometer, was invented 40 years later. Rather thanmeasuring a voltage difference, a bolometer measured a change in electricalresistance related to temperature. In 1880 Longley and Abbot used abolometer to detect a cow over 1,000 feet away!



Herschels son, Sir John, using a device called an evaporograph, producedthe first infrared image, crude as it was, in 1840. The thermal image wascaused by the differential evaporation of a thin film of oil. The image wasviewed by light reflecting off the oil film. During World War I, Case becamethe first to experiment with photoconducting detectors (thallium sulfide) thatproduced signals not as a result of being heated, but by their direct interactionwith photons. The result was a faster, more sensitive detector. During WorldWar II, the technology began to expand and resulted in a number of militaryapplications and developments. The discovery by German scientists thatcooling the detector increased performance was instrumental in the rapidexpansion of the infrared technology. It was not until the 1960s that infraredthermal imaging began to be used for nonmilitary applications. Althoughsystems were cumbersome, slow to acquire data, and had poor resolution, they proved useful for the inspection of electrical systems. Continued advances in the 1970s, again driven mainly by the military, produced the first portable systems usable for thermal nondestructive testing (TNDT).



These systems, utilizing a cooled scanned detector, proved both rugged and reliable. However, the quality of the image, although often adequate, waspoor by todays standards. By the end of the decade, infrared was beingwidely used in mainstream industry, for building inspections, and for a varietyof medical applications. It became practical to calibrate systems and producefully radiometric images, meaning that radiometric temperatures could bemeasured throughout the image. Adequate detector cooling, which hadpreviously been accomplished through cryogenicsusing either compressedor liquefied gaseswas now done much more conveniently usingthermoelectric coolers. Less expensive tube-based pyroelectric vidicon (PEV)imaging system were also developed and produced. Although not radiometric,PEVs were lightweight, portable and could operate without cooling. In the late1980s, a new technology, the focal plane array (FPA), was released from themilitary into the commercial marketplace. The FPA employs a large array ofthermally sensitive semiconductor detectors, similar to those in chargecoupled device (CCD) visual cameras. This was a significant improvementover the single-element, scanned detector and the result was a dramaticincrease in image quality and spatial resolution. Arrays of 320 240 and 256 256 elements are now the norm, and for specialized applications, arrays are available with densities up to 1000 1000.



Development of the FPA technology has exploded in the last decade. Both long-and short-wave sensing systems are now available in fully radiometric versions, as are systems with data capture rates as high as 500 frames per second and sensitivities of 0.18F (0.1C) or less. While the cost of radiometric systems has not dropped significantly, the quality has increased dramatically. The digital storage of data has allowed 12 and 14-bit digital files; the result is access to far more image and radiometric detail than ever imagined. Figure 9-1, taken with a typical FPA system, shows the clarity of detail available and its appeal for specialized work like this biological study of thermoregulation in ravens. (Because this book is printed with black ink only, much of the information conveyed by the original color images has been lost. Interested readers may obtain the originalimages from the authors website, www.snellinfrared.com.) Although not radiometric, pyroelectric FPA systems have also been developed that provide excellent imagery at prices below $15,000. Costs have dropped so much that afixed-lens pyroelectric system is now being installed in some model year 2001, luxury automobiles to enhance the safety of driving at night. Concurrently, the use of computers and image processing software has grown tremendously. Nearly all systems commercially available today offer software programs that facilitateanalysis and report writing. Reports can be stored digitally and sent electronicallyover the Internet.



FIGURE 9-1.



With so many advances being made so rapidly, it is difficult to imagine what is next. Certainly, we will see continued development of new detector systems,most notably the quantum well integrated processing (QWIP) systems, with apromise of higher data capture speeds and greater sensitivities. Tunablesensors capable of capturing and processing images at several wavelengthswill also become commercially available. Probably the greatest area of growthwill be in the development of automated systems in which the infraredinstrument, combined with a data processing computer, will capture andanalyze data and provide control feedback to a process or system beingmonitored. Systems are already in use for certain processes in the cement,metals, and chemical industries and have significantly reduced failures andcosts, while dramatically increasing quality and profits.



As equipment has improved, so has the need to qualify inspection personnel and standardize inspection methodologies. In the late 1980s, members of the American Society for Nondestructive Testing (ASNT) met to discuss a strategy for having infrared adopted as a standard test method. ASNT, a volunteer professional organization, is responsible for developing personnel qualification programs. Certification, which is conducted by the employer, is based on the practitioner having had the required training and experience, as well as passing written and practical examinations.

In 1992, infrared testing was officially adopted by ASNT and thermographers could for the first time be certified to a widely used, professionally recognized standard. Since that time, a number of large companies in the United States have begun certifying their thermographers in compliance with ASNT recommendations.



The development of inspection standards over the past two decades has also been an interesting process. The American Society of Heating, Refrigeration,and Air Conditioning Engineers (ASHRAE) developed a building inspectionstandard, which was later modified and adopted by the InternationalStandards Organization (ISO). The American Society for Testing andMaterials (ASTM) also developed several standards used for determining theperformance of infrared systems, as well as standards for roof moisture andbridge deck inspections. Concurrently, a number of relevant ASTM standardswere developed for radiation thermometry, which also has application toinfrared thermography.



The National Fire Protection Association (NFPA) has developed a standard, NFPA 70-B - the maintenance of electrical systems, which incorporates the use of thermography. Even though inspection standards continue to be developed, there are many applications at this time that lack inspection standards. Without standards, inspection results may lack repeatability, which often leads to misleading, perhaps even dangerous, analysis of data.

Access to the technology has become easy as the value of products has improved. Companies investing in developing solid programs, including inspection procedures and qualified personnel, have a distinct advantage due to access to the remarkable long-term benefits provided by infrared.



Keywords: Radiometric ( as contrast with photometric) Less expensive tube-based pyroelectric vidicon (PEV) imaging system focal plane array (FPA) charge coupled device (CCD) visual cameras. quantum well integrated processing (QWIP) systems



Radiometry is a set of techniques for measuring electromagnetic radiation,including visible light. Radiometric techniques in optics characterize the distribution of the radiation's power in space, as opposed to photometric techniques, which characterize the light's interaction with the human eye. Radiometry is distinct from quantum techniques such as photon counting.

The use of radiometers to determine the temperature of objects and gasses by measuring radiation flux is called pyrometry. Handheld pyrometer devices are often marketed as infrared thermometers. Radiometry is important in astronomy, especially radio astronomy, and plays a significant role in Earth remote sensing. The measurement techniques categorized as radiometry in optics are called photometry in some astronomical applications, contrary to the optics usage of the term. Spectroradiometry is the measurement of absolute radiometric quantities in narrow bands of wavelength

Charlie Chong/ Fion Zhang http://en.wikipedia.org/wiki/Radiometry


Vidicon A vidicon tube is a video camera tube design in which the target material is a photoconductor. The Vidicon was developed in the 1950s at RCA by P. K. Weimer, S. V. Forgue and R. R. Goodrich as a simple alternative to the structurally and electrically complex Image Orthicon. While the initial photoconductor used was selenium, other targetsincluding silicon diode arrayshave been used.

The vidicon is a storage-type camera tube in which a charge-density pattern is formed by the imaged scene radiation on a photoconductive surface which is then scanned by a beam of low-velocity electrons. The fluctuating voltage coupled out to a video amplifier can be used to reproduce the scene being imaged. The electrical charge produced by an image will remain in the face plate until it is scanned or until the charge dissipates. Pyroelectric photocathodes can be used to produce a vidiconsensitive over a broad portion of the infrared spectrum.

Prior to the design and construction of the Galileo probe to Jupiter in the late 1970s to early 1980s, NASA used Vidicon cameras on most of their unmanned deep space probes equipped with the remote sensing ability.

Vidicon tubes were popular in 1970's and 1980's after which they were rendered obsolete by CCD and CMOS sensors.



Schematic of vidicon tube.

Charlie Chong/ Fion Zhang http://en.wikipedia.org/wiki/Video_camera_tube#Vidicon


focal-plane array (FPA)A staring array, staring-plane array, focal-plane array (FPA), or focal-plane is an image sensing device consisting of an array (typically rectangular) of light-sensing pixels at the focal plane of a lens. FPAs are used most commonly for imaging purposes (e.g. taking pictures or video imagery), but can also be used for non-imaging purposes such as spectrometry, lidar, and wave-front sensing.

In radio astronomy the term "FPA" refers to an array at the focus of a radio-telescope (see full article on Focal Plane Arrays). At optical and infrared wavelengths it can refer to a variety of imaging device types, but in common usage it refers to two-dimensional devices that are sensitive in the infrared spectrum. Devices sensitive in other spectra are usually referred to by other terms, such as CCD (charge-coupled device) and CMOS image sensor in the visible spectrum. FPAs operate by detecting photons at particular wavelengths and then generating an electrical charge, voltage, or resistance in relation to the number of photons detected at each pixel. This charge, voltage, or resistance is then measured, digitized, and used to construct an image of the object, scene, or phenomenon that emitted the photons.

Applications for infrared FPAs include missile or related weapons guidance sensors, infrared astronomy, manufacturing inspection, thermal imaging for firefighting, medical imaging, and infrared phenomenology (such as observing combustion, weapon impact, rocket motor ignition and other events that are interesting in the infrared spectrum).

Charlie Chong/ Fion Zhang http://en.wikipedia.org/wiki/Staring_array


A charge-coupled device (CCD) is a device for the movement of electrical charge, usually from within the device to an area where the charge can be manipulated, for example conversion into a digital value. This is achieved by "shifting" the signals between stages within the device one at a time. CCDsmove charge between capacitive bins in the device, with the shift allowing for the transfer of charge between bins.

The CCD is a major piece of technology in digital imaging. In a CCD image sensor, pixels are represented by p-doped MOS capacitors. These capacitors are biased above the threshold for inversion when image acquisition begins, allowing the conversion of incoming photons into electron charges at the semiconductor-oxide interface; the CCD is then used to read out these charges. Although CCDs are not the only technology to allow for light detection, CCD image sensors are widely used in professional, medical, and scientific applications where high-quality image data is required. In applications with less exacting quality demands, such as consumer and professional digital cameras, active pixel sensors (CMOS) are generally used; the large quality advantage CCDs enjoyed early on has narrowed over time.

Charlie Chong/ Fion Zhang http://en.wikipedia.org/wiki/Charge-coupled_device


CMOSComplementary metaloxidesemiconductor (CMOS) is a technology for constructing integratedcircuits. CMOS technology is used in microprocessors, microcontrollers, static RAM, and other digital logic circuits. CMOS technology is also used for several analog circuits such as image sensors (CMOS sensor), data converters, and highly integrated transceivers for many types of communication. In 1963, while working for Fairchild Semiconductor, Frank Wanlass patented CMOS (US patent 3,356,858).

CMOS is also sometimes referred to as complementary-symmetry metaloxidesemiconductor (or COS-MOS). The words "complementary-symmetry" refer to the fact that the typical design style with CMOS uses complementary and symmetrical pairs of p-type and n-type metal oxide semiconductor field effect transistors (MOSFETs) for logic functions.

Two important characteristics of CMOS devices are high noise immunity and low static power consumption. Since one transistor of the pair is always off, the series combination draws significant power only momentarily during switching between on and off states. Consequently, CMOS devices do not produce as much waste heat as other forms of logic, for example transistortransistor logic (TTL) or NMOS logic, which normally have some standing current even when not changing state. CMOS also allows a high density of logic functions on a chip. It was primarily for this reason that CMOS became the most used technology to be implemented in VLSI chips.

The phrase "metaloxidesemiconductor" is a reference to the physical structure of certain field-effect transistors, having a metal gate electrode placed on top of an oxide insulator, which in turn is on top of a semiconductor material. Aluminium was once used but now the material is polysilicon. Other metal gates have made a comeback with the advent of high-k dielectric materials in the CMOS process, as announced by IBM and Intel for the 45 nanometer node and beyond

Charlie Chong/ Fion Zhang http://en.wikipedia.org/wiki/CMOS


Quantum well integrated processing (QWIP) systemsA quantum well is a potential well with only discrete energy values.One technology to create a quantum well is to confine particles, which were originally free to move in three dimensions, to two dimensions, by forcing them to occupy a planar region. The effects of quantum confinement take place when the quantum well thickness becomes comparable to the de Broglie wavelength of the carriers (generally electrons and holes), leading to energy levels called "energy subbands", i.e., the carriers can only have discrete energy values.



2. THEORY AND PRINCIPLESTo use todays infrared equipment, it is essential to understand the basics of both heat transfer and radiation physics. As powerful as modern equipment is,for the most part, it still cannot think for itself. Its value, therefore, depends onthe thermographers ability to interpret the data, which requires a practicalunderstanding of the basics.



Electromagnetic Radiation - Light


https://vimeo.com/51901088


Thermal EnergyEnergy can be changed from one form to another. For instance, a car engine converts the chemical energy of gasoline to thermal energy. That, in turn,produces mechanical energy, as well as electrical energy for lights or ignition,and heat energy for the defroster or air conditioner. During these conversions,although the energy becomes more difficult to harness, none of it is lost. This is the First Law of Thermodynamics. A byproduct of nearly all energy conversions is heat or thermal energy. When there is a temperature difference between two objects, or when an object is changing temperature, heat energy is transferred from the warmer areas to the cooler areas until thermal equilibrium is reached. This is the Second Law of Thermodynamics. A transfer of heat energy results either in (1) electron transfer or (2) increased atomic or molecular vibration.

Heat energy can be transferred by any of three modes: conduction, convection, or radiation. Heat transfer by conduction occurs primarily in solids, and to some extent in fluids, as warmer molecules transfer their energy directly to cooler, adjacent ones. Convection takes place in fluids and involves the mass movement of molecules. Radiation is the transfer of energy between objects by electromagnetic radiation. Because it needs no transfer medium, it can take place even in a vacuum.



Heat energy can be transferred by any of three modes: conduction, convection, or radiation.



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 samedirection over time. A fully warmed-up machine under constant load transfersheat at a steady-state rate to its surroundings. In reality, there is no such thingas true steady-state heat flow! Although we often ignore them, there arealways small transient fluctuations. A more accurate term is quasi-steady state heat transfer. When heat transfer and temperatures are constantly and significantly changing with time, heat flow is said to be transient. A machine warming up or cooling down is an example. Because thermographers are often concerned with the movement of heat energy, it is vital to understand what type of heat flow is occurring in a given situation.

Heat energy is typically measured in British thermal units (Btu) or calories (c). A Btu is defined as the amount of energy needed to raise the temperature ofone pound of water one degree Fahrenheit. A calorie is the amount of heatenergy needed to raise the temperature of one gram of water one degree Celsius. One wooden kitchen match, burned entirely, gives off approximately one Btu or 252 calories of heat energy.



One wooden kitchen match, burned entirely, gives off approximately one Btu or 252 calories of heat energy.


Charlie Chong/ Fion ZhangCharlie Chong/ Fion Zhang


The units to describe the energy content of food are also termed calories.These calories are actually kilocalories (Kcal or C) and are equal to onethousand calories (c). Various devices, such as a calorimeter or a heat flowmeter can measure the flow of heat energy. These devices are not commonlyused, except by scientists in research laboratories. Temperature is a measureof the relative hotness of a material compared to some known reference.

There are many ways to measure temperature.

The most common is to use our sense of touch. We also use comparisons of various material properties, including

expansion (liquid and bimetal thermometers), a change in electrical voltage (thermocouple), and a change in electrical resistance (bolometers).

Infrared radiometers infer a temperature measurement from detected infraredradiation. (?)



Extracted From Text Answering to aforementioned Puzzle!In the history and development section, it was noted that over the years three main types of infrared imaging systems were developed: (1) scanners orscanning systems, (2) pyroelectric vidicons (PEVs), and (3) focal plane arrays(FPAs). While all three types are still in use today, the overwhelming majorityof new equipment sales are the FPA systems.

It is useful to have a general understanding of how imaging systems work. The purpose of the imager is to detect the infrared radiation given off by the object being inspected, commonly called the target. The targets radiosity is focused by the thermally transparent optics onto a detector, which is sensitive to this invisible heat radiation. A response from the detector produces a signal, usually a voltage or resistance change, which is read by the electronics of the system and creates a thermal image on a display screen, typically a video viewfinder. In this seemingly simple process, we are able, without contact, to view the thermal image, or thermogram, that corresponds to the thermal energy coming off the surface of the target.



Regardless of how heat energy is transferred, thermographers must understand that materials also change temperatures at different rates due totheir thermal capacitance. Some materials, like water, heat up and cool downslowly, while others, like air, change temperature quite rapidly. The thermalcapacitance or specific heat of a material describes this rate of change.

Without an understanding of these concepts and values, thermographers willnot be able to properly interpret their findings, especially with regard totransient heat flow situations. Although potentially confusing, these propertiescan also be used to our advantage. Finding liquid levels in tanks, for example, is possible because of the differences between the thermal capacitance of the air and the liquid.

Keywords:The thermal capacitance or specific heat of a material describes this rate of change.



Heat capacity, or thermal capacity, is a measurable physical quantity equal to the ratio of the heat added to (or removed from) an object to the resulting temperature change. The SI unit of heat capacity is joule per Kelvin J/(K) [J/(KgK)]? and the dimensional form is L2T2 M1. Specific heat is the amount of heat needed to raise the temperature of a certain mass 1 degree Celsius.

Heat capacity is an extensive property of matter, meaning it is proportional to the size of the system. When expressing the same phenomenon as an intensive property, the heat capacity is divided by the amount of substance, mass, or volume, so that the quantity is independent of the size or extent of the sample. The molar heat capacity is the heat capacity per unit amount (SI unit: mole) of a pure substance and the specific heat capacity, often simply called specific heat, is the heat capacity per unit mass of a material. Occasionally, in engineering contexts, the volumetric heat capacity is used.

Charlie Chong/ Fion Zhang http://en.wikipedia.org/wiki/Heat_capacity


Temperature reflects the average randomized kinetic energy of constituent particles of matter (e.g. atoms or molecules) relative to the centre of mass of the system, while heat is the transfer of energy across a system boundary into the body other than by work or matter transfer. Translation, rotation, and a combination of the two types of energy in vibration (kinetic and potential) of atoms represent the degrees of freedom of motion which classically contribute to the heat capacity of matter, but loosely bound electrons may also participate. On a microscopic scale, each system particle absorbs thermal energy among the few degrees of freedom available to it, and at sufficient temperatures, this process contributes to the specific heat capacity that classically approaches a value per mole of particles that is set by the DulongPetit law. This limit, which is about 25 joules per Kelvin for each mole of atoms, is achieved by many solid substances at room temperature.

For quantum mechanical reasons, at any given temperature, some of these degrees of freedom may be unavailable, or only partially available, to store thermal energy. In such cases, the specific heat capacity is a fraction of the maximum. As the temperature approaches absolute zero, the specific heat capacity of a system also approaches zero, due to loss of available degrees of freedom. Quantum theory can be used to quantitatively predict the specific heat capacity of simple systems.



An object's heat capacity (symbol C) is defined as the ratio of the amount of heat energy transferred to an object and the resulting increase in temperature of the object,

C = Q/Tassuming that the temperature range is sufficiently small so that the heat capacity is constant. More generally, because heat capacity does depend upon temperature, it should be written as:

C (T) = Q/dTwhere the symbol is used to imply that heat is a path function. In the International System of Units, heat capacity has the unit joules per Kelvin.

Heat capacity is an extensive property, meaning it depends on the extent or size of the physical system in question. A sample containing twice the amount of substance as another sample requires the transfer of twice the amount of heat (Q ) to achieve the same change in temperature (T ).



For many experimental and theoretical purposes it is more convenient to report heat capacity as an intensive property an intrinsic characteristic of a particular substance. This is most often accomplished by expressing the property in relation to a unit of mass. In science and engineering, such properties are often prefixed with the term specific. International standards now recommend that specific heat capacity always refer to division by mass. The units for the specific heat capacity are [c] = J/ KgKIn chemistry, heat capacity is often specified relative to one mole, the unit of amount of substance, and is called the molar heat capacity. It has the unit [ Cmol ] = J/ MolKFor some considerations it is useful to specify the volume-specific heat capacity, commonly called volumetric heat capacity, which is the heat capacity per unit volume and has SI units [s] = J/ m3K. This is used almost exclusively for liquids and solids, since for gases it may be confused with specific heat capacity at constant volume.



Tank Level Finding liquid levels in tanks, for example, is possible because of the differences between the thermal capacitance of the air and the liquid.



Tank Level



Latent HeatAs materials change from one state or phase (solid, liquid or gas) to another, heat energy is released or absorbed. When a solid changes state to a liquid,energy is absorbed in order to break the bonds that hold it as a solid. Thesame thing is true as a liquid becomes a gas; energy must be added to breakthe bonds. As gases condense into liquids, and as liquids freeze into solids,the energy used to maintain these high-energy states is no longer neededand is released. This energy, which can be quite substantial, is called latentenergy because it does not result in the material changing temperature. Theimpact of energy released or absorbed during phase change often affectsthermographers. The temperature of a roof surface, for instance, can changevery quickly as dew or frost forms, causing problems during a roof moisturesurvey. A wet surface or a rain-soaked exterior wall will not warm up until it isdry, thus masking any subsurface thermal anomalies. On the positive side,state changes enable thermographers to see thermal phenomena, such aswhether or not solvents have been applied evenly to a surface.



Latent Heat



Latent Heat Soaked Roof Top with pools of Water



ConductionConduction is the transfer of thermal energy from one molecule or atom directly to another adjacent molecule or atom with which it is in contact. Thiscontact may be the result of physical bonding, as in solids, or a momentarycollision, as in fluids. Fouriers law describes how much heat is transferred byconduction:

Q = k (L A T) -1

whereQ = heat transferredk = thermal conductivityL = thickness of materialsA = area normal to flowT = temperature difference



The thermal conductivity (k) is the quantity of heat energy that is transferred through one square foot of a material, which is one inch thick, during one hourwhen there is a one-degree temperature difference across it. The metricequivalent (in watts) is W/(m x C) and assumes a thickness of one meter.Materials with high thermal conductivities, such as metals, are efficientconductors of heat energy. We use this characteristic to our advantage bymaking such things as cooking pans and heat sinks from metal. Differences inconductivity are the basis for many thermographic applications, especially theevaluation of flaws in composite materials or the location of insulationdamage. Materials with low thermal conductivity values, such as wool,fiberglass batting, and expanded plastic foams, do not conduct heat energyvery efficiently and are called insulators. Their insulating value is dueprimarily to the fact that they trap small pockets of air, a highly inefficientconductor.



The term R-value, or thermal resistance, is a measure of the resistance to conductive heat flow. It is defined, as the inverse of conductivity, or 1/k. R value is a term that is generally used when describing insulating materials. The following are the thermal conductivities and R-values of some common materials:

Another important material property is thermal diffusivity. Thermal diffusivity is the rate at which heat energy moves throughout the volume of a material. Diffusivity is determined by the ratio of the materials thermal conductivity to its thermal capacitance. Differences in diffusivity and consequent heat flow are the basis for many active thermography applications in TNDT (?) .



Fouriers law - The One Dimensional Heat Conduction EquationHeat conduction is one of the three modes of heat transfer. In 1822, a French physicist published a treatise giving a complete theory and mathematical model. Who was he, and which mathematical formulation is he most famously known for?

IntroductionHeat conduction is transfer of heat from a warmer to a colder object by direct contact. A famous example is shown in A Christmas Story, where Ralphiedares his friend Flick to lick a frozen flagpole, and the latter subsequently gets his tongue stuck to it.

The mathematical model was first formulated by the French physicist Jean Baptiste Joseph Fourier, he of the eponymous Fourier Series. He found that heat flux is proportional to the magnitude of a temperature gradient. His equation is called Fourier's Law.



Fourier's Law Of Heat Conduction (Joseph Fourier)In a one dimensional differential form, Fourier's Law is as follows:

1) The heat flux per unit area, q = Q/A = - kdT/dx.

The symbol q is the heat flux, which is the heat per unit area, and it is a vector. Q is the heat rate. dT/dx is the thermal gradient in the direction of the flow. The minus sign is to show that the flow of heat is from hotter to colder. If the temperature decreases with x, q will be positive and will flow in the direction of x. If the temperature increases with x, q will be negative, and will flow opposite to the direction of x. In the International System of Units SI, q is watts per meter squared (w/m2).



The constant k is the thermal conductivity, and is used to show that not all materials heat up or retain heat equally well. In SI units, k is W/m K, where W is watts, m is meters, and K is Kelvin. It may also be J/m s K, where J is joules and s is seconds. In the English system, it is Btu/h ft F, or British thermal units per horsepower foot Fahrenheit.

The thermal conductivity is larger for conductors than insulators. Silver is an excellent conductor at 428 W/m k, and so is copper with its value of 401 W/m*k. Air and wool are insulators and are poor conductors; they are 0.026 and 0.043 W/m k, respectively.The heat transfer or conduction rate is a scalar and is

2) The total heat flow, Q = -kA T/L,where L is the length of the slab. T is the temperature difference between two different surfaces.

Equations 1 and 2 show that heat can be considered to be a flow. The flow of heat depends upon the thickness of the material, the area, and the conductivity, all of which combine to retard or resist this flow.



ExampleA slab that is made from copper has a length of 10 cm and an area that is 90 cm2. The front side is heated to 150C and the back to 10C. Find the heat flux q and the heat flow rate Q in the slab once steady state is reached. Assume dT/dx is constant.

From equation 1, we have q = -kA (Tback - Tfront) / L = - (401 W/m*K) * (10C -150C)/0.1 m = 5.6 x 105 W/m2. The heat rate is Q = qA = q(90 x 10-4 m2) = 5.1 x 103 watts or J/s.



Fouriers LawAn empirical relationship between the conduction rate in a material and the temperature gradient in the direction of energy flow, first formulated by Fourier in 1822 [see Fourier (1955)] who concluded that "the heat flux resulting from thermal conduction is proportional to the magnitude of the temperature gradient and opposite to it in sign". For a unidirectional conduction process this observation may be expressed as:

q = kdT/dxwhere the vector q is the heat flux (W/m2) in the positive x-direction, dT/dx is the (negative) temperature gradient (K/m) in the direction of heat flow (i.e., conduction occurs in the direction of decreasing temperature and the minus sign confirms this thermodynamic axiom) and the proportionality constant k is the Thermal Conductivity of the material (W/mK). Fourier's Law thus provides the definition of thermal conductivity and forms the basis of many methods of determining its value. Fourier's Law, as the basic rate equation of the conduction process, when combined with the principle of conservation of energy, also forms the basis for the analysis of most Conduction problems.

Charlie Chong/ Fion Zhang http://thermopedia.com/content/781/


ConvectionHeat energy is transferred in fluids, either gases or liquids, by convection. During this process, heat is transferred by conduction from one molecule toanother and by the subsequent mixing of molecules. In natural convection,this mixing or diffusing of molecules is driven by the warmer (less dense)molecules tendency to rise and be replaced by more dense, coolermolecules.

Cool cream settling to the bottom of a cup of hot tea is a good example of natural convection. Forced convection is the result of fluid movement caused by external forces such as wind or moving air from a fan. Natural convection is quickly overcome by these forces, which dramatically affect the movement of the fluid. Figure 9-2 shows the typical, yet dramatic, pattern associated, in large part, with the cooling effect of convection on a persons nose.



FIGURE 9-2.



Newtons Law of Cooling describes the relationship between the various factors that influence convection:

Q = h A TwhereQ = heat energyh = coefficient of convective heat transferA = areaT = Temperature differenceThe coefficient of convective heat transfer is often determined experimentally or by estimation from other test data for the surfaces and fluids involved. The exact value depends on a variety of factors, of which the most important are velocity, orientation, surface condition, geometry, and fluid viscosity.



Changes in h can be significant due merely to a change in orientation. The topside of a horizontal surface can transfer over 50% more heat by naturalconvection than the underside of the same surface. In both natural and forcedconvection, a thin layer of relatively still fluid molecules adheres to the transfer surface. This boundary layer, or film coefficient, varies in thicknessdepending on several factors, the most important being the velocity of the fluid moving over the surface. The boundary layer has a measurable thermal resistance to conductive heat transfer. The thicker the layer, the greater the resistance. This, in turn, affects the convective transfer as well. At slow velocities, these boundary layers can build up significantly. At higher velocities, the thickness of this layer and its insulating effect are both diminished. Why should thermographers be concerned with convection? As forced convection, such as the wind, increases, heat transfer increases and can have a significant impact on the temperature of a heated or cooled surface. Regardless of velocity, this moving air has no affect on ambientsurfaces (?) . Thermographers inspect a variety of components where an increase in temperature over ambient is an indication of a potential problem. Forced convection is capable of masking these indications.



RadiationIn addition to heat energy being transferred by conduction and convection it can also be transferred by radiation. Thermal infrared radiation is a form ofelectromagnetic energy similar to light, radio waves, and x-rays. All forms ofelectromagnetic radiation travel at the speed of light, 186,000 miles/second (3 108 meters/second). All forms of electromagnetic radiation travel in a straight line as a waveform; they differ only in their wavelength. Infrared radiation that is detected with thermal imaging systems has wavelengths between approximately 2 and 15 microns (m). Electromagnetic radiation can also travel through a vacuum, as demonstrated by the suns warming effect from a distance of over 94 million miles of space. All objects above absolute zero radiate infrared radiation. The amount and the exact wavelengths radiated depend primarily on the temperature of the object. It is this phenomenon that allows us to see radiant surfaces with infrared sensing cameras.



The Suns Electromagnetic SpectrumThe energy that reaches the Earth is known as solar radiation. Although the sun emits radiation at all wavelengths, approximately 44% falls within visible-light wavelengths. The region of the spectrum referred to as visible light (light our eyes can detect) is composed of relatively short wavelengths in the range 400 nanometers (nm), or 0.4 micrometers (m), through 700 nm, or 0.7 m.



Infrared radiation that is detected with thermal imaging systems has wavelengths between approximately 2 and 15 microns (m). (2 x 10-6 m ~15 x 10-6 m)



Due to atmospheric absorption, significant transmission through air occurs in only two windows or wavebands: the short (2~6 m) and long (8~15 m)wavebands. Both can be used for many thermal applications. With someapplications, one waveband may offer a distinct advantage or make certainapplications feasible. These situations will be addressed in subsequentsections. The amount of energy emitted by a surface depends on severalfactors, as shown by the StefanBoltzmann formula:

Q = T4 absoluteWhere: Q = energy transmitted by radiation (per unit area?) = the StefanBoltzmann constant (0.1714 10-8 Btu/hr ft2 R4) = the emissivity value of the surfaceT = the absolute temperature of the surface

The rate of change of energy transmitted by radiation;Q/ t = A x T4 absolute



Thermal Radiation and Stefan-Boltzmann Equation


https://www.youtube.com/embed/93-_JhGNn1Y

https://www.youtube.com/watch?v=93-_JhGNn1Y


Due to atmospheric absorption, significant transmission through air occurs in only two windows or wavebands: the short (2~6 m) and long (8~15 m)wavebands. (1m = 1x 10-6 m, 1nm= 1 x 10-9 m)


0.7 ~ 1.4 m1.4 ~ 3.0 m3.0 ~ 1.0 x 106 m


D

u

e

t

o

a

t

m

o

s

p

h

e

r

i

c

a

b

s

o

r

p

t

i

o

n

,

s

i

g

n

i

f

i

c

a

n

t

t

r

a

n

s

m

i

s

s

i

o

n

t

h

r

o

u

g

h

a

i

r

o

c

c

u

r

s

i

n

o

n

l

y

t

w

o

w

i

n

d

o

w

s

o

r

w

a

v

e

b

a

n

d

s

:

t

h

e

s

h

o

r

t

(

2

~

6

m

)

a

n

d

l

o

n

g

(

8

~

1

5

m

)

w

a

v

e

b

a

n

d

s

.

(

1

m

=

1

x

1

0

-

6

m

,

1

n

m

=

1

x

1

0

-

9

m

)



Due to atmospheric absorption, significant transmission through air occurs in only twowindows or wavebands: the short (2~6 m) and long (8~15 m) wavebands.

0.7 m ~ 1 mm


When electromagnetic radiation interacts with a surface several events may occur. Thermal radiation may be reflected by the surface, just like light on amirror. It can be absorbed by the surface, in which case it often causes achange in the temperature of the surface. In some cases, the radiation canalso be transmitted through the surface; light passing through a window is agood example. The sum of these three components must equal the total amount of energy involved.

This relationship, known as the conservation of energy, is stated as follows:

R + A + T = 1

whereR = Reflected energyA = Absorbed energyT = Transmitted energy



Radiation is never perfectly transmitted, absorbed, or reflected by a material. Two or three phenomena are occurring at once. For example, one can seethrough a window (transmission) and also see reflections in the window at thesame time. It is also known that glass absorbs a small portion of the radiationbecause the sun can cause it to heat up. For a typical glass window, 92% ofthe light radiation is transmitted, 6% is reflected, and 2% is absorbed. Onehundred percent of the radiation incident on the glass is accounted for.Infrared radiation, like light and other forms of electromagnetic radiation, alsobehaves in this way. When a surface is viewed, not only radiation that hasbeen absorbed may be seen, but also radiation that is being transmittedthrough the target and/or reflected by it. Neither the transmitted nor reflectedradiation provides any information about the temperature of the surface.



The combined radiation reflecting from a surface to the infrared system is called its radiosity. The job of the thermographer is to distinguish the emittedcomponent from the others so that more about the target temperature can beunderstood. Only a few materials transmit infrared radiation very efficiently.The lens material of the camera is one. Transmissive materials can be usedas thermal windows, allowing viewing into enclosures. The atmosphere isalso fairly transparent, at least in two wavebands. In the rest of the thermalspectrum, water vapor and carbon dioxide absorb most thermal radiation. Ascan be seen from Figure 9-3, radiation is transmitted quite readily in both theshort (26 m) and long (814 (15) m) wavebands. Infrared systems have been optimized to one of these bands or the other. Broadband systems are also available and have some response in both wavebands.

A transmission curve for glass would show us that glass is somewhat transparent in the short waveband and opaque in the long waveband. It is surprising to try to look thermally through a window and not be able to see much of anything!



FIGURE 9-3.


The atmosphere is also fairly transparent, at least in two wavebands. In the rest of the thermal spectrum, water vapor and carbon dioxide absorb most thermal radiation. (6m ~ 8 m)

(6m ~ 8m)

(26 m) (814 (15) m)


Radiosity:The combined radiation reflecting from a surface to the infrared system is called its radiosity.

Radiosity (radiometry)In radiometry, radiosity is the radiant flux leaving (emitted, reflected and transmitted by) a surface per unit area, and spectral radiosity is the radiosity of a surface per unit frequency or wavelength, depending on whether the spectrum is taken as a function of frequency or of wavelength

The SI unit of radiosity is the watt per square metre (W/m2), while that of spectral radiosity in frequency is the watt per square metre per hertz (Wm2 Hz1) and that of spectral radiosity in wavelength is the watt per square metre per metre (Wm3) - commonly the watt per square metre per nanometre (Wm2 nm1). The CGS unit erg per square centimeter per second (ergcm2 s1) is often used in astronomy. Radiosity is often called "intensity" in branches of physics other than radiometry, but in radiometry this usage leads to confusion with radiant intensity.

radiant intensity (radiometry)In radiometry, radiant intensity is the radiant flux emitted, reflected, transmitted or received, per unit solid angle, and spectral intensity is the radiant intensity per unit frequency or wavelength, depending on whether the spectrum is taken as a function of frequency or of wavelength. These are directional quantities. The SI unit of radiant intensity is the watt per steradian (W/sr), while that of spectral intensity in frequency is the watt persteradian per hertz (Wsr1 Hz1) and that of spectral intensity in wavelength is the watt per steradian per metre (Wsr1 m1) - commonly the watt per steradian per nanometre (Wsr1 nm1). Radiant intensity is distinct from irradiance and radiant exitance, which are often called intensity in branches of physics other than radiometry. In radio-frequency engineering, radiant intensity is sometimes called radiation intensity.



Many thin plastic films are transparent in varying degrees to infrared radiation. A thin plastic bag may be useful as a camera cover in wet weather or dirtyenvironments. Be aware, however, that all thin plastic films are not the same!While they may look similar, it is important to test them for transparency and measure the degree of thermal attenuation. Depending on the exact atomicmakeup of the plastic, they may absorb strongly in very narrow, specificwavebands. Therefore, to measure the temperature of a thin plastic film, afilter must be used to limit the radiation to those areas where absorption (andemission) occurs.



The vast majority of materials are not transparent. Therefore, they are opaque to infrared radiation. This simplifies the task of looking at them thermally by leaving one less variable to deal with. This means that the only radiation we detect is that which is reflected and absorbed by the surface (R + A = 1).

If R = 1, the surface would be a perfect reflector. Although there are no suchmaterials, the reflectivity of many polished shiny metals approaches this value.They are like heat mirrors. Kirchhoffs law says that for opaque surfaces theradiant energy that is absorbed must also be reemitted, or A = E. By substitution, it is concluded that the energy detected from an opaque surface is either reflected or emitted (R + E = 1). Only the emitted energy provides information about the temperature of the surface.

In other words, an efficient reflector is an inefficient emitter, and vice versa. For thermographers, this simple inverse relationship between reflectivity and emissivity forms the basis for interpretation of nearly all of that is seen. Emissive objects reveal a great deal about their temperature. Reflective surfaces do not. In fact, under certain conditions, very reflective surfaces typically hide their true thermal nature by reflecting the background and emitting very little of their own thermal energy.



If E = 1, all energy is absorbed and reemitted. Such an object, which exists only in theory, is called a blackbody. Human skin with an emissivity of 0.98, isnearly a perfect blackbody, regardless of skin color. Emissivity is acharacteristic of a material that indicates its relative efficiency in emittinginfrared radiation. It is the ratio of thermal energy emitted by a surface to thatenergy emitted by a blackbody of the same temperature. Emissivity is a valuebetween zero and one. Most nonmetals have emissivities above 0.8. Metals,on the other hand, especially shiny ones, typically have emissivities below 0.2.Materials that are not blackbodiesin other words everything!are calledreal bodies. Real bodies always emit less radiation than a blackbody at thesame temperature. Exactly how much less depends on their emissivity.Several factors can affect what the emissivity of a material is. Besides thematerial type, emissivity can also vary with surface condition, temperature,and wavelength. The emittance of an object can also vary with the angle ofview.



It is not difficult to characterize the emissivity of most materials that are not shiny metals. Many of them have already been characterized, and theirvalues can be found in tables such as Table 9-2. These values should beused only as a guide. Because the exact emissivity of a material may vary from these values, skilled thermographers also need to understand how to measure the actual value. It is interesting to note that cracks, gaps, and holes emit thermal energy at a higher rate than the surfaces around them. The same is true for visible light. The pupil of your eye is black because it is a cavity, and the light that enters it is absorbed by it. When all light is absorbed by a surface, we say it is black. The emissivity of a cavity will approach 0.98 when it is seven times deeper than it is wide.



From an expanded statement of the StefanBoltzmann law, the impact that reflection has on solving the temperature problem for opaque materials can be seen:

Q = T4 + ( (1 ) T 4 background)



The second part of the equation (in boldface) represents that portion of the radiosity that comes from the reflected energy. When using a radiometricsystem to make a measurement, it is important to characterize and accountfor the influence of the reflected background temperature. Consider these twopossible scenarios:

When the object being viewed is very reflective, the temperature of the reflected background becomes quite significant.

When the background is at a temperature that is extremely different from the object being viewed, the influence of the background becomes more pronounced.



It becomes clear that repeatable, accurate radiometric measurements can be made only when emissivities are high. This is a fundamental limitation withinwhich all thermographers work. Generally, it is not recommended to maketemperature measurements of surfaces with emissivities below approximately0.50, in other words all shiny metals, except under tightly controlled laboratoryconditions. However, with a strong understanding of how heat energy movesin materials and a working knowledge of radiation, the value of infraredthermography as a noncontact temperature measurement tool fornondestructive evaluation is remarkable.



Fouriers Law

Charlie Chong/ Fion Zhang https://www.youtube.com/watch?v=7U9tza1DaqI

https://www.youtube.com/embed/7U9tza1DaqI


StefanBoltzmann formula:Q = T4 absolute



StefanBoltzmann law


https://vimeo.com/51844267


StefanBoltzmann law


https://www.youtube.com/embed/8hJx2Kjtz0U

https://www.youtube.com/watch?v=8hJx2Kjtz0U


3. EQUIPMENT AND ACCESSORIESInfrared systems can generally be classified as either thermal imaging systems or point measuring systems. Thermal imaging systems can befurther subdivided into quantitative or radiometric (measuring temperatures)and qualitative or nonradiometric systems (thermal images only). Pointmeasuring systems are also often termed spot or point radiometers andcan be either hand-held or fixed mounted.

Qualitative thermal imaging systems are useful when radiometric temperatures are not required. Using these lower-cost instruments, it is possible to make thermal comparisons and thus distinguish thermal anomalies in equipment and materials.

Quantitative thermography involves the use of calibrated instruments. By using correction factors, accurate radiometric temperatures can be determined from the thermal image. Calibration, which is expensive to achieve or maintain, is best made for the specific temperature ranges and accuracy required.



Nonimaging radiometric measuring systems, called point or spot radiometers, are lower- cost instruments, which, although they do not give an image, canbe valuable for many types of NDT. For limited areas and pieces ofequipment, and when used properly, this equipment can provide accurate,real-time radiometric measurement across a wide range of temperatures.Typically, spot radiometers have emissivity correction capabilities.

Without an image, it can be difficult knowing the exact area the spot radiometer is measuring, possibly resulting in erroneous data. Some models include a laser-aiming device to help aim them. It should be carefully noted, however, that the area being measured is always much larger than the small spot illuminated by the laser. Newer models are available with a projected laser beam that also outlines the exact measurement area, alleviating any misunderstanding.



The waveband response of spot system detectors is typically wide enough to allow them to be filtered to detect narrow wavebands for specialized applications. This is particularly useful for such applications as measuring temperatures of thin plastic films or glass during manufacturing. Spot systems can also be fixed or mounted to constantly monitor process temperatures unattended. The data is typically fed back to a computer or control system. Such thermal data can be correlated not only to temperature, but also thickness, moisture content, material integrity, material type, or parts presence detection. When combined with visual automated systems, a powerful information system is available.

Even though spot radiometers are much less expensive than thermal imaging systems, they should not be thought of as a substitute for an imaging system. Both types of systems have their place in most NDT programs.



A variation of the spot radiometer is the line scanner. It is also very useful for many fixed mounted NDT inspections. This instrument employs a detector orsmall array of detectors that is then scanned along a line. If the line scannerviews a moving object, an image can be created. Line scanners areparticularly useful for viewing equipment that is moving at a constant ratebecause the image is built up one line at a time as the product passes by.Two common applications are viewing the refractory in rotating limekilns orthe cross-roll moisture profile on the sheet in a paper machine. Line scanners,which can produce very high-quality images with excellent radiometrics, areless expensive than full imaging systems and lend themselves well to thesetypes of automated inspections.



In the history and development section, it was noted that over the years three main types of infrared imaging systems were developed: (1) scanners orscanning systems, (2) pyroelectric vidicons (PEVs), and (3) focal plane arrays(FPAs). While all three types are still in use today, the overwhelming majorityof new equipment sales are the FPA systems.

It is useful to have a general understanding of how imaging systems work. The purpose of the imager is to detect the infrared radiation given off by the object being inspected, commonly called the target. The targets radiosity is focused by the thermally transparent optics onto a detector, which is sensitive to this invisible heat radiation. A response from the detector produces a signal, usually a voltage or resistance change, which is read by the electronics of the system and creates a thermal image on a display screen, typically a video viewfinder. In this seemingly simple process, we are able, without contact, to view the thermal image, or thermogram, that corresponds to the thermal energy coming off the surface of the target.



Scanners or scanning systemsThe first infrared imaging systems used a technique called scanning to create a full screen image using a single, small detector. A series of rotatingor oscillating mirrors or prisms inside the scanner direct the infrared radiationfrom a small part of the scene being viewed onto the detector. The detectorresponse results in the display of a portion of the field of view. Very quickly,the mirror or prism moves to scan another part of the scene with the resultantdisplay. This process of making a series of single measurements happensthousands of times per second, so that a complete image is built up andviewed in what appears to be real time, i.e., at 30 frames per second orgreater.



For some types of detectors to respond quickly, accurately, and in a repeatable fashion (particularly photon detectors), they must be cooled toextremely low temperatures. Systems with the highest-quality images useliquid nitrogen (LN2) at -320.8F (-196C). The nitrogen is simply poured intoan insulated container, called a Dewar, that surrounds the detector. Under normal conditions, the detector will stay cold for between 2 ~ 4 hours before it must be refilled. Other cooling systems have employed a cylinder of very high pressure (6000 psi) argon gas as a coolant. As it is slowly released, the gas expands and cools the detector to -292F (-180C). Obviously, great care must be used in handling cryogenics. Because of these safety problems and the problems of dealing with cryogenic substances in the field, cryogenically cooled systems are seldom used anymore except in laboratory situations. Several types of non-cryogenic cooling schemes have replaced them. While non-cryogenic devices require a fair amount of power and have a limited life, their benefits make them much more attractive than cryogenic devices.



Pyroelectric vidiconHistorically, pyroelectric vidicon (PEV) designs have offered a lower-cost alternative to scanners, and many are still in use. While PEVs are notradiometric, they can produce a good quality image. In addition, they do notrequire cooling. The detector in a PEV system is a monolithic crystal that,when heated by the incoming radiation, changes polarity. An electron-canning gun, similar to those used in older television cameras, reads thischange, which then produces an electronic image of the thermal scene. PEVsare slower than scanners in their response by an order of magnitude. PEVsalso need to be constantly moved or panned, or have their signal chopped.At this time, PEV tubes are no longer being manufactured.



Focal plane array (FPA) systemsFocal plane array (FPA) systems, which use a matrix of detectors to sense the infrared radiation, have a much higher spatial resolution than scanners or PEV systems. Typical detectors have pixel resolutions of 256 256 or 320 240. Resolutions of 512 512 pixels may soon be the norm. Although early FPA systems had to be cooled and were only sensitive in the short waveband, uncooled, long-wave sensing systems are now available.



There are several types of FPA systems. Short-wave instruments typically use photon detectors. These are cooled detectors, which actually count thenumber of photons received. Photon detecting systems now have veryreliable radiometrics and excellent imagery.

Long-wave systems use thermal detectors, which do not have to be cooled, although they must be temperature stabilized. While some are classified as (1) bolometers and others (2) pyroelectric or (3) ferroelectric, in all cases, thermal detectors are actually heated up by the incoming radiation. In the former this results in a resistance change, whereas in the latter there is a change in polarization. Very reliable radiometrics and excellent imagery is now possible from bolometers, but pyroelectric systems, while producinggood quality images, have proven extremely difficult to be used radiometrically.



Pyroelectricity is the ability of certain materials to generate a temporary voltage when they are heated or cooled. The change in temperature modifies the positions of the atoms slightly within the crystal structure, such that the polarization of the material changes. This polarization change gives rise to a voltage across the crystal. If the temperature stays constant at its new value, the pyroelectric voltage gradually disappears due to leakage current (the leakage can be due to electrons moving through the crystal, ions moving through the air, current leaking through a voltmeter attached across the crystal, etc.).

Pyroelectricity should not be confused with thermoelectricity: In a typical demonstration of pyroelectricity, the whole crystal is changed from one temperature to another, and the result is a temporary voltage across the crystal. In a typical demonstration of thermoelectricity, one part of the device is kept at one temperature and the other part at a different temperature, and the result is a permanent voltage across the device as long as there is a temperature difference.

Charlie Chong/ Fion Zhang http://en.wikipedia.org/wiki/Pyroelectricity


Ferroelectricityis a property of certain materials that have a spontaneous electric polarization that can be reversed by the application of an external electric field. The term is used in analogy to ferromagnetism, in which a material exhibits a permanent magnetic moment. Ferromagnetism was already known when ferroelectricity was discovered in 1920 in Rochelle salt by Valasek. Thus, the prefix ferro, meaning iron, was used to describe the property despite the fact that most ferroelectric materials do not contain iron.

Polarization:When most materials are polarized, the polarization induced, P, is almost exactly proportional to the applied external electric field E; so the polarization is a linear function. This is called dielectric polarization (see figure). Some materials, known as paraelectric materials, show a more enhanced nonlinear polarization (see figure). The electric permittivity, corresponding to the slope of the polarization curve, is not constant as in dielectrics but is a function of the external electric field.

In addition to being nonlinear, ferroelectric materials demonstrate a spontaneous nonzero polarization (after entrainment, see figure) even when the applied field E is zero. The distinguishing feature of ferroelectrics is that the spontaneous polarization can be reversed by a suitably strong applied electric field in the opposite direction; the polarization is therefore dependent not only on the current electric field but also on its history, yielding a hysteresis loop. They are called ferroelectrics by analogy to ferromagnetic materials, which have spontaneous magnetization and exhibit similar hysteresis loops.

Typically, materials demonstrate ferroelectricity only below a certain phase transition temperature, called the Curie temperature, Tc, and are paraelectric above this temperature.

Charlie Chong/ Fion Zhang http://en.wikipedia.org/wiki/Ferroelectricity


Infrared systems vary widely in price. At this time, spot radiometer systems start at under $500. Simple pyroelectric FPA instruments are in the range of$10,000 to $20,000. A complete radiometric system with software and normallens is typically priced between $45,000 and $55,000. Although specificationsand configurations vary, all these systems are designed to be used in typicalindustrial environments. Infrared imaging systems are generally made up ofseveral common components, including:

Lens, Detector, Processing electronics, Controls, Display, Data storage, Data processing and Report generation software, and Filter.



LensLenses serve to focus the incoming infrared radiation on the detector. Some infrared lens materials are actually opaque to visible light. Materialscommonly used for lenses are germanium (Ge), silicon (Si), and zinc selenide(ZnSe). Lenses can be of different focal lengths. A normal lens (with a field ofview from 16 to 25, approximately) is useful for most applications. Whereinspection space is limited or a wide view required, the use of a wide-anglelens is recommended. Over longer distances, such as when viewing a disconnect switch in a substation, a telephoto lens may be warranted. Lens selection also impacts spatial and measurement resolution. Coatings used on some lenses may contain small amounts of radioactive material (thorium). When using the system in a nuclear power plant, make sure to test the system for its baseline characteristics prior to entry.

Keywords:germanium (Ge), silicon (Si), and zinc selenide (ZnSe).



DetectorThe radiation is focused on the detector, where it produces a measurable response. Materials commonly used for detectors are platinum silicide (PtSi),mercury cadmium telluride (HgCdTe), and indium antimonide (InSb).

Processing ElectronicsThe response from the detector is processed electronically to produce a thermal image, a temperature measurement, or both.

ControlsVarious controls on the system allow for adjustments to control the input of infrared radiation or the output of data. These typically include adjustments torange and span, thermal level, polarity, emissivity, and other temperaturemeasurement functions.



Platinum silicidePlatinum silicide (PtSi) is a semiconductor material used in infrared detectors. It is used in detectors for infrared astronomy. Platinum silicide is capable of operating at 1-5 m wavelength range. It has a good sensitivity (up to 0.05 C) and high stability. Its manufacturing process is relatively simple, with good yields, resulting in reasonable cost.

Platinum silicide is made by ion implantation of platinum into silicon surface, forming a Schottky diode structure. Due to silicon-based technology, large detector devices with low noise and good quality are available, and the manufacturing is relatively simple. However their quantum efficiency is abysmally () low (typically under 1%), and therefore platinum silicidedevices are currently used only rarely, being displaced by better materials.

Charlie Chong/ Fion Zhang http://www.chemeurope.com/en/encyclopedia/Platinum_silicide.html


Platinum silicide devices offer high uniformity of the imaging arrays, avoiding the need of on-line image corrections, and simplifying the device construction.They are in use in some thermal imaging applications, particularly for measurement, as PtSi is very stable and has low drift of parameters with time. The low cost and high stability makes it a suitable material for imaging devices for preventive maintenance and scientific IR imaging.

A well-formed platinum silicide layer is opaque to infrared light, and the barrier height of the resulting Schottky diodes should be 0.84 0.3 V. Mercury cadmium telluride HgCdTe and indium antimonide InSb are materials with similar uses.

Charlie Chong/ Fion Zhang http://www.chemeurope.com/en/encyclopedia/Platinum_silicide.html


Platinum Silicide SPM-ProbesHighly conductive and wear-resistant SPM probes NANOSENSORSPlatinum Silicide AFM probes are designed for conductive AFM imaging where the combinations of excellent conductivity, high wear resistance and a small tip radius are required. Until recently scientists interested in conductive AFM could choose between two types of SPM probes for these applications: metal coated probes (mostly platinum coated) and conductive diamond coated SPM probes. These two probe types have certain advantages and disadvantages. Metal coated probes offer their user high conductivity and a relatively small tip radius. Their disadvantage is that the coating wears relatively fast. In contrast silicon probes covered with a conductive diamond coating offer a high wear resistance, however their conductivity is ten to hundred times lower than that of metal coated probes and due to the thickness of the conductive diamond coating the tip radius is relatively large.

Charlie Chong/ Fion Zhang http://www.nanosensors.com/pdf/platinum_silicide.pdf


With the introduction of the NANOSENSORS Platinum Silicide SPM probes we successfully combine the advantages of wear resistance, high resolution imaging and metal-like conductivity in one conductive SPM probe. While not as robust as diamond Platinum Silicide has an substantially increased hardness over platinum or other metals. The conductivity of Platinum Silicideis close to the conductivity of platinum and more than one order of magnitude better than diamond.


Conductive diamond probes. Advantages: hard, long-lasting coating. Disadvantages: large radius, low conductivity

Platinum (Iridium) coated probes. Advantages: relatively small radius, excellent conductivity. Disadvantages: fast wear-out

http://www.nanosensors.com/pdf/platinum_silicide.pdf


Tip Features at a GlanceHard, solid and conductive silicide apex versus traditional conductive probes with only a few tens of nanometers metal coating at the tip apex Smaller tip radius (nominal 25nm) than normal metal coated probes (nominal 30nm). About five to six times smaller radius when compared to diamond coated tips (nominal 150nm) Almost metal like conductivity. More than ten times better conductivity than conductive diamond probes Dramatically improved wear rates compared to silicon and PtIr coated tips



Platinum Silicide probe. Advantages: small radius, excellent conductivity, good wear-out behavior.



Field of ApplicationNANOSENSORS Platinum Silicide probes are ideally suited for: Conductive AFM (CAFM) Tunneling AFM (TUNA) Scanning Capacitance Microscopy (SCM) Kelvin Probe Force Microscopy (KPFM) Electrostatic Force Microscopy (EFM)

Due to the small radius / high aspect ratio tip apex NANOSENSORSlatinum Silicide probes are designed to withstand the high forces required for Scanning Spreading Resistance Microscopy (SSRM).



Mercury(II) cadmium(II) tellurideHgCdTe or Mercury cadmium telluride (also Cadmium Mercury Telluride, MCT or CMT) is an alloy of CdTe and HgTe and is sometimes claimed to be the third semiconductor of technological importance after Silicon and Gallium(III) arsenide. The amount of cadmium (Cd) in the alloy (the alloy composition) can be chosen so as to tune the optical absorption of the material to the desired infrared wavelength. CdTe is a semiconductor with a bandgap of approximately 1.5 eV at room temperature. HgTe is a semimetal, hence its bandgap energy is zero. Mixing these two substances allows one to obtain any bandgap between 0 and 1.5 eV.

HgCdTe is usually referred to as MerCad Telluride, or simply MerCad in the Infrared sensors community.

Charlie Chong/ Fion Zhang http://www.chemeurope.com/en/encyclopedia/Mercury%28II%29_cadmium%28II%29_telluride.html


Properties ElectronicThe electron mobility of HGCdTe with a large Hg content is very high. At room temperature only InSb and InAs of common semiconductors used for infrared detection surpass HgCdTe's electron mobility. At 80K the electron mobility of Hg0.8Cd0.2Te can be several hundred thousand cm/V/s. Electrons also have a long ballistic length at this temperature; their mean free path can be several m. MechanicalHgCdTe is a soft material due to the weak bonds Hg forms with tellurium. It is a softer material than any common III-V semiconductor. The Mohs hardness of HgTe is 1.9, CdTe is 2.9 and Hg0.5Cd0.5Te is 4. The hardness of lead salts is lower still.



ThermalThe thermal conductivity of HgCdTe is low; at low cadmium concentrations it is as low as 0.2W.K-1m-1. This means that it is unsuitable for high power devices. Although, infrared light-emitting diodes and lasers have been made in HgCdTe they must be operated cold to be efficient. The specific heat capacity is 150 J.kg-1K-1. [1]

OpticalHgCdTe is transparent in the infrared at photon energies below the energy gap. The refractive index is high, reaching nearly 4 for high Hg content HgCdTe.



Infrared detectionHgCdTe is the only common material that can detect infrared radiation in both of the accessible atmospheric windows. These are from 3 to 5 m (the mid-wave infrared window, abbreviated MWIR) and from 10 to 12 m (the long-wave window, LWIR). Detection in the MWIR and LWIR windows is obtained using 30% [(Hg0.7Cd0.3)Te] and 20% [(Hg0.8Cd0.2)Te] cadmium respectively. HgCdTe can also detect in the short wave infrared SWIR atmospheric windows of 2.2 to 2.4 m and 1.5 to 1.8 m.

Owing to its cost, the use of HgCdTe has so far been restricted to the military field and infrared astronomy research. Military technology depends on HgCdTe for night vision. In particular, the US air force makes extensive use of HgCdTe on all aircraft, and to equip airborne smart bombs. A variety of heat-seeking missiles are also equipped with HgCdTe detectors. HgCdTe detector arrays can also be found at most of the worlds major research telescopes including several satellites. Many HgCdTe detectors (such as Hawaii and NICMOS detectors) are named after the astronomical observatories or instruments for which they were originally developed.



The main limitation of LWIR HgCdTe-based detectors is that they need cooling to temperatures near that of liquid nitrogen (77K), to reduce noise due to thermally excited current carriers (see cooled infrared camera). MWIR HgCdTe cameras can be operated at temperatures accessible to thermoelectric coolers with a small performance penalty. Hence, HgCdTe detectors are heavy and require maintenance. On the other side, HgCdTe enjoys much higher speed of detection and is much more sensitive than some of its cheaper competitors.

HgCdTe is often a material of choice for detectors in Fourier Transform Infrared Spectrometer (FTIR) instruments. This is because of the large spectral range of HgCdTe detectors and also the high quantum efficiency.

HgCdTe can be used as a heterodyne detector, in which the interference between a local source and returned laser light is detected. In this case it can detect sources such as CO2 lasers. In heterodyne detection mode HgCdTe can be uncooled, although greater sensitivity is achieved by cooling. Photodiodes, photoconductors or photoelectromagnetic (PEM) modes can be used. A bandwidth in excess of 1GHz can be achieved with photodiode detectors.



The main competitors of HgCdTe are less sensitive Si-based bolometers (see uncooled infrared camera), InSb and photon-counting superconducting tunnel junction (STJ) arrays. Quantum Well Infrared Photodetectors (QWIP), manufactured from III-V semiconductor materials such as GaAs and AlGaAs, are another possible alternative, although their theoretical performance limits are inferior to HgCdTe arrays at comparable temperatures and they require the use of complicated reflection/diffraction gratings to overcome certain polarization exclusion effects which impact array Responsivity. In the future, the primary competitor to HgCdTe detectors may emerge in the form of Quantum Dot Infrared Photodetectors (QDIP), based on either a colloidal or type-II superlattice structure. Unique 3-D Quantum Confinement effects, combined with the unipolar (non-exciton based photoelectric behavior) nature of quantum dots could allow comparable performance to HgCdTe at significantly higher operating temperatures. Initial laboratory work has shown promising results in this regard and QDIPs may be one of the first significant nanotechnology products to emerge.



In HgCdTe, detection occurs when an infrared

understanding infrared thermography-note 1

Documents