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0.4 0.78 110,000 5,000 2,500

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15 C Air Temperature (59 F)5.9mm Precipitable Water (46% Relative Humidity)

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1998 Putman Publishing Company and OMEGA Press LLC.

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TRANSACTIONS Volume 1 05

Non-Contact Temperature MA Technical Reference Series Brought to You by OMEGA

1

VOLUME

I N M E A S U R E M E N T A N D C O N T R O L


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06 Volume 1 TRANSACTIONS

A Historical Perspective1 IR Through the Ages From Newton to Einstein Todays Applications

Figure 1-1: The First IR Thermometer

Lens RetinaLightDetector

To Brain

Eye

Theoretical Development2

10

I N M E A S U R E M E N T A N D C O N T R O L

TABLE OF CONTENTS

VOLUME 1NON-CONTACT TEMPERATURE MEASUREMENTSection Topics Covered Page

Radiation Basics

Blackbody Concepts From Blackbodies to Real Surfaces

Figure 2-1: Radiation Energy Balance

Transmitted Energy

Radiant Energy

AbsorbedEnergy

ReflectedEnergy

17

IR Thermometers & Pyrometers3 The N Factor Types of Radiation Thermometers Design & Engineering

Figure 3-6: Ratio Pyrometry Via a Filter Wheel

Viewing MicroscopeTemperatureControlled

Cavity

Objective Lens

Target Mirror AndFirst Field Stop

Lens

Aperture Stop Second Field Stop

Lens

Sensor

Rotating

Filter Wheel

Eye

24

Infrared Thermocouples4 Thermocouple Basics Self-Powered Infrared Thermocouples Installation Guidelines

0 200 400 600 800

8

6

4

2

0

m V

F

6.68mV

4 mV

2.68 mV

Figure 4-1: Thermocouple Operation

1 3

2 38


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Fiber Optic Extensions5 Fiber Advantages Fiber Applications Component Options

Section Topics Covered Page

Figure 5-2: Typical IR Fiber Optic Probe

Lens

Low TemperatureOptical FiberSingle Crystal

Sapphire (Al2O3)

NarrowbandFilter

AnalyzerOptical

Detector

Coupler

Thin FilmMetal Coating

Al2O3 ProtectiveFilm

BlackbodyCavity

Linescanning & Thermography6

43

Calibration of IR Thermometers7

Products & Applications8

Infrared Linescanners

2-D Thermographic Analysis Enter the Microprocessor

Figure 6-3: 2-D Thermographic Camera

Two Dimensional Thermograph

False Color Image

Object

AreaBeing

Scanned

Why Calibrate? Blackbody Cavities Tungsten Filament Lamps

Schematic of the Infrared Spectrum 02

Table of Contents 06

Editorial 08About OMEGA 09

Alternative Configurations Application Guidelines Accessories & Options

46

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t y ,

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0.8

0.7

0.5

0.3

0.1

Figure 7-2: Effective Emissivity of Spherical Cavities

Cavity Surface Emissivity

Figure 8-2: Sighting on a Specular Surface

Target

Hot Furnace Walls

Thermometer

53

56

REFERENCE SECTIONS

68 Information Resources

72 Emissivity of Common Materials

77 Glossary

80 Index


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Editorial


Welcome to TransaSince its founding in 1962, OMEGA has grown from a manufacturer of a single producta finegauge thermocoupleinto being an established global leader in the scientific and technical

marketplace, offering more than 68,000 state-of-the-art instrumentation and controldevices. And although OMEGAs staff, facilities, and client services are the finest anywhere,OMEGAs legendary handbooks and encyclopedias have become its hallmarkengineersthroughout the world daily rely on these reference tools of unprecedented value not only forOMEGA product data, but for the vital technical information necessary to effectively employtodays sophisticated instruments and process control devices.

But were not resting on our laurels. We realize that your need for basic and relevant scientificdata and information continues to grow, and that you require all the information you can get tokeep up with rapidly advancing and ever-more-complex instrumentation and control technology.

Thats why weve launched OMEGAs Transactions in Measurement & Control , the firstissue of which you now hold in your hands. Conceived as a practical thesis, a

technical reference series for everyday users of instrumentation andcontrols, rather than a series of erudite essays, each issue of

Transactions will be jam-packed with information on a differentmeasurement and control technology topic. This issue, for

instance, delves deeply into the issue of non-contacttemperature measurement, providing a historical andtheoretical context, engineering and design principles,plus selection and application guidelines for devicesranging from low-cost infrared thermocouples tosophisticated linescanners. Neither advertisements orpromotionals will be present in the Transactions series.

Future issues of Transactions , to be published on aquarterly basis, will systematically cover other aspects of

temperature, humidity, pressure, strain, flow, level, pH, andconductivity instrumentation as well as other measurement,

data acquisition, and control topics.

We hope Transactions finds a permanent home on your referenceshelf, and that it proves itself of great value now and in the future.

S


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About OMEGA


OMEGAs Transactions in Measurement & Control series, as well as our legendary set of handbooks andencyclopedias, are designed to provide at-your-fingertips access to the technical information you needto help meet your measurement and control requirements. But when your needs exceed the printed

wordwhen technical assistance is required to select among alternative products, or when no off-the-shelfproduct seems to fill the billwe hope youll turn to OMEGA. There is no advertising orpromotional materials in the Transactions series. There will be none.

Our people, our facilities, and our commitment to customer service set thestandard for control and instrumentation. A sampler of our comprehensive resources

and capabilities: OMEGAs commitment to leading-edge research and development and

state-of-the-art manufacturing keeps us firmly at the forefront of technology.OMEGAs Development and Engineering Center, located on our Stamford, Conn.,campus, is home to OMEGAs design and engineering laboratories. All productdesigns are tested and perfected here prior to marketing. This facility housesOMEGAs metrology lab and other quality control facilities. The testing that takes place here assuresthat you receive the best products for your applications.

On the manufacturing side, our Bridgeport, N.J., vertically integrated manufacturing facility nearPhiladelphia houses advanced thermocouple wire production equipment along with a host of othercomputerized CNC milling machines, injection molding equipment, screw machines, braiders, extruders,punch presses and much, much more.

If our broad range of standard products dont quite match your needs, OMEGA is proud to offer the mostsophisticated and extensive custom engineering capabilities in the process measurement and control industry.Whether you need a simple modification of a standard product or complete customized system, OMEGA canaccommodate your special request. Free CAD drawings also are supplied with customized product orders or anew design built to your specifications at no obligation.

We believe in active versus reactive customer service. To complement our current business andmanufacturing operations, OMEGA continues to strive toward new levels of quality by pursuing ISO 9000quality standards. This systematic approach to quality strengthens OMEGAs competitive edge. Ourcalibration services and quality control test center are trustworthy resources that help satisfy our customers

needs for accuracy on an initial and ongoing basis. The companys technical center welcomes many corporate groups of engineers and scientists who turn

to OMEGA for training. Our 140-seat auditorium, equipped with the latest in multimedia presentationtechnologies, provides an ideal learning environment for training tailored to your companys needsfrombasic refreshers to in-depth courses.

In short, it is our commitment to quality instrumentation and exceptional customer service that remainsthe cornerstone of our success. OMEGAs priority is clear: we exist to facilitate solutions to your needs.

For more information about Transactions or OMEGA Technologies, look us up on the Internet atwww.omega.com.

Exceeding Your ExpectationsO


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Our eyes only see the tinyfraction of energy emittedby the sun in the form of visible light. However, if

we could see the infrared rays emit-ted by all bodiesorganic and inor-ganicwe could effectively see inthe dark. Though invisible to the

human eye, infrared radiation canbe detected as a feeling of warmthon the skin, and even objects thatare colder than ambient tempera-ture radiate infrared energy. Someanimals such as rattlesnakes, havesmall infrared temperature sensorslocated under each eye which cansense the amount of heat beinggiven off by a body. These sensorshelp them to locate prey and pro-tect themselves from predators.

Non-contact temperature sensorsuse the concept of infrared radiantenergy to measure the temperatureof objects from a distance. Afterdetermining the wavelength of the

energy being emitted by an object,the sensor can use integrated equa-tions that take into account thebodys material and surface qualitiesto determine its temperature. In thischapter, we will focus on the historyof radiation thermometry and thedevelopment of non-contact tem-

perature sensors.

IR Through the AgesAlthough not apparent, radiationthermometry has been practiced forthousands of years. The first practicalinfrared thermometer was the humaneye (Figure 1-1). The human eye con-tains a lens which focuses emittedradiation onto the retina. The retina isstimulated by the radiation and sends

a signal to the brain, which serves asthe indicator of the radiation. If prop-erly calibrated based on experience,the brain can convert this signal to ameasure of temperature.

People have been using infraredheat to practical advantage for thou-sands of years. There is proof fromclay tablets and pottery dating backthousands of years that the sun wasused to increase the temperature of materials in order to produce moldsfor construction. Pyramids were

built from approximately 2700-2200B.C. of sun-dried bricks. TheEgyptians also made metal toolssuch as saws, cutting tools, andwedges, which were crafted by theexperienced craftsmen of their time.The craftsmen had to know how hotto make the metal before they couldform it. This was most likely per-formed based on experience of thecolor of the iron.

Because fuel for firing was scarce,

builders of Biblical times had todepend on the suns infrared radiationto dry the bricks for their temples andpyramids. The Mesopotamian remainsof the Tower of Babel indicate that it


IR Through the Ages

From Newton to Einstein

Today's Applications

NON-CONTACT TEMPERATURE MEASUREMENTA Historical Perspective

1

OA Historical Persp

Figure 1-1: The First IR Thermometer

LensRetinaLightDetector

To Brain

Eye


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was made of sun-dried brick, faced

with burnt brick and stone. In India,a sewer system dating back to 2500B.C. carried wastewater throughpottery pipes into covered brickdrains along the street and dis-charged from these into brick cul-verts leading into a stream.

In ancient Greece, as far back as2100 B.C., Minoan artisans producedthings such as vases, statues, textiles.By using sight, they could approximatewhen a piece of material could be

shaped. Terra-cotta pipes were builtby heating them to a certain tempera-ture and casting them into a mold.

In more recent years, specialcraftsmen have relied on their ownsenses to visualize when a material isthe correct temperature for moldingor cutting. Sight has been used forsteel working, glass working, waxmolding, and pottery. From experi-ence, skilled craftsmen learned toestimate the degree of heat required

in the kiln, smelter, or glass furnaceby the color of the interior of theheating chamber. Just as a classicalblacksmith, for example, might judgethe malleability of a horseshoe by itscherry-red color.

In countries around the world, thetechnique of sight is still being used.In Europe, glass molding craftsmenuse sight to determine when glass isready to be shaped (Figure 1-2). Theyput a large piece of glass in a heating

furnace by use of a large metal rod.When the glass reaches the desiredcolor and brightness, they pull it outof the oven and immediately form itinto the shape they want. If the glasscools and loses the desired color orbrightness, they put it back in theoven or dispose of it. The glass mak-ers know when the glass is ready, bysight. If you have a chandelier madeof glass, or hand-made glasses from

Europe, most likely they wereformed in this way.

From Newton to EinsteinThe thermometer was invented inItaly by Galileo Galilei (1564-1642),about two hundred years before theinfrared light itself was discovered in1800, and about 100 years before thegreat English scientist Sir IsaacNewton (1642-1727) investigated thenature of light by experimentationwith prisms.As published in Opticks in 1704,Newton used glass prisms to showthat white light could be split up into

a range of colors (Figure 1-3). The leastbent portion of the light consisted of red, and then following in order,orange, yellow, green, blue, indigo, andviolet, each merging gradually into thenext. Newton also show that the dif-ferent colors could be fed backthrough another prism to producewhite light again. Newtons workmade it clear that color was an inher-ent property of light and that white

light was a mixture of different colors.Matter affected color only by absorb-ing some kinds of light and transmit-ting or reflecting others.

It was also Newton who, in 1675,proposed that light was made up of small particles, or corpuscles. Withthis theory, Newton set out to mea-sure the relative sizes of these corpus-cles. From observations of theeclipses of the moons of Jupiter,Newton realized that all light traveledat the same speed. Based on thisobservation, Newton determined therelative sizes of the different colorlight particles by the refraction angles.

In 1678, Christiaan Huygens (1629-1695), a mathematician, astronomer,and natural scientist, challengedNewtons corpuscular theory propos-ing that light could be better under-stood as consisting of waves. Throughthe 1800s, the theory was well accept-ed, and it eventually became importantin James Clerk Maxwells theory of electromagnetic radiation.

Ironically for the field of infrared

A Historical Perspective1


Figure 1-2: Glass Manufacture Using Visual IR Temperature Measurement


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thermometry, infrared radiation was

first discovered by using a conven-tional thermometer. FriedrickWilliam Herschel (1738-1822), a scien-tist and astronomer, is known as thefather of sidereal astronomy. Hestudied the planets and was the firstscientist to fully describe the MilkyWay galaxy. He also contributed tothe study of the solar system and thenature of solar radiation. In 1800,England, he was experimenting withsunlight. While using colored glasses

to look at the Sun, Herschel noticedthat the sensation of heat was notcorrelated to visible light (Figure 1-4).This led him to make experimentsusing mercury thermometers andglass prisms and to correctlyhypothesize the existence of theinvisible infrared heat waves. UntilHerschel, no one had thought toput a thermometer and a prismtogether to try to measure theamount of heat in each color.

In 1800, Herschel had formed a sun-light spectrum and tested differentparts of it with a thermometer to seeif some colors delivered more heatthan others. He found that the tem-

perature rose as he moved toward the

red end of the spectrum, and itseemed sensible to move the ther-mometer just past the red end in orderto watch the heating effect disappear.It did not. Instead, the temperaturerose higher than ever at a spotbeyond the red end of the spectrum(Figure 1-4). The region was calledinfrared, which means below the red.

How to interpret the region wasnot readily apparent. The firstimpression was that the sun deliv-

ered heat rays as well as light raysand that heat rays refracted to a less-er extent than light rays. A half-cen-tury passed before it was establishedthat infrared radiation had all theproperties of light waves except thatit didnt affect the retina of the eyein such a way as to produce a sensa-tion of light.

The German physicist Joseph vonFraunhofer (1787-1826) investigatedthe solar spectrum in the early 1800s.

His spectroscope introduced parallelrays of white light by passing sunlightthrough a slit. The light contacted aprism, where the prism broke thelight into its constituent rays. He pro-

duced an innumerable amount of

lines, each an image of the slit andeach containing a very narrow bandof wavelengths. Some wavelengthswere missing however. The slitimages at those wavelengths weredark. The result was that the solarspectrum was crossed by dark lines.These lines would later becomeimportant to the study of emissionand radiation.

In 1864, James Clerk Maxwell (1831-1879) brought forth for the first time

the equations which comprise thebasic laws of electromagnetism. Theyshow how an electric charge radiateswaves through space at various defi-nite frequencies that determine thecharges place in the electromagneticspectrumnow understood toinclude radio waves, microwaves,infrared waves, ultraviolet waves,X-rays, and gamma rays.

In addition, Maxwells equationsmost profound consequence was a

theoretical derivation of the speedof electricity300,000 km/sec.extremely close to the experimen-tally derived speed of light. Maxwellobserved and wrote, The velocity is



Figure 1-3: Newton Splits, Recombines White Light


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so nearly that of light, that it seemswe have strong reason to concludethat light itselfis an electromag-netic disturbance in the form of waves propagated through the elec-tromagnetic field according to elec-tromagnetic laws. Maxwell wasable to predict the entire electro-

magnetic spectrum.Another German, physiologist and

physicist Hermann von Helmholtz(1821-1894), accepted Maxwells theo-ry of electromagnetism, recognizingthat the implication was a particletheory of electrical phenomena. If we accept the hypothesis that theelementary substances [elements]are composed of atoms, statedHelmholtz in 1881, we cannot avoidconcluding that electricity, also, pos-

itive as well as negative, is dividedinto elementary portions whichbehave like atoms of electricity.

Gustav Robert Kirchhoff (1824-1887), a physicist and mathematician,worked with Robert Bunsen (1811-1899), an inorganic chemist and aphysicist, in 1859 on a spectrometerthat contained more than one prism.The spectroscope permitted greaterseparation of the spectral lines than

could be obtained by Fraunhofersspectroscope. They were able toprove that each chemical elementemits a characteristic spectrum of light that can be viewed, recorded,and measured. The realization thatbright lines in the emission spectraof the elements exactly coincided

in wavelength with the dark lines inthe solar spectrum indicated thatthe same elements that were emit-ting light on earth were absorbinglight in the sun. As a consequence of this work, in 1859, Kirchhoff devel-oped a general theory of emissionand radiation known as Kirchhoffslaw. Simply put, it states that a sub-stances capacity to emit light isequivalent to its ability to absorb itat the same temperature.

The following year, Kirchhoff, setforth the concept of a blackbody.This was one of the results of Kirchhoffs law of radiation. A black-body is defined as any object thatabsorbs all frequencies of radiationwhen heated and then gives off allfrequencies when cooled. This devel-opment was fundamental to thedevelopment of radiation thermom-etry. The blackbody problem arose

because of the observation thatwhen heating an iron rod, for exam-ple, it gives off heat and light. Itsradiation may be at first invisible, orinfrared, however it then becomesvisible and red-hot. Eventually itturns white hot, which indicates thatit is emitting all colors of the spec-

trum. The spectral radiation, whichdepends only on the temperature towhich the body is heated and not onthe material of which it is made,could not be predicted by classicalphysics. Kirchhoff recognized that itis a highly important task to find thisuniversal function. Because of itsgeneral importance to the under-standing of energy, the blackbodyproblem eventually found a solution.

An Austrian physicist, Josef Stefan

(1835-1893) first determined the rela-tion between the amount of energyradiated by a body and its tempera-ture. He was particularly interested inhow hot bodies cooled and howmuch radiation they emitted. Hestudied hot bodies over a consider-able range of temperatures, and in1879 determined from experimentalevidence that the total radiationemitted by a blackbody varies as the



Figure 1-4: Herschel Discovers Infrared Light

I n f r a r e d

Thermometers

Prism

R e d

O r a n g e

Y e l l o w

G r e e n B l u e

V i o l e t


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fourth power of its absolute temper-

ature (Stefans law). In 1884, one of hisformer students, Ludwig Boltzmann(1844-1906), determined a theoreticalderivation for Stefans experimentallyderived law of blackbody radiationbased on thermodynamic principlesand Maxwells electromagnetic theo-ry. The law, now known as the Stefan-Boltzmann fourth-power law, formsthe basis for radiation thermometry.It was with this equation that Stefanwas able to make the first accurate

determination of the surface tem-perature of the sun, a value of approximately 11,000F (6,000C).

The next quandary faced by theseearly scientists was the nature of the thermal radiation emitted byblackbodies. The problem was chal-lenging because blackbodies did notgive off heat in the way the scien-tists had predicted. The theoreticalrelationship between the spectralradiance of a blackbody and its

thermodynamic temperature wasnot established until late in thenineteenth century.

Among the theories proposed toexplain this inconsistency was one bythe German physicist Wilhelm Wienand the English physicist JohnRayleigh. Wilhelm Wien (1864-1928)measured the wavelength distributionof blackbody radiation in 1893. A plotof the radiation versus the wavelengthresulted in a series of curves at differ-

ent temperatures. With this plot, hewas able to show that the peak valueof wavelength varies proportionallywith the amount of energy, andinversely with absolute temperature.As the temperature increases, notonly does the total amount of radia-tion increase, in line with Stefans find-ings, but the peak wavelengthdecreases and the color of the emit-ted light changes from red to orange

to yellow to white.

Wien attempted to formulate anempirical equation to fit this rela-tionship. The complex equationworked well for high frequencyblackbody radiation (short wave-lengths), but not for low frequencyradiation (long wavelengths).Rayleighs theory was satisfactory forlow frequency radiation.

In the mid-1890s, Max Karl ErnstLudwig Planck (1858-1947), a Germanphysicist and a former student of

Kirchhoff, and a group of Berlinphysicists were investigating the lightspectrum emitted by a blackbody.Because the spectrometer emitteddistinct lines of light, rather thanbroad bands, they hypothesized thatminute structures were emitting thelight and began to develop an atom-ic theory that could account forspectral lines.

This was of interest to Planckbecause in 1859 Kirchhoff had dis-

covered that the quality of heatradiated and absorbed by a black-body at all frequencies reached anequilibrium that only depended on

temperature and not on the nature

of the object itself. But at any giventemperature, light emitted from aheated cavitya furnace, for exam-pleruns the gamut of spectral col-ors. Classical physics could not pre-dict this spectrum.

After several false starts, beginningin 1897, Planck succeeded in finding aformula predicting blackbody radia-tion. Planck was able to arrive at a for-mula that represented the observedenergy of the radiation at any given

wavelength and temperature. He gavethe underlying notion that light andheat were not emitted in a steadystream. Rather, energy is radiated indiscrete units, or bundles. Planck dis-covered a universal constant, Plancksconstant, which was founded onphysical theory and could be used tocompute the observed spectrum. Thisassumed that energy consisted of thesum of discrete units of energy hecalled quanta, and that the energy

emitted, E, by each quantum is givenby the equation E = h = hc/ , where (sec-1) is the frequency of the radia-tion and h is Plancks constantnow



Figure 1-5: The Sidewinder Missle's IR Guidance System

Reticle Locationand Centerof Rotation

Dome

Figure 1-5: The Sidewinder Missiles IR Guidance System


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known to be a fundamental constant

of nature. By thus directly relating theenergy of radiation to its frequency, anexplanation was found for the obser-vation that higher energy radiation hasa higher frequency distribution.Plancks finding marked a new era inphysics.

Before Plancks studies, heat wasconsidered to be a fluid composedof repulsive particles capable of combining chemically with materialatoms. In this theory, the particles of

heat entered a system and movedbetween the particles. A mutualrepulsion of the particles of heat cre-ated a pressure. A thermometerdetected this pressure. Plancks con-stant became known as a fortunateguess. It allowed for theoreticalequations which agreed with theobservable range of spectral phe-nomena, and was fundamental in thetheory of blackbody radiation.

Albert Einstein (1879-1955) studied

the works of Maxwell and Helmholtz.In 1905, Einstein used the quantum as atheoretical tool to explain the photo-electric effect, showing how light cansometimes act as a stream of particles.He published three papers in volumeXVII of Annalen der Physik. In one, heset forth his now famous theory of rel-ativity, but another showed that a fun-damental process in nature is at workin the mathematical equation whichhad resolved the problem of black-

body radiation.Light, Einstein showed, is a stream

of particles with a computableamount of energy using Plancksconstant. Within a decade, this pre-diction confirmed experimentallyfor visible light.

Max Karl Ernst Ludwig Planck initi-ated quantum theory at the turn of the twentieth century and changedthe fundamental framework of

physics. Wrote Einstein, He has given

one of the most powerful of allimpulses to the progress of science.

Todays ApplicationsThe first patent for a total radiationthermometer was granted in 1901.The instrument used a thermoelec-tric sensor; it had an electrical out-put signal and was capable of unat-tended operation. In 1931, the firstcommercially-available total radia-

tion thermometers were introduced.These devices were widely usedthroughout industry to record andcontrol industrial processes. They arestill used today, but mainly used forlow temperature applications.

The first modern radiation ther-mometers were not available untilafter the second World War. Originallydeveloped for military use, lead sulfidephotodetectors were the first infraredquantum detectors to be widely used

in industrial radiation thermometry.Other types of quantum detectorsalso have been developed for militaryapplications and are now widelyapplied in industrial radiation ther-mometry. Many infrared radiationthermometers use thermopile detec-tors sensitive to a broad radiationspectrum and are extensively used inprocess control instrumentation.

Infrared thermometers currently are

being used in a wide range of industri-

al and laboratory temperature controlapplications. By using non-contacttemperature sensors, objects that aredifficult to reach due to extreme envi-ronmental conditions can be moni-tored. They can also be used for prod-ucts that cannot be contaminated by acontact sensor, such as in the glass,chemical, pharmaceutical, and foodindustries. Non-contact sensors can beused when materials are hot, moving,or inaccessible, or when materials can-

not be damaged, scratched, or torn bya contact thermometer.

Typical industries in which non-contact sensors are used includeutilities, chemical processing, phar-maceutical, automotive, food pro-cessing, plastics, medical, glass, pulpand paper, construction materials,and metals. Industrially, they areused in manufacturing, quality con-trol, and maintenance and havehelped companies increase produc-

tivity, reduce energy consumption,and improve product quality.

Some applications of radiationthermometry include the heat treat-ing, forming, tempering, and anneal-ing of glass; the casting, rolling, forg-ing, and heat treating of metals; qual-ity control in the food and pulp andpaper industry; the extrusion, lamina-tion, and drying of plastics, paper,and rubber; and in the curing process



Figure 1-6: IR Optics for Missile Guidance

Reimaging Lens

Detector

Main Optics

Objects atInfinity

Reticle


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of resins, adhesives, and paints.

Non-contact temperature sensorshave been used and will continue tobe valuable for research in military,medical, industrial, meteorological,ecological, forestry, agriculture, andchemical applications.

Weather satellites use infraredimaging devices to map cloud pat-terns and provide the imagery seen inmany weather reports. Radiationthermometry can reveal the temper-ature of the earths surface even

through cloud cover.Infrared imaging devices also are

used for thermography, or thermalimaging. In the practice of medicine,for example, thermography has beenused for the early detection of breastcancer and for the location of thecause of circulatory deficiencies. Inmost of these applications, theunderlying principle is that patholo-gy produces local heating andinflammation which can be found

with an infrared imager. Other diag-nostic applications of infrared ther-mography range from back problemsto sinus obstructions.

Edge burning forest fires have beenlocated using airborne infraredimagers. Typically, the longer wave-lengths of the emitted infrared radia-tion penetrate the smoke better thanthe visible wavelengths, so the edgesof the fire are better delineated.

On the research front, one sophis-

ticated infrared thermometry appli-cation is in the study of faults in met-als, composites, and at coating inter-faces. This technique is known aspulsed video thermography. A com-posite material consisting of a car-bon-fiber skin bonded to an alu-minum honeycomb is subjected to

pulses of heat from a xenon flash

tube. Infrared cameras record aframe-by-frame sequence of heatdiffusion through the object, whichis displayed on screen. Defects showup as deviations in the expected pat-terns for the material being tested.

Among the military applications of radiation thermometry are night-visionand the heat-seeking missile. In thelatter case, the operator simply launch-es the missile in the general directionof the target. On-board detectors

enable the missile to locate the targetby tracking the heat back to the source.The most widely known militaryinfrared missile applications are theSidewinder air-to-air missile and asatellite-borne intercontinental ballis-tic missile (ICBM) detection system.

Both rely on detecting the infrared

signature of an emission plume or very

hot exhaust engine. The Sidewindermissile guidance system is shownschematically in Figure 1-5. A specialinfrared dome protects the opticalsystem inside. The optical system con-sists of a primary and secondary mirrorand a set of correction lenses to causean image to focus onto a special reti-cle. All the light from the reticle isfocused onto a detector (Figure 1-6).The reticle can modulate the radiationto distinguish between clouds and

provide directional information.Portable surface-to-air missiles,

SAMs, are effective defense unitsthat guide themselves to a target bydetecting and tracking the heat emit-ted by an aircraft, particularly theengine exhaust. T



References and Further Reading Album of Science, The 19th Century , Pearce L. Williams, Charles ScribnersSons, 1978.

Asimovs Chronology of Science and Discovery , Isaac Asimov,HarperCollins Publishers, 1994. The Biographical Dictionary of Scientists , 2nd ed., Oxford University Press,1994. Dictionary of Scientific Biography , Vols. 9, 10, 11, Charles C. Gillispile,Charles Scribners Sons, 1973. Engineering in History , Richard S. Kirby and Sidney Withington, Arthur B.Darling, Frederick G. Kilgour, McGraw-Hill, 1956. The Invisible World of the Infrared , Jack R. White, Dodd, Mead & Company, 1984. The McGraw-Hill Encyclopedia of Science and Technology , 8th ed., Vol. 9,McGraw-Hill, 1997.

Notable Twentieth-Century Scientists , Emily J. McMurray, Gale ResearchInc., 1995. Pioneers of Modern Science, The World of Science , Bill MacKeith,Andromeda Oxford Limited, 1991. The Scientific 100. A Ranking of the Most Influential Scientists, Past and Present , John Simmons, Carol Publishing Group, 1996. Theory and Practice of Radiation Thermometry , David P. DeWitt., and


17/86

All matteranimate or inani-mate, liquid, solid, or gasconstantly exchanges ther-mal energy in the form of

electromagnetic radiation with itssurroundings. If there is a tempera-ture difference between the objectin question and its surroundings,

there will be a net energy transfer inthe form of heat; a colder object willbe warmed at the expense of its sur-roundings, a warmer object cooled.And if the object in question is at thesame temperature as its surrounding,the net radiation energy exchangewill be zero.

In either case, the characteristicspectrum of the radiation dependson the object and its surroundingsabsolute temperatures. The topic of

this volume, radiation thermometry,or more generally, non-contact tem-perature measurement, involves tak-ing advantage of this radiationdependence on temperature tomeasure the temperature of objectsand masses without the need fordirect contact.

.Radiation BasicsThe development of the mathemat-

ical relationships to describe radia-tion were a major step in the devel-opment of modern radiation ther-mometry theory. The ability toquantify radiant energy comes,appropriately enough, from Plancksquantum theory. Planck assumedthat radiation was formed in dis-crete energy packages called pho-tons, or quanta, the magnitude of which are dependent on the wave-

length of the radiation. The totalenergy of a quantum, E, is found bymultiplying Plancks constant, h =6.6256 x 10-34, and, the radiation fre-quency, , in cycles per second.

In 1905, Albert Einstein postulat-ed that these quanta are particlesmoving at the speed of light, c =

2.9979 x 108

m/s. If these photonstraveled at the speed of light, thenthey must obey the theory of rela-tivity, stating E2 = c2p2 , and eachphoton must have the momentum p= E/c = h/ . The frequency can befound by dividing the speed of lightby its particle wavelength = c/ .Substituting for momentum:

E = h = hc/

From this equation, it is apparentthat the amount of energy emitteddepends on the wavelength (or fre-quency). The shorter the wave-length, the higher the energy.

Emitted radiation consists of a

continuous, non-uniform distributionof monochromatic (single-wave-length) components, varying widelywith wavelength and direction. Theamount of radiation per unit wave-length interval, referred to as thespectral concentration, also varieswith wavelength. And the magnitude

of radiation at any wavelength aswell as the spectral distribution varywith the properties and temperatureof the emitting surface. Radiation isalso directional. A surface may prefera particular direction to radiate ener-gy. Both spectral and directional dis-tribution must be considered instudying radiation.

Wavelength can be thought of as atype of address to find where a raysenergy is located. The map contain-

ing all the wavelengths of electro-magnetic radiation is called the elec-tromagnetic spectrum (see the insidefront cover of this volume). The shortwavelengths are the gamma rays,X-rays, and ultraviolet (UV) radiation,


Radiation Basics

Blackbody Concepts

From Blackbodies to Real Surfa

NON-CONTACT TEMPERTheoretical Developm

2

ATheoretical Development

Figure 2-1: Radiation Energy Balance

Transmitted Energy

Radiant Energy

AbsorbedEnergy

ReflectedEnergy


18/86

containing the highest amount of

energy emitted. The intermediateportion of the spectrum, the heatregion, extends from approximately0.1 to 1000 m (micrometers ormicrons: 1,000,000 microns = 1 meter),and includes a portion of the ultravio-let and all of the visible (VIS) andinfrared (IR) waves. This portion istermed thermal radiation, and isimportant in the study of heat trans-fer and radiation thermometry.

Non-contact temperature sensors

work in the infrared portion of thespectrum. The infrared range fallsbetween 0.78 microns and 1000microns in wavelength, and is invisi-ble to the naked eye. The infrared isregion can be divided into threeregions: near-infrared (0.78-3.0microns); middle infrared (3-30microns); and far infrared (30-300microns). The range between 0.7microns and 14 microns is normallyused in infrared temperature mea-

surement. The divisions have been

related to the transmission of theatmosphere for different types of applications.

Blackbody ConceptsIncident energy striking an objectfrom the surroundings, can beabsorbed by the object, reflected bythe object, or transmitted throughthe object (if it is not opaque) asseen in Figure 2-1. If the object is at

a constant temperature, then therate at which it emits energy mustequal the rate at which it absorbsenergy, otherwise the object wouldcool (emittance greater thanabsorption), or warm (emittanceless than absorption). Therefore,for bodies at constant temperature,the emittance (absorption), thereflection and the transmittance of energy equals unity.

Central to radiation thermometry

is the concept of the blackbody. In

1860, Kirchhoff defined a blackbodyas a surface that neither reflects ortransmits, but absorbs all incidentradiation, independent of directionand wavelength. The fraction of radi-ation absorbed by a real body iscalled absorptivity, . For an idealblackbody, the absorptivity is 1.0 ( b= 1). For non-blackbodies, the absorp-tion is a fraction of the radiation heattransfer incident on a surface, or 0 1. Hence, in term of radiation heat

transfer, q:

qabsorbed = qincident

In addition to absorbing all inci-dent radiation, a blackbody is a per-fect radiating body. To describe theemitting capabilities of a surface incomparison to a blackbody,Kirchoff defined emissivity () of areal surface as the ratio of the ther-mal radiation emitted by a surface

at a given temperature to that of ablackbody at the same temperatureand for the same spectral anddirectional conditions.

This value also must be consideredby a non-contact temperature sensorwhen taking a temperature measure-ment. The total emissivity for a realsurface is the ratio of the total amountof radiation emitted by a surface incomparison to a blackbody at thesame temperature. The tables begin-

ning on p. 72 give representative emis-sivity values for a wide range of mate-rials. If precise temperature measure-ments are required, however, the sur-faces actual emittivity value should beobtained. (Although often used inter-changeably, the terms emissivity andemittivity have technically differentmeanings. Emissivity refers to a prop-erty of a material, such as cast iron,whereas emittivity refers to a property



Figure 2-2: Spectral Distributions

R e

l a t i v

e E n e r g y

Wavelength, Microns

=1.0 (Blackbody)

=0.9 (Graybody)

varies withwavelength

(Non-graybody)


19/86

of a specific surface.)

In 1879, Stefan concluded basedon experimental results that theradiation emitted from the surfaceof an object was proportional tothe fourth power of the absolutetemperature of the surface. Theunderlying theory was later devel-oped by Boltzmann, who showedthat the radiation given off by ablackbody at absolute temperatureTs (K) is equal to:

q = Ts4

where ( is the Stefan-Boltzmann con-stant ( = 5.67 x 10-8 W/m 2 K4 ). Theheat transfer rate by radiation for anon-blackbody, per unit area isdefined as:

q = (Ts4 - Tsur4)

where T s is the surface temperatureand Tsur is the temperature of the

surroundings.Although some surfaces come

close to blackbody performance, allreal objects and surfaces have emis-sivities less than 1. Non-blackbodyobjects are either graybodies, whoseemissivity does vary with wave-length, or non-graybodies, whoseemissivities vary with wavelength.Most organic objects are graybodies,with an emissivity between 0.90 and0.95 (Figure 2-2).

The blackbody concept is impor-tant because it shows that radiantpower depends on temperature.When using non-contact tempera-ture sensors to measure the energyemitted from an object, dependingon the nature of the surface, theemissivity must be taken intoaccount and corrected. For example,an object with an emissivity of 0.6 isonly radiating 60% of the energy of a

blackbody. If it is not corrected for,

the temperature will be lower thanthe actual temperature. For objectswith an emissivity less than 0.9, theheat transfer rate of a real surface isidentified as:

q = Ts4

The closest approximation to ablackbody is a cavity with an interi-or surface at a uniform temperatureTs, which communicates with the

surroundings by a small hole havinga diameter small in comparison tothe dimensions of the cavity (Figure2-3). Most of the radiation enteringthe opening is either absorbed orreflected within the cavity (to ulti-mately be absorbed), while negligi-ble radiation exits the aperture. Thebody approximates a perfectabsorber, independent of the cavi-tys surface properties.

The radiation trapped within the

interior of the cavity is absorbed andreflected so that the radiation withinthe cavity is equally distributedsome radiation is absorbed and somereflected. The incident radiant ener-gy falling per unit time on any sur-face per unit area within the cavity isdefined as the irradiance G (W/m

2 m). If the total i rradiation G (W/m 2)represents the rate at which radiationis incident per unit area from alldirections and at all wavelengths, it

follows that:

G = 0G (d)

If another blackbody is brought intothe cavity with an identical tempera-ture as the interior walls of the cavi-ty, the blackbody will maintain itscurrent temperature. Therefore, therate at which the energy absorbed bythe inner surface of the cavity will

equal the rate at which it is emitted.

In many industrial applications,transmission of radiation, such asthrough a layer of water or a glassplate, must be considered. For a spec-tral component of the irradiation,portions may be reflected, absorbed,and transmitted. It follows that:

G = G,ref + G,abs + G,tran

In many engineering applications,however, the medium is opaque to

the incident radiation. Therefore,G,tran = 0, and the remaining absorp-tion and reflection can be treated assurface phenomenon. In otherwords, they are controlled byprocesses occurring within a frac-tion of a micrometer from the irra-diated surface. It is therefore appro-priate to say that the irradiation isabsorbed and reflected by the sur-face, with the relative magnitudesof G ,ref and G,abs depending on the

wavelength and the nature of thesurface.

Radiation transfer by a non-black-body encompasses a wide range of wavelengths and directions. Thespectral hemispherical emissivepower, E (W/m

2 m) is defined asthe rate at which radiation is emit-ted per unit area at all possiblewavelengths and in all possibledirections from a surface, per unitwavelength d about and per unit

surface area.Although the directional distribu-

tion of surface emission variesdepends on the surface itself, manysurfaces approximate diffuse emit-ters. That is, the intensity of emittedradiation is independent of thedirection in which the energy is inci-dent or emitted. In this case, thetotal, hemispherical (spectral) emis-sive power, E (W/m

2) is defined as:




20/86

E() = ,e()

or

E =e

where e is the total intensity of theemitted radiation, or the rate at whichradiant energy is emitted at a specificwavelength, per unit area of the emit-ting surface normal to the direction,per unit solid angle about this direc-

tion, and per unit wavelength. Noticethat E is a flux based on the actualsurface area, where e is based on theprojected area. In approximating ablackbody, the radiation is almostentirely absorbed by the cavity. Anyradiation that exits the cavity is due tothe surface temperature only.

The spectral characteristics of blackbody radiation as a function of temperature and wavelength weredetermined by Wilhelm Wien in

1896. Wien derived his law for thedistribution of energy in the emissionspectrum as:

E,b(,T) = 2h2/ 5 [exp(hc o/ kT)]

where E,b (b for blackbody) representsthe intensity of radiation emitted by ablackbody at temperature T, and wave-length per unit wavelength interval,per unit time, per unit solid angle, perunit area. Also, h = 6.626 x 10-24 Js and

k = 1.3807 x 10-23

JK-1

are the universalPlanck and Boltzman constants,respectively; c o = 2.9979 x 10

8 m/s isthe speed of light in a vacuum, and T isthe absolute temperature of the black-body in Kelvins (K).

Due to the fact that deviationsappeared between experimentalresults and the equation, Planck sug-gested in 1900 a refinement thatbetter fit reality:

E,b(,T) = 2h2

/ 5

[exp(hc o/ kT) - 1]

It is from this equation that Planckpostulated his quantum theory. Amore convenient expression for thisequation, referred to as the Planckdistribution law (Figure 2-4), is:

E,b(,T) =I,b(,T)=C1/ 5[exp(C2/ T) - 1]

where the first and second radiation

constants are C 1 = 2hco2

= 3.742 x 108

W m4/m 2 and C2 = (hco/k) = 1.439x 104 m K.

Plancks distribution shows that aswavelength varies, emitted radiationvaries continuously. As temperatureincreases, the total amount of energyemitted increases and the peak of the curve shifts to the left, or towardthe shorter wavelengths. In consider-ing the electromagnetic spectrum, itis apparent that bodies with very

high temperatures emit energy in the

visible spectrum as wavelengthdecreases. Figure 2-4 also shows thatthere is more energy difference perdegree at shorter wavelengths.

From Figure 2-4, the blackbodyspectral distribution has a maximumwavelength value, max, whichdepends on the temperature. By dif-ferentiating equation 2.12 withrespect to and setting the resultequal to zero:

maxT = C3

where the third radiation constant,C3 = 2897.7m K. This is known asWiens displacement law. Thedashed line in Figure 2-4 defines thisequation and locates the maximumradiation values for each tempera-ture, at a specific wavelength.Notice that maximum radiance isassociated with higher temperaturesand lower wavelengths.



,

Figure 2-3: An Isothermal Blackbody Cavity


21/86

From Blackbodies to Real SurfacesAt first it would seem that a radiation

thermometer would utilize the entirespectrum, capturing most of the radi-ant emission of a target in its particu-lar temperature range. There are sev-eral reasons why this is not practical.

In the equations for infrared radia-tion derived above, it was found thatat very low wavelengths, the radianceincreases rapidly with temperature, incomparison to the increase at higherwavelengths, as shown in Figure 2-4.Therefore, the rate of radiance change

is always greater at shorter wave-lengths. This could mean more precisetemperature measurement and tightertemperature control. However, at agiven short wavelength there is alower limit to the temperature thatcan be measured. As the process tem-perature decreases, the spectral rangefor an infrared thermometer shifts tolonger wavelengths and becomes lessaccurate.

The properties of the material atvarious temperatures must also be

considered. Because no materialemits as efficiently as a blackbody ata given temperature, when measuringthe temperature of a real target,other considerations must be made.Changes in process material emissivi-ty, radiation from other sources, andlosses in radiation due to dirt, dust,smoke, or atmospheric absorptioncan introduce errors.

The absorptivity of a material isthe fraction of the irradiation

absorbed by a surface. Like emission,it can be characterized by both adirectional and spectral distribution.It is implicit that surfaces may exhib-it selective absorption with respectto wavelength and direction of theincident radiation. However, for mostengineering applications, it is desir-able to work with surface propertiesthat represent directional averages.The spectral, hemispherical absorp-

tivity for a real surface ()isdefined as:

() G,abs()/G ()

where G ,abs is the portion of irradia-tion absorbed by the surface. Hence, depends on the directional distri-bution of the incident radiation, aswell as on the wavelength of theradiation and the nature of theabsorbing surface. The total, hemi-spherical absorptivity, , representsan integrated average over both

directional and wavelength. It isdefined as the fraction of the totalirradiation absorbed by a surface, or:

Gabs/G

The value of depends on thespectral distribution of the incidentradiation, as well as on its direc-tional distribution and the natureof the absorbing surface. Although



S p e c t r a l E m i s s i v e

P o w

e r ,

E , b

, W / m

2 . m

Wavelength, , m

0.1

109

0.2 0.4 0.6 1.0 2.0 4.0 6.0 2010 40 60 100

108

107

106

105

104

103

102

101

100

10-1

10-2

10-3

10-4

5800 K

100 K

300 K

800 K1000 K

2000 K

50 K

Visible Spectral Region

maxT=2898 m . K

Solar Radiation

Figure 2-4: Planck Prediction of Blackbody Emissive Power


22/86

is independent on the tempera-

ture of the surface, the same maynot be said for the total, hemi-spherical emissivity. Emissivity isstrongly temperature dependent.

The reflectivity of a surfacedefines the fraction of incidentradiation reflected by a surface. Itsspecific definition may take severaldifferent forms. We will assume areflectivity that represents an inte-grated average over the hemisphereassociated with the reflected radia-

tion to avoid the problems fromthe directional distribution of thisradiation. The spectral, hemispheri-cal reflectivity (), then, isdefined as the spectral irradiationthat is reflected by the surface.Therefore:

() G,ref ()/G ()

where G ,ref is the portion of irradia-tion reflected by the surface. The

total, hemispherical reflectivity isthen defined as:

Gref /G

If the intensity of the reflected

radiation is independent of thedirection of the incident radiationand the direction of the reflectedradiation, the surface is said to bediffuse emitter. In contrast, if theincident angle is equivalent to thereflected angle, the surface is aspecular reflector. Although no sur-face is perfectly diffuse or specular,specular behavior can be approxi-mated by polished or mirror-likesurfaces. Diffuse behavior is closely

approximated by rough surfacesand is likely to be encountered inindustrial applications.

Transmissivity is the amount of radiation transmitted through a sur-face. Again, assume a transmissivitythat represents an integrated aver-age. Although difficult to obtain aresult for transparent media, hemi-spherical transmissivity is defined as:

= G,tr()/G ()

where G ,tr is the portion of irradia-tion reflected by the surface. Thetotal hemispherical transmissivity is:

= Gtr /G

The sum of the total fractions of energy absorbed ( ), reflected (),and transmitted ( ) must equal thetotal amount of radiation incidenton the surface. Therefore, for anywavelength:

+ + = 1

This equation applies to a semitrans-

parent medium. For properties thatare averaged over the entire spec-trum, it follows that:

+ + = 1

For a medium that is opaque, thevalue of transmission is equal to zero.Absorption and reflection are sur-face properties for which:

+ = 1

and

+ = 1



S p e c t r a

l T r a n s m i t t a n c e

,

Wavelength, , m2.5 3 4 5 6 7 8

1.0

0.8

0.6

0.4

0.2

0109

3.43 Microns 4.8 to 5.3 Microns 7.9 Microns

0.009 In. Thick

0.027 In. Thick

0.061 In. Thick

0.124 In. Thick

0.231 In. Thick

Figure 2-5: Soda-Lime Glass Spectral Transmittance


23/86

For a blackbody, the transmitted and

reflected fractions are zero and theemissivity is unity.

An example of a material whoseemissivity characteristics change rad-ically with wavelength is glass. Soda-lime glass is an example of a materialwhich drastically changes its emissiv-ity characteristics with wavelength(Figure 2-5). At wavelengths belowabout 2.6 microns, the glass is highlytransparent and the emissivity isnearly zero. Beyond 2.6 microns, the

glass becomes increasingly moreopaque. Beyond 4 microns, the glassis completely opaque and the emis-sivity is above 0.97. T



References and Further Reading Temperature Measurement in Engineering , H. Dean Baker, E.A. Ryder, andN.H. Baker, Omega Press, 1975. Heat and Thermodynamics , 6th ed., Mark W. Zemansky, and Richard H.Dittman, McGraw-Hill, 1981. Industrial Temperature Measurement , Thomas W. Kerlin and Robert L.Shepard, Publishers Creative Series, Inc., ISA. Introduction to Heat Transfer , 2nd ed., Frank P. Incropera, and David P.DeWitt, John Wiley & Sons, 1990. The Invisible World of the Infrared , Dodd, Jack R. White, Mead & Company, 1984. Process/Industrial Instruments and Controls Handbook , 4th ed., DouglasM. Considine, McGraw-Hill, 1993.

Theory and Practice of Radiation Thermometry , David P. DeWitt andGene D. Nutter, John Wiley & Sons, 1988. Thermodynamics , 5th ed., Virgil M. Faires, The Macmillan Company, 1971.


24/86


25/86

There is, however, a technical dis-

tinction. Emissivity refers to theproperties of a material; emittanceto the properties of a particularobject. In this latter sense, emissivi-ty is only one component in deter-mining emittance. Other factors,including shape of the object, oxi-dation and surface finish must betaken into account.

The apparent emittance of amaterial also depends on the tem-perature at which it is determined,

and the wavelength at which themeasurement is taken. Surface condi-tion affects the value of an objectsemittance, with lower values forpolished surfaces, and higher valuesfor rough or matte surfaces. In addi-tion, as materials oxidize, emittancetends to increase, and the surfacecondition dependence decreases.Representative emissivity values for arange of common metals and non-metals at various temperatures are

given in the tables starting on p. 72.The basic equation used to

describe the output of a radiationthermometer is:

V (T) = K TN

Where: = emittivity V(T) =thermometer output with

temperatureK =constant

T = object temperatureN = N factor ( = 14388/( T )) = equivalent wavelength

A radiation thermometer with thehighest value of N (shortest possibleequivalent wavelength) should beselected to obtain the least depen-dence on target emittance changes.The benefits of a device with a highvalue of N extends to any parameter

that effects the output V. A dirty

optical system, or absorption of energy by gases in the sighting path,has less effect on an indicated tem-perature if N has a high value.

The values for the emissivities of

almost all substances are known andpublished in reference literature.

However, the emissivity determinedunder laboratory conditions seldomagrees with actual emittance of anobject under real operating condi-tions. For this reason, one is likely touse published emissivity data whenthe values are high. As a rule of thumb,most opaque non-metallic materialshave a high and stable emissivity (0.85to 0.90). Most unoxidized, metallicmaterials have a low to medium emis-sivity value (0.2 to 0.5). Gold, silver

and aluminum are exceptions, withemissivity values in the 0.02 to 0.04range. The temperature of these met-als is very difficult to measure with aradiation thermometer.

One way to determine emissivityexperimentally is by comparing theradiation thermometer measurementof a target with the simultaneousmeasurement obtained using a ther-mocouple or RTD. The difference in

readings is due to the emissivity,

which is, of course, less than one. Fortemperatures up to 500F (260C)emissivity values can be determinedexperimentally by putting a piece of black masking tape on the target sur-

face. Using a radiation pyrometer setfor an emissivity of 0.95, measure the

temperature of the tape surface(allowing time for it to gain thermalequilibrium). Then measure the tem-perature of the target surface with-out the tape. The difference in read-ings determines the actual value forthe target emissivity.

Many instruments now have cali-brated emissivity adjustments. Theadjustment may be set to a value of emissivity determined from tables,such as those starting on p. 72, or

experimentally, as described in thepreceding paragraph. For highestaccuracy, independent determina-tion of emissivity in a lab at thewavelength at which the thermome-ter measures, and possibly at theexpected temperature of the target,may be necessary.

Emissivity values in tables havebeen determined by a pyrometersighted perpendicular to the target.

IR Thermometers & Pyromet3


Figure 3-2: Effect of Non-Blackbody Emissivity on IR Thermometer Error

1500 2000 2500 3000 3500 4000 4500 5000

800

700

600

500

400

300

200

100

E r r o r -

F

True Temperature, F

E = 0 .

3

E = 0. 5

E = 0 . 7

E=0 . 9


26/86

If the actual sighting angle is more

than 30 or 40 degrees from the nor-mal to the target, lab measurementof emissivity may be required.

In addition, if the radiation pyrom-eter sights through a window, emissiv-ity correction must be provided forenergy lost by reflection from the twosurfaces of the window, as well asabsorption in the window. For exam-ple, about 4% of radiation is reflectedfrom glass surfaces in the infraredranges, so the effective transmittance

is 0.92. The loss through other materi-als can be determined from the indexof refraction of the material at thewavelength of measurement.

The uncertainties concerningemittance can be reduced usingshort wavelength or ratio radiationthermometers. Short wavelengths,around 0.7 microns, are useful

because the signal gain is high in this

region. The higher response outputat short wavelengths tends toswamp the effects of emittance vari-ations. The high gain of the radiatedenergy also tends to swamp theabsorption effects of steam, dust orwater vapor in the sight path to thetarget. For example, setting thewavelength at such a band will causethe sensor to read within 5 to 10degrees of absolute temperaturewhen the material has an emissivity

of 0.9 ( 0.05). This represents about1% to 2% accuracy.

Types of Radiation ThermometersHistorically, as shown in Figure 3-1, aradiation thermometer consisted of an optical system to collect theenergy emitted by the target; a

detector to convert this energy to an

electrical signal; an emittivity adjust-ment to match the thermometer cal-ibration to the specific emittingcharacteristics of the target, and anambient temperature compensationcircuit, to ensure that temperaturevariations inside the thermometerdue to ambient conditions did notaffect accuracy.

The modern radiation thermome-ter is still based on this concept.However the technology has

become more sophisticated towiden the scope of the applicationsthat can be handled. For example,the number of available detectorshas greatly increased, and, thanks toselective filtering capabilities, thesedetectors can more efficiently bematched to specific applications,improving measurement perfor-mance. Microprocessor-based elec-tronics can use complex algorithmsto provide real time linearization and

compensation of the detector out-put for higher precision of measuredtarget temperature. Microprocessorscan display instantaneous measure-ments of several variables (such ascurrent temperature, minimum tem-perature measured, maximum tem-perature measured, average tempera-ture or temperature differences) onintegral LCD display screens.

A convenient classification of radi-ation thermometers is as follows:

Broadband radiation ther-mometers/pyrometers; Narrow band radiation ther-mometers/pyrometers; Ratio radiation thermometers/pyrometers; Optical pyrometers; and Fiber optic radiation ther-mometers/pyrometers.

These classifications are not rigid.For example, optical pyrometers can

IR Thermometers & Pyrometers3


I n t e n s i t y

Wavelength

Figure 3-3: Blackbody Radiation in the Infrared

5 m 10 m

200F

400F

600F

800F

1200F

Ultraviolet

VisibleLight

Infrared Radio Waves

1n m .77 1mm 1m 1km

B l u e

R e

d X - R

a y s

Wavelength


27/86

be considered a subset of narrow

band devices. Fiber optic radiationthermometers, to be discussed indetail in another section, can beclassified as wide band, narrowband, or ratio devices. Likewise,infrared radiation thermometers canbe considered subsets of several of these classes.

Broadband RadiationBroadband radiation thermometers

typically are the simplest devices,cost the least, and can have aresponse from 0.3 microns wave-length to an upper limit of 2.5 to 20microns. The low and high cut-offsof the broadband thermometer are afunction of the specific optical sys-tem being used. They are termedbroadband because they measure asignificant fraction of the thermalradiation emitted by the object, in thetemperature ranges of normal use.

Broadband thermometers aredependent on the total emittance of the surface being measured. Figure 3-2 shows the error in reading for vari-ous emissivities and temperatureswhen a broadband device is calibrat-ed for a blackbody. An emissivitycontrol allows the user to compen-sate for these errors, so long as theemittance does not change.

The path to the target must beunobstructed. Water vapor, dust,

smoke, steam and radiation absorp-tive gases present in the atmospherecan attenuate emitted radiation fromthe target and cause the thermome-ter to read low.

The optical system must be keptclean, and the sighting window pro-tected against any corrosives in theenvironment.

Standard ranges include 32 to1832F (0 to 1000C), and 932 to 1652F

(500 to 900C). Typical accuracy is

0.5 to 1% full scale.

Narrow Band RadiationAs the name indicates, narrow bandradiation thermometers operate overa narrow range of wavelengths.Narrow band devices can also bereferred to as single color ther-

mometers/pyrometers (see OpticalPyrometers). The specific detectorused determines the spectral

response of the particular device.For example, a thermometer using asilicon cell detector will have aresponse that peaks at approximate-ly 0.9 microns, with the upper limitof usefulness being about 1.1microns. Such a device is useful formeasuring temperatures above1102F (600C). Narrow band ther-mometers routinely have a spectralresponse of less than 1 micron.

Narrow band thermometers use

filters to restrict response to aselected wavelength. Probably themost important advance in radiationthermometry has been the introduc-tion of selective filtering of theincoming radiation, which allows aninstrument to be matched to a par-ticular application to achieve highermeasurement accuracy. This was

made possible by the availability of more sensitive detectors andadvances in signal amplifiers.

Common examples of selectivespectral responses are 8 to 14microns, which avoids interferencefrom atmospheric moisture over longpaths; 7.9 microns, used for the mea-surement of some thin film plastics; 5microns, used for the measurementof glass surfaces; and 3.86 microns,which avoids interference from car-bon dioxide and water vapor inflames and combustion gases.



Figure 3-4: The 'Two-Color' IR Thermometer

5

4

3

2

1

R e

l a t i v e E n e r g y R a

d i a t e

d ,

H

Wavelength, .4 .8 1.2 1.6 Microns

1 2

T1

T2

T3

T4

T5

Ratio H 1/H 2 changes as a function of temperature (T 1...T5)


28/86

The choice of shorter or longer

wavelength response is also dictatedby the temperature range. The peaksof radiation intensity curves move

towards shorter wavelengths as tem-perature increases, as shown in Figure3-3. Applications that dont involvesuch considerations may still bene-fit from a narrow spectral responsearound 0.7 microns. While emissivi-ty doesnt vary as much as you

decrease the wavelength, the ther-mometer will lose sensitivitybecause of the reduced energyavailable.

Narrow band thermometers withshort wavelengths are used to measurehigh temperatures, greater than 932F(500C), because radiation energy con-tent increases as wavelengths getshorter. Long wavelengths are used forlow temperatures -50F (-45.5C).

Narrow band thermometers range

from simple hand-held devices, tosophisticated portables with simul-taneous viewing of target and tem-perature, memory and printout capa-bility, to on-line, fixed mounted sen-sors with remote electronics havingPID control.

Standard temperature ranges varyfrom one manufacturer to the next,but some examples include: -36 to1112F (-37.78 to 600C), 32 to 1832F (0

to 1000C), 1112 to 5432F (600 to

3000C) and 932 to 3632F (500 to2000C). Typical accuracy is 0.25% to2% of full scale.

Ratio RadiationAlso called two-color radiation ther-mometers, these devices measurethe radiated energy of an objectbetween two narrow wavelength

bands, and calculates the ratio of thetwo energies, which is a function of the temperature of the object.Originally, these were called twocolor pyrometers, because the twowavelengths corresponded to differ-ent colors in the visible spectrum(for example, red and green). Manypeople still use the term two-colorpyrometers today, broadening the

term to include wavelengths in the

infrared. The temperature measure-ment is dependent only on the ratioof the two energies measured, andnot their absolute values as shown inFigure 3-4. Any parameter, such astarget size, which affects the amountof energy in each band by an equalpercentage, has no effect on thetemperature indication. This makes aratio thermometer inherently moreaccurate. (However, some accuracyis lost when youre measuring small

differences in large signals). The ratiotechnique may eliminate, or reduce,errors in temperature measurementcaused by changes in emissivity, sur-face finish, and energy absorbingmaterials, such as water vapor,between the thermometer and thetarget. These dynamic changes mustbe seen identically by the detector atthe two wavelengths being used.

Emissivity of all materials doesnot change equally at different

wavelengths. Materials for whichemissivity does change equally atdifferent wavelengths are calledgray bodies. Materials for which thisis not true are called non-gray bod-ies. In addition, not all forms of sight path obstruction attenuate theratio wavelengths equally. For exam-ple, if there are particles in the sightpath that have the same size as one



Figure 3-5: Beam-Splitting in the Ratio IR Thermometer

OutputRatio

BeamSplitter

Colimator

Target 1

2

Figure 3-6: Ratio Pyrometry Via a Filter Wheel

Viewing Microscope

TemperatureControlled

Cavity

Objective Lens

Target Mirror AndFirst Field Stop

Lens

Aperture Stop Second Field Stop

Lens

Sensor

RotatingFilter Wheel

Eye


29/86

of the wavelengths, the ratio canbecome unbalanced.

Phenomena which are non-dynamic in nature, such as the non-gray bodiness of materials, can bedealt with by biasing the ratio of thewavelengths accordingly. This adjust-ment is called slope. The appropriateslope setting must be determinedexperimentally.

Figure 3-5 shows a schematic dia-gram of a simple ratio radiation ther-

mometer. Figure 3-6 shows a ratiothermometer where the wavelengthsare alternately selected by a rotatingfilter wheel.

Some ratio thermometers usemore than two wavelengths. A multi-wavelength device is schematicallyrepresented in Figure 3-7. Thesedevices employ a detailed analysis of the targets surface characteristicsregarding emissivity with regard towavelength, temperature, and sur-

face chemistry. With such data, acomputer can use complex algo-rithms to relate and compensate foremissivity changes at various condi-tions. The system described in Figure3-7 makes parallel measurement pos-sible in four spectral channels in therange from 1 to 25 microns. Thedetector in this device consists of anoptical system with a beam splitter,and interference filters for the spec-

tral dispersion of the incident radia-tion. This uncooled thermometerwas developed for gas analysis.Another experimental system, usingseven different wavelengths demon-strated a resolution of 1C measur-ing a blackbody source in the rangefrom 600 to 900C. The same systemdemonstrated a resolution of 4 Cmeasuring an object with varyingemittance over the temperaturerange from 500 to 950C.

Two color or multi-wavelengththermometers should be seriouslyconsidered for applications whereaccuracy, and not just repeatability, is

critical, or if the target object is under-

going a physical or chemical change.Ratio thermometers cover widetemperature ranges. Typical commer-cially available ranges are 1652 to5432 F (900 to 3000C) and 120 to6692F (50 to 3700C). Typical accura-cy is 0.5% of reading on narrowspans, to 2% of full scale.

Optical PyrometersOptical pyrometers measure the

radiation from the target in a narrowband of wavelengths of the thermalspectrum. The oldest devices use theprinciple of optical brightness in thevisible red spectrum around 0.65microns. These instruments are alsocalled single color pyrometers.Optical pyrometers are now avail-able for measuring energy wave-lengths that extend into the infraredregion. The term single color pyrom-eters has been broadened by some

authors to include narrow band radi-ation thermometers as well.

Some optical designs are manual-ly operated as shown in Figure 3-8.



Aperture

IncidentRadiation

Infrared Window

Beam Splitter

Filters

Responsive Elements

Preamplifiers

O u t p u t s

Figure 3-7: Schematic of a Multispectral IR Thermometer

Typical configuration of an industrial infrared temperature probe.


30/86

The operator sights the pyrometer

on target. At the same time he/shecan see the image of an internallamp filament in the eyepiece. Inone design, the operator adjusts thepower to the filament, changing itscolor, until it matches the color of the target. The temperature of thetarget is measured based uponpower being used by the internal fil-ament. Another design maintains aconstant current to the filamentand changes the brightness of the

target by means of a rotatable ener-gy-absorbing optical wedge. Theobject temperature is related to theamount of energy absorbed by thewedge, which is a function of itsannular position.

Automatic optical pyrometers,sensitized to measure in the infraredregion, also are available. Theseinstruments use an electrical radia-tion detector, rather than the

human eye. This device operates by

comparing the amount of radiationemitted by the target with thatemitted by an internally controlledreference source. The instrumentoutput is proportional to the differ-ence in radiation between the targetand the reference. A chopper, drivenby a motor, is used to alternatelyexpose the detector to incomingradiation and reference radiation. Insome models, the human eye is usedto adjust the focus. Figure 3-9 is a

schematic of an automatic opticalpyrometer with a dichroic mirror.Radiant energy passes through thelens into the mirror, which reflectsinfrared radiation to the detector,but allows visible light to passthrough to an adjustable eyepiece.The calibrate flap is solenoid-oper-ated from the amplifier, and whenactuated, cuts off the radiationcoming through the lens, and focus-

es the calibrate lamp on to the

detector. The instrument may have awide or narrow field of view. All thecomponents can be packaged into agun-shaped, hand-held instrument.Activating the trigger energizes thereference standard and read-outindicator.

Optical pyrometers have typicalaccuracy in the 1% to 2% of fullscale range.

Fiber Optic RadiationAlthough not strictly a class untothemselves, these devices use a lightguide, such as a flexible transparentfiber, to direct radiation to thedetector, and are covered in moredetail in the chapter beginning on p.43. The spectral response of thesefibers extends to about 2 microns,and can be useful in measuringobject temperatures to as low as210F (100C). Obviously, these

devices are particularly useful whenit is difficult or impossible to obtaina clear sighting path to the target, asin a pressure chamber.

Design and ConstructionThe manufacturer of the radiationthermometer selects the detectorand optical elements to yield theoptimum compromise based uponthe conflicting parameters of cost,

accuracy, speed of response, andusable temperature range. The usershould be cognizant of how the dif-ferent detectors and optical ele-ments affect the range of wave-lengths over which a thermometerresponds. The spectral response of apyrometer will determine whether ausable measurement is possible,given the presence of atmosphericabsorption, or reflections from



Figure 3-8: Optical Pyrometry By Visual Comparison

EyepieceRed Filter

CalibratedTungsten Lamp

Lens

Lens

Target

Target

Battery

Battery

Ammeter Is CalibratedIn Units Of Temperature

Slide Wire

ConstantCurrent

Flow

Eyepiece

Red Filter Standard Lamp

SlidingGate

Sliding GateOpening Is

Calibrated in UnitOf Temperature


31/86

other objects, or trying to measure

the temperature of materials likeglass or plastics.

DetectorsThermal, photon, and pyroelectricdetectors are typically used in radi-ation pyrometers. Radiation detec-tors are strongly affected by ambi-ent temperature changes. Highaccuracy requires compensation forthis ambient drift.

The responsivity of a radiation

detector may be specified in termsof either the intensity of radiation, orthe total radiant power incidentupon the detector.

When the image formed by thetarget surface area is larger than theexposed area of the detector, theentire detector surface is subjectedto a radiation intensity proportionalto the brightness of the target. Thetotal radiant power absorbed by thedetector then depends on the area

of its sensitive surface. The actualsize of the effective target area isdetermined by the magnification of

the optical system. Sensitivity typi-

cally is not uniform over the surfaceof a detector, but this has no effectif the target brightness is uniform. If substantial temperature differencesoccur on the target surface withinthe patch imaged on the detector,an ambiguously weighted averagewill result.

In the case of total radiant power,the area of the target surfaceimaged on the detector is limited bya stop optically conjugate to the

detector. This area can be madearbitrarily small. As a result, localtemperatures can be measured onthe target body surface. The respon-sivity of the detector may dependon the location of this target sourceimage on the detector surface.Constancy of calibration willdepend on maintaining the elementin a fixed position with respect tothe optical system.

Thermal detectors are the most

commonly used radiation ther-mometer detectors. Thermal detec-tors generate an output because

they are heated by the energy they

absorb. These detectors have lowersensitivity compared to otherdetector types, and their outputsare less affected by changes in theradiated wavelengths. The speed of response of thermal detectors islimited by their mass.

Thermal detectors are blackenedso that they will respond to radiationover a wide spectrum (broadbanddetectors). They are relatively slow,because they must reach thermal

equilibrium whenever the targettemperature changes. They can havetime constants of a second or more,although deposited detectorsrespond much faster.

A thermopile consists of one ormore thermocouples in series, usual-ly arranged in a radial pattern so thehot junctions form a small circle,and the cold junctions are main-tained at the local ambient temper-ature. Advanced thin film ther-

mopiles achieve response times inthe 10 to 15 millisecond range.Thermopiles also increase the out-put signal strength and are the bestchoice for broadband thermome-ters. Ambient temperature compen-sation is required when thermopiledetectors are used. A thermostati-cally controlled thermometer hous-ing is used to avoid ambient temper-ature fluctuations for low tempera-ture work. Self-powered infrared

thermocouples are covered in thechapter beginning on p. 38.

Bolometers are essentially resis-tance thermometers arranged forresponse to radiation. A sensing ele-ment with a thermistor, metal film,or metal wire transducer is oftencalled a bolometer.

Photon detectors release electriccharges in response to incident radi-ation. In lead sulfide and lead



Figure 3-9: An Automatic Optical Pyrometer

Adjustable EyepieceCircleReticleDichroicMirrorLens

Calibration Lamp

Optical Chopper

Filter

Detector

Amplifier

CalibrateFlag

Eye

Sync. Motor


32/86

selenide detectors, the release of

charge is measured as a change inresistance. In silicon, germanium, andindium antimonide, the release of charge is measured as a voltage out-put. Photon detectors have a maxi-mum wavelength beyond which theywill not respond. The peak responseis usually at a wavelength a littleshorter than the cutoff wavelength.Many radiation thermometers usephoton detectors rather than ther-mal detectors, even though they

measure over a narrower band of wavelength. This is because withinthe range of useful wavelengths, thephoton detectors have a sensitivity1000 to 100,000 times that of thethermal detector. Response time of these detectors is in microseconds.They are instable at longer wave-lengths and higher temperatures.They are often used in narrow bandthermometers, or broadband ther-mometers at medium temperatures

(200 to 800F/93 to 427C), and oftenprovided with cooling.

Pyroelectric detectors change sur-face charge in response to receivedradiation. The detector need notreach thermal equilibrium when thetarget temperature changes, since itresponds to changes in incomingradiation. The incoming radiationmust be chopped, and the detectoroutput cannot be used directly. Achopper is a rotating or oscillating

shutter employed to provide ACrather than DC output from the sen-sor. Relatively weak AC signals aremore conveniently handled by con-ditioning circuitry. The detectorchange can be likened to a changein charge of a capacitor, which mustbe read with a high impedance cir-cuit. Pyroelectric detectors haveradiation absorbent coatings so theycan be broadband detectors.

Response can be restricted by

selecting the coating material withappropriate characteristics.Photon and pyroelectric detectors

have thermal drift that can be over-come by temperature compensation(thermistor) circuitry, temperatureregulation, auto null circuitry, chop-

ping, and isothermal protection.Figure 3-10 shows the different

sensitivity for various radiationdetectors. PbS has the greatest sensi-tivity, and the thermopile the least.

Optical SystemsAs shown in Figure 3-11, the opticalsystem of a radiation pyrometer maybe composed of lenses, mirrors, orcombinations of both. Mirror sys-tems do not generally determine thespectral response of the instrument,as the reflectivity is not dependenton wavelength over the range usedfor industrial temperature measure-

ment. A mirror system must be pro-

tected from dirt and damage by awindow. Copper, silver and gold arethe best materials for mirrors in theinfrared range. Silver and copper sur-faces should be protected againsttarnish by a protective film.

The characteristics of the window

material will affect the band of wavelengths over which the ther-mometer will respond. Glass doesnot transmit well beyond 2.5microns, and is suited only for highertemperatures. Quartz (fused silica)

transmits to 4 microns, crystallinecalcium fluoride to 10 microns, ger-manium and zinc sulfide can transmitinto the 8 to 14 micron range. Moreexpensive materials will increase thetransmission capability even more, asshown in Figure 3-12.

Windows and filters, placed infront of or behind the optical sys-tem, and which are opaque outside agiven wavelength range, can alter the



Figure 3-10: Relative Sensitivity of IR Detectors

106

105

104

103

102

101

1

R e

l a t i v e

S e n s i t i v i

t y

Wavelength, m.1 .2 .3 .5 .7 1 2 3 5 7 10 20

Chopped Unchopped

Metal Thermopile

Thin FilmThermopile

Thermistor Bolometer

Pyro ElectricDetector

InSb

InAsSi

PbS

Ge


33/86

transmission properties greatly, and

prevent unwanted wavelengths fromreaching the detector.

Mirror systems are generally usedin fixed focus optical instruments.Varying the focus of the instrumentrequires moving parts, which is lesscomplicated in a lens system. Theselection of lens and window mate-rial is a compromise between theoptical and physical properties of the material, and the desired wave-length response of the instrument.

The essential design characteristicsof materials suitable for lenses,prisms, and windows includeapproximate ref

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