infrared and thermal testing glossary - asnt.org

Herbert Kaplan, Honeyhill Technical Company,Norwalk, Connecticut

Jean-Claude Krapez, French National AerospaceResearch Establishment (ONERA), Châtillon, France

Minh Phong Luong, École Polytechnique, Paris, France

Xavier P.V. Maldague, University Laval, Quebec,Quebec, Canada

Gary L. Orlove, FLIR Systems, North Billerica,Massachusetts

Nik Rajic, Defence Science and TechnologyOrganisation, Melbourne, Australia

Andres E. Rozlosnik, SI Termografia Intrarroja, BuenosAires, Argentina

20C H A P T E R

Infrared and ThermalTesting Glossary

From Nondestructive Testing Handbook, Third Edition: Volume 3, Infrared and Thermal Testing © 2001. Reprinted with permission of The American Society for Nondestructive Testing Inc.

680 Infrared and Thermal Testing

IntroductionMany of the definitions in this glossaryare adapted from the Nondestructive TestingHandbook, second edition: Volume 10,Nondestructive Testing Overview.1 These andother definitions in this glossary havebeen modified to satisfy peer review andeditorial style. References given in thisglossary should be considered notattributions but rather acknowledgmentsand suggestions for further reading.

The definitions in this NondestructiveTesting Handbook volume should not bereferenced for inspections performedaccording to standards or specifications orin fulfillment of contracts. Standardswriting bodies take great pains to ensurethat their documents are definitive inwording and technical accuracy. Peopleworking to written contracts orprocedures should consult definitionsreferenced in real standards whenappropriate.

This glossary is provided forinstructional purposes. No other use isintended.

Aabsolute temperature: Temperature

measured from absolute zerotemperature, expressed in kelvin (K)in SI.1

absolute temperature scale: Temperaturemeasurement scale based on coldestpossible temperature equal to 0. (Seerankine and kelvin).2

absolute zero: Temperature that is zeroon the kelvin or rankine temperaturescales. The temperature at which nomolecular motion takes place in amaterial.3

absorptivity (absorptance): Proportion(as a fraction of 1) of the radiantenergy impinging on a material’ssurface that is absorbed into thematerial. For a blackbody, this is unity(1.0). Technically, absorptivity is theinternal absorptance per unit pathlength. In thermography, the twoterms have sometimes been usedinterchangeably.3

acceptance level: In contrast to rejectionlevel, test level above or below which,depending on the test parameter, testobjects are acceptable.1 Comparerejection level.

accuracy: Degree of conformity of ameasurement to a standard or truevalue.1

adaptive thresholding: Threshold valuevarying with inconstant backgroundgray level.1

agency: Organization selected by anauthority to perform nondestructivetesting, as required by a specificationor purchase order.1

algorithm: Prescribed set of well definedrules or processes for the solution of amathematical problem in a finitenumber of steps.1,5

ambient light: Light in the environmentas opposed to illumination providedby a visual testing system.1

ambient operating range: Range ofambient temperatures over which aninstrument is designed to operatewithin published performancespecifications.3

ambient temperature: Temperature ofimmediate surroundings andenvironment where a test ormeasurement takes place. A parameterused to compensate for radiationreflected from test object and air inthe field of view.

ambient temperature compensation:Correction built into an instrument toprovide automatic compensation inthe measurement for variations ininstrument ambient temperature.3

amplitude response: That property of atest system whereby the amplitude ofthe detected signal is measuredwithout regard to phase.1,4

analog-to-digital converter: Circuitwhose input is information in analogform and whose output is the sameinformation in digital form.1,5

anisotropy: A material’s characteristic ofexhibiting different values of aproperty (acoustic velocity, forexample) in different directions in thematerial because of differentarrangements of atoms.1

annealing: Process of heating and coolinga material, usually to reduce residualstresses or to make it softer.1

PART 1. Terminology


anomaly: Discontinuity. A variation fromnormal in product quality or materialproperty.1

AOQ: Average outgoing quality.AOQL: Average outgoing quality limit.AQL: See acceptable quality level.apparent temperature: Target surface

temperature indicated by an infraredpoint sensor, line scanner or imager,generally taking the emissivity intoaccount.3

arc: Luminous high temperature dischargeproduced when an electric currentflows across a gap.1

arcing: Electric current flow through agap, often accompanied by intenseheat and light.1

arc welding: See electric arc welding.artifact: In nondestructive testing, an

indication that may be interpretederroneously as a discontinuity.1

artificial discontinuity standard: Seeacceptance standard.

artificial discontinuity: Reference point,such as a hole, groove, implant ornotch, that are introduced into areference standard to provideaccurately reproducible sensitivitylevels for nondestructive testequipment. A manufactured materialanomaly.1

ASNT: The American Society forNondestructive Testing.

ASNT Recommended Practice No. SNT-TC-1A: Set of guidelines for employersto establish and conduct anondestructive testing personnelqualification and certificationprogram. SNT-TC-1A was first issued in1968 by the Society for NondestructiveTesting (SNT, now ASNT) and has beenrevised every few years since.1

atmospheric temperature: Temperatureof atmosphere sensed by scanner.

atmospheric windows (infrared):Spectral intervals within the infraredspectrum in which the atmospheretransmits radiant energy well(atmospheric absorption is aminimum). These are roughly definedas 2 to 5 µm and 8 to 14 µm.3

attenuation: Decrease in signalmagnitude during energy transmissionfrom one point to another. This lossmay be caused by absorption,reflection, scattering of energy orother material characteristics or maybe caused by an electronic or opticaldevice such as an attenuator.1

automated system: Acting mechanismthat performs required tasks at adetermined time and in a fixedsequence in response to certainconditions and instructions.1

Bbackground noise: Signals that originate

from the test object, the testinstrument and their surroundingsand that interfere with test signals ofinterest. It may have electrical ormechanical origins. Sometimes calledgrass or hash.1

background signal: Steady or fluctuatingoutput signal of a test instrumentcaused by the presence of acoustic,chemical, electrical or radiationconditions to which the sensingelement responds.1

background temperature, instrument:Apparent ambient temperature of thescene behind and surrounding theinstrument, as viewed from the target.The reflection of this background mayappear in the image and affect thetemperature measurement. Mostquantitative thermal sensing andimaging instruments provide a meansfor correcting measurements for thisreflection.3

background temperature, target:Apparent ambient temperature of thescene behind and surrounding theinstrument, as viewed from theinstrument. When the FOV of a pointsensing instrument is larger than thetarget, the target backgroundtemperature will affect the instrumentreading.3

backscattering, infrared: Reflection ofthermal energy — e.g., generated bythe ground and reflecting off theunderside of clouds or inversionlayers, or unwanted front surfacereflections from a transparent opticalelement.

black body: See blackbody.blackbody: Hypothetical radiation source

that yields the maximum radiationenergy theoretically possible at a giventemperature. A blackbody will absorball incident radiation falling on it. Bydefinition it has an emissivity of 1.0.See also emissivity.1

bolometer, infrared: Thermal infrareddetector in which electricalconductivity changes withtemperature.

borescope: Industrial endoscope.borescopy: Technology of the borescope

and its application to nondestructivetesting.

burning: Extreme overheating of a metal.Makes grains excessively large andcauses the more fusible constituents ofsteel to melt and run into the grainboundaries or it may leave voidsbetween the grains. Steel may beoxidized to the extent that it is nolonger usable and cannot be correctedby heat treating but may be remelted.1

681Infrared and Thermal Testing Glossary


Ccalibration: Adjusting an instrument so

that its readings agree with astandard.3

calibration accuracy: Accuracy to whicha calibration is performed, usuallybased on the accuracy and sensitivityof the instruments and references usedin the calibration.3

calibration check: Routine check of aninstrument against a reference toensure that the instrument has notdeviated from calibration since its lastuse.3

calibration reflector: Reflector with aknown dimensioned surfaceestablished to provide an accuratelyreproducible reference level.1

calibration source, infrared: Blackbodyor other target of known temperatureand effective emissivity used as acalibration reference.3

candela: Base unit of measure in SI formeasuring luminous intensity. Theluminous intensity in a givendirection of a source that emitsmonochromatic radiation of frequency540 × 1012 Hz and that has a radiantintensity in that direction of1.4641 mW·sr–1. Symbolized cd.Formerly known as candle.1

candle: Former name for candela.1capacitance, thermal: Amount of heat

that an object can store. The termthermal capacitance is used to describeheat capacity in terms of an electricalanalogy, where loss of heat isanalogous to loss of charge on acapacitor. Structures with high thermalcapacitance change temperature moreslowly than those with low thermalcapacitance.3

capacity, heat: Ability of a material orstructure to store heat. The product ofthe specific heat and the density ofthe material. This means that densermaterials generally will have higherheat capacities than porous materials.3Heat capacity is the amount of energy(J·m–3·K–1) required to elevate by onedegree a given volume of material.Among common materials, water hasone of the highest heat capacities; air,one of the lowest.

casting: Object of shape obtained bysolidification of a substance in a mold.

CCD: See charge coupled device.celsius (centigrade): Temperature scale

based on 273 K (0 °C = +32 °F) as thefreezing point of water and 373 K(100 °C = 212 °F) as the boiling pointof water at standard atmosphericpressure. A relative scale related to thekelvin scale (0 °C = 273.12 K;1 °C = 1 K).3

certification: Process of providing writtentestimony that an individual isqualified. See also certified.1

certified: Having written testimony ofqualification. See also certification.1

charge coupled device (CCD): Solid stateoptical sensor widely used in imaginginspection systems for its accuracy,high speed scanning and long servicelife.1 Incoming radiation induceselectrical charges stored in acapacitor-like semiconductorstructures and later transferred toidentical neighbor structures, ready forreading.

closing: In image processing, dilationfollowed by erosion. A single pixelclosing connects a broken featureseparated by one pixel.1 See alsomathematical morphology and opening.

code: Standard enacted or enforced as alaw.1

coefficient of thermal expansion: Linearexpansion or contraction per unitlength per degree of temperaturechange between specified lower andupper temperature limits.1

coefficients of the filter: Values in amask that serves as a filter in imageprocessing.1

color: Aspect of visible light sometimesused to identify wavelength or spectralband, as in two-color radiometry(meaning a method that measures intwo spectral bands); also usedconventionally (visual color) as ameans of displaying a thermal image,as in color thermogram.3

colored body: See nongraybody.complete testing: Testing of an entire

production lot in a prescribed manner.Sometimes complete testing entailsthe inspection of only the criticalregions of a part. One hundredpercent testing requires the inspectionof the entire part by prescribedmethods. Compare sampling, partial.1

conduction: Heat transfer occurring whenmore energetic particles collide with— and thus impart some of their heatenergy to — adjacent less energetic(slower moving) particles. This actionis passed on from one atom (or freeelectron) to the next in the directionof cooler regions. Thus, heat alwaysflows from a warmer to a coolerregion.1

conductivity, thermal (k): Materialproperty defining the relativecapability to carry heat by conductionin a static temperature gradient.Conductivity varies slightly withtemperature in solids and liquids andwith temperature and pressure ingases. It is high for metals (copper hasa k of 380 W·m–1·K–1) and low forgases and porous materials (concretehas a k of 1.0 W·m–1·K–1).3



continuous annealing furnace: Furnacein which castings are heat treated, bybeing passed through different heatzones kept at constant temperatures.1

continuous casting: Casting technique inwhich an ingot, billet, tube or othershape is continuously solidified whilebeing poured so that its length is notdetermined by mold dimensions.1

contrast: Difference in visibility(brightness, color or temperature)between an indication and thesurrounding surface.1

convection: Type of heat transfer thattakes place in a moving medium andis almost always associated withtransfer between a solid (surface) and amoving fluid (such as air), wherebyenergy is transferred from highertemperature sites to lower temperaturesites.3

cooling stresses: Residual stressesresulting from nonuniformdistribution of temperature duringcooling.1

corrosion: Deterioration of a metal bychemical or electrochemical reactionwith its environment. Removal ofmaterial by chemical attack, such asthe rusting of automobilecomponents.1

crack, cold: Cracks that occur in a castingafter solidification, due to excessivestress generally resulting fromnonuniform cooling.1

crack, cooling: Cracks in bars of alloy ortool steels resulting from unevencooling after heating or hot rolling.They are usually deep and lie in alongitudinal direction, but are usuallynot straight.1

crack, grinding: Thermal cracks causedby local overheating of the surfacebeing ground.1

crack, hot: Cracks that develop before thecasting has completely cooled, ascontrasted with cold cracks, thatdevelop after solidification.1 Alsocalled hot tear.

crack, transverse: Cracks at right anglesto the length of the test object.1

crack, weld: Cracks in weld fusion zonesor adjacent base metal. Usually a resultof thermal expansion or contractionstresses related to temperature changesduring welding.1

crack: (1) A break, fissure or rupture,usually V shaped and relatively narrowand deep. A discontinuity that has arelatively large cross section in onedirection and a small or negligiblecross section when viewed in adirection perpendicular to the first.1(2) Propagating discontinuities causedby stresses such as heat treating orgrinding. Difficult to detect unaidedbecause of fineness of line and pattern(may have a radial or latticedappearance).1

crater: (1) In machining, a depression inthe cutting tool face eroded by chipcontact. (2) In arc or gas fusionwelding, a cavity in the weld beadsurface, typically occurring when theheat source is removed andinsufficient filler metal is available tofill the cavity.1

C-scan: Data presentation techniqueapplied to reflection and transmissiontechniques. It yields atwo-dimensional plan view of theobject but no depth indications unlessspecial gating procedures are used.1

cutoff frequency: Upper or lowerfrequency corresponding to thespectral response of a filter oramplifier, at a specified amount less(usually 3 dB power or 6 dBvoltage/amplitude) than themaximum response.1

DD* (detectivity star): Detectivity

expressed inversely so that higher D*sindicate better performance.Sensitivity figure of merit of aninfrared detector. D* is taken atspecific test conditions of choppingfrequency and information bandwidthand displayed as a function of spectralwavelength.3 D* is the detectivityscaled to the unit sensitive detectionarea, with detectivity corresponding tothe inverse of the noise equivalentflow.

defect: Discontinuity whose size, shape,orientation or location make itdetrimental to the useful service of itshost object or which exceeds anaccept/reject criterion of an applicablespecification.1 Note that somediscontinuities may not affectserviceability and are therefore notdefects.1 Compare discontinuity andindication.1

deformation: Change of shape underload.1 See also creep and elasticdeformation.

delamination: Laminar discontinuity,generally an area of unbonded layersof materials.1



depth of field: In photography orthermography, the range of distanceover which an imaging system givessatisfactory definition when its lens isin the best focus for a specificdistance.1

depth of fusion: Depth to which the basemetal melted during welding.1

detector, infrared: Transducer elementthat converts incoming infraredradiant energy impinging on itssensitive surface to a useful electricalsignal.3

diffuse reflector: Surface that reflects aportion of the incident radiation insuch a manner that the reflectedradiation is equal in all directions. Amirror is not a diffuse reflector.3

diffusion, heat: See thermal diffusion.diffusion, mass: Process by which

molecules intermingle as a result ofconcentration gradients or thermalmotion.1 Spreading of a gas throughother gases within a volume.

diffusivity, thermal: See thermaldiffusivity.

dilation: In image processing, thecondition of a binary image where thepixel in the output image is a 1 if anyof its eight closest neighbors is a 1 inthe input image. See also closing,erosion, mathematical morphology andopening.1

directional properties, material:Properties whose magnitudes dependon the relation of the test axis to thespecific direction in the metal,resulting from preferred orientation orfrom fibering in the structure. Seeanisotropy.1

directional properties, radiation:Radiation properties (emissivity,absorptivity, reflectivity) as referencedto a particular direction. See alsohemispherical properties, radiation.1

direct viewing: Viewing of a test object inthe viewer’s immediate presence. Theterm direct viewing is used in the fieldsof robotics and surveillance todistinguish conventional from remoteviewing.1

discernible image: Image capable ofbeing recognized by sight without theaid of magnification.1

discontinuity: Intentional orunintentional interruption in thephysical structure or configuration of apart.1,4 After nondestructive testing,unintentional discontinuitiesinterpreted as detrimental in the hostobject may be called flaws or defects.Compare defect, dislocation andindication.1

discontinuity, artificial: Referencediscontinuities such as holes,indentations, cracks, grooves ornotches that are introduced into areference standard to provideaccurately reproducible indications fordetermining sensitivity levels.1

discontinuity, primary processing: Inmetals processing, a material anomalyproduced from the hot or coldworking of an ingot into forgings, rodand bar.1

discontinuity, service induced: Materialanomaly caused by the intended useof the part.1

display resolution, thermal: Precisionwith which an instrument displays itsassigned measurement parameter(temperature), usually expressed indegrees, tenths of degrees, hundredthsof degrees and so forth.3

dissipation: Generation of heat by plasticdeformation.

distal: In a manipulative or interrogatingsystem, of or pertaining to the endopposite from the eyepiece andfarthest from the person using thesystem. Objective; tip.1

Eeffective emissivity (εε*): Measured

emissivity value of a particular surfaceunder existing measurementconditions (rather than the generictabulated value for the surfacematerial) that can be used to correct aspecific measuring instrument toprovide a correct temperaturemeasurement.3

effusivity, thermal: Ability of heat toescape from a body, expressed as acharacteristic of that body.3 Squareroot of the product of thermalconductivity, mass density and specificheat.

elasticity: Ability of a material to resumeits former shape after deformation.1

electric arc welding: Joining of metals byheating with electric arc. Also calledarc welding.1

electromagnetic interference: SeeEMI/RFI noise.

EMI/RFI noise: Disturbances to electricalsignals caused by electromagneticinterference (EMI) or radio frequencyinterference (RFI). In thermography,this may cause noise patterns toappear on the display.3



emissivity: Variable ratio of the totalenergy radiated by a given surface at agiven temperature to the total energyradiated by a blackbody at the sametemperature. Emissivity can be total,directional or hemispherical. Emissivityis a surface phenomenon dependingon surface condition and composition.Smooth materials have loweremissivities than rough or corrodedmaterials.1 Emissivity values rangebetween 0 for a perfect reflector to 1.0for a blackbody.

endoscope: Device for viewing theinterior of objects. From the Greekwords for inside view, the termendoscope is used mainly for medicalinstruments. Nearly every medicalendoscope has an integral light source;many incorporate surgical tweezers orother devices. Compare borescope.1

environmental rating: Rating given anoperating unit (typically an electricalor mechanical enclosure) to indicatethe limits of the environmentalconditions under which the unit willfunction reliably and within publishedperformance specifications.3

erosion: (1) Loss of material ordegradation of surface quality throughfriction or abrasion from movingfluids, made worse by solid particles inthose fluids or by cavitation in themoving fluid. (2) In image processing,condition of a binary image where thepixel in the output image is a 1 if eachof its eight neighbors is a 1 in theinput image. See also closing, dilation,mathematical morphology and opening.1

eutectic liquid: Liquid having aproportion of metals such that two ormore solid phases form at the sametemperature during cooling.1

eutectic point: Temperature andproportion of metals at which two ormore phases of a eutectic liquid form.Compare eutectoid.1

eutectoid: Similar to eutectic but in asolid system during cooling.1

evaluation: Process of determining themagnitude and significance of adiscontinuity after the indication hasbeen interpreted as relevant.Evaluation determines if the testobject should be rejected, repaired oraccepted. See indication andinterpretation.1

examination: Process of testing materials,interpreting and evaluating testindications to determine if the testobject meets specified acceptancecriteria.1

exfoliation: Corrosion that progressesapproximately parallel to the outersurface of the metal, causing layers ofthe metal to be elevated by theformation of corrosion product.1

exitance, radiant: See radiosity.

external discontinuities: Discontinuitieson the outside or exposed surface of atest object.1

Ffahrenheit: Temperature scale based on

32 °F as the freezing point of waterand 212 °F as the boiling point ofwater at standard atmosphericpressure; a relative scale related to therankine scale [0 °F = 459.67 R; 1 °F(DT) = 1 R (DT)].3

false indication: Test indication thatcould be interpreted as originatingfrom a discontinuity but whichactually originates where nodiscontinuity exists.1 Distinct fromnonrelevant indication.1 Comparedefect.1

feature extraction: From an enhancedimage, derivation of some featurevalues, usually parameters fordistinguishing objects in the image.1

fiber optic, infrared: Flexible fiber madeof a material that transmits infraredenergy, used for making noncontacttemperature measurements whenthere is not a direct line of sightbetween the instrument and thetarget.3

fiber optics: Technology of lighttransmission through fibers such asplastic, glass or quartz.1

field: In video technology, one of twovideo picture components thattogether make a frame. Each picture isdivided into two parts called fieldsbecause a frame at the rate of thirtyframes per second in a standard videooutput would otherwise produce aflicker discernible to the eye. Eachfield contains one half of the totalpicture elements. Two fields arerequired to produce one completevisible light picture or frame so thefield frequency is sixty fields persecond and the frame frequency isthirty frames per second.1 In infraredtechnology there can be four fields.

field of view: Range or area where thingscan be seen through an imagingsystem, lens or aperture.1 Angularsubtense (expressed in angular degreesor radians per side if rectangular, andangular degrees or radians if circular)over which an instrument willintegrate all incoming radiant energy.In a radiation thermometer, the field ofview is the target spot size; in ascanner or imager the field of view isthe scan angle or picture size or totalfield of view (TFOV).3 Compare depthof field.



field of vision: Range or area wherethings can be perceivedorganoleptically at a point in time,assuming the eye to be immobile.1

filter: (1) Network or device that passeselectromagnetic wave energy over adescribed range of frequencies andattenuates energy at all otherfrequencies.1,4 (2) Processingcomponent or function that excludesa selected kind of signal or part of asignal.1 (3) In optics, a sequence ofmaterials so arranged to pass selectedwavelengths and to block or attenuateothers.

filtering: See low pass filtering.filter, spectral: Optical element, usually

transmissive, used to restrict thespectral band of energy received by aninstrument’s detector.

fine crack: Discontinuity in a solidmaterial with a very fine opening tothe surface, but possessing length anddepth greater than the width of thisopening. Usually the depth is manytimes the width.1

finite element analysis: Numericaltechnique for the analysis of a systemwhereby that system is decomposedinto a collection of finite sizedelements.1

focal plane array (FPA): Linear or two-dimensional matrix of detectorelements, typically used at the focalplane of an instrument. Inthermography, rectangular FPAs areused in staring (nonscanning) infraredimagers. These are called IRFPAimagers.3

focal point: Point at which theinstrument optics image the infrareddetector at the target plane. In aradiation thermometer, this is wherethe spot size is the smallest. In ascanner or imager, this is where theinstantaneous field of view (IFOV) issmallest.3

focal zone: Distance before and after thefocal point in which the intensitydiffers a specified amount (usually6 dB) from the focal intensity. Alsocalled depth of field or depth of focus.1

focus: Position of a viewed object and alens system relative to one another tooffer a distinct image of the object asseen through the lens system. Seeaccommodation and depth of field.1

focusing, automatic: (1) Feature ofcamera, usually incorporating a rangefinder, whereby the lens systemadjusts to focus on an object in part ofthe field of view. (2) Metaphoricalattribute of a borescopic instrument’sdepth of field (the range of distance infocus). The depth of field is so great inthe case of video borescopes thatfocusing is unnecessary for mostapplications.1 Longer depths of fieldare obtained by reducing the opticalaperture.

focusing, primary: Focusing of an imageby the lens onto a fiber optic bundleat the tip of a probe.1

focusing, secondary: Focusing at theeyepiece of a borescope or otheroptical instrument, specifically themanual refocusing needed when theviewing distance changes.1

focus, principal plane of: Single planeactually in focus in a photographicscene.1

foreground temperature (see instrumentambient background): Temperature ofthe scene behind and surrounding theinstrument as viewed from the target.

foreign materials: They may appear asisolated, irregular or elongatedvariations not corresponding tovariations in thickness of material orto cavities. May be sand, slag, oxide ordross metal or any material includedin the material being examined.1

forging crack: Discontinuity formedduring mechanical shaping of metal.1

foundry: Establishment or building wheremetal castings are produced.1

frame: Complete raster scan projected ona video screen. In North America,there are 30 frames per second in astandard video output, eitherElectronics Industry Association (EIA)RS-170 or National TelevisionStandards Committee (NTSC) format;in Europe, 25 frames per second inphase alternation line (PAL) orSystème Electronique Couleur avecMémoire (SECAM) formats. A framemay be comprised of two interlacedfields, each displaying part of the totalframe. See also field.1

frame repetition rate: Time it takes aninfrared imager to scan (update) everythermogram picture element (pixel);in frames per second.3

full scale: The span between theminimum value and the maximumvalue that any instrument is capableof measuring. In a thermometer, thiswould be the span between thehighest and lowest temperature thatcan be measured.3



Ggas tungsten arc welding (GTAW): Inert

gas shielded arc welding using atungsten electrode. Also called tungsteninert gas (TIG) welding.1

general examination: Test orexamination of a person’s knowledge,typically (in the case of nondestructivetesting personnel qualification) awritten test on the basic principles of anondestructive testing method andgeneral knowledge of basic equipmentused in the method. (According toASNT’s guidelines, the generalexamination should not addressknowledge of specific equipment,codes, standards and procedurespertaining to a particular application.)Compare practical examination andspecific examination.1

gray body: See graybody.graybody: Radiator whose spectral

emissivity is uniform for allwavelengths (but not 1.0) and hasvalue less than 1.0. See blackbody.1

gray level: Integer number representingthe brightness or darkness of a pixelor, as a composite value, of an imagecomprised of pixels.1

Hheat: Energy associated with the random

and chaotic motions of the atomicparticles from which matter iscomposed. All materials (hot or cold)contain heat and radiate infraredenergy. The unit for measuring heat isthe joule (J), equal to about 0.24calorie (cal) or 9.481 × 10–4 Britishthermal units (BTUs). Compareinfrared radiation and temperature.1

heat checking: Surface cracking causedwhen metal rapidly heated (or cooledand heated repeatedly) is preventedfrom expanding freely by colder metalbelow the surface. Friction mayproduce the heat. Sometimes calledthermal fatigue.1

heat treatment: Heating and cooling ametal or alloy in such a way as toobtain desired conditions orproperties. Heating for the solepurpose of working is excluded fromthe meaning of this definition.1

heat wave: Thermally produced variationin flue gas density that distorts imagesof objects in a firebox.1

hemispherical properties, radiation:Radiation properties (emissivity,absorptivity, reflectivity) as referencedto all directions of hemisphericalspace.

hertz (Hz): A unit of measurement ofsignal frequency; 1 Hz = 1 cycle persecond.3

holes: Voids remaining in an object as aresult of improper manufacturingprocessing. Often called gas holes,cavities or air locks.1

hot tear: See crack, hot.hue: Characteristic of light at a particular

bandwidth that gives a color itsname.1

hundred percent testing: See one hundredpercent testing.

hyperthermia: Heating so excessive thatit can damage or kill plant or animalcells.1

Iilluminance: Density of luminous flux on

a surface. Measured in the SI systemby lux.1

illuminate: Shed light on.1illumination: Act of illuminating or state

of being illuminated. See alsoilluminate. Compare illuminance.1,6

image: Visual representation of a testobject or scene.1

image display tone: Gray shade or colorhue on a thermogram.3

image enhancement: Any of a variety ofimage processing steps, used singly orin combination to improve thedetectability of objects in an image.1

image guide: Fiber bundle that carries thepicture formed by the objective lens atthe distal end of a fiber opticborescope back to the eyepiece.1

image, infrared: See thermogram.image orthicon: Television tube that uses

photoemission to create an image.image processing: Actions applied singly

or in combination to an image, inparticular the measurement andalteration of image features bycomputer. Also called pictureprocessing.1

image processing, thermal: Analysis ofthermal images, usually by computer;enhancing the image to prepare it forcomputer or visual analysis.3 In thecase of an infrared image orthermogram, this could includetemperature scaling, spot temperaturemeasurements, thermal profiles, imagemanipulation, subtraction,discontinuity detection, qualitativeassessment, quantitativecharacterization and storage. Seesignal processing.

image segmentation: Process in whichthe image is partitioned into regions,each homogeneous.1



imager, infrared: An infrared instrumentthat collects the infrared radiantenergy from a target surface andproduces an image in monochrome(black and white) or color, where thegray shades or color hues correspondrespectively to target exitance.3

imaging radiometer: Infrared thermalimager that provides quantitativethermal images.3

impurities: Elements or compoundswhose presence in a material isunintentional.1

incandescence: Emission of visibleradiation due to thermal excitation.1

incandescent: Emitting visible radiationas a result of heating.1

inclusion: Foreign particles or impurities,usually oxides, sulfides, silicates andsuch retained in metal (welds orcastings), forming during solidificationor subsequent reaction of the solidmetal.1

in control: Within prescribed limits ofprocess control.1

indication: Nondestructive testingdiscontinuity response that requiresinterpretation to determine itsrelevance. Compare defect, discontinuityand false indication.1

indication, discontinuity: Visibleevidence of a material discontinuity.Subsequent interpretation is requiredto determine the significance of anindication.1

indication, false: Indication produced bysomething other than a discontinuity.Can arise from improper testprocedures.1

indication, nonrelevant: Indication dueto misapplied or improper testing.May also be an indication caused byan actual discontinuity that does notaffect the usability of the object (achange of section, for instance).1

indication, relevant: Indication from adiscontinuity (as opposed to anonrelevant indication) requiringevaluation by a qualified inspector,typically with reference to anacceptance standard, by virtue of thediscontinuity’s size or location.1,4

indium antimonide (InSb): Materialfrom which fast, sensitivephotodetectors used in infraredscanners and imagers are made. Suchdetectors usually requiring coolingwhile in operation. Operation is in theshort wave band (2 to 5 µm).

inertia, thermal: See thermal effusivity.infrared: Beyond infrared, referring to

radiation with frequency lower than,and wavelength greater than, that ofthe color red. See infrared radiation.

infrared and thermal testing:Nondestructive testing that uses heatdiffusion and infrared radiation asinterrogating energy.

infrared camera: Radiometer that collectsinfrared radiation to create an imageof the infrared radiance field. Thisimage may then be transformed to animage of the temperature field.1

infrared focal plane array (IRFPA):Linear or two-dimensional matrix ofindividual infrared detector elements,typically used as a detector in aninfrared imaging instrument.3

infrared radiation: Radiant energybeyond the color red, of wavelengthsfrom the red visible (0.75 µm) toabout 300 µm, between the visible andmicrowave regions of theelectromagnetic spectrum.1,6

infrared radiation thermometer:Instrument that converts incominginfrared radiant energy from a spoton a target surface to a measurementvalue that can be related to thetemperature of that spot.3

infrared thermal imager: Instrument orsystem that converts incominginfrared radiant energy from a targetsurface to a thermal map, orthermogram, on which color hues orgray shades can be related to thetemperature distribution on thatsurface.3

infrared thermography: Imaging of atemperature field through the emittedinfrared radiation. See infraredradiation. Compare thermography.

inspection: See examination.instantaneous field of view (IFOV):

Angular subtense (expressed in angulardegrees or radians per side ifrectangular and angular degrees orradians if round) over which aninstrument will integrate all incomingradiant energy; the projection of thedetector at the target plane. In aradiation thermometer this defines thetarget spot size; in a line scanner orimager it represents one resolutionelement in a scan line or athermogram and is a measure ofspatial resolution.3

intensity, radiant: Luminous flux persteradian emanating from a visiblesource, measured in lm·sr–1. Also, theradiant flux per steradian emanatingfrom that source and measured inW·sr–1.

interface: Boundary between twoadjacent media.1

interpretation: Determination of thesignificance of test indications fromthe standpoint of their relevance orirrelevance. The determination of thecause of an indication or theevaluation of the significance ofdiscontinuities from the standpoint ofwhether they are detrimental orinconsequential.1



IR: (1) Abbreviation for infrared. (2) InASNT, an abbreviation for the infraredand thermal testing method.

IRFPA: See infrared focal plane array.IRFPA imager or camera: Infrared

imaging instrument that incorporatesa two-dimensional infrared focal planearray and produces a thermogramwithout mechanical scanning.3

irradiance: Power of electromagneticradiant energy incident on or radiatedfrom the surface of a given unit area.Compare radiance.1

isotherm: Locus or pattern superimposedon a thermogram or on a line scanthat includes or highlights all pointsthat have the same apparenttemperature.3

isotropy: Condition in which significantmedium properties (velocity, forexample) are the same in alldirections.1

Kkelvin: Absolute temperature scale related

to the celsius (or centigrade) relativescale. The kelvin unit is equal to 1 °C;0 kelvin = -273.16 °C; the degree signand the word degrees are not used indescribing kelvin temperatures.3

Kirchoff’s law: Principle that thesummation of all flux exchanges(aborbed, reflected, transmitted) on asemitransparent object equal unity.

Llack of fusion: Discontinuity due to lack

of union between weld metal andparent metal or between successiveweld beads.1 Also called incompletepenetration.

lambertian: Having a surface that emitsuniformly in all directions. A blackbodyis a lambertian source.

lamination: Discontinuity in plate, sheetor strip caused by pipe, inclusions orblowholes in the original ingot. Afterrolling, laminations are usually flatand parallel to the outside surface.Laminations may also result frompipe, blisters, seams, inclusions orsegregation elongated and are madedirectional by working. Laminationdiscontinuities may also occur inmetal powder compacts.1 May appearin the form of rectangles or plates asinclusion stringers between rolledsurfaces. Short, intermittentlaminations may be detrimental if theobject is subjected to high bendingstresses in service.1

laser: Acronym (light amplification bystimulated emission of radiation). Thelaser produces a highly mono-chromatic and coherent (spatial andtemporal) beam of radiation. A steadyoscillation of nearly a singleelectromagnetic mode is maintainedin a volume of an active materialbounded by highly reflecting surfaces,called a resonator. The frequency ofoscillation varies according to thematerial used and by the methods ofinitially exciting or pumping thematerial.1,6

laser pyrometer: Infrared radiationthermometer that projects a laserbeam to the target, uses the reflectedlaser energy to compute targeteffective emissivity and automaticallycomputes target temperature(assuming that the target is a diffusereflector). Not to be confused withlaser aided aiming devices on someradiation thermometers.3

leak: Opening that allows the passage of afluid.1

leakage: Measurable quantity of fluidescaping from a leak.1

leakage rate: Quantity of leakage fluid perunit time that flows through a leak ata given temperature as a result of aspecified pressure difference across theleak.1

leak testing (LT): Nondestructive testingmethod for detecting, locating ormeasuring leaks or leakage inpressurized or evacuated systems orcomponents.1

lens: Translucent object that refracts lightpassing through it in order to focusthe light on a target.1

lens optics: See geometrical optics.light: Radiant energy that can excite the

retina and produce a visual sensation.The visible portion of theelectromagnetic spectrum, from about380 to 770 nm.1,6

light, speed of: See speed of light.limited certification: Individuals who are

certified only for specific operationsare usually called limited Level (I, II orIII) or are designated as having limitedcertification because they are notqualified to perform the full range ofactivities expected of personnel at thatlevel of qualification.1

line pair: Pair of adjacent, parallel linesused to evaluate the resolution of aspecific imaging system. See alsominimum line pair.1



line scanner, infrared: Instrument thatscans an infrared field of view along astraight line at the target plane inorder to collect infrared radiant energyfrom a line on the target surface,usually done by incorporating onescanning element within theinstrument. If the target (such as asheet or web process) moves at a fixedrate normal to the line scan direction,the result can be displayed as athermogram.3

line scan rate: Number of target linesscanned by an infrared scanner orimager in one second.3

liquid crystals: Liquids (generallycholesterol esters) whose opticalproperties cause them to reflect vividspectral colors for temperaturechanges. Their adjustable response issensitive and can be made to changefrom red to blue over a temperaturegradient as small as 1 K (1 °C =1.8 °F).1

lot tolerance percent defective: Inquality control, the percent defectiveat which there is a 10 percentprobability of acceptance in aproduction run.1

low pass filtering: Passage of lowfrequencies or long wavelengths, withattenuation of high frequencies orshort wavelengths.

lumen: Luminous flux per steradian froma source whose luminous intensity is 1candela. Symbolized lm.1

luminance: Ratio of a surface’s luminousintensity in a given direction to a unitof projected area. Measured in candelaper square meter.1

luminosity: Luminous efficiency ofradiant energy.1

lux: Unit of measure for illuminance inSI. Equivalent to lumens per squaremeter and symbolized lx. Formerlyknown as meter-candle.1

Mmachine vision: Automated system

function of acquiring, processing andanalyzing images to evaluate a testobject or to provide information forhuman interpretation. A typicalsystem consists of a light source, avideo camera, a video digitizer, acomputer and an image display.1

magnitude: Absolute value of a complexnumber.

manual zero: Control on a testinstrument that allows the user to zerothe instrument panel meter.1

mask: (1) Spatial filter in the sensing unitof a surface inspection system. (2) Ann × n matrix that serves as a filter inimage processing.1

material noise: Random signals caused bythe material structure of the testobject. A component of backgroundnoise.1

mathematical morphology: Imageprocessing technique of expandingand shrinking. The basic operators inmathematical morphology are dilation(expanding), erosion (shrinking),opening and closing.1

matte: Tending to diffuse light ratherthan reflect it; not shiny. The termmatte is generally applied to smoothsurfaces or coatings. Comparespecular.1

measurement spatial resolution,IFOVmeas: Smallest target spot size onwhich an infrared imager can producea measurement, expressed in terms ofangular subtense (mrad per side). Theslit response function (SRF) test is usedto measure IFOVmeas.3

measurement system: Entire system fromsensor to display inclusive.1

mechanical properties: Properties of amaterial that reveal its elastic andinelastic behavior where force isapplied, thereby indicating itssuitability for mechanical applications(for example, modulus of elasticity,tensile strength, elongation, hardnessand fatigue limit).1

medium, transmitting medium:Composition of the measurement pathbetween a target surface and themeasuring instrument through whichthe radiant energy propagates. Thiscan be vacuum, gaseous (such as air),solid, liquid or any combination ofthese.3

melting point coatings: Coatings thatmelt at some specific temperature.Anomalies are usually associated witha temperature increase, so thematerials melt first over anomalies.Melting point compounds also arecomparatively insensitive and requirerelatively high surface temperatures.1

mercury cadmium telluride (HgCdTe):Material used for fast, sensitiveinfrared photodetectors used ininfrared sensors, scanners and imagersthat requires cooled operation.3Operation is in the long wavelengthregion (8 to 12 µm).

micrometer (µm): One millionth of ameter (1 × 10–6 m); a unit used toexpress wavelength in the infraredband.

micron: Disfavored term for micrometer.milliradian (mrad): One thousandth of a

radian (1 radian = 180·π–1); a unit usedto express instrument angular field ofview; ann angle whose tangent isequal to 0.001; 1 mrad =0.057 295 78°).3



minimum line pair: Closest distance thata specific imaging system can resolvebetween a pair of adjacent, parallellines (line pair) used to evaluate spatialsystem resolution.1

minimum resolvable temperaturedifference: Thermal resolution;thermal sensitivity: the smallesttemperature difference that aninstrument can clearly distinguish outof the noise, taking into accountcharacteristics of the display and thesubjective interpretation of theoperator.3

MKSA: System of measurement unitsbased on the meter, kilogram, secondand ampere.

modulation: In general, the changes inone wave train caused by another; inthermal scanning and imaging, imageluminant contrast(Lmax – Lmin)·(Lmax + Lmin)–1.3

modulation transfer function (MTF):Measure of the ability of an imagingsystem to reproduce the image of atarget. A formalized procedure is usedto measure MTF. It assesses the spatialresolution of a scanning or imagingsystem as a function of distance to thetarget.3

Nnarrow banded: Relative term denoting a

restricted range of frequencyresponse.1

National Institute of Standards andTechnology: See NIST.

NBS: See NIST.NDC: Nondestructive characterization.NDE: (1) Nondestructive evaluation. (2)

Nondestructive examination.NDI: Nondestructive inspection.NDT: Nondestructive testing.NIST: National Institute of Standards and

Technology (formerly National Bureauof Standards), United StatesDepartment of Commerce,Gaithersburg, Maryland.3

NIST traceability: Traceability to theNational Institute of Standards andTechnology (NIST) is a means ofensuring that reference standards(such as calibrated leaks or heatsources) remain valid and that theircalibration remains current.3

noise: Any undesired signals that tend tointerfere with normal reception orprocessing of a desired signal. Theorigin may be an electrical, photonic,thermal or acoustic source —indicating either small discontinuitiesor abrupt changes in properties of thetest material.1

noise equivalent temperaturedifference: Temperature differencethat is just equal to the noise signal; ameasure of thermal resolution, but nottaking into account characteristics ofthe display and the subjectiveinterpretation of the operator.3

nondestructive characterization (NDC):Branch of nondestructive testingconcerned with the description andprediction of material properties andwith the behaviors of components andsystems.

nondestructive evaluation (NDE):Another term for nondestructive testing.In research and academiccommunities, the word evaluation isoften preferred because it emphasizesinterpretation by knowledgeablepersonnel.1

nondestructive examination (NDE):Another term for nondestructive testing.In the utilities and nuclear industry,examination is sometimes preferredbecause testing can imply performancetrials of pressure containment orpower generation systems.1

nondestructive inspection (NDI):Another term for nondestructive testing.In some industries (utilities, aviation),the word inspection often impliesmaintenance for a component thathas been in service.1

nondestructive testing (NDT):Determination of the physicalcondition of an object withoutaffecting that object’s ability to fulfillits intended function. Nondestructivetesting techniques typically use aprobing energy to determine materialproperties or to indicate the presenceof material discontinuities. See alsonondestructive characterization,nondestructive evaluation, nondestructiveexamination and nondestructiveinspection.1

nongraybody: Radiating object that doesnot have a spectral radiationdistribution similar to a blackbody andcan be partly transparent to infrared(transmits infrared energy at certainwavelengths); also called a colored body.Glass and plastic films are examples ofnongraybodies.3 The emissivity of acolored body has a spectraldependence.

nonrelevant indication: See indication,nonrelevant.

numerical analysis: Technique togenerate numbers as the solution to amathematical model of a physicalsystem. Used in place of a closed formanalytic expression. Usually requiresdigital computation.1



Oobjective: In discussion of a lens system

(camera, borescope, microscope,telescope), of or pertaining to the endor lens closest to the object ofexamination — at the end oppositefrom the eyepiece. Distal; tip.1

objective lens: Primary lens of an opticalsystem, On an infrared instrument,usually the interchangeable lens thatdefines the total field of view.3

one hundred percent testing: Testing ofall parts of an entire production lot ina prescribed manner. Sometimes,complete testing entails the testing ofonly the critical portions of the part.Compare sampling, partial.1

opaque: Impervious to radiant energy. Inthermography, an opaque material isone that does not transmit thermalinfrared energy.3

optical element, infrared: Any elementthat collects, transmits restricts orreflects infrared energy as part of aninfrared sensing or imaginginstrument.3

opening: Image processing operation oferosion followed by dilation. A singleopening eliminates isolated singlepixels. See also closing.1

optimum frequency: Frequency thatprovides the highest signal-to-noiseratio compatible with the detection ofa specific discontinuity. Eachcombination of discontinuity type andmaterial may have a differentoptimum frequency.1

organoleptic: Relying on or using senseorgans, such as the human eye.1

orientation: Angular relationship of asurface, plane, discontinuity or axis toa reference plane or surface.1

orthicon: See image orthicon.

Pparts per million (ppm): Expression of

concentration of a substance as a ratio.For example, a tracer gasconcentration might be 10 ppm in airor nitrogen. The more specific termsµL·L–1 and µg·g–1 are often used forproportions of volume and mass,respectively.

pass: In welding, a single bead along theentire weld length or the process oflaying down that bead.1

peak hold: Feature of an instrumentwhereby an output signal ismaintained at the peak instantaneousmeasurement for a specified duration.3

period: Absolute value of the minimuminterval after which the samecharacteristics of a periodic waveformor a periodic feature recur.1,5

photoconduction: Method by which avidicon television camera tubeproduces an electrical image, in whichconductivity of the photosensitivesurface changes in relation to intensityof the light from the scene focusedonto the surface. Comparephotoemission.1

photodetector (photon detector): Typeof infrared detector that has fastresponse (on the order ofmicroseconds), limited spectralresponse and usually requires cooledoperation; photodetectors are used ininfrared radiation thermometers,scanners and imagers,3 because, unlikethermal detectors, direct photoninteraction obviates external heatingof the detector for the signal to besensed.

pyroelectric detector: Type of thermalinfrared detector that acts as a currentsource with its output proportional tothe rate of change of its temperature.3

photoemission: Method by which animage orthicon television camera tubeproduces an electrical image, in whicha photosensitive surface emitselectrons when light from a viewedobject is focused on that surface.Compare photoconduction.1

photometer: Basic measuring instrumentof photometry. Accurate metersmeasuring radiant energy incident ona receiver, producing measurableelectrical quantities.1

photometric brightness: Luminance of alight source.1

photometry: Science and practice of themeasurement of light or photon-emitting electromagnetic radiation.See also relative photometry.1

photon: Particle of electromagneticradiation (not limited to visible light).

photoreceptor: Light sensor.1physical properties: Nonmechanical

properties such as density, electricalconductivity, heat conductivity andthermal expansion.1

picture element: See pixel.picture processing: See image processing.Planck’s distribution law: Fundamental

law that relates the emitted energyspectral radiance to wavelength and toemitted surface temperature.

porosity: Discontinuity in metal resultingfrom the creation or coalescence ofgas.1

practical examination: In certification ofnondestructive testing personnel, ahands-on examination using testequipment and sample test objects.Compare general examination andspecific examination.1



process: Repeatable sequence of actions tobring about a desired result.1

process control: Application of qualitycontrol principles to the managementof a repeated process.1

process testing: Initial product testing toestablish correct manufacturingprocedures and then by periodic teststo ensure that the process continues tooperate correctly.1

propagation: Advancement of energythrough a medium.

pseudocolor: Image enhancementtechnique wherein colors are assignedto pixels in an image according totheir intensity.

psychophysics: Interaction betweenvision performance and physical orpsychological factors. One example isthe so-called vigilance decrement, thedegradation of reliability based onperforming visual and/or repetitiveactivities over a period of time.1

pulsed thermography: See thermal waveimaging.

pyroelectric detector: Type of thermalinfrared detector that acts as a currentsource with its output proportional tothe rate of change of its temperature.3(Heating or cooling of pyroelectricmaterial creates charge accumulation.)

pyroelectric vidicon (PEV): Video cameratube with its receiving elementfabricated of pyroelectric material andsensitive to wavelengths from about2 to 20 µm; used in infrared thermalviewers. Sometimes called pyroelectricvidicon.3

pyrometer: Optical radiationthermometer for remote measurementof spot radiation levels, rather thanimaging a scene in the manner of aninfrared video camera.1,2 Usually adevice with digital outputs. It may bemounted in place or hand held. Seepyrometry.

pyrovidicon: See pyroelectric vidicon.pyrometry: Measurement of fire or of hot

objects, such as the monitoring offurnace or foundry conditions. Seepyrometer.

Qquadrature: Relation between two

periodic functions when the phasedifference between them is 90 degrees(that is, the time delay is one-fourth ofa period).

qualification: Process of demonstratingthat an individual has the requiredamount and the required type oftraining, experience, knowledge andcapabilities. See also qualified.1

qualified: Having demonstrated therequired amount and the requiredtype of training, experience,knowledge and abilities. See alsoqualification.1

quality: Ability of a process or product tomeet specifications or to meet theexpectations of its users in terms ofefficiency, appearance, longevity andergonomics.1

quality assurance: Administrative actionsthat specify, enforce and verify aquality program.1

quality control: Physical andadministrative actions required toensure compliance with the qualityassurance program. May includenondestructive testing in themanufacturing cycle.1

quality of lighting: Level of distributionof luminance in a field of view or in avisual environment.1

Rradian: Angle equal to 180·π–1 degrees or

57.295 78 angular degrees.3radiance: Radiant flux per unit solid

angle and per unit projected area ofthe source. Measured in watt persquare meter steradian. Compareirradiance.1

radiant energy: Energy transmittedthrough a medium by electromagneticwaves. Also known as radiation.1 Seealso radiance.

radiant flux: Radiant energy’s rate offlow, measured in watt.1

radiant intensity: Electromagnetic energyemitted per unit time per unit solidangle. Measured in watt per steradian.1

radiant power: Total radiant energyemitted per unit time.1

radiation, thermal: Mode of heat flowthat occurs by emission andabsorption of electromagneticradiation, propagating at the speed oflight and, unlike conductive andconvective heat flow, capable ofpropagating across a vacuum; the formof heat transfer that allows infraredthermography to work becauseinfrared energy travels from the targetto the detector by radiation.3

radiation reference source: Blackbody orother target of known temperatureand effective emissivity used as areference to obtain optimummeasurement accuracy, ideally,traceable to the National Institute ofStandards and Technology.3

radiation thermometer: See infraredradiation thermometer.

radio frequency display: Presentation ofunrectified signals on a displayscreen.1 See also video presentation



radio frequency interference: SeeEMI/RFI noise.

radiometer: Instrument for measuringradiant power of specified frequencies.Different radiometers exist fordifferent frequencies.1

radiosity: Total infrared energy (radiantflux) leaving a target surface. This iscomposed of radiated, reflected andtransmitted components. Only theradiated component is related to targetsurface temperature. Also calledexitance.3

radiometric photometer: Radiometer formeasuring radiant power over avariety of wavelengths.1

rankine: Absolute temperature scalerelated to the fahrenheit relative scale.The rankine unit (°R) is equal to 1 °F;0 °R = –459.72 °F; the degree sign andthe word degrees are not used indescribing rankine temperatures.3

raster: Repetitive pattern whereby adirected element (a robotic arm or aflying dot on a video screen) followsthe path of a series of adjacent parallellines, taking them successively in turn,always in the same direction (from topto bottom or from left to right),stopping at the end of one line andbeginning again at the start of thenext line. Following a raster patternmakes it possible for electron beams toform video pictures or frames and fora sensor bearing armature to cover apredetermined part of the surface of atest object.1

ratio pyrometer: Infrared thermometerthat uses the ratio of incominginfrared radiant energy at twonarrowly separated wavelengths todetermine a target’s temperatureindependent of target emittance; thisassumes graybody conditions and isnormally limited to relatively hottargets, above about 420 K (150 °C ≅300 °F).3

Recommended Practice SNT-TC-1A: SeeASNT Recommended Practice No. SNT-TC-1A.

recommended practice: Set of guidelinesor recommendations.1

reference junction: In a thermocouple,the junction of the dissimilar metalsthat is not the measurement junction.This is normally maintained at aconstant reference temperature.3

reflectance or spectral reflectance: Ratioof wave energy (radiant flux) reflectedfrom a material to incident waveenergy (incident radiant flux) per unitarea.1,3 See also reflectivity.

reflection: General term for the processby which the incident flux leaves asurface or medium from the incidentside, without change in frequency.Reflection is usually a combination ofspecular and diffuse reflection.1,6

reflectivity: Ratio ρ of the intensity of thetotal energy reflected from a surface tototal radiation on that surface:

For a perfect mirror, reflectivity ρapproaches 1.0; for a blackbody thereflectivity is 0.3 See also reflectanceand reflection.

reflectometer: Photometer used tomeasure diffuse, specular and totalreflectance.1

reject: Instrument function or controlused for minimizing or eliminatinglow amplitude signals (electrical ormaterial noise) so that other signalsmay be further amplified. Use of thiscontrol can reduce vertical linearity.Also called suppression.1

rejection level: Value established for atest signal above or below which,depending on the test parameter, testobjects are rejectable or otherwisedistinguished from the remainingobjects.1 Compare acceptance level.

relative humidity: Ratio (in percent) ofthe water vapor content in the air tothe maximum content possible at thattemperature and pressure.3

relevant indication: See indication,relevant.

remote viewing: Viewing of a test objectnot in the viewer’s immediatepresence. The word remote previouslyimplied either closed circuit televisionor fiber optic systems remote enoughso that, for example, the eyepiece andthe objective lens could be in differentrooms. High resolution video anddigital signals can now be transmittedaround the world with little loss ofimage quality. Compare direct viewing.1

repeatability: Ability to reproduce adetectable indication in separateprocessings and tests from a constantsource.1 For thermal measurements,expressed in degrees difference or apercentage of full scale.3

resistance, thermal (R): Measure of amaterial’s resistance to the flow ofthermal energy, inversely proportionalto its thermal conductivity k, wherek = 1·R–1.3

resistance temperature device (RTD):Sensor that measures temperature by achange in resistance as a function oftemperature.3

resolution: Aspect of image qualitypertaining to a system’s ability toreproduce objects, often measured byresolving a pair of adjacent objects orparallel lines. See also minimum linepair and resolving power.1

ρ ε τ= 1 −− −−



resolution, discontinuity: Property of atest system that enables the separationof indications due to discontinuitieslocated in close proximity to eachother in a test object.1

resolution test: Procedure wherein a lineis detected to verify a system’ssensitivity.1

resolution threshold: Minimum distancebetween a pair of points or parallellines when they can be distinguishedas two, not one, expressed in minutesof arc. Vision acuity in such a case isthe reciprocal of one half of the periodexpressed in minutes.1,6

resolving power: Ability of detectionsystems to separate two points in timeor distance. Resolving power dependson the angle of vision and thedistance of the sensor from the testsurface. Resolving power in visionsystems is often measured usingparallel lines. Compare resolution.1

response time: Time it takes for aninstrument output signal or display torespond to a temperature step changeat the target; expressed in seconds(typically, to 95 percent of the finalvalue, and approximately equal to 5time constants).3

retina: In the eye, the tissue that senseslight.1

robotic system: Automated systemprogrammed to perform purposefulmovements in variable sequences.1

Ssample and hold: Feature of an

instrument whereby an output signalis maintained at an instantaneousmeasurement value for a specifiedduration after a trigger or until anexternal reset is applied.3

sampling, partial: Testing of less thanone hundred percent of a productionlot. See one hundred percent testing.1

sampling, random partial: Partialsampling that is fully random.1

sampling, specified partial: Partialsampling in which a particularfrequency or sequence of sampleselection is prescribed. An example ofspecified partial sampling is the testingof every fifth unit.1

scan angle: For a line scanner, the totalangular scan possible at the targetplane, typically 90 degrees.3

scanning: Movement of a sensor over thesurface of a test object in a controlledmanner so as to achieve completecoverage.1

scan position accuracy: For a linescanner, the precision with whichinstantaneous position along the scanline can be set or measured.3

sector: For a line scanner, a portion of thetotal scan angle over whichmeasurement is made at the targetplane.3

seebeck effect: See thermoelectric effect.sensitivity: Measure of a sensor’s ability

to detect small signals. Limited by thesignal-to-noise ratio.1 See minimumresolvable temperature difference.

setpoint: Any temperature setting atwhich an activating signal or closurecan be preset so that, when themeasured temperature reaches thesetpoint, a control signal, pulse orrelay closure is generated.3

shock: Sudden application of force, for aspecific time duration; also thetemporary or permanent damage to asystem as a result of a shock.3

shrink: Internal rupture occurring incastings due to contraction duringcooling, sometimes caused byvariations in solidification rates in themold. Includes shrinkage sponge,small voids (stringers or bunches) or afingerprint pattern of semifused seams.Also applied to surface shrinkagecracks.1

SI: The International System of units ofmeasurement. An international systemof measurement based on seven units:meter (m), kilogram (kg), second (s),kelvin (K), ampere (A), candela (cd)and mole (mol).1,5

signal: Response containing relevantinformation.1,4

signal processing: (1) Acquisition,storage, analysis, alteration and outputof digital data through a computer.1(2) In infrared and thermal testing,manipulation of temperature signal orimage data for purposes of enhancingor controlling a process. Examples forinfrared radiation thermometers arepeak hold, valley hold, averaging andsample and hold. Examples for scannersand imagers are usually referred to asimage processing and includequalitative characterization,quantitative characterization,alignment, isotherm enhancement,image subtraction, image averaging,and image filtering.3

signal-to-noise ratio: Ratio of signalvalues (responses that contain relevantinformation) to baseline noise values(responses that contain nonrelevantinformation). See noise.1,4

slit response function: Measure of themeasurement spatial resolution(IFOVmeas) of an infrared scanner orimager.3 See modulation transferfunction.

smoothing: In image processing, use ofpositive coefficients in a linearcombination of pixel values tosmoothen abrupt transitions in adigital image.1



SNT-TC-1A: See ASNT RecommendedPractice No. SNT-TC-1A.

spatial resolution: Spot size in terms ofworking distance. In an infraredradiation thermometer this isexpressed in milliradians or as a ratio(D/d) of the target spot size(containing 95 percent of the radiantenergy, according to common usage)to the working distance. In scannersand imagers it is most often expressedin milliradians.3

specular reflector: Smooth reflectingsurface that reflects all incidentradiant energy at an anglecomplementary (equal around thenormal) to the angle of incidence. Amirror is a specular reflector.3

spectral response: Spectral wavelengthinterval over which an instrument orsensor responds to infrared radiantenergy, expressed in micrometer (µm):also, the relative manner (spectralresponse curve) in which it respondsover that interval.3

spectral reflectance: See reflectance.spectral transmittance: See transmittance,

spectral.spot: Instantaneous size (diameter unless

otherwise specified) of the area at thetarget plane that is being measured bythe instrument. In infraredthermometry, this is specified by mostmanufacturers to contain 95 percentof the radiant energy of an infinitelylarge target of the same temperatureand emissivity.3

storage operating range: Temperatureextremes over which an instrumentcan be stored and, subsequently,operate within published performancespecifications.3

subtense, angular: Angular diameter ofan optical system or subsystem,expressed in angular degrees ormilliradians. In thermography, theangle over which a sensing instrumentcollects radiant energy.3

specification: Set of instructions orstandards invoked by a specificcustomer to govern the results orperformance of a specific set of tasksor products.1

specific examination: In certification ofnondestructive testing personnel, awritten examination that addresses thespecifications and products pertinentto the application. Compare generalexamination and practical examination.1

spectral power distribution: Radiantpower per unit wavelength as afunction of wavelength. Also knownas spectral energy distribution, spectraldensity and spectral distribution.1 SeePlanck’s law.

spectral reflectance: See reflectance.spectral transmittance: See transmittance.

spectrophotometer: Instrument used forspectrophotometry.1

spectrophotometry: Measurement ofelectromagnetic radiant energy as afunction of wavelength, particularly inthe ultraviolet, visible and infraredwavelengths.1

spectroradiometer: Instrument used forspectroradiometry.1

spectroradiometry: Measurement ofelectromagnetic radiant power andspectral emittance, used particularly toexamine colors and to measure thespectral emittance of light sources.1

spectroscope: Instrument used forspectroscopy.1

spectroscopy: Spectro photometry orspectroradiometry in which thespectrum, rather than being analyzedonly by a processing unit, is presentedas a digital signal for computeranalysis or in a visible form to theoperator for organolepticexamination.1

spectrum: (1) Amplitude distribution offrequencies in a signal.1(2) Representation of radiant energy inadjacent bands of hues in sequenceaccording to the energy’s wavelengthsor frequencies. A rainbow is a wellknown example of a visible spectrum.1

spectrum response: Amplification (gain)of a receiver over a range offrequencies or wavelengths.1

specular: Of a mirrorlike surface,reflective or shiny, with little or nodiffusion.

speed of light: Speed of all radiantenergy, including light: 2.997 925 ×108 m·s–1 in vacuum (about 186 000mi·s–1). In all materials the speed isless in practical measurements andvaries with the material’s index ofrefraction, which itself varies withwavelength.1,6

standard: (1) Physical object with knownmaterial characteristics used as a basisfor comparison or calibration. (2)Concept established by authority,custom or agreement to serve as amodel or rule in the measurement ofquantity or the establishment of apractice or procedure.1 (3) Documentto control and govern practices in anindustry or application, applied on anational or international basis andusually produced by consensus. Seealso acceptance standard, workingstandard and reference standard.1,4

standard atmospheric conditions:Atmospheric pressure of 101.325 kPa(14.6959 lbf·in.–2). Temperature of293.15 K (20 °C, 68 °F or 527.67 °R).The density of dry air at theseconditions is 1.2041 kg·m–3 (0.075 17lb·ft–3).1



Stefan-Boltzmann law: Relationshipgoverning the wavelengthindependent rate of emission ofradiant energy per unit area. The lawrelates the total radiation intensity tothe fourth power of absolutetemperature and emissivity of thematerial surface. For example,intensity (heat flow) from a copperblock at 100 °C (212 °F) is 300 W·m–2

(95 BTU·ft–2·h–1). (Stefan-Boltzmannconstant for photon emission =1.52041 × 1015 photon·s–1·m–2·K–2.)1

stereo photography: Close rangephotogrammetric technique involvingthe capture and viewing of two imagesof the same object in order toreconstruct a three dimensional imageof the object.1

subsurface discontinuity: Anydiscontinuity that does not extendthrough the surface of the object inwhich it exists.1 See near surfacediscontinuity.

Ttarget: Object surface to be measured or

imaged.3temperature: Measure of the intensity of

particle motion in kelvin (K), degreescelsius (°C) or degrees fahrenheit (°F)or, in the absolute scale, kelvin (K) ordegrees rankine (°R), where anincrement of 1 K = 1 °C = 1.8 °R =1.8 °F. Compare heat.1

temperature measurement drift: Readingchange (error), with time, of a targetwith nonvarying temperature, whichmay be caused by a combination ofambient changes, line voltage changesand instrument characteristics.3

temperature resolution: See minimumresolvable temperature (difference),MRT(D).3

test surface: Exposed surface of a testobject.1

thermal: Physical phenomenon of heatinvolving conduction, convection orradiation.

thermal conductivity: See conductivity,thermal.

thermal conductivity vacuum gage:Instrument that operates on principlethat as gas molecules are removedfrom a system, the amount of heattransfer by conduction is reduced. Thisrelationship is used to indicateabsolute pressure.1

thermal detector, infrared: Type ofinfrared detector that changeselectrical characteristics as a functionof temperature; typically, thermaldetectors have slow response, (on theorder of milliseconds) broad spectralresponse and usually operate at roomtemperature; thermal detectors arecommonly used in infrared radiationthermometers and in some imagers.3

thermal diffusion: Process by whichthermal energy is transferred from hotor cold regions and finally is spreadout. See also conduction and thermaldiffusivity.

thermal diffusivity: Speed at which heatdiffuses through an object. Expressedas the rate α of temperature changewith time. Each material has its owncharacteristic value of diffusivity,combining the overall influence ofthermal conductivity k, density ρ andspecific heat Cp.

In a practical sense, thermal diffusivitydetermines how fast a material willheat up or cool down. The rate oftemperature change with time is morerapid in a material with a highthermal diffusivity (for example,metals) and slower in a material with alower diffusivity (for example,plastics).1

thermal effusivity: See effusivity, thermal.thermal equilibrium: Condition of an

object wherein temperaturesthroughout the object remainconstant.1

thermal viewer: Nonmeasuring thermalimager that produces qualitativethermal images related to thermalradiant distribution over the targetsurface.3

thermal wave imaging: Term used todescribe an active technique forinfrared nondestructive materialtesting, in which a sample isstimulated with a pulse of thermalenergy and in which the time basedreturned thermal images are processedto determine flaw depth and severity.3Also called pulse stimulated imaging orpulsed thermography.3

thermistor: Temperature detector, usuallya semiconductor, whose electricalresistance decreases predictably andnonlinearly with increasingtemperature.3 The coefficient ofelectrical resistance with temperatureis typically on the order of–4 percent K–1.

thermistor bolometer, infrared:Thermistor so configured as to collectradiant infrared energy; a type ofthermal infrared detector.3

αρ

== kCp



thermoelectric effect: Phenomenon thatexplains the operation ofthermocouples; that in a closedelectrical circuit made up of twojunctions of dissimilar metalconductors, a direct current will flowas long as the two junctions are atdifferent temperatures. Thephenomenon is reversible; if thetemperatures at the two junctions arereversed, the flow of current reverses.3Also called seebeck effect.

thermocouple: Device for measuringtemperature based on the fact thatopposite junctions between certaindissimilar metals develop an electricalpotential when placed at differenttemperatures.3

thermogram: Thermal map or image of atarget where the gray tones or colorhues correspond to the distribution ofinfrared thermal radiant energy overthe surface of the target (qualitativethermogram); when correctlyprocessed and corrected, a graphicrepresentation of surface temperaturedistribution (quantitativethermogram).3

thermograph: Another word used todescribe an infrared thermal imager.3

thermography: Imaging or viewing of anobject or process through sensing ofinfrared radiation emitted by it. Thetemperature patterns on the materialsurface produce correspondingradiation patterns. Thus, heat flow byboth conduction and radiation may beobserved and used to locate materialdiscontinuities.1 Most often,thermography is based on sensing ofinfrared radiation.

thermomechanical coupling: interactionbetween mechanical and thermalbehaviors of materials.

thermometer: Any device used formeasuring temperature.3

thermopile: Device constructed by thearrangement of thermocouples inseries to add the thermoelectricvoltage. A radiation thermopile is athermopile with junctions so arrangedas to collect infrared radiant energyfrom a target, a type of thermalinfrared detector.3

threshold: See adaptive thresholding,resolution threshold and threshold level.

thresholding: Digital data processingtechnique that reduces a gray levelimage into a binary image byapplication of a threshold.1

threshold level: Setting of an instrumentthat causes it to register only thosechanges in response greater or lessthan a specified magnitude.1,4

time constant: Time it takes for anysensing element to respond to 63.2percent of a step change at the targetbeing sensed. In infrared sensing andthermography, the time constant of adetector is a limiting factor ininstrument performance, as it relatesto response time.3

total field of view (TFOV): In imagers,the total solid angle scanned, usuallyrectangular in cross section. See alsofield of view.3

transducer: Any device that can convertenergy from one form to another. Inthermography, and infrared detector isa transducer that converts infraredradiant energy to some usefulelectrical quantity.3

transfer calibration: Technique forcorrecting a temperature measurementor a thermogram for various errors byplacing a radiation reference standardadjacent to the target.3

transfer standard: Precision radiometricmeasurement instrument with NISTtraceable calibration used to calibrateradiation reference sources.3

transient heat flow: Heat flow occurringduring the time it takes an object toreach thermal equilibrium or steadystate.1

transmissivity: Proportion τ of infraredradiant energy impinging on anobject’s surface, for any given spectralinterval, that is transmitted throughthe object.

where τ is transmissivity, ε isemissivity and ρ is reflectivity. For ablackbody, transmissivity = 0.Transmissivity is the internaltransmittance per unit thickness of anondiffusing material.3 See alsotransmittance, spectral.

transmittance, spectral: Radiant fluxpassing through a medium divided bythe wavelength of the incident radiantflux.1

two-color pyrometer: See ratio pyrometer.

Uunity: One (1.0).3

Vvalley hold: Feature of an instrument

whereby an output signal ismaintained at the lowestinstantaneous measurement for aspecified duration; opposite of peakhold.3

τ ε ρ= 1 −− −−



visual testing: Method of nondestructivetesting using electromagnetic radiationat visible frequencies.1

voids: Hollow spots, depressions orcavities. See also discontinuity anddislocation.1

Wwavelength: Distance in the propagation

direction that a wave travels incompleting a full cycle.

white light: Light combining allfrequencies in the visible spectrum.1

Wien’s displacement law: For practicalinfrared imaging, Wien’s displacementlaw gives the wavelength of maximumemittance.1

working distance: Distance from thetarget to the instrument, usually tothe primary optic.3

working standard: Work piece or energysource calibrated and used in place ofexpensive reference standards. In thecalibrating of photometers, thestandard would be a light source.1

Zzone: In line scanners, a scanned area

created by the transverse linearmotion of the product or processunder a measurement sector of thescanner.3



infrared and thermal testing glossary - asnt.org

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