Visual Testing Glossary - ASNT

Visual Testing Glossary

13C H A P T E R

From Nondestructive Testing Handbook, Third Edition: Volume 9, Visual Testing © 2010. Reprinted with permission of The American Society for Nondestructive Testing Inc.

Introduction

PurposeStandards writing bodies take great painsto ensure that their standards aredefinitive in wording and technicalaccuracy. People working to writtencontracts or procedures should consultdefinitions referenced in standards whenappropriate. For example, persons whowork in accordance with standardspublished by ASTM International areencouraged to refer to definitions in theASTM standards.1The definitions in this Nondestructive

Testing Handbook volume should not bereferenced for tests performed accordingto standards or specifications or infulfillment of contracts. This glossary isprovided for instructional purposes. Noother use is intended.

On ReferencesMany definitions in this glossary areadapted from other volumes of theNondestructive Testing Handbook series,especially from the second edition’s Visualand Optical Testing (1993).2Some terms apply generally to

nondestructive testing and are not specificto visual testing — terms on subjects suchas metallurgy, quality control andpersonnel qualification. Many of thesedefinitions come from the second editionvolume Nondestructive Testing Overview(1996)3; some are rephrased in the thirdedition’s most recent volume, MagneticTesting (2008).4Entries from other volumes in the

Nondestructive Testing Handbook series arereprinted but generally not referencedbelow.

Definitions

A

acceptable quality level (AQL):Maximum percent defective (or themaximum percentage of units withrejectable discontinuities) that, for thepurposes of sampling tests, can beconsidered satisfactory as a processaverage.

acceptance criterion: Benchmark againstwhich test results are to be comparedfor purposes of establishing thefunctional acceptability of a part orsystem being examined.4

acceptance level: Measured value orvalues above or below which testobjects are acceptable, in contrast torejection level.4

acceptance standard: (1) Specimen,similar to the product to be tested,containing natural or artificialdiscontinuities that are well definedand similar in size or extent to themaximum acceptable in the product.4(2) Document defining acceptablediscontinuity size limits. See alsostandard.

accommodation: Of the eye, adjustmentof the lens’ focusing power bychanging the thickness and curvatureof the lens through its movement bytiny muscles.

ACGIH: American Conference ofGovernmental Industrial Hygienists.

acuity: See neural acuity, vision acuity.adaptive thresholding: Threshold value

varying with inconstant backgroundgray level.

adhesive wear: See wear, adhesive.alpha ferrite: Form of pure iron that has

a body centered cubic structure stablebelow 910 °C (1670 °F). Also calledalpha iron.

alpha iron: See alpha ferrite.ambient light: Light in the environment

as opposed to illumination providedby a visual testing system.

ampere (A): SI unit of electric current.4angle: See field angle.angstrom (Å): Disused unit of length. 1 Å

= 0.1 nm.

304 Visual Testing

Terms


anomaly: (1) In nondestructive testing, anonrelevant indication. (2) Innondestructive testing, anunintentional or undesired materialcondition that may qualify as a defect.Compare defect; discontinuity. Someanomalies, such as inadequate casehardening or rough surface finish,may be defects but, because there is nointerruption in the material structure,are not discontinuities.

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

ASNT: American Society forNondestructive Testing.

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

axial: Of or pertaining to a directionalong the length of an oblong objectand perpendicular to its radius — forexample, down the length of acylinder. Compare radial.

B

background cylinder and differencecylinder: Two devices used tocalculate illuminance by using theequivalent sphere illuminationtechnique.2,5

binary system: In metallurgy, atwo-element alloy system. See alsoisomorphous binary system.

birefringence: Splitting of a light beaminto two parts through a translucentmaterial.

black body: Theoretical object thatradiates more total power and morepower at any wavelength than anyother source operating at the sametemperature.2,5

blackbody: See black body.black light: Term sometimes used for

ultraviolet radiation, particularly inthe near ultraviolet range of about320 to 400 nm.

blacklight: See black light.blind spot: Portion of the retina where

the optic nerve enters, without rodsand cones and hence insensitive tolight.2,6

blister: Discontinuity in metal, on or nearthe surface, resulting from theexpansion of gas in a subsurface zone.Very small blisters are called pinheadsor pepper blisters.4

blotch: (1) An irregularly spaced area ofcolor change on a surface. (2) Thenonuniform condition of a surfacecharacterized by such blotches.

blowhole: Hole in a casting or a weldcaused by gas entrapped duringsolidification.4

blue light hazard: Danger posed to theeye by long term exposure to highfrequency visible light at intensitiesand durations that may damage theretina.

borescope: Industrial endoscope; aperiscope or telescope using mirrors,prisms, lenses, optic fibers ortelevision wiring to transmit imagesfrom inaccessible interiors for visualtesting. Borescopes are so calledbecause they were originally used inmachined apertures and holes such asgun bores. There are both flexible andrigid borescopes.

borescope, angulated: Borescope bent forviewing at forward oblique, right angleor retrospective angles for visualtesting of surfaces not accessible withconventional borescopes.

borescope, calibrated: Borescope withgage on external tube to indicate thedepth of insertion during a test.Borescopes with calibrated reticles areused to determine angles or sizes ofobjects in the field when held at apredetermined working distance.

borescope, cave: Multiangulated,periscopic borescope used for remoteobservation of otherwise inaccessibleareas.

borescope, fiber optic: Industrialendoscope, or fiber optic borescope,that uses glass or quartz fibers totransmit light and the optical path toand from the test object.

borescope, indexing: Borescope that canbe bent 90 degrees by rotation of aknob after the instrument has beeninserted through an aperture. A knobat the eyepiece can rotate the objectivehead through 360 degrees forscanning a circumferential weld seam.

borescope, micro-: Borescope with anoutside diameter generally from 1 to5 mm (0.04 to 0.2 in.), typically usingquartz filaments. Compare miniatureborescope.

borescope, miniature: Borescope with anoutside diameter generally less than13 mm (0.5 in.). Sometimes calledminiborescope. See also microborescope.

borescope, panoramic: Borescope with ascanning mirror mounted in front ofthe objective lens system. Rotation ofthe mirror is adjusted at the ocularend of the instrument to scan inforward oblique, right angle andretrospective directions.

borescope, retrospective: Borescope thatlooks backward more than 90 degreesfrom the distal line of interrogationnormal to the plane of a conventionalobjective lens.

borescope, rigid: Borescope that does notbend, typically in order to keep thegeometrical optics in alignmentthrough a light train system.

305Visual Testing Glossary


borescope, ultraviolet: Borescopeequipped with ultraviolet lamps, filtersand special transformers to transmitradiation of ultraviolet wavelengths.

borescope, video: Borescope transmittingimage electronically.

borescope, waterproof/vaporproof:Borescope completely sealed andimpervious to water or other types offluid, used for internal tests of liquid,gas or vapor environments.

borescope, wide field: Borescope withrotating objective prism to providefields of view up to 120 degrees.

borescopy: Viewing or inspection with aborescope.

brinelling: Repeated stripe indentationsmade by a spherical object. Falsebrinelling refers to a type of surfacewear.

burr: Raised or turned over edge occurringon a machined part and resulting fromcutting, punching or grinding.2,5

burst: In metal, external or internalrupture caused by entrapped gas.

butt weld:Weld that joins the edges oftwo work pieces in the same plane.

C

calibration: (1) Ratio of the output from adevice to a reference input. Knowledgeof this ratio helps to infer a device’sinput from its output. (2) Act ofreturning an instrument to theparameters and settings of the originalequipment manufacturer.(3) Statement of the scale of a device.

candela (cd): Base SI unit of luminousintensity, in a given direction, of amonochromatic radiation source thathas a frequency of 5.4 × 1014 Hz andthat has a radiant intensity in thatdirection of 1.464 mW·sr–1.4

candle: Former name for candela.cavitation fatigue: Form of pitting,

caused by erosion from vibration andmovement in liquid environments.

CCD: See charge coupled device.cementite: Iron carbide (Fe3C), a hard

and brittle substance present in steels.certification:With respect to

nondestructive test personnel, theprocess of providing written testimonythat an individual has met thequalification requirements of a specificpractice or standard. See also certifiedand qualified.

certified:With respect to nondestructivetest personnel, having writtentestimony of qualification. See alsocertification and qualification.

CGS system: Obsolete system ofmeasurement units based on thecentimeter, gram and second.Compare SI.

chafing: See wear, fretting.

channels: In biology, mechanismsfunctioning as band pass filters in thevisual cortex of mammals, causingsensitivity to visual stimuli inparticular frequencies and ranges.

charge coupled device (CCD): Solid stateimage sensor. Charge coupled devicesare widely used in inspection systemsbecause of their accuracy, high speedscanning and long service life.

check cracking: Surface crack caused byoverheating and having cross hatchedpattern. See grinding crack.

closing: In image processing, dilationfollowed by erosion. A single pixel byclosing connects a broken featureseparated by one pixel. See alsoopening.

closure: Process by which a personcognitively completes patterns orshapes that are incompletelyperceived.

cocoa: Debris (usually oxides of thecontacting metals) of fretting wear,retained at or near the site of itsformation — a condition easilyidentified during visual tests. Withferrous metals, the debris is brown, redor black, depending on the type ofiron oxide formed. For this reason,ferrous debris is called cocoa or, whenmixed with oil or grease, red mud.

code: Standard enacted or enforced as alaw.

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

cold light: Disused word for fluorescence.color: Visual sensation by means of which

humans distinguish light of differinghue (predominant wavelengths),saturation (degree to which thoseradiations predominate over others)and lightness.

color blindness: Deficiency in ability toperceive or distinguish hues.

color discrimination: Perception ofdifferences between two or more hues.

color temperature: Rating of a lightsource for color vision.

compound microscope: See microscope,compound.

cone: Part of the eye; color sensitivephotoreceptor at the inner region ofthe retina. Cones assist with mesopicvision and are responsible forphotopic vision. Compare rod.

confidence level: Level of assurance fordetecting a specified discontinuity sizewith a specified probability. See alsoprobability of detection.

constitution diagram: See phase diagram.contrast: (1) Difference in color or

brightness between a test indicationand background. (2) Differencebetween the amount of light reflectedor transmitted by an object and by thebackground in the field of view.

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control: See in control, process control andquality control.

corrosion: Loss or degradation of metalbecause of chemical reaction.

corrosion, crevice: Corrosion found intight crevices or pores (pits) andaccelerated by galvanic activity fromhigh ion concentrations.

corrosion-erosion: Simultaneousoccurrence of erosion and corrosion.

corrosion, fretting: Corrosion facilitatedby fretting, particularly where aprotective surface has been chafed in acorrosive environment.

corrosion, poultice: Corrosion occurringunder a layer of foreign material (forexample, under mud in automobilerocker panels).

cosine law: Physical law stating that theillumination of a surface varies as thecosine of the incidence angle.Maximum illumination is obtainedwhere the cosine equals one and whenthe source is perpendicular to thesurface.

crack: (1) Stress induced break, fissure orrupture, sometimes V shaped in crosssection and relatively narrow. Byconvention, a crack is called linear if itis at least three times longer than it iswide. (2) Propagating discontinuitycaused by fatigue, corrosion or stressessuch as heat treating or grinding. Maybe difficult to detect unaided becauseof fineness of line and pattern (mayhave a radial or latticed appearance).

creep: Gradual and permanent change ofshape in a metal under constant load,usually at elevated temperature.Occurs in three stages: primary creep,secondary creep and tertiary creep. Seealso deformation.

crevice corrosion: See corrosion, crevice.

D

dark adaptation: Process by which theretina becomes adapted to aluminance less than about0.034 cd·m–2.2,6 In dark adaptation,the pupils dilate and the two types ofphotoreceptors in the retina changechemical balance. After a finiteamount of time, possibly 10 min,vision will change from photopicvision to mesopic or scotopic lowillumination vision.4

dark adapted vision: See accommodation,visual; scotopic vision.

defect: Discontinuity whose size, shape,orientation or location (1) makes itdetrimental to the useful service of itshost object or (2) exceeds anaccept/reject criterion of an applicablespecification. Some discontinuities donot exceed an accept/reject criterionand are therefore not defects.Compare crack; discontinuity; indication.

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

delta ferrite: Solid solution with bodycentered cubic structure and iron assolvent. Also called delta iron.

delta iron: See delta ferrite.depth of field: In photography, the range

of distance over which an imagingsystem gives satisfactory definitionwhen its lens is in the best focus for aspecific distance.

dewetting: In soldering, the flow andretraction of solder, caused bycontaminated surfaces, dissolvedsurface coatings or overheating beforesoldering.

difference cylinder: See backgroundcylinder.

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 and opening.

diopter: Unit used to express theresolving power of lenses, equal to theinverse of the length (in meters) of theoptical axis.

directional lighting: Lighting providedon the work plane or objectpredominantly from a preferreddirection.2,6

direct photometry: Simultaneouscomparison of a standard lamp and anunknown light source.2,6

direct substitution alloy: Alloy in whichthe atoms of the alloying element canoccupy the crystal lattice spacesnormally occupied by the atoms of theparent metal.

direct viewing: (1) Viewing of a testobject in the viewer’s immediatepresence. The term direct viewing isused in the fields of robotics andsurveillance to distinguishconventional from remote viewing.(2) Viewing of a test object duringwhich the light image is not mediatedthrough a system of two or morelenses (as in a borescope) ortransduced through an electronicsignal (as with a charge coupledcamera). The term direct viewing is usedin some specifications to meanviewing possibly with a mirror ormagnifier but not with a borescope.Compare indirect viewing; remoteviewing.

direct vision instrument: Device offeringa view directly forward. A typicalscene is about 20 mm (0.75 in.) wideat 25 mm (1 in.) from the objectivelens.



discontinuity: Interruption in thephysical structure or configuration of atest object. After nondestructivetesting, a discontinuity indication canbe interpreted to be a defect.4,7Compare defect; indication.

discontinuity, artificial: Referencediscontinuity such as hole,indentation, crack, groove or notchintroduced into a reference standardto provide accurately reproducibleindications for determining testsensitivity levels.4

discontinuity, inherent: Materialanomaly originating fromsolidification of cast metal. Pipe andnonmetallic inclusions are the mostcommon inherent discontinuity andcan lead to other types ofdiscontinuities in fabrication.2,5

discontinuity, primary processing:Discontinuity produced from the hotor cold working of an ingot intoforgings, rods, bars and othershapes.2,5

discontinuity, secondary processing:Discontinuity produced duringmachining, grinding, heat treating,plating or other finishingoperations.2,5

discontinuity, service induced:Discontinuity caused by the intendeduse of the part.

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

E

elastic deformation: Temporary changein shape under a load. The materialreturns to its original size and shapeafter the load is removed. Elasticdeformation is the state in which mostmetal components are used in service.

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

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

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. See borescope.

equilibrium diagram: Phase diagramshowing the phases present atequilibrium in a material system.

equivalent 20/20 near vision acuity:Vision acuity with remote viewing orother indirect viewing thatapproximates 20/20 direct viewingclosely enough to be considered thesame for visual testing purposes.

equivalent sphere illumination: Level ofperfectly diffuse (spherical)illuminance that makes the visual taskas photometrically visible within acomparison test sphere as it is in thereal lighting environment.

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. See wear. (2) In imageprocessing, condition of a binaryimage where the pixel in the outputimage becomes a 1 if each of its eightneighbors is a 1 in the input image.See also closing, dilation and opening.

erosion-corrosion: Simultaneousoccurrence of erosion and corrosion.

etch crack: Shallow crack in hardenedsteel containing high residual surfacestresses, produced in an embrittlingacid environment.2,5

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

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

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

evaluation: Process of deciding theseverity of a condition after anindication has been interpreted, todetermine whether it meetsacceptance criteria.

eye sensitivity curve: Graphic expressionof vision sensitivity characteristics ofthe human eye. In the case of aphysical photometer, the curve shouldbe equivalent to the standard observer.The required match is typicallyachieved by adding filters between thesensitive elements of the meter andthe light source. See photopic vision.

F

false brinelling: Fretting wearindentations. Compare brinelling.

false indication: Test indication thatcould be interpreted as originatingfrom a discontinuity but that actuallyoriginates where no discontinuityexists in the test object. Compareindication, nonrelevant; defect.4

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farsightedness: Vision acuity functionallyadequate for viewing objects at adistance, generally farther than arm’slength. Also called hyperopia. Comparenearsightedness.

far vision: Vision of objects at a distance,generally beyond arm’s length.Compare near vision.

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

ferrite: Solid solution of one or moreother elements in alpha iron.

fiber optic borescope: See borescope, fiberoptic.

fiber optics: Technology of efficienttransmission of light throughtransparent fibers such as glass, quartzand plastic by means of total internalreflection.

fiberscope: Jargon for fiber opticborescope.

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, then, arerequired to produce one completepicture or frame so the field frequencyis sixty fields per second and the framefrequency is thirty frames per second.

field angle: Included angle between thosepoints on opposite sides of a beamaxis at which the luminous intensity is10 percent of the maximum value.This angle may be determined from anilluminance curve or may beapproximated by use of an incidentlight meter.2,6

field of view: Range or area where thingscan be seen through an imagingsystem, lens or aperture. Comparedepth of field.

field of vision: Range or area wherethings can be perceived by eyesight ata point in time, assuming the eye tobe immobile.

fillet weld:Weld of approximatelytriangular cross section joining twosurfaces approximately at a right angleto each other.

filter: Processing component or functionthat excludes a selected kind of signalor part of a signal.

filtering: See low pass filtering.fit up: To secure one or more joint

members with special externalfixturing in order to preventmovement during welding.2,5

flakes: Short discontinuous internalfissures in ferrous metals attributed tostresses produced by localizedtransformation and/or decreasedsolubility of hydrogen during coolingusually after hot working. On afractured surface, flakes appear asbright silvery areas; on an etchedsurface, flakes appear as short,discontinuous cracks.2,5

flaw: Rejectable anomaly.4 See also defect.fluorescence: Phenomenon of absorption

of electromagnetic radiation and itsreemission at a lower energy (longerwavelength). In visual testing,fluorescence is typically a response toultraviolet radiation.

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.

focus, principal plane of: Single plane infocus in a photographic scene.

focusing, automatic: (1) Feature of acamera whereby the lens systemadjusts to focus on an object in thefield 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. Despite the name, nomechanism is actively adjusted. Thelarge depth of field is due both to thesmall diameter of the lens apertureand to the proximity of the lens to thecharge coupled device.

focusing, primary: Focusing by the lensof the image onto a fiber optic bundleat the tip of a probe.

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

footcandle (ftc): Disfavored unit ofilluminance, where1 ftc = 1 lm·ft–2 = 10.76 lx.

footlambert (ftl): Disfavored unit ofluminance, where 1 ftl = 3.426 cd·m–2.

forging crack: Stress induceddiscontinuity formed duringmechanical shaping of metal; seecrack.

fovea centralis: Region of sharpest visionin the retina, where the layer of bloodvessels, nerve fibers and cells abovethe rods and cones is far thinner thanin peripheral regions.

foveal vision: See photopic vision.fracture mechanics: Field of solid

mechanics that deals with behavior ofcracked bodies subjected to stress andstrain.



frame: Complete raster scan projected ona video screen. There are thirty framesper second in a standard video output.A frame may be comprised of twofields, each displaying part of the totalframe. See also field.

frequency: Number of times per secondthat a cyclical waveform repeats. Theunit of frequency is hertz (Hz).

fretting corrosion: See corrosion, fretting.fretting wear: See wear, fretting.friction oxidation: See wear, fretting.

G

galling: Type of surface damage caused byfriction.

galvanic series: List of metals, alloys andgraphite (a nonmetal) in sequencewith the most anodic (easily corroded)in liquids at one end of the list andthe most cathodic (least easilycorroded) at the other end. Forpractical reasons, this sequence iscompiled using seawater as theelectrolyte — 3 to 5 percent sodiumchloride and other salts dissolved inwater.

gamma iron: Iron with face centeredcubic structure formed by slow coolingof delta ferrite. This characteristiclattice structure is stable between906 °C (1663 °F) and 1391 °C(2535 °F). Also called austenite.

gas metal arc welding (GMAW): Inertgas shielded metal joining process thatuses a continuous and consumablewire electrode. Also called MIG (metalinert gas) welding. Compare gastungsten arc welding and shielded metalarc welding.

gas tungsten arc welding (GTAW): Inertgas shielded metal joining process thatuses a nonconsumable tungstenelectrode. Filler material, whenneeded, is manually fed into themolten weld puddle. Also calledtungsten inert gas (TIG) welding.Compare gas metal arc welding; shieldedmetal arc welding.

gasket seal: Resilient ring, usually virginpolytetrafluoroethylene (PTFE), in apiping or tubing connection. Compareinterference sealing thread.

general examination: In personnelqualification, a test or examination ofa person’s knowledge, typically (in thecase of nondestructive testingpersonnel qualification) a written teston the basic principles of anondestructive test 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.

geometrical optics: Mathematical studyof how light rays are reflected andrefracted and practical techniquesbased on such understanding,including the transmission of imagesby lenses and mirrors. Also called lensoptics.

glare: Excessive brightness (or brightnessvarying by more than 10:1 within thefield of view) that interferes withobservation or interpretation of a testresponse. Glare may be caused byreflection, whether specular (smoothsurface) or diffuse (rough surface), oflight or radiation sources.

glare, blinding: Glare so intense that foran appreciable length of time after ithas been removed, no object can beseen.2,6

glare, direct: Glare resulting from highluminances or insufficiently shieldedlight sources in the field of view.Direct glare is usually associated withbright areas, such as luminaires,ceilings and windows which areoutside the visual task or region beingviewed.2,6

glare, reflected: Glare resulting fromspecular reflections of highluminances in polished or glossysurfaces in the field of view. It usuallyis associated with reflections fromwithin a visual task or nearby areas.2,6

gloss meter: Reflectometer used tomeasure specular reflectance.2,6

gnomon: Artifact intended to cast ashadow. The shadow may be used tomeasure time or distance.

gouge: Surface indentation caused byforceful abrasion or impact or flamecutting. Also called nick. Compare toolmark.

grain: Solid particle or crystal of metal. Asmolten metal solidifies grains growand lattices intersect, forming irregulargrain boundaries.

grain boundary: Interface that formsbetween grains of solidifying metal asthe random oriented crystal latticesmeet. See grain.

310 Visual Testing


gray body: Radiator whose spectralemissivity is uniform for allwavelengths.

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

green rot: Form of attack due tosimultaneous carburization andoxidation of stainless heating elementscommon to nickel chromium andnickel chromium iron alloys,especially in furnace environments.

grinding crack: Shallow crack formed inthe surface of relatively hard materialsbecause of excessive grinding heat orthe brittleness of the material.Grinding cracks typically are oriented90 degrees to the direction ofgrinding.4

H

Hadfield’s steel: Austenitic manganesespecialty steel, easily work hardened.

halitation: Rings of light visible around aspot on a video screen where anelectron scanning beam is held.

heading: Upsetting wire, rod or bar stockin dies to form parts having some ofthe cross sectional area larger than theoriginal. Examples are bolts, rivets andscrews.1

heat affected zone: Portion of base metalnot melted during brazing, cutting orwelding but with mechanicalproperties altered by the heat.4

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. Heat checking issometimes called thermal fatigue.

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

hot tear: Fracture formed in a cast metalduring solidification and due toextensive tensile stress associated withvolumetric shrinkage. Hot tears oftenoccur where areas of differentthicknesses adjoin.

hue: Characteristic of light at a particularbandwidth; commonly associated withthe color’s name.

human factors: Factors in the overall testsensitivity based upon mental andphysical condition of the inspector,training, experience level and thephysical conditions under which theinspector must work.

hyperopia: See farsightedness.hyperthermia: Heating so excessive that

it can damage or kill plant or animalcells.

I

illuminance: Intensity of radiant energy(density of luminous flux) on asurface, in the visible light spectrum.Illuminance is measured in lux.

illuminate: Cast light on (something).illumination: Act of illuminating or state

of being illuminated. See alsoilluminate. Compare illuminance.2,6

image: Visual representation of an objector scene.

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

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

image orthicon: Television tube that usesthe photoemission method. Comparevidicon tube.

image processing: Actions applied singlyor in combination to an image, inparticular the measurement andalteration of image features bycomputer. Also called picture processing.

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

in control:Within prescribed limits ofprocess control.

incandescence: Emission of visibleradiation as a result of heating.

indication: Nondestructive test responsethat requires interpretation todetermine its relevance.4 See alsodefect; discontinuity; false indication;nonrelevant indication.

indication, nonrelevant: Indication thathas no relation to a discontinuity thatmight constitute a defect.1 Testresponse caused by geometry or by aphysical condition that is not adiscontinuity.

indication, relevant: Indication from adiscontinuity (as opposed to a falseindication) requiring evaluation by aqualified inspector, typically withreference to an acceptance standard,by virtue of the discontinuity’s size orlocation.1

indirect viewing: Viewing of a test objectduring which the light image ismediated through a system of two ormore lenses (as in a borescope) ortransduced through an electronicsignal (as with a charge coupledcamera). Compare direct viewing; remoteviewing.

infrared radiation: Electromagneticradiant energy of wavelengths longerthan 770 nm.2,6

interference fitted thread: See interferencesealing thread.



interference objective: In a microscope, asmall, metallized glass mounted incontact with the test object andadjustable for tilt to control fringespacing.

interference sealing thread: Piping sealusing a tapered connection made upunder great pressure, forcing themating surfaces together more tightlythan is possible with a spiral thread.Compare gasket seal.

interlaced scanning: Process whereby thepicture appearing on a video screen isdivided into two parts. Interlacedscanning reduces flicker by increasingthe electron beam’s downward rate oftravel so that every other line is sent.When the bottom is reached, thebeam is returned to the top and thealternate lines are sent. The odd andeven line scans are each transmitted at1/60 s, totaling 1/30 s per frame andretaining the standard rate of30 frames per second. The eye’spersistence of vision allows the oddand even lines to appear as a singleimage without flicker.

interpretation: Determination of thecause, significance and relevance oftest indications.

interstitial alloy: Alloy in which theatoms of the alloying element fit intothe spaces between the atoms of theparent metal.

inverse square law: Physical law for apoint source of energy. The quantityor strength is inversely proportional tothe square of the distance from theorigin.

iris: Ring of variable area around thepupil and in front of the lens of theeye. The surface area of the iris adjustsspontaneously to change the amountof light entering the eye.

irradiance: Radiant power falling upon aknown surface area at a given angle.Compare radiance. See also radiometer.

IshiharaTM plates: Trade name for a kindof pseudoisochromatic plates, used forcolor differentiation vision testing.

isomorphous binary system: Twoelement alloy system in which bothelements are completely soluble ineach other in the liquid and the solidstates, in all proportions at alltemperatures.

J

jaeger eye chart: Eye chart used for nearvision acuity examination.

K

kinetic vision acuity: Vision acuity witha moving target. Studies indicate that10 to 20 percent of visual efficiencycan be lost by target movement.

L

laboratory microscope: Conventionalcompound microscope. See microscope.

lambert cosine law: See cosine law.lambertian: Having a surface that reflects

light diffusely and uniformly ratherthan specularly. See matte. Mostobjects have a lambertian surface.Compare specular.

lap: Forging discontinuity caused by afolding over of metal. Laps are foundin rolled bar stock and at or neardiameter changes.2,5

laser: Acronym (light amplification bystimulated emission of radiation). Adevice, the laser that produces ahighly monochromatic and coherent(spatial and temporal) beam ofradiation.2

leaked visible light: Unwantedelectromagnetic radiation that has awavelength between 400 and 800 nmthat is generated by a UV-A source butnot filtered out of the emissionspectrum. Leaked visible light isgenerally perceived as purple or darkblue light and not accurately measuredusing a photometric sensor.4

lens: Transparent object that refracts lightpassing through it in order to focusthe light.

lens optics: See geometrical optics.light adapted vision: See photopic vision.light guide bundle: Bundle of filaments,

usually glass, that carries noncoherentlight from a high intensity sourcethrough a fiber optic borescope toilluminate an object.

light: Radiant energy that can excite theretina and produce a visual sensation.The visible portion of theelectromagnetic spectrum extendsfrom about 400 to 800 nm.2,6

lighting, back: Placement of light sourceand image sensor on opposite sides ofthe test object, used when thesilhouette of a feature is important.

lighting, flash: See lighting, strobe.lighting, front: Placement of light source

and image sensor on the same side ofthe test object.

lighting, strobe: Lighting that flashesintermittently at a rate that may beadjusted and is often perceived as aflicker, used to image moving objectsor still objects with potentialmovement.

312 Visual Testing


lighting, structured: Combining a lightsource with optical elements to form aline or sheet of light.

light meter: See photometer. Compareradiometer.

limited certification: Of a person,certified only for specific operations;usually called limited Level (I or II) ordesignated as having limitedcertification because they are notqualified to perform the full range ofactivities expected of personnel at thatlevel of qualification, for a givenmethod.

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

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

low pass filtering: In image processing,linear combination of pixel values tosmoothen abrupt transitions in adigital image. Also called smoothing.

lumen (lm): SI photometric unit ofluminous flux, weighted according tothe photopic vision response. Onelumen equals the light emitted by onecandela (cd) point source into onesteradian (sr) solid angle (1 lm =1 cd·sr–1).

lumen method: Lighting designprocedure used for predetermining therelation between the number andtypes of lamps or luminaires, the roomcharacteristics and the averageilluminance on the work plane. Ittakes into account both direct andreflected flux. Also called fluxmethod.2,6

luminance: Photometric brightness of alight source defined by the density ofits luminous intensity, measured asluminous flux per unit solid angle perunit area in a given direction.Reported in candela per square meter(cd·m–2).

luminosity: Luminous efficiency ofradiant energy.

luminous efficacy: Ratio of the totalluminous flux of a light source to thetotal radiant flux or to the powerinput. Sometimes called luminousefficiency.

luminous efficiency: See luminousefficacy.

luminous flux: Radiant energy’s time rateof flow. Measured in lumens.

luminous intensity: Measure of a lightsource’s power output per unit solidangle emitted or reflected from apoint, when weighted by the photopicspectral luminous efficiency responsecurve. Luminous intensity is measuredin candela. Compare luminance.4

lux (lx): SI unit of illuminance, equal toone lumen per square meter(1 lx = 1 lm·m–2).

M

machine vision: Automated systemfunction of acquiring, processing andanalyzing images to evaluate a testobject or to provide information orinterpretation for humaninterpretation. A typical machinevision system consists of a lightsource, a video camera, a videodigitizer, a computer and an imagedisplay.

macula lutea: Irregular, diffuse ring ofyellow pigment which partly overlapsthe fovea and surrounds it out toaround 10 degrees and which absorbsblue light, thus changing the color ofthe light reaching receptors beneath.

martensite: (1) Acicular (needlelike)microstructure produced by fastcooling or quenching of metals andalloys such as steel. (2) The hard steelwith such microstructure produced byfast cooling of austenite. Martensite isa constituent commonly found inquenched steel.

martensite finish temperature:Temperature at which martensiteformation is completed as steel cools.

martensite start temperature:Temperature at which martensite startsto form as steel cools.

mask: (1) A spatial filter in the sensingunit of a surface inspection system.(2) An n × n square matrix withdifferent values that serves as a filterin image processing.

match bend effect: Optical illusionwhereby an area of uniform brightnessappears to be nonuniform because ofcontrast with the brightness of anadjacent area.

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

matte: Having a surface that reflects lightdiffusely rather than at an angle equalto the angle of incidence; not shiny.Also called lambertian. The term matteis generally applied to smooth surfacesor coatings. Compare specular.

mesopic vision: Vision adapted to a levelof light between photopic at3.4 × 10–2 cd·m–2 (3.2 × 10–3 cd·ft–2)and scotopic at 3 × 10–5 cd·m–2

(2.7 × 10–6 cd·ft–2).metallograph: Short term for

metallographic microscope.metallographic microscope: See

microscope, metallographic.



metallography: Science and practice ofmicroscopic testing, inspection andanalysis of a metal’s structure,typically at magnifications from 50×to 2500×.

metallurgical microscope: See microscope,metallurgical.

microborescope: See borescope, micro-.microscope: Instrument that provides

enlarged images of small objects.microscope, compound: Conventional

microscope, using geometrical opticsfor magnification. Also calledlaboratory microscope.

microscope, interference: Magnifierusing the wavelength of light as a unitof measure for surface contour andother characteristics.

microscope, metallographic:Metallurgical microscopeincorporating a camera. Also called ametallograph. Most metallographicmicroscopes share these features:(a) stand with concealed shockabsorbers, (b) intense light source,(c) inverted stand so that the testobject is face down, (d) viewingscreens for prolonged tasks such as dirtcount or grain size measurements,(e) bright, dark and polarizedillumination options.

microscope, metallurgical: Microscopedesigned with features suited formetallography.

microscope, phase contrast: Laboratorymicroscope with two additionaloptical elements to transmit bothdiffracted and undiffracted light,revealing refractive indexdiscontinuities in a completelytransparent test object.

microscope, polarizing: Microscope withpolarizing elements to restrict lightvibration to a single plane for studyingmaterial with directional opticalproperties. As fibers, crystals, sheetplastic and materials under strain arerotated between crossed polarizers onthe microscope stage, they changecolor and intensity in a way that isrelated to their directional properties.

MIG welding: See gas metal arc welding.miniature borescope: See borescope,

miniature.miniborescope: Jargon for miniature

borescope.minimum line pair: Closest distance that

a specific imaging system can resolvebetween a pair of adjacent, parallellines (line pair) used to evaluatesystem resolution.

modulus of elasticity: Ratio betweenstress and strain in a materialdeformed within its linear elasticrange.

monochromatic: Light from a very smallportion of the spectrum.

monochromator: Device that uses prismsor gratings to separate or disperse thewavelengths of the spectrum into oneband.

morphology: See mathematicalmorphology.

mottle: Apparently random positioning ofmetallic flakes that creates anaccidental pattern.

multipass weld:Weld made by manypasses, one pass at a time.

multiphase alloy: Alloy in which severalphases are present.

N

NDE: (1) Nondestructive evaluation.(2) Nondestructive examination. Seenondestructive testing.

NDI: Nondestructive inspection. Seenondestructive testing.

NDT: See nondestructive testing.near ultraviolet radiation: Ultraviolet

radiation with wavelengths rangingfrom about 320 to about 400 nm.Formerly called black light.

near vision: Vision of objects nearby,generally within arm’s length.Compare far vision.

nearsightedness: Vision acuityfunctionally adequate for viewingobjects nearby, generally within arm’slength. Also called myopia. Comparefarsightedness.

necking down: Localized reduction inarea of a specimen or structuralmember during welding underoverload.2,5

negative sliding: Rolling and sliding ofmeshing gears or rollers when therolling and sliding are in oppositedirections.

neural acuity: Ability of the eye andbrain together to discriminate patternsfrom background. Discrimination isinfluenced by knowledge of the targetpattern, by the scanning techniqueand by the figure-to-groundrelationship of a discontinuity. Thefigure/ground relationship can bereferred to as having a level of visualbackground noise.

nick: Surface indentation caused byforceful abrasion or impact. Also calledgouge. Compare tool mark.

nit: Obsolete unit for measuringluminance, equivalent to one candelaper square meter. Abbreviated nt.

noble metals: Cathodic metals (such asgold, platinum and silver), whichstrongly resist corrosion.

314 Visual Testing


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

nondestructive examination (NDE):Another term for nondestructivetesting. In the utilities and nuclearindustry, examination is sometimespreferred because testing can implyperformance trials of pressurecontainment or power generationsystems.

nondestructive inspection (NDI):Another term for nondestructivetesting. In some industries (utilities,aviation), the word inspection oftenimplies maintenance for a componentthat has been in service.

nondestructive testing (NDT):Determination of the physicalcondition of an object withoutaffecting that object’s ability to fulfillits intended function. Nondestructivetest methods typically use anappropriate form of energy todetermine material properties or toindicate the presence of materialdiscontinuities (surface, internal orconcealed). Sometimes callednondestructive evaluation, nondestructiveexamination or nondestructive inspection.

nonferromagnetic material: Material notmagnetizable and essentially notaffected by magnetic fields.4

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.4

O

objective: 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.

OCTG: Oil country tubular goods.oil country tubular goods: Hollow

cylindrical components, such as pipes,used in petroleum wells to case thehole and to convey petroleum andrelated products.4

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

opsin: See visual purple.

optic disk: Area in the retina throughwhich the fibers from the variousreceptors cross the inner (vitreoushumor) side of the retina and passthrough it together in the optic nervebundle. This transitional area iscompletely blind.

optics: Physical science of thetransmission of radiation, especially oflight. See geometrical optics.

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

orthicon: See image orthicon.

P

parafoveal vision: See scotopic vision.parallax: Apparent difference in position

of an imaged point according to twodifferently positioned sensors.

pass: In welding, a single bead of weldmetal along the entire joint or theprocess of laying down that bead.

pearlite: Platelet mixture of cementiteand ferrite in steels or in alpha andbeta phases in nonferrous alloys.

peripheral vision: Seeing of objectsdisplaced from the primary line ofsight and outside the central visualfield.2,6

phase: In metallurgy, a physicallyhomogeneous portion of a materialsystem, specifically the portion of analloy characterized by itsmicrostructure at a particulartemperature during melting orsolidification.

phase contrast microscope: Seemicroscope, phase contrast.

phase diagram: Graph showing thetemperature, pressure and compositionlimits of phase fields in a materialsystem. Also called a constitutiondiagram. Compare equilibrium diagram.

photochromic lens: Eyeglass materialthat automatically darkens to reducelight transmission when exposed toultraviolet radiation.

photoconduction: Method by which avidicon television camera tubeproduces an electrical image, in whichthe conductivity of the photosensitivesurface changes in relation to theintensity of the light reflected fromthe scene focused onto the surface.Compare photoemission.

photoelasticity: Effect of a material’selastic properties on the way that itrefracts or reflects light.

photoelectric effect: Emission ofelectrons from a surface bombarded bysufficiently energetic photons. Suchemissions may be used in anilluminance meter and can becalibrated in lux.2,6



photoemission: Method by which animage orthicon television camera tubeproduces an electrical image, in whicha photosensitive surface emitselectrons when light reflected from aviewed object is focused on thesurface. Compare photoconduction.

photometer: Device used to measureilluminance. The sensor is filteredsuch that its response closely matchesthe spectral responsivity curve of thehuman eye. In nondestructive testing,photometers measure lux. Compareradiometer.

photometric brightness: Luminance of alight source.

photometry: Study and measurement ofelectromagnetic radiation withapproximate wavelengths between 400and 800 nm, within the human eye’sspectral responsivity. See alsophotometer; photopic vision; radiometry;relative photometry.

photon: Particle of light, hypothesized toexplain those behaviors of light inwhich its behavior is corpuscularrather than wavelike.

photopic vision: Average spectralresponsivity curve of the human eyewhen adapted to well lit conditions(greater than 0.034 cd·m–2). Thephotopic spectral luminous efficiencyresponse curve is governed by anaveraged retinal cone response withsensitivity peaks centered at about555 nm. Also known as foveal visionand light adapted vision. Comparemesopic vision and scotopic vision.2,6

photoreceptor: Light sensor.picture element: See pixel.picture processing: See image processing.pipe: Longitudinal centerline

discontinuity inherent in ingots,imparted to some rolled metal andconsisting of a concavity or voids.

pitting: Discontinuity consisting ofsurface cavities. See also cavitationfatigue.

pixel: Addressable point on the screen ofa digital image. The image from aconventional computer is an array ofpixels, each of which has a numericalvalue. The higher the number for apixel, the brighter it is. Formerly calledpicture element.

plane of focus: See focus, principal planeof.

platelet: Flat crystallites in certain phasesof steel.

polarizing microscope: See microscope,polarizing.

porosity: Discontinuity in metal resultingfrom the creation or coalescence ofgas. Very small pores open to thesurface are called pinholes.2,5

positive sliding: Rolling and sliding ofmeshing gears or rollers when thedirections of rolling and sliding arethe same.

poultice corrosion: See corrosion, poultice.practical examination: In certification of

nondestructive testing personnel, ahands-on examination using testequipment and sample test objects.Compare general examination; specificexamination.

primary creep: First stage of creep,marked by elastic strain plus plasticstrain.

principal plane of focus: See focus,principal plane of.

probability of detection (PoD): Statisticalstatement from a specific testprocedure indicating how likely agiven discontinuity length may bereliably found.

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

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

pseudocolor: Image enhancementtechnique wherein colors are assignedto an image at several gray scaleintervals.

pseudoisochromatic plate: Image usedfor color vision examinations. Eachplate bears an image which may bedifficult for the examinee to see if hisor her color vision is impaired. Seealso Ishihara™ plates.

psychophysics: Interaction betweenvision performance and physical orpsychological factors. One example isthe so-called vigilance decrement, thedegradation of reliability based onperforming visual activities over aperiod of time.

pupil: Black aperture in the center of theeye’s lens, through which light entersthe lens to impinge on the retina.

purple: See visual purple.

Q

qualification: Process of demonstratingthat an individual has the requiredamount and the required type oftraining, experience, knowledge andabilities.4-6 See also certification andqualified.

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

quality: Ability of a process or product tomeet specifications or to meet theexpectations of its users in terms ofefficiency, appearance, reliability andergonomics.4-6

316 Visual Testing


quality assurance: Administrative actionsthat specify, enforce and verifyquality.4-6

quality control: Physical andadministrative actions required toensure compliance with a qualityassurance program. Quality controlmay include nondestructive testing inthe manufacturing cycle.4-6

quality of lighting: Level of luminance ina visual task or environment.

R

radial: Of or pertaining to direction fromcenter of a circle (or a sphere or crosssection of a cylindrical object) to itssurface, and perpendicular to its axis.Compare axial.

radiance: Radiant flux per unit solidangle and per unit projected area ofthe source. Measured in watts persquare meter steradian. Compareirradiance.

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

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

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

radiant power: Total radiant energyemitted per unit time.

radiometer: Device used to measureirradiance. In nondestructive testing,radiometers are used to measure UV-Aoutput, or leaked visible light, inmicrowatt per square centimeter(µW·cm–2). See also irradiance.Compare photometer.

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

radiometry: Study and measurement ofelectromagnetic radiation emitted by asource or falling upon a surface.

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.

rat’s tooth principle: (1) The tendencyfor hard material on a tooth’s frontsurface to wear more slowly than softmaterial on the back surface, keepingthe edge sharp. (2) Mechanism of wearwhereby adjacent hard and softsurfaces wear at different rates,producing a self-sharpening edge.

Recommended Practice No. SNT-TC-1A:Set of guidelines published by theAmerican Society for NondestructiveTesting, for employers to establish andconduct a qualification andcertification program fornondestructive testing personnel.4-6

recommended practice: Set of guidelinesor recommendations.4-6

recovery: Reduced stress level andincreased ductility of metal after workhardening. See creep.

recrystallization: Changes inmicrostructure and properties uponheating of cold worked metal.

red mud: Debris (usually oxides of thecontacting metals) of fretting wear,mixed with oil or grease and retainedat or near the site of its formation. Seealso cocoa.

reference standard: Object containingknown discontinuities at knowndistances and representing accept orreject criteria.

reflectance: Ratio of reflected waveenergy to incident wave energy. Alsoknown as reflectivity.

reflection: General term for the processby which the incident energy leaves asurface or medium from the incidentside, without change in frequency.Reflection is usually a combination ofspecular and diffuse reflection.2,6

reflectometer: Photometer used tomeasure diffuse, specular and totalreflectance.

reflector: Device used to redirect theluminous flux from a source by theprocess of reflection.2,6

refraction: Reorientation of radiation’spath by the medium through which itpasses.

relative photometry: (1) Evaluation of adesired photometric characteristicbased on an assumed lumen output ofa test lamp. (2) Measurement of anuncalibrated light source relative toanother uncalibrated light source.



remote viewing: (1) Indirect viewing of atest object far from the viewer’simmediate presence — for example,viewing with telemetry or crawlers.The term remote viewing is used in thefields of robotics and surveillance todistinguish conventional from distantviewing tasks. (2) Viewing of a testobject during which the light image ismediated through a system of two ormore lenses (as in a borescope) ortransduced through an electronicsignal (as with a charge coupledcamera). This use of the term remoteviewing in some specifications is amisnomer, intended merely todistinguish borescopy from directviewing. Compare borescope; directviewing; indirect viewing.

replica: Piece of malleable material, suchas polyvinyl or polystyrene plasticfilm, molded to a test surface for therecording or analysis of the surfacemicrostructure.

replica, metallographic: Replica suitablefor microscopic examination. Seemetallography.

replication: Method for copying thetopography of a surface by making itsimpression in a plastic or malleablematerial.

reserve vision acuity: Ability of anindividual to maintain vision acuityunder poor viewing conditions. Avisual system with 20/20 near visionacuity under degraded viewingconditions has considerable reservevision acuity compared to that of anindividual with 20/70 near visionacuity.

resolution: Aspect of image qualitypertaining to a system’s ability todepict objects, often measured bydistinguishing (resolving) a pair ofadjacent objects or parallel lines.

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

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.2,6

resolving power: Ability of vision orother detection system to separate twopoints. Resolving power depends onthe angle of vision and the distance ofthe sensor from the test surface.Resolving power is often measuredusing parallel lines. Compareresolution.

retina: In the eye, the tissue on back,inside surface of the eyeball, oppositethe pupil, where light sensitive rodsand cones sense light.

retinene: See visual purple.

rhodopsin: See visual purple.robotic system: Automated system

programmed to perform purposefulactions in variable sequences.

rod: Retinal receptor that responds at lowlevels of luminance even down belowthe threshold for cones. At these levelsthere is no basis for perceivingdifferences in hue and saturation. Norods are found in the fovea centralis.2,6Concentrated toward the outer regionof the retina, rods assist with mesopicvision and are responsible for scotopicnight vision. Compare cone.

root mean square (rms): Statisticalmeasure of the magnitude of a varyingquantity, such as current. Square rootof the mean square of a set ofmeasures, usually a time series.

S

sampling, partial: Testing of less than100 percent of a production lot.

sampling, random partial: Partialsampling that is fully random.

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

saturation: Relative or comparative colorcharacteristic resulting from a hue’sdilution with white light.

scalar: Quantity completely specified by asingle number and unit.4

scaling: (1) Forming a layer of oxidationproduct on metals, usually at hightemperature. (2) Deposition ofinsoluble constituents on a metalsurface, as in cooling tubes and waterboilers.2,5

scoring: (1) Marring or scratching of anyformed part by metal pickup on apunch, die or guide. (2) Reducingthe thickness of a part along a lineto weaken it purposely at a specificlocation.2,5

scotopic vision: Dark adapted vision,using only the rods in the retina,where differences in brightness can bedetected but differences in huecannot. Vision is wholly scotopicwhen the luminance of the testsurface is below 3 × 10–5 cd·m–2

(2.7 × 10–6 cd·ft–2). Also known asparafoveal vision. Compare mesopicvision and photopic vision.

scuffing: Type of adhesive wear.seam: Linear discontinuity formed by a

lack of metal from folds produced byan underfilled pass during metalrolling. Squeezed tight on subsequentpasses, the underfill runs parallel tothe longitudinal axis of the bar.

318 Visual Testing


second stage replica: Positive replicamade from the first cast to produce aduplicate of the original surface.

secondary creep: Second stage of creep,in which deformation proceeds at aconstant rate and less rapidly than inprimary creep. Essentially anequilibrium condition between themechanisms of work hardening andrecovery. See also defect anddiscontinuity.4

sensitivity: Ability of a sensor or systemto distinguish a signal or indicationfrom background noise. See alsoprobability of detection.

sensitization: Precipitation of chromiumcarbides in the grain boundaries of acorrosion resistant alloy, resulting inintergranular corrosion that wouldotherwise be resisted.

sensor: Device that detects a materialproperty or mechanical behavior (suchas radiation or displacement) andconverts it to an electrical signal.

shadow casting: Nondestructivetechnique of vapor depositing a thinmetal film onto a replica at an obliqueangle in order to obtain a micrographof a test surface of an opaque testobject.

shear break: Open break in metal at theperiphery of a bolt, nut, rod ormember at approximately a 45 degreeangle to the applied stress. Shearbreaks occur most often with flangedproducts. Also called shear crack.2,5

shear crack: See shear break.shielded arc welding: Joining of metals

by heating them with an electric arcbetween electrode(s) and the workpiece, using an inert gas to shield theelectrode(s). See also gas tungsten arcwelding.

shoulder: Cylindrical metal component(pipe) surface, machined to receivethreading indentations but in fact notthreaded, where the thread stops onthe outside surface of the pipe.

SI (International System of Units):Measurement system in which thefollowing seven units are basic: meter,mole, kilogram, second, ampere,kelvin and candela.

signal electrode: Transparent conductingfilm on the inner surface of a vidicon’sfaceplate and a thin photoconductivelayer deposited on the film.

signal processing: Acquisition, storage,analysis, alteration and output ofdigital or analog data.

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

signal: Physical quantity, such as voltage,that contains relevant information.4

simple magnifier: Microscope having asingle converging lens.

smoothing: In image processing, linearcombination of pixel values to smoothabrupt transitions in a digital image.Also called low pass filtering.

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

spalling fatigue: See subcase fatigue.specific examination: In certification of

nondestructive testing personnel, awritten examination that addresses thespecifications and products pertinentto the application. Compare generalexamination and practical examination.

specific gravity: Unitless ratio of thedensity of a material divided by thedensity of water. Water has a densityof about 1 g·cm–3, or 1000 kg·m–3.

specification: Set of instructions orstandards invoked to govern theproperties, results or performance of aspecific set of tasks or products.4-6

spectral irradiance: Measure of energyemitted by a radiation source asfunction of wavelength. Units ofspectral irradiance are watts per squaremeter and are often plotted versuswavelength.

spectral power distribution: Radiantpower per unit wavelength as afunction of wavelength. Also knownas spectral energy distribution, spectraldensity and spectral distribution.

spectral reflectance: Radiant fluxreflected from a material divided bythe incident radiant flux.

spectral responsivity: Measure of aphotometric or radiometric sensor’ssensitivity over a wavelength range ofinterest, often presented as percentversus wavelength. Photometricsensors should exhibit a bell shapedspectral responsivity curve over thevisible light range, whereasradiometric sensors may exhibit a flator other response curve.

spectral transmittance: Radiant fluxpassing through a medium divided bythe incident radiant flux.

spectrophotometer: Instrument used forspectrophotometry.

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

spectroradiometer: Instrument used forspectroradiometry.

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

spectroscope: Instrument used forspectroscopy.



spectroscopy: Spectrophotometry orspectroradiometry in which thespectrum, rather than being analyzedonly by a processing unit, is presentedin a visible form to the operator fororganoleptic examination.

spectrum: Representation of radiantenergy in adjacent bands of hues insequence according to the energy’swavelengths or frequencies. A rainbowis a well known example of a visiblespectrum.

specular: Pertaining to a mirrorlikereflective finish, as of a metal.Compare lambertian.

specular reflection:When reflectedwaves and incident waves form equalangles at the reflecting surface.

speed of light: Speed of all radiantenergy, including light, is299792458 × 108 m·s–1 in vacuum. Inall materials the speed is less andvaries with the material’s index ofrefraction, which itself varies withwavelength.2,6

speed of vision: Reciprocal of theduration of the exposure time requiredfor something to be seen.2,6

standard: Object, document or conceptestablished by authority, custom oragreement to serve as a model or rulein the measurement of quantity or theestablishment of a practice orprocedure.4,8 See also reference standardand acceptance standard.

standardization, instrument: Adjustmentof instrument readout before use to aspecified reference value.4

standard observer response curve: Seeeye sensitivity curve.

steel: Iron alloy, usually with less thantwo percent carbon.

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

stick welding: See shielded metal arcwelding.

strain: Deflection or alteration of theshape of a material by external forces.

stress: (1) In physics, the action in amaterial that resists external forcessuch as tension and compression.(2) Load per unit of area.

stress concentration: Region where forceper unit area is elevated, often becauseof geometric factors or cracks. Alsoknown as a stress raiser.

stress raiser: Contour or property changethat locally increases stress magnitude.

stress riser: See stress raiser.

stringer: In wrought materials, anelongated configuration ofmicroconstituents or foreign materialaligned in the direction of working.Commonly, the term is associatedwith elongated oxide or sulfideinclusions in steel.

subcase fatigue: Fatigue originating belowthe case depth.

subcase origin fatigue: See subcasefatigue.

subsurface fatigue: Fatigue cracking thatoriginates below the surface. Usuallyassociated with hard surfaced or shotpeened parts but may occur any timesubsurface stresses exceed surfacestresses.

T

tarasov etching technique:Way ofvisually inspecting for the presence ofdeleterious effects in hardened steelsby using specific etching solutions andmethods of inspection.

temperature diagram: See timetemperature transformation (TTT)diagram.

tempering: Process of heating a material,particularly hardened steel to belowthe austenite transformationtemperature, to improve ductility.

tertiary creep: Third stage of creep,marked by steady increase in strain tothe point of fracture under constantload.

test object: Physical part or specimensubject to nondestructive testing.

threshold: (1) A value in a phenomenonwhere a large change of output occurs.(2) Setting of an instrument thatcauses it to register only those changesin response greater or less than aspecified magnitude.4 See adaptivethresholding, resolution threshold.

thresholding: Digital data processingtechnique that reduces a gray levelimage into a binary image.

throat, actual: Shortest distance from theroot of a fillet weld to its face, asopposed to theoretical throat or weldsize.

throat, effective: In fillet welds, the weldthroat including the amount of weldpenetration but ignoring excess metalbetween the theoretical face and theactual face.

throat, theoretical: Distance from thebeginning of the root of a fillet weldperpendicular to the hypotenuse ofthe largest right triangle that can beinscribed within the cross section ofthe fillet weld. Compare weld size.

320 Visual Testing


throat, weld: Distance from the root of afillet weld to its face. Compare weldsize and throat, actual.

TIG welding: Tungsten inert gas welding.See gas tungsten arc welding.

time temperature transformation (TTT)diagram: Graph showing timerequired at any temperature totransform austenite to pearlite, bainiteor martensite.

tip: Distal or objective end of a borescope.tool mark: Shallow indentation or groove

made by the movement ofmanufacturing tools over a surface.Compare gouge or nick.

trace: Line formed by electron beamscanning from left to right on a videoscreen to generate a picture.

tracer: In leak testing, a gas that is sensedas it escapes from confinement.

transformation diagram: See timetemperature transformation (TTT)diagram.

troland: Unit of retinal illuminance equalto that produced by a surface whoseluminance is 1 cd·m–2 when the pupilmeasures 1 mm2.

tubing string: Pipe with which oil or gashas contact as it is brought to theEarth’s surface.4

tungsten inert gas (TIG) welding: See gastungsten arc welding.

U

ultraviolet borescope: See borescope,ultraviolet.

ultraviolet radiation: Electromagneticradiation with wavelengths between40 and 400 nm. See also irradiance andUV-A.

ultraviolet radiometer: Meter, usuallycalibrated at 365 nm, used influorescent liquid penetrant andmagnetic particle testing to measureoutput of ultraviolet lamp.

undercut: Undesirable groove left unfilledby weld metal, created during weldingand located in base plate at the toe ofa weld.

Unified Numbering System:Alphanumeric system for identifyingalloys according to a registrymaintained by ASTM Internationaland SAE International.4

UV-A: Electromagnetic radiation withwavelengths between 315 and400 nm. Fluorescent nondestructivetesting has historically used ultravioletenergy centered at 365 nm. See alsoirradiance and radiometer.

V

video: Pertaining to the transmission anddisplay of images in an electronicformat that can be displayed on amonitor or screen.

videoscope: Jargon for video borescope. Seeborescope, video.

vidicon tube: Analog television tube thatuses the photoconduction method.Compare image orthicon.

vigilance decrement: Degradation ofreliability during performance ofvisual activities over a period of time.See also psychophysics.

visibility: Quality or state of beingperceivable by the eye. In manyoutdoor applications, visibility isdefined in terms of the distance atwhich an object can be just perceivedby the eye. In indoor applications itusually is defined in terms of thecontrast or size of a standard testobject, observed under standardizedview conditions, having the samethreshold as the given object.2,6

visible light: Radiant energy with awavelength between 400 and 800 nmas measured in photometric units oflux.

vision: Perception by eyesight. See farvision, machine vision, mesopic vision,near vision, peripheral vision, photopicvision, scotopic vision, and speed ofvision.

vision acuity: Ability to distinguish finedetails visually at a given distance.Quantitatively, it is the reciprocal ofthe minimum angular separation inminutes of two lines of widthsubtending one minute of arc whenthe lines are just resolvable asseparate.2,6

visual acuity: See vision acuity.visual angle: Angle formed by lines

drawn from center of eye subtendedby an object or detail at the point ofobservation. It usually is measured inminutes of arc.2,6

visual background noise: Formations onor signals from a test object thatconstitutes the background to adiscontinuity. The higher the level ofvisual background noise, the moredifficult it is to distinguish adiscontinuity.

visual efficiency: Reliability of a visualsystem. The term visual efficiency uses20/20 near vision acuity as a baselinein the United States for 100 percentvisual efficiency.



visual field: Locus of objects or points inspace that can be perceived when headand eyes are fixed. The field may bemonocular or binocular.2,6

visual perception: Interpretation ofimpressions transmitted from theretina to the brain in terms ofinformation about a physical worlddisplayed before the eye. Visualperception involves any one or moreof the following: recognition of thepresence of something (object,aperture or medium); identifying it;locating it in space; noting its relationto other things; identifying itsmovement, color, brightness orform.2,6

visual performance: Quantitativeassessment of the performance of avisual task, taking into considerationspeed and accuracy.2,6

visual purple: Chromoprotein calledrhodopsin, the photosensitive pigmentof rod vision. The mechanism ofconverting light energy into nerveimpulses is a photochemical process inthe retina. Chromoprotein istransformed by the action of radiantenergy into a succession of products,finally yielding the protein called opsinplus the carotenoid known as retinene.

visual task: Appearance and immediatebackground of those details andobjects that must be seen for theperformance of a given activity. Theterm visual task is a misnomer becauseit refers to the visual display itself andnot the task of extracting informationfrom it. See visual field.

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

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

volt (V): Measurement unit of electricpotential.4

VT: Visual testing.

W

wavelength: Distance between repeatingvalues of a wave. For example, thedistance from one peak to the nextpeak on a sine wave.

wear: See erosion; rat’s tooth principle; wear,adhesive; and wear, fretting.

wear oxidation: See wear, fretting.wear, adhesive: Degradation of a surface

by microwelding and consequentfracture due to the sliding of onesurface against another. Types includefretting, galling and scuffing.

wear, fretting: Surface degradation bymicrowelding and microfractures onsurfaces rubbing each other. Alsocalled chafing, friction oxidation andwear oxidation. See also cocoa and falsebrinelling.

weld size: Thickness of weld metal — in afillet weld the distance from the rootto the toe of the largest isosceles righttriangle that can be inscribed in across section of the weld.

weld throat: See throat, weld.welder’s flash: Clinical condition,

specifically keratoconjunctivitis,commonly caused by overexposure toultraviolet radiation of welding arc.

white light: Light combining allfrequencies in the visible spectrum(wavelengths from 380 to 780 nm)and in equal proportions.

work hardening: Increase in hardnessaccompanying plastic deformation ofa metal. Usually caused in a metal byrepeated impacting, bending orflexing. Compare creep and recovery.

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

322 Visual Testing


1. ASTM E 1316, Standard Terminology forNondestructive Examinations. WestConshohocken, PA: ASTMInternational (2007).

2. Nondestructive Testing Handbook,second edition: Vol. 8, Visual andOptical Testing. Columbus, OH:American Society for NondestructiveTesting (1993).

3. Nondestructive Testing Handbook,second edition: Vol. 10, NondestructiveTesting Overview. Columbus, OH:American Society for NondestructiveTesting (1996).

4. Nondestructive Testing Handbook, thirdedition: Vol. 8, Magnetic Testing.Columbus, OH: American Society forNondestructive Testing (2008).

5. EPRI Learning Modules. Charlotte, NC:Electric Power Research Institute.

6. IES Lighting Handbook: ReferenceVolume. New York, NY: IlluminatingEngineering Society of North America(1984).

7. Lovejoy, D. Magnetic Particle Inspection:A Practical Guide. New York, NY:Chapman & Hall (1993).

8. TO33B-1-1 (NAVAIR 01-1A-16)TM43-0103, Nondestructive TestingMethods. Washington, DC: UnitedStates Department of Defense, UnitedStates Air Force (June 1984).


References



Documents

Visual Testing Glossary - ASNT