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NDT&E International 40 (2007) 62–70

www.elsevier.com/locate/ndteint

Infrared thermography (IRT) and ultraviolet fluorescence (UVF) for thenondestructive evaluation of ballast tanks’ coated surfaces$

Mohammed A. Omara, Belal Gharaibehb, Abraham J. Salazarb, Kozo Saitob,�

aDepartment of Mechanical Engineering, Clemson University, Clemson, SC 29634-5101, USAbMechanical Engineering Department, University of Kentucky, 151 RGAN Bldg., Lexington-KY 40506-0503, USA

Received 25 January 2006; received in revised form 5 July 2006; accepted 14 July 2006

Available online 11 September 2006

Abstract

This paper quantifies the detectability limits of infrared thermography (IRT) and ultraviolet fluorescence (UVF) techniques in the

thickness measurements of the naval protective coatings applied on ship ballast tanks. The change in signal per unit thickness (25 mm) is

used to describe each technique resolution, when using a pulsed-thermographic procedure and a UVF spectroscopic analysis.

Furthermore, Tanimoto criterion in addition to a local signal-to-noise ratio (SNR) computation measure the revealability of pinholes,

the IRT technique provides an 83% revealability while the UVF imaging has a 66% value. The goal of this work is to benchmark the

novel UVF approach against that of the IRT procedures.

r 2006 Elsevier Ltd. All rights reserved.

Keywords: Ultraviolet fluorescence; Infrared imaging; Marine coating; Spectroscopy

1. Introduction

Coatings are applied on material surfaces to provide twomain attributes. (1) Appearance that relates to the colordimensions and the geometrical aspect that features hazeand gloss. (2) Protection, represented by its ability toprotect against corrosion. The protection features arecontrolled by paint coverage, thickness and adhesionintegrity issues. Coat coverage with the designed layerthickness is vital toward breaking the corrosion cycle(anode, cathode and electrolyte) and thus preventing thesubstrate material loss. This requires complete paintdeposition on the substrate to cover its different geometriespreventing the substrate from being the anode. Thethickness decides on the paint shielding performance withtime, thick and thin paint layers hinder the protectionaction; thin paint represented by lesser thicknesses and

e front matter r 2006 Elsevier Ltd. All rights reserved.

teint.2006.07.013

n original paper which has neither previously, nor

in whole or in part been submitted anywhere else.

ing author. Tel.: +1859 257 6336x80639;

3304.

esses: omar@engr.uky.edu (M.A. Omar),

y.edu (K. Saito).

pinholes do not shield the substrate from the electrolyteleading to corrosion initiation. Thick paint affects the coatstructure adhesion leading to coat peeling. In otherapplications, it hinders the curing process in releasing thesolvent content, creating pop-ups and surface bubbles thatmay break cracking the paint layer as in Fig. 1,consequently exposing the substrate. From the above, theoptimal primer inspection tool should have both coveragemonitoring and thickness evaluation capabilities. Tradi-tional paint thickness gauges operate in a contact, point-by-point operation that limit their use, while there is noavailable tool for pinhole detection.Coming section discusses the details of the problem

under study, and Sections 3 (UVF) and 4 (IRT) constructthe experimental data needed for the feasibility studyintroduced in Section 5.

2. Problem description

This paper focuses on the evaluation of newly coatedships’ ballast tanks with marine primers that range inthickness from 5 to 10 mil (125–250 mm). Oil tankers andother medium to large size ships use the ballast tanks to

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Fig. 1. Surface bubbles resulted from thick paint deposition.

Table 1

Various applied and proposed techniques applicability matrix against IRT

(relative to baseline range 0 (worse)–1 (best))

Measurement tech. applicability Wet sponge Visual tech. UVF IRT

Insensitivity to ambient light 1.0 0.0 0.5a 1.0

Insensitivity to geometry 0.5 0.0 1.0 1.0

Speed and coverage 0.0 1.0 1.0 1.0

Portability 0.5 1.0 0.5b 1.0

Contact-less operation 0.0 1.0 1.0 1.0

Ease of automation 0.0 1.0 1.0 1.0

Operator training and acceptance 0.0 1.0 1.0 1.0

aFrom previous study.bDue to UV-light source weight.

M.A. Omar et al. / NDT&E International 40 (2007) 62–70 63

stabilize its floating equipments, by filling it with seawater.Corroded ballast tanks not only affect their own perfor-mance but also the safety of the whole ship. While,currently used tools are applied in an on-site (e.g. wetsponge) and/or an off-site modes (e.g. wave tank intro-duced by Marintek, Norway) [1], it still require: (1) a drypaint film, which delays the inspection and separate it fromthe application step, even though, what is desired is tomerge the inspection and application in a single step ‘‘do itright the first time’’, to (a) save time and effort and (b)facilitate an immediate correction because, the cost of atouch job is about $22/m2. (2) Physical contact, thisrequires accessibility to corners and cervices that areconsidered most critical for corrosion initiation, whilethose tanks complicated geometries and uniqueness chal-lenge this accessibility. Those two shortcomings, inaddition to the recently issued regulations [2] that enforcemore stringent standards on the finished ballast tanks’ andthe increase in ships sizes, the tankers size went from100,000 to 500,000 dwt in 10 years [3], motivate thedevelopment of a nondestructive, non contact and realtime tool to evaluate the paint deposition quality in termsof coverage and layer thickness.

Visible techniques in the form of machine vision systemsor human naked eye inspection offer a contact-lessoperation, however still inadequate for the detection ofcoating pinholes due to (1) variability and availability ofenvironments lighting, as in dark tanks and (2) itssensitivity to the inspected surfaces’ profile and geometry,due to its dependence on reflection. Those two factorshinder the repeatability and the reliability of this test. Inaddition, visual inspection is a facial technique not capableof providing thickness information. Those issues motivatethe utilization of the higher (infrared) and the lower(ultraviolet) bounds of the visible spectrum because, oftheir emission-based operation.

Infrared thermography or, IRT nondestructive applica-tions constitute the thermal mapping of material surfacesto investigate its facial and/or its subsurface status thatmay affect its performance. This is done using photonic orbolometric arrays sensitive in the (1–13 mm) band of the

electromagnetic spectrum [4]. IRT has been successfullyused as a real-time thermal machine vision system fordetecting thin paint spots on automotive fuel containers in[5]. Further, it has effectively been used to detect latentcorrosion in different steel structures including bridges [6]and petroleum reservoirs [7].The UVF is a novel technique based on detecting the

visible fluorescence emission upon exciting the embeddedoptical pigments within the coat structure by shinning aUV light. This technique is applied to highlight the missedor uncoated spots in naval coatings by tracing thosepigments emission, we introduced more about this techni-que principles in [8]. Several companies adapted the UVFtechnology upon the NAVSEA recommendation andregulations, current regulation require the inclusion offluorescent pigments in naval primers. Table 1 presents theapplicability matrix for the various applied and proposedtechniques for the ballast tanks inspection, produced uponon-site testing.

3. Ultraviolet fluorescence (UVF)

The primer coat under study is an epoxy mixed withfluorescence optical pigments or fluorophore, the chemistryof this fluorophore is a 2,5-thiophenediylbis(5-tert-butyl-1,3-benzoxazole) in powder form. This coat structure isdesigned to absorb the UV stimulation in the form of light,to fluoresce, where fluorescence is defined as the relaxationof an atom or molecule from an excited state with theemission of a photon. The energy of this emitted photon isoften detected as visible light known as fluorescence, detailsof fluorescence theory and chemistry could be found in[9,10]. Fluorescence found applications in the textileindustry as a brightener; such brighteners are known underdifferent names such as fluorescent whitening agents(FWA), fluorescent brightening agents (FBA), or fluores-cent bleachers, and applied effectively in a variety ofpolymer substrates like molded thermoplastic, fibers,adhesives, high-quality paper for color printing, andsynthetic leather [11].

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The idea behind using the optically active paint is twofold: (1) monitor the coat coverage by tracing thefluorescence emission; and (2) the fluorescence intensitylevels decide on the pigment loading consequently thefilm thickness, complete theory and verification are foundin [12].

The experimental work utilizes two sets of samples; (1)Samples with three different thicknesses, prepared usingdrawdown blades with gaps allowing a controlled wet filmthickness, the final dry film thickness is 5, 7 and 10mils,measured using ultrasound gauge (commercial nameElcometer 456, product of UK). (2) One spray-paintedsample with drilled pinholes configuration displayed inFig. 2a, as representative of flat areas and larger structuredsample featuring uncoated edges and corners as in Fig. 3.These set of samples utilized a steel substrate with 2.0mil(50 mm) roughness profile, standard in this field.

Fig. 2. (a) Photograph of the pinhole sample, (b) fluorescent image in a, (c) pr

bar indicates temperature readings and (e) processed thermal image.

First, we present the thickness study using a spectro-scopic analysis of the fluorescence emission. This is done toillustrate the fluorescence spectral emission and helpexplain the fluorescence emission theory. The spectroscopeused is UV–VIS utilizing a 248 pixel detector sensitive inthe 0.20–0.85 mm band, with 600 mm terminated fiber optic(commercial name Ocean 2000 USB product of Oceanoptics, US). To stimulate the fluorophore pigments, a UVlight source with a collimated 60W Xenon metal Halidelamp, peaking at 0.37 mm (commercial name ATI-UVproduct of Advanced Technology Inc. US) was used, asdisplayed in the experimental setup Fig. 4. The experi-mental procedure includes shinning the UV stimulation onthe thickness samples and acquiring the spectral emissionusing the fiber optic connection to the spectroscope thatcommunicates the spectrum through a USB connection tothe processing unit. Fig. 6 shows the spectra of the three

ocessed fluorescent image, (d) thermal pinhole image (thermogram), color-

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thickness samples, indicating the bands of interest; (1) UVsource reflection, which occupies the 0.35–0.40 mm band, inother words centered at the UV source wavelength(0.37 mm). This band does not provide any depth informa-tion but relates to the samples surface profile and can beeasily eliminated using suitable cut filters. (2) The emissionband, centered at 0.44 mm with band intensity levels thatdescribe the optical pigments loading; high loadingsproduce high emission levels. The shift in wavelength from0.37 mm (UV source) to 0.44 mm (fluorescence) is caused bythe loss of energy accompanied when the electron returnsto its original state after the excitation; this is pictoriallyexplained by the absorption–emission mirror image spec-trum shown in Fig. 7. Fig. 6 presents the intensity levelsrelative to a 40W incandescent lamp emission, thisintensity profile is an average spectrum of several record-

Fig. 3. Large structured sample, featuring uncoated spots on corners,

region under study is shown in red.

6

82.5

Acq

3.5Tested sample

Fig. 4. The UVF spectrosco

ings across the area covered by the UV-lamp. This figureshows that the 5, 7 and 10mil samples have 1.217, 1.413and 1.559 relative intensity, respectively. This indicatesthe UVF capability of measuring the film thickness,more analysis of the UVF resolution follow in Section 5(Figs. 4–8).In brief, the fluorescence could be quantified in terms of

2 coefficients: (I) fluorescence rate G defined in Eq. (1) asthe integration of the absorption spectrum e(u), and (II) thequantum yield Q described mathematically in Eq. (2), moredetails about this are in [13];

G ¼ 2:88� 10�9 n2 hu�3i�1Z�ðuÞdu

u, (1)

Q ¼G

Gþ kth, (2)

where u ¼ 2p=l is the wave number, n is the refractiveindex of the medium (paint), and kth is the radiation-lossdecay, i.e. energy lost thermally during the relaxation.The second testing scheme is intended to investigate the

UVF pinhole detection capability. The test procedureincludes shinning the sample with a portable UV lamp withGaussian distribution, followed by acquiring a static imageof the fluorescence emission. The detector used is a high-resolution CMOS 3152� 2068 array (Canon D60, Japan)while the UV lamp was complimented with a diffuser toreduce the reflected portion and to avoid the speculareffects in the grabbed images, this setup is in Fig. 5. Fig. 2bdisplays the fluorescent image of the sample in 2a,indicating the pinholes as dark spots, this image is acquiredwithout ambient lighting contribution to present the soleemission of fluorescence. The fluorescent image of thehighlighted portion of sample 3 is displayed in 8a. Inprevious study [14], the effect of narrow band filters andthe ambient light contribution on the acquired imagequality was demonstrated, some results are displayed inFig. 9. The processing of 2b and 8a images and the

6

uisitionUnit

UV SourceUSB2000

Lightguiding

cord

DirectingLens

py experimental setup.

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Fig. 5. UV pinhole detection setup.

1.65 mil

1.47 mil

1.2

10 mil

1

0.8

0.6

0.4

0.2

0100 200 300 400 500 600 700 800 900

Wavelength nanometer

Inte

nsity

arb

. Uni

ts

Reflection band

Emission band

Fig. 6. The spectra of the thickness samples, the intensity units are relative

to a 40W incandescent lamp emission.

1.20

1.00

0.80

0.60

0.40

0.20

0.00275 300 325 350 375 400 425 450 475 500 525 550 575 600 625

Wavelength (nm)

Rel

ativ

e In

tens

ity

Absorption Emission

Fig. 7. Absorption–emission mirror image spectrum for the 2,5-thiophe-

nediylbis fluorophore.

M.A. Omar et al. / NDT&E International 40 (2007) 62–7066

computation of the SNR and contrast levels are insection five.

4. IRT experimental work

This part of the manuscript discusses the IRT testing ofthe samples discussed in previous section, to evaluate thismethodology’s performance against that of the UVFapproach in detecting the coat pinholes and evaluatingthe film thickness.

Transient or time-resolved thermography is used to inferinformation about the material subsurface status. Forthickness evaluation, we use a pulsed thermography

routine that consists of stimulating the thickness sampleswith short duration high-power radiation source andrecording the surface thermal profile. To achieve this, weutilize a bi-tube pulse unit with 6.4 kJ intensity deposited in15ms duration (Balcar source, product of France), and aphotonic InSb detector 256� 256 array, sensitive in the3–5 mn band of the infrared spectrum with 120Hzacquisition rate with a mean NETD of 25mK (TVS8500, CMC Cincinnati, OH). The detector is equipped witha 14.51 lens that provides a FOV of 8� 8 cm and IFOV of0.03 cm; the experimental setup is in Fig. 10. Thetemperature history recording for the three thicknesssamples is shown in Fig. 11; this profile represents theaverage temperature evolution for an area of 5� 5 cm toavoid local spikes. The thickness values are determinedbased on the onset time of departure for each profile using

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90

80

70

60

50

40

30

200 100 200 300 400 500 600 700

(a)

(c)

Distance in pixels

Distance in pixels

(b)

(d)

Rel

ativ

e In

tens

ityR

elat

ive

Inte

nsity

180

160

140

120

100

80

600 50 100 150 200 250

Fig. 8. (a) fluorescence image of the highlighted potion of Fig. 3, (b) intensity profile drawn across the dotted line of a. (c) thermogram of the highlighted

potion, (d) intensity profile for the dotted line in b.

M.A. Omar et al. / NDT&E International 40 (2007) 62–70 67

Fourier number Fo ¼ a t=d2, this time indicates when thesubstrate start affecting the thermal behavior of thestructure; the thinner the coat the lesser time it takesfor the substrate effect to appear. This behavior conformsto the solution of the thermal conduction in two-layermedia upon a pulse [15]. For a sample with unknownthickness, there is a need for a calibration curvethat includes high and low thickness samples inaddition to the substrate (semi-infinite) thermalprofile. As in the case of UV, the data analysis follows inSection 5.

For the pinholes detection, we use a static mode of IRTthrough acquiring snap shot thermograms after heating thepinhole sample. Two quartz-heating lamps with 400-Wintensity, installed along with the IR cam in a detectionhead (Fig. 12), was used for 4 s to heat the paint layerstimulating a facial thermal emission. The pinholes behaveaccording to the substrate properties, i.e. ksteel, rsteel and

Cpsteel, while the well-coated surroundings have thethermal properties of the primer-coat. This creates thedifference in the temperature reading across a missedcoated region and another well-coated one, pictorially thisis shown in Figs. 2d and 8c, and mathematically explainedthrough the thermal mismatch factor or, the thermalreflection coefficient in Eq. (3), which is found to be 0.92for this paint–substrate combination; more details could befound in [16]:

C ¼ecoat � esteel

ecoat þ esteel, (3)

where, e ¼ffiffiffiffiffiffiffiffiffiffiffiffikrCp

pis the thermal inertia, the subscripts

coat and steel refers to the use of the thermal properties(thermal conductivity k, density r and specific Cp) of thesteel and the primer-coat, respectively.The IRT thickness measurement is introduced as an

empirical testing that does not include a calibration effort

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Fig. 10. The experimental setup for the IR pulse routine.

Fig. 9. The effect of ambient light and narrow band filters on the quality of acquired fluorescent acquired images.

M.A. Omar et al. / NDT&E International 40 (2007) 62–7068

using the heat conduction solution, to match the UVFempirical nature.

5. Data analysis and discussion

This section describes the UVF and the IRT dataanalysis, produced in Sections 3 and 4, respectively. Thisstudy introduces an initial investigation for the UVF andIRT detection and sensitivity limits in this application.

To compare the thickness signal produced by twodifferent devices (UVF spectrometer and IRT camera),we devise b and D, where D is the change in the detectorreading upon encountering a difference of one unit

thickness and b is the ratio of signal produced per unitthickness (D) to the equipment minimum detectable signal(resolution). The resolution of the UV–VIS spectrometerused (relative to a 40-W incandescent lamp emission) is0.01, and the intensity difference generated for the 5–7mildifference is D ¼ 0:196, and for the 7–10mil is D ¼ 0:147,averaging Davg ¼ 0.03/mil and providing a b value of 3.The unit thickness is chosen to be 1mil (25 mm) conformingto the standards and the traditional regulations andtolerances in this application.For the IRT, the temporal signal determines the

thickness reading. The IR cam temporal resolution is theacquisition rate of the cam used that is 120Hz or 0.00833 s

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60

55

50

45

40

35

5 mil

30

7 mil

25

10 mil

20

Time in seconds

Tem

pera

ture

Evo

lutio

n °C

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35

Fig. 11. The temperature response of the three thickness samples upon the

pulse, averaged over a 5� 5 cm area.

Fig. 12. The detection head used for pinhole detection.

M.A. Omar et al. / NDT&E International 40 (2007) 62–70 69

to acquire a single thermogram at full frame. The timeelapsed from 5 to 7mil is 0.04 and from 7 to 10 is 0.05 s,averaging a 0.018 s/mil. This gives a b value of 2.3. Thereliability and repeatability of the readings are insuredthrough the averaging of experimental readings and therepetition of each test; a sequence of three repetitions isdone with an standard deviation of 0.044 and 1.32 for theUVF and IRT, respectively. From this analysis, both theUVF and the IRT techniques are capable of providing adetectable thickness signal (above the equipments’ noisethreshold) using the described procedure to resolve a unitthickness change, i.e. 25 mm.

For processing the pinholes, we utilize an in-housedeveloped code that operates by isolating the backgroundcontribution using a local thresholding approach. Thedetails and the performance of this code are found in [17].This processing aims at the automated detection ofpinholes from the acquired images. This code is capableof eliminating any spurious background effects (e.g.corroded regions). Processing the UVF image in 2b resultsin Fig. 2c, and for the IRT image in 2d results in 2e. These

two processed images show the effect of this code in (a)eliminating the non uniformities (UV light Gaussiandistribution and IRT heating non uniformities) and (b)highlighting the pinholes. To quantify the processed imagesin 2c and 2e, we utilize Tanimoto criterion (or, revealability Ren.) described in Eq. (4), and first introduced in[18]. Applying this criterion provides 66% reveal ability(detecting 4 out of 6 without false positives) for the UVFimage and 83% (detecting 5 out of 6 without falsepositives) for the IRT.

Ren: ¼Nreal �Nmissed

Nreal �N false. (4)

where N is the number of defects and the subscripts real,missed, false refer to real defects, missed defects and falsepositives found in the processed image. One should bear inmind that applying Tanimoto’s criterion for a single imagewith 6 defectives is considered highly conservative, and sothose number (83% and 66%) are considered good.The usage of this code is verified because, it was effective

in the automated detection of defects in several applica-tions; real-time detection of thin paint in [5], and in thesuccessful processing of thermal sequences for the non-destructive evaluation of high density polyethylene(HDPE) joints in [19]. Nevertheless, this code does notprovide any information about the signal consistencyacross the defective area but judges the defective spotbased on the maximum signal attained. To investigate theconsistency issue, we devise a local computation of SNRfor the structure images displayed in Figs. 8a and c. Thisnoise study is aimed at investigating the uniformity of thesignal across the defectives using an approach proposed byLee et al. [20] and reported in [4,21]. Eq. (5) describes thiscomputation approach.

SNR ¼

Pi

Pj½imageði; jÞ�2=ðncol � nrowÞ

1=2½sðNði; jÞÞ�2, (5)

where SNR is the signal-to-noise ratio, the numerator is theaverage power of the image, (i, j) the coordinates, and ncol,nrow the number of columns and rows, respectively. Thedenominator represents average power of noise and s is thestandard deviation of noise N(i,j). The noise N(i,j) iscalculated as the difference between two images acquiredfor a single scene simultaneously. To utilize Lee’s et al.approach in studying the noise from single image, wemodify Eq. (5) into (6).

SNR ¼

Pki ½Pf ðiÞ�2=k

½sðNðiÞÞ�2, (6)

where Pf(i,j) is the intensity profile along the defective as inFigs. 8b and d, and noise N(i) is the average differencebetween group of profiles such as Pf(i) for each image.Using Eq. (6) to compute this local version of the SNR

results in a value of 5.1 for the fluorescent image in 8a anda value of 6.5 for the thermal image in 8c.

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From the above analysis, it is clear that both the IRTand UVF techniques can provide a detectable (above noisethreshold) thickness signal to resolve a 1-mil (25 mm)thickness difference. For the pinhole detection, both werefound compatible with the in-house developed code,detecting 5 and 4 defects out of 6 without any falsepositives. Finally, IRT and UVF output images providedgood signal uniformity for the elongated defectives (SNRvalues of 6.5 and 5.1, respectively), indicating their abilityof retrieving and preserving defects different shapes withresolvable contrast from their surroundings.

The transfer of the UVF and the IRT technologies fromlaboratory testing to on-site measurement requires addres-sing the packaging issues such as the weight, the size, andthe interface of any proposed apparatus; field measure-ments using current setups are underway.

6. Conclusion

This paper discussed the successful application of thenonvisible (UVF and IRT) imaging for the evaluation ofthe naval primers used in protecting the ballast tanks’surfaces. This is considered the first reported study for thenaval usage of the UVF technique. The thermal pinholedetection is discussed through the development of acompact detection head; furthermore, a novel processingcode was used to automate the inspection process, thisdevelopment indicates a prototype potential for bothtechniques. The local computation of the SNR along withTanimoto criterion were used to describe the UVF and theIRT experimental data in a quantative objective manner toprove their signal integrity and usefulness. In addition, thiswork motivates the application of the emission (nonvisible)approach in inspecting complicated geometries due to itsindependence of scattering.

Acknowledgments

This study was sponsored in part by Toyota MotorManufacturing. Dr. P. Gossen is acknowledged for thetechnical discussion and the access to the National SurfaceTreatment Center (NSTC) Labs.

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