Name: subsurface defect localization by structured heating using laser projected photothermal thermography
Uploaded: 2017-05-15T14:00:00
Description: Watch this Scientific Journal Video about Subsurface Defect Localization by Structured Heating Using Laser Projected Photothermal Thermography at JoVE.com

We would like to thank Taarna Studemund and Hagen Wendler for taking photographs of the experimental setup as well as preparing them for figure publication. Furthermore, we would like to thank Anne Hildebrandt for the sample preparation and Sreedhar Unnikrishnakurup, Alexander Battig and Felix Fritzsche for proof-reading.

NOTE: Caution: Please pay attention to laser safety because the setup uses a class 4 laser. Please wear the correct protective glasses and clothes. Also, handle the pilot laser with care.
1. Couple the Diode Laser to the Projector Development Kit (PDK)
<ol>
	<li>Prepare the breadboard.
	<ol>
		<li>Preassemble all devices to the breadboard as shown in Figure 3. Place the breadboard with all preassembled devices in a laser laboratory.</li>
	</ol>
	</li>
	<li>Position the l...

<ol>
	<li>Thiel, E., Kreutzbruck, M., Ziegler, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Laser-projected+photothermal+thermography+using+thermal+wave+field+interference+for+subsurface+defect+characterization.">Laser-projected photothermal thermography using thermal wave field interference for subsurface defect characterization.</a> Appl. Phys. Lett. 109 (12), 123504 (2016).</li><li>Ibarra-Castanedo, C., Tarpani, J. R., Maldague, X. P. V. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Nondestructive+testing+with+thermography.">Nondestructive testing with thermography.</a> Eur. J. Phys. 34 (6), 91-109 (2013).</li><li>Maldague, X. P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Introduction+to+NDT+by+active+infrared+thermography.">Introduction to NDT by active infrared thermography.</a> Mater. Eval. 60 (9), 1060-1073 (2002).</li><li>Li, T., Almond, D. P., Rees, D. A. S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Crack+imaging+by+scanning+pulsed+laser+spot+thermography.">Crack imaging by scanning pulsed laser spot thermography.</a> Ndt&amp;E Int. 44 (2), 216-225 (2011).</li><li>Lugin, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Detection+of+hidden+defects+by+lateral+thermal+flows.">Detection of hidden defects by lateral thermal flows.</a> Ndt&amp;E Int. 56, 48-55 (2013).</li><li>Li, T., Almond, D. P., Rees, D. A. S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Crack+imaging+by+scanning+laser-line+thermography+and+laser-spot+thermography.">Crack imaging by scanning laser-line thermography and laser-spot thermography.</a> Meas. Sci. Technol. 22 (3), (2011).</li><li>Pech-May, N. W., Oleaga, A., Mendioroz, A., Salazar, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Fast+Characterization+of+the+Width+of+Vertical+Cracks+Using+Pulsed+Laser+Spot+Infrared+Thermography.">Fast Characterization of the Width of Vertical Cracks Using Pulsed Laser Spot Infrared Thermography.</a> Journal of Nondestructive Evaluation. 35 (2), 22 (2016).</li><li>Thiel, E., Kreutzbruck, M., Ziegler, M., Douglass, M. R., King, P. S., Lee, B. L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Proc. SPIE 9761. , (2016).</li><li>Thiel, E., Kreutzbruck, M., Ziegler, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Proc. WCNDT 2016. , 6 (2016).</li><li>Mandelis, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Diffusion-Wave Fields: mathematical methods and Green functions. , (2001).</li><li>Almond, D., Patel, P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Photothermal Science and Techniques. 10, (1996).</li><li>Salazar, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Energy+propagation+of+thermal+waves.">Energy propagation of thermal waves.</a> Eur. J. Phys. 27 (6), 1349-1355 (2006).</li><li>Bennett, C. A., Patty, R. R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Thermal+wave+interferometry:+a+potential+application+of+the+photoacoustic+effect.">Thermal wave interferometry: a potential application of the photoacoustic effect.</a> Appl. Opt. 21 (1), 49-54 (1982).</li><li>Busse, G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Stereoscopic+depth+analysis+by+thermal+wave+transmission+for+nondestructive+evaluation.">Stereoscopic depth analysis by thermal wave transmission for nondestructive evaluation.</a> Appl. Phys. Lett. 42 (4), 366 (1983).</li><li>Holtmann, N., Artzt, K., Gleiter, A., Strunk, H. 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SPIE 9861. , (2016).</li><li>Ravichandran, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Spatial and temporal modulation of heat source using light modulator for advanced thermography. , (2015).</li><li>. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> DLP 0.55 XGA Series 450 DMD. , (2015).</li><li>. DLP LightCommander Control Software - User Manual Available from: <a target="_blank" href="https://support.logicpd.com/ProductDownloads/LegacyProducts/DLPLightCommander.aspx?_sw_csrfToken=318b0448">https://support.logicpd.com/ProductDownloads/LegacyProducts/DLPLightCommander.aspx?_sw_csrfToken=318b0448</a> (2011)</li><li>Moench, H., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=High-power+VCSEL+systems+and+applications.">High-power VCSEL systems and applications.</a> Proc. SPIE 9348. , (2015).</li></ol>

The presented protocol describes how to locate artificial subsurface defects oriented perpendicular to the surface. The main idea of the method is to create interfering thermal wave fields which interact with the subsurface defect. The most important steps are (i) to combine an SLM with a diode laser in order to create two alternating high-power illumination patterns at the sample surface; these patterns are photothermally converted into coherent thermal wave fields, (ii) to let them destructively interfere whilst intera...

The laser projected photothermal thermography (LPPT) method is used to locate subsurface defects that are embedded in the volume of the test specimen and oriented predominantly perpendicular to its surface.
The method uses the destructive interference of two anti-phased thermal wave fields of the same elongation and frequency as shown in Figure 1b. In isotropic defect-free materials, the thermal waves neutralize destructively (i.e. zero temperature oscillation) at the symmetry plane by coherent superposition. In case of a material with a subsurface defect, the method takes advantage of the interaction of the lateral (i.e. in-plane) components between the transient heat flow and this defect. This interaction can be measured in a recreated oscillating temperature elongation at the symmetry line on the sample surface. Now, the defect containing sample is scanned by the superposed thermal wave field and the level of temperature elongation is measured in relation to the sample position. Due to symmetry, the destructive interference condition is satisfied once again when the defect crosses the symmetry plane; this enables us to locate the defect very sensitively. Moreover, since the level of maximal disturbance of the destructive interference correlates with the depth of the defect, it is possible to determine its depth by analyzing the temperature scan1.
The LPPT can be assigned to the active thermography methodology, a well-established non-destructive method, where transient heating is actively generated and the resulting, also transient, temperature distribution is measured via a thermal IR camera. In general, the sensitivity of this methodology is limited to defects which are oriented essentially perpendicular to the transient heat flow. Moreover, since the governing transient heat conduction equation is a parabolic partial differential equation, the heat flow into the volume is strongly damped. As a consequence, the probing depth of the active thermography methodology is limited to a near surface region, usually in the millimeter-range. Two of the most common active thermography techniques are pulsed and lock-in thermography. They are fast due to planar optical surface illumination2, but lead to a transient heat flow perpendicular to the surface. Therefore, the sensitivity of these techniques is limited to defects predominantly oriented parallel (e.g. delaminations or voids) to the heated sample surface. An empirical rule for pulsed thermography states that &#34;the radius of the smallest detectable defect should be at least one to two times larger than its depth under the surface&#34;3. To increase the effective interaction area between a perpendicularly oriented defect (e.g. a crack) and the heat flow, the direction of the heat flow needs to be changed. Local excitation, by using a focused laser with a linear or circular spot for instance, generates a heat flow with an in-plane component that is able to effectively interact with the perpendicular defect4,5,6,7.
In the presented method, we also use the lateral heat flow components to detect subsurface defects, but we use the fact that thermal waves can be superposed, whereas defects, especially vertically oriented ones, disturb this superposition. In this way, the presented method resembles a reference-free, symmetric and very sensitive method, as it is possible to detect artificial subsurface defects at a width/depth ratio of far below one8,9. Until now, it was difficult to create two anti-phased thermal wave fields supplying sufficient energy. We achieved this by coupling a spatial light modulator (SLM) to a high-power diode laser, which enabled us to merge the high optical power of the laser system with the spatial and temporal resolution of the SLM (see Figure 2) into a high-power projector. The thermal wave fields are now created by photothermal conversion of two anti-phased sinusoidally modulated line patterns via the pixel brightness of the projected image (see Figure 2, Figure 1a). This leads to structured heating of the sample surface and results in well-defined destructively interfering thermal wave fields. In order to find a subsurface defect, the disturbance of the destructive inference is measured as a temperature oscillation at the surface using an IR camera.
The term thermal wave, is controversially discussed because thermal waves do not transport energy due to the diffusive character of the heat propagation. Still, there is wave-like behavior when heating periodically, allowing us to use similarities between real waves and diffusion processes10,11,12. Thus, a thermal wave can be understood as highly damped in the propagation direction but periodic over time (Figure 1b). The characteristic thermal diffusion length <img alt="Equation 1" src="/files/ftp_upload/55733/55733eq1.jpg" /> is hereby described by its material properties (thermal conductivity k, heat capacity cp and density &#961;), and the excitation frequency &#402;. Although the thermal wave is decaying strongly, its wave nature can be applied to gain insight into the properties of the sample. The first application of thermal wave interference was used to determine the thickness of layers. In contrast to our method, the interference effect was used in the depth dimension (i.e. perpendicular to the surface)13. Extending the idea of interference to a second dimension by splitting up a laser beam, thermal wave interference was used to size subsurface defects14. Still this method was applied in transmission configuration, which means that it was limited by the penetration depth of the thermal wave. Furthermore, because only one laser source has been used, this method applies constructive interference, meaning that a defect-free reference is needed. Apart from the idea of using thermal wave interference, the first technical approach to spatially and temporally controlled heating was performed by Holtmann et al. using an unmodified liquid crystal display (LCD) projector with the built-in light source, which was severely limited in its optical output power15. Further approaches by Pribe and Ravichandran aimed at increasing the optical output power by also coupling a laser to a SLM16,17.
The protocol presented herein describes how to apply the LPPT method to locate subsurface defects oriented perpendicularly to the surface of steel samples. The method is at an early stage, yet powerful enough to validate the proposed approach; however, it is still limited in terms of the achievable optical output power of the experimental setup. Since the increase of the optical output power remains a challenge, the presented method is applied to coated steel containing artificial electrically discharge machined notches. Nevertheless, the most important and most critical steps of the protocol, generating a homogeneous structured illumination, meeting prerequisites for destructive thermal wave interference, and locating the defect, still hold for more demanding defects as well. Since the governing quantity is the thermal diffusion length &#956;, the LPPT method can be applied to numerous different materials as well.
<img alt="Figure 1" src="/files/ftp_upload/55733/55733fig1.jpg" /> 
Figure 1: Principle of destructive interference effect. (a) Schematic of the illumination pattern used during experiments. The sample is spatially and temporally heated by two periodically illuminated patterns with a phase shift of &#960;. The dashed line represents the symmetry line between both patterns. This line will be used for evaluation as a &#34;depletion line&#34;. (b) Diagram of the spatially and temporally resolved alternating thermal result as calculated from the analytical solution of the thermal heat conduction equation. It shows the responding thermal waves to the illumination of (a) with an irradiance of the two patterns with Popt1 = 1.5 W sin(2&#960; 0.125 Hz t) + 1.5 W and Popt2 = 1.5 W sin(2&#960; 0.125 Hz t + &#960;) + 1.5 W for constructional steel &#961; = 7,850 kg/m3, cp = 461 J/(kg&#183;K), k = 54 W/(m&#183;K). The temperature profile at the dashed line shows no thermal oscillation for homogeneous, isotropic material. <a href="http://cloudflare.jove.com/files/ftp_upload/55733/55733fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" src="/files/ftp_upload/55733/55733fig2.jpg" /> 
Figure 2: Schematic of the measurement principle of structured heating used in active thermography. A Gaussian beam homogenized to a top hat profile is applied to a Spatial Light Modulator (SLM). The SLM resolves the beam spatially by its switchable elements and temporally by its switching speed. Each element represents an SLM pixel. In this experiment, the SLM is a digital micro mirror device (DMD). By modulating the pixel brightness A with a time deterministic control software, the sample surface is heated in a structured way. In case of the presented experiment, we modulate two anti-phased lines (phases: &#966; = 0, &#960;), which are the origin of coherently interfering thermal wave fields at the angular frequency &#969;. The wave fields interact with the sample&#39;s inner structure also influencing the temperature field at the surface. This is measured via its thermal radiation by a mid-wave infrared camera. <a href="http://cloudflare.jove.com/files/ftp_upload/55733/55733fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

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		<tr height="84">
			<td height="84" style="height:84px;width:269px;">500 W diode laser system, 940 nm</td>
			<td style="width:88px;">Laserline</td>
			<td style="width:104px;">LDM 500 - 20</td>
			<td style="width:220px;">Pilot laser class 2 @ 650 nm, diode laser is a class 4 laser system --&#62; special laboratory needed</td>
		</tr>
		<tr height="84">
			<td height="84" style="height:84px;width:269px;">Laser control box</td>
			<td style="width:88px;">Laserline</td>
			<td style="width:104px;">Laser control box LDM</td>
			<td style="width:220px;">Add on to the laser system, used to switch electronically, laser threshold, shutter, laser on 0 V ..5 V TTL</td>
		</tr>
		<tr height="84">
			<td height="84" style="height:84px;width:269px;">Control box scanner</td>
			<td style="width:88px;">Laserline</td>
			<td style="width:104px;"></td>
			<td style="width:220px;">Add on to the laser system, used to adjust the optical output power via analog signal from 0 V..10 V</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:269px;">Fiber Laser Mount 2&#34;, f = 80 mm</td>
			<td style="width:88px;">Laserline</td>
			<td style="width:104px;"></td>
			<td style="width:220px;">Add on to the laser system</td>
		</tr>
		<tr height="84">
			<td height="84" style="height:84px;width:269px;">Multifunction Data Aquisition (DAQ) Device + BNC Terminal</td>
			<td style="width:88px;">National Instruments</td>
			<td style="width:104px;">NI-USB 6251</td>
			<td style="width:220px;">The DAQ card is used to trigger the IR camera,&#160; the DLP Light Commander 5500, control Laser and diode PDA 36A</td>
		</tr>
		<tr height="63">
			<td height="63" style="height:63px;width:269px;">Standard - PC&#160;</td>
			<td style="width:88px;"></td>
			<td style="width:104px;"></td>
			<td style="width:220px;">Control PC - graphic card for two screens, at least 4 x USB, Windows based</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:269px;">BNC cabel</td>
			<td style="width:88px;"></td>
			<td style="width:104px;"></td>
			<td style="width:220px;">Standard cable</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:269px;">HDMI cable</td>
			<td style="width:88px;"></td>
			<td style="width:104px;"></td>
			<td style="width:220px;">Standard cable</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:269px;">Micro USB to USB cable</td>
			<td style="width:88px;"></td>
			<td style="width:104px;"></td>
			<td style="width:220px;">Standard cable</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:269px;">LabVIEW 2013 SP1 Development System</td>
			<td style="width:88px;">National Instruments</td>
			<td style="width:104px;"></td>
			<td style="width:220px;">Development environment for device control</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:269px;">LPPT control software</td>
			<td style="width:88px;">BAM</td>
			<td style="width:104px;"></td>
			<td style="width:220px;">part of the LPPT software package by LabVIEW 2013 SP1</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:269px;">LPPT intensity&#160; software</td>
			<td style="width:88px;">BAM</td>
			<td style="width:104px;"></td>
			<td style="width:220px;">part of the LPPT software package by LabVIEW 2013 SP1</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:269px;">LPPT laser control software</td>
			<td style="width:88px;">BAM</td>
			<td style="width:104px;"></td>
			<td style="width:220px;">part of the LPPT software package by LabVIEW 2013 SP1</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:269px;">Matlab 2016b</td>
			<td style="width:88px;">MathWorks</td>
			<td style="width:104px;"></td>
			<td style="width:220px;">Postprocessing of the measurement data</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:269px;">LPPT postprocessing software</td>
			<td style="width:88px;">BAM</td>
			<td style="width:104px;"></td>
			<td style="width:220px;">Postprocessing of the measurement data</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:269px;">IR camera control PC&#160;</td>
			<td style="width:88px;">InfraTec</td>
			<td style="width:104px;"></td>
			<td style="width:220px;">Control PC is supplied by camera distributor</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:269px;">IR camera control software</td>
			<td style="width:88px;">InfraTec</td>
			<td style="width:104px;">Irbis 3 Professional</td>
			<td style="width:220px;"></td>
		</tr>
		<tr height="84">
			<td height="84" style="height:84px;width:269px;">InfraTec SDK</td>
			<td style="width:88px;">InfraTec</td>
			<td style="width:104px;"></td>
			<td style="width:220px;">Dynamic Link Library as interface between the native data aquisition format of Infratec and Matlab</td>
		</tr>
		<tr height="105">
			<td height="105" style="height:105px;width:269px;">IR camera</td>
			<td style="width:88px;">InfraTec</td>
			<td style="width:104px;">Image IR 8300</td>
			<td style="width:220px;">640 x 512, cooled InSb detector, wavelength 2 &#181;m..5.7 &#181;m, noise = 20 mK + accessories (LAN cable, Digital in/out cable, space ring, power supply, case)&#160;</td>
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		<tr height="21">
			<td height="21" style="height:21px;width:269px;">Tripod</td>
			<td style="width:88px;">Manfrotto</td>
			<td style="width:104px;">161MK2B</td>
			<td style="width:220px;"></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:269px;">IR camera mount</td>
			<td style="width:88px;">Manfrotto</td>
			<td align="right" style="width:104px;">405</td>
			<td style="width:220px;"></td>
		</tr>
		<tr height="84">
			<td height="84" style="height:84px;width:269px;">Projector development kit (PDK) for digital light processing (DLP) technology (DLP Light Commander 5500)</td>
			<td style="width:88px;">Logic PD</td>
			<td style="width:104px;">DLP-LC-DLP5500-10R</td>
			<td style="width:220px;">DLP5500 Digital Micromirror Device from Texas Instruments included , light engine and case need to be disassembed</td>
		</tr>
		<tr height="63">
			<td height="63" style="height:63px;width:269px;">PDK control software</td>
			<td style="width:88px;">Logic PD</td>
			<td style="width:104px;"></td>
			<td style="width:220px;">Included when delivered, DLP Light Commander control software</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:269px;">Mechanical platform for the PDK</td>
			<td style="width:88px;">BAM</td>
			<td style="width:104px;"></td>
			<td style="width:220px;">Self made (140 x 230 x 420) mm</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:269px;">Power meter control unit</td>
			<td style="width:88px;">Ophir</td>
			<td style="width:104px;">Vega</td>
			<td style="width:220px;">USB Interface</td>
		</tr>
		<tr height="63">
			<td height="63" style="height:63px;width:269px;">30 W power meter head&#160;</td>
			<td style="width:88px;">Ophir</td>
			<td style="width:104px;">30(150)A-LP1-18</td>
			<td style="width:220px;">Power meter head to determine Transmission of the projector system</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:269px;">500 W power meter head</td>
			<td style="width:88px;">Ophir</td>
			<td style="width:104px;">FL500A</td>
			<td style="width:220px;">Power meter for process supervision</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:269px;">Motion controller</td>
			<td style="width:88px;">Newport</td>
			<td style="width:104px;">ESP301</td>
			<td style="width:220px;">with USB Interface</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:269px;">Translation stage</td>
			<td style="width:88px;">Newport</td>
			<td style="width:104px;">M-ILS200CC</td>
			<td style="width:220px;">Connected to ESP301</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:269px;">Photodiode with amplifier</td>
			<td style="width:88px;">Thorlabs</td>
			<td style="width:104px;">PDA 36A-EC</td>
			<td style="width:220px;">1&#34; mount</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:269px;">Reflective filter ND1</td>
			<td style="width:88px;">Thorlabs</td>
			<td style="width:104px;">ND10A</td>
			<td style="width:220px;">to be mounted to the PDA 36A</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:269px;">Pinhole 1&#34;</td>
			<td style="width:88px;">Thorlabs</td>
			<td style="width:104px;">P1000S</td>
			<td style="width:220px;">to be mounted to the PDA 36A</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:269px;">Optical aluminium breadboard&#160;</td>
			<td style="width:88px;">Thorlabs</td>
			<td style="width:104px;">MB60120/M</td>
			<td style="width:220px;">(1200 mm x 900 mm) base&#160;</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:269px;">Plano Convex Lens f = 200 mm</td>
			<td style="width:88px;">Thorlabs</td>
			<td style="width:104px;">LA1979-B</td>
			<td style="width:220px;">Coated for IR, first telescope lens</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:269px;">Plano Convex Lens f = 75 mm</td>
			<td style="width:88px;">Thorlabs</td>
			<td style="width:104px;">LA1145-B</td>
			<td style="width:220px;">Coated for IR, second telescope lens</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:269px;">xy-translation stage</td>
			<td style="width:88px;">Newport</td>
			<td style="width:104px;">M401</td>
			<td style="width:220px;">Used for adjusting the telecope</td>
		</tr>
		<tr height="63">
			<td height="63" style="height:63px;width:269px;">Beamsampler</td>
			<td style="width:88px;">Thorlabs</td>
			<td style="width:104px;">BSF20-B&#160;</td>
			<td style="width:220px;">Splits the optical output, used to reduce the optical input for the projector system</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:269px;">Mirror</td>
			<td style="width:88px;">Thorlabs</td>
			<td style="width:104px;">BB2-E03</td>
			<td style="width:220px;">Mirror for coupling the beam to the DLP Light Commander</td>
		</tr>
		<tr height="63">
			<td height="63" style="height:63px;width:269px;">Heavy duty lab jack</td>
			<td style="width:88px;">Thorlabs</td>
			<td style="width:104px;">L490</td>
			<td style="width:220px;">Used for the fiber mount and on top of the linear stage to position the sample (2x)</td>
		</tr>
		<tr height="63">
			<td height="63" style="height:63px;width:269px;">PDK-objective&#160;</td>
			<td style="width:88px;">Nikon</td>
			<td style="width:104px;">Nikon AF Nikkor 50 mm 1:1:8:D&#160;</td>
			<td style="width:220px;">Objective for DLP Light Commander, 50 mm</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:269px;">Plano Convex Lens f = 100 mm</td>
			<td style="width:88px;">Thorlabs</td>
			<td style="width:104px;">LA1050 -B</td>
			<td style="width:220px;">Lens is attached to the Nikon Objective</td>
		</tr>
		<tr height="105">
			<td height="105" style="height:105px;width:269px;">Bi-Convex Lens f = 60 mm</td>
			<td style="width:88px;">Thorlabs</td>
			<td style="width:104px;">LB1723 -B</td>
			<td style="width:220px;">Lens to be attached to the Nikon objective in order to determine the optical transmission with the 30 W measurement head</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:269px;">Square protected gold mirror</td>
			<td style="width:88px;">Thorlabs</td>
			<td style="width:104px;">PFSQ20-03-M01</td>
			<td style="width:220px;"></td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:269px;">High power IR sensor card</td>
			<td style="width:88px;">Newport</td>
			<td style="width:104px;">F-IRC-HP-M</td>
			<td style="width:220px;">Sensor card to check the optical pathway</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:269px;">2&#34; crosshairs</td>
			<td style="width:88px;">BAM</td>
			<td style="width:104px;"></td>
			<td style="width:220px;">Selfmade</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:269px;">1&#34; crosshairs</td>
			<td style="width:88px;">BAM</td>
			<td style="width:104px;"></td>
			<td style="width:220px;">Selfmade</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:269px;">Bullseye level</td>
			<td style="width:88px;">Thorlabs</td>
			<td style="width:104px;">LCL01</td>
			<td style="width:220px;"></td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:269px;">Translation Stage</td>
			<td style="width:88px;">Newport</td>
			<td style="width:104px;">M-UMR8.25</td>
			<td style="width:220px;">Used for measuring the beam profile</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:269px;">Micrometer screw</td>
			<td style="width:88px;">Newport</td>
			<td style="width:104px;">DM17-25</td>
			<td style="width:220px;">Used with translation stage M-UMR8.25</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:269px;">Mounted Zero Aperture Iris</td>
			<td style="width:88px;">Thorlabs</td>
			<td style="width:104px;">&#160;ID75Z/M</td>
			<td style="width:220px;">used to check the optical pathway</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:269px;">Bases and Post Holders Essentials Kit, Metric and Universal Components</td>
			<td style="width:88px;">Thorlabs</td>
			<td style="width:104px;">ESK01/M</td>
			<td style="width:220px;">Basis&#160;</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:269px;">Posts &#38; Accessories Essentials Kit, Metric and Universal Components</td>
			<td style="width:88px;">Thorlabs</td>
			<td style="width:104px;">ESK03/M</td>
			<td style="width:220px;"></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:269px;">M6 Cap Screw and Hardware Kit</td>
			<td style="width:88px;">Thorlabs</td>
			<td style="width:104px;">HW-KIT2/M</td>
			<td style="width:220px;"></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:269px;">Construction Rails</td>
			<td style="width:88px;">Thorlabs</td>
			<td style="width:104px;">XE25L700/M</td>
			<td style="width:220px;"></td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:269px;">1&#34; Construction Cube</td>
			<td style="width:88px;">Thorlabs</td>
			<td style="width:104px;">RM1G</td>
			<td style="width:220px;">Used to mount construction rails</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:269px;">Electrical discharge machining</td>
			<td style="width:88px;">Sodick</td>
			<td style="width:104px;">AG60L</td>
			<td style="width:220px;">www.sodick.de</td>
		</tr>
		<tr height="84">
			<td height="84" style="height:84px;width:269px;">St37 block of steel (100 x 100 x 40) mm</td>
			<td style="width:88px;">BAM</td>
			<td style="width:104px;"></td>
			<td style="width:220px;">selfmade, hidden defect with remaining wall thicknesses of 0.25 mm, 0.5 mm, 0.70 mm 1.25 mm (shown in Fig. 5)</td>
		</tr>
		<tr height="84">
			<td height="84" style="height:84px;width:269px;">St37 block of steel (100 x 100 x 40) mm</td>
			<td style="width:88px;">BAM</td>
			<td style="width:104px;"></td>
			<td style="width:220px;">selfmade, hidden defect with remaining wall thicknesses of 1 mm, 1.5 mm, 1.75 mm, 2 mm (shown in Fig. 5)</td>
		</tr>
		<tr height="63">
			<td height="63" style="height:63px;width:269px;">Graphite spray</td>
			<td style="width:88px;">CRC Industries Europe NV</td>
			<td style="width:104px;">GRAPHIT 33</td>
			<td style="width:220px;">Ref. 20760, 200 ml aerosol (Kontakt-Chemie)</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:269px;">Protective tape</td>
			<td style="width:88px;">Tesa</td>
			<td style="width:104px;">tesakrepp 4348</td>
			<td style="width:220px;">used to protect the hidden defects while coating</td>
		</tr>
	</tbody>
</table>

subsurface defect localization by structured heating using laser projected photothermal thermography

The presented method is used to locate subsurface defects oriented perpendicularly to the surface. To achieve this, we create destructively interfering thermal wave fields that are disturbed by the defect. This effect is measured and used to locate the defect. We form the destructively interfering wave fields by using a modified projector. The original light engine of the projector is replaced with a fiber-coupled high-power diode laser. Its beam is shaped and aligned to the projector&#39;s spatial light modulator and optimized for optimal optical throughput and homogeneous projection by first characterizing the beam profile, and, second, correcting it mechanically and numerically. A high-performance infrared (IR) camera is set up according to the tight geometrical situation (including corrections of the geometrical image distortions) and the requirement to detect weak temperature oscillations at the sample surface. Data acquisition can be performed once a synchronization between the individual thermal wave field sources, the scanning stage, and the IR camera is established by using a dedicated experimental setup which needs to be tuned to the specific material being investigated. During data post-processing, the relevant information on the presence of a defect below the surface of the sample is extracted. It is retrieved from the oscillating part of the acquired thermal radiation coming from the so-called depletion line of the sample surface. The exact location of the defect is deduced from the analysis of the spatial-temporal shape of these oscillations in a final step. The method is reference-free and very sensitive to changes within the thermal wave field. So far, the method has been tested with steel samples but is applicable to different materials as well, in particular to temperature sensitive materials.

Following the protocol, side 1 of the steel sample with a subsurface defect at a depth of 0.25 mm was chosen to generate representative results. The defect was initially positioned approximately at the center of the illuminated area. The sample was then moved from -5 mm to 5 mm via the linear stage at a speed of 0.05 mm/s. Using these parameters, Figure 11a shows the scan data after extracting them from the depletion line. At this stage, the success of the experiment can ...

Watch this Scientific Journal Video about Subsurface Defect Localization by Structured Heating Using Laser Projected Photothermal Thermography at JoVE.com

Subsurface Defect Localization by Structured Heating Using Laser Projected Photothermal Thermography

This method aims at locating vertical subsurface defects. Here, we couple a laser with a spatial light modulator and trigger its video input to heat a sample surface deterministically with two anti-phased modulated lines while acquiring highly resolved thermal images. The defect position is retrieved from evaluating thermal wave interference minima.

The presented method is used to locate subsurface defects oriented perpendicularly to the surface. To achieve this, we create destructively interfering ...

Research

JoVE Journal

Engineering

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Fabrication of Nano-engineered Transparent Conducting Oxides by Pulsed Laser Deposition

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Multifunctional Hybrid Fe2O3-Au Nanoparticles for Efficient Plasmonic Heating

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One of the most widely used methods for manufacturing colloidal gold nanospherical particles involves the reduction of chloroauric acid (HAuCl4) to ...

Electroactive Polymer Nanoparticles Exhibiting Photothermal Properties

A method for the synthesis of electroactive polymers is demonstrated, starting with the synthesis of extended conjugation monomers using a three-step ...

Automation of Mode Locking in a Nonlinear Polarization Rotation Fiber Laser through Output Polarization Measurements

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Measuring Spray Droplet Size from Agricultural Nozzles Using Laser Diffraction

When making an application of any crop protection material such as an herbicide or pesticide, the applicator uses a variety of skills and information to ...

Quantitative Analysis of Vacuum Induction Melting by Laser-induced Breakdown Spectroscopy

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Pore-scale Imaging and Characterization of Hydrocarbon Reservoir Rock Wettability at Subsurface Conditions Using X-ray Microtomography

In situ wettability measurements in hydrocarbon reservoir rocks have only been possible recently. The purpose of this work is to present a protocol to ...