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. P., 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=Iterative+improvement+of+Lockin-thermography+results+by+temporal+and+spatial+adaption+of+optical+excitation.">Iterative improvement of Lockin-thermography results by temporal and spatial adaption of optical excitation.</a> Qirt J. 9 (2), 167-176 (2012).</li><li>Pribe, J. D., Thandu, S. C., Yin, Z., Kinzel, E. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Toward+DMD+illuminated+spatial-temporal+modulated+thermography.">Toward DMD illuminated spatial-temporal modulated thermography.</a> Proc. 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>

<table border="0" cellpadding="0" cellspacing="0" width="681">
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		<col />
		<col />
	</colgroup>
	<tbody>
		<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>
		</tr>
		<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 ...

The overall goal of this method is to use structured heating and highly resolved thermal imaging in a non-destructive and non-contact manner to locate subsurface defects perpendicularly oriented to a steel sample surface. This method can help answer key questions in the thermal imaging field. For instance, how small and how deep a defect can be in order to be detected.The main advantage of this technique is that we can generate thermal wave fields that propagate in the plane of observation, making the approach highly sensitive to perpendicularly oriented defects. This laser-projected photothermal thermography system is arranged on a bench top breadboard. This system has undergone most of the preparatory steps required for use in an experiment.At the head of the beam path is the laser source. This laser fiber is supported by a laser fiber mount. Next, a telescope reduces the beam diameter of the laser to an appropriate size for later in the beam line.Behind the beam sampler, a 500 watt power meter head absorbs much of the beam energy to allow the laser to operate at full power. From beam sampler, the beam continues via a mirror to a projector development kit. This is a disassembled commercial projector with its light engine and lenses removed.For the experiment, collimate the beam to enter the projector. After passing through the projector, the beam will encounter the sample which will be mounted on a computer control translation stage. To complete this setup, obtain a 100 millimeter focal length lens for the projector.Attach the lens to the projector objective just before the translation stage. Next, use an LED flashlight as an input light source to the projector. Position a white sheet of paper in front of the objective and move it until there is a sharp illuminated rectangle on the sheet indicating the position of the image-plane.At this point, obtain a sample for use in the experiment. Mount the sample in the beam path on the linear translation stage equipped with a lab jack. Raise the sample with the lab jack so its top is in line with the top of the projected rectangle.Ensure that a defect is within the illuminated area in the image-plane. Next, arrange to do infrared photography by first obtaining a gold mirror on a post. The mirror will reflect the scattered beam to the camera.Mount the mirror on a post holder near the projector. It should reflect the upper edge of the sample and be angled to see as much of the sample surface as possible. Light reflected from the mirror will enter an infrared camera that is mounted on a tripod.Position it at the height of the projector objective so that it sees the projected white image via the gold mirror. Set up the camera to be controlled by computer, and let it warm up. After connecting the camera to its control software, obtain a steel ruler.Hold the ruler at the surface of the sample and manually focus the camera on it. The temperature contrast to the steel ruler aids in focusing. Work to achieve the sharpest image.One of the most critical steps is to achieve sufficient lateral resolution at the sample surface. This is important because the line of depletion must be resolved. Use the laser software to set the laser voltage to 10 volts and start the laser.Work with the camera software to the relationship between the projector and the camera. Select Measure from the options along the top. Go to the Measure areas toolbar and choose the cross tool option.When the laser is on there will be a thermal image. Use the tool to mark the corners of the image by left clicking on the frame and then note the coordinates. The camera control software must be configured for the experiment.Begin by switching to the Camera panel. There, click the Remote button to open the remote control panel. There in the drop down menu, choose the option Process-IO.Also, go on to click on the Sync In option and the Gate option. After this close the menu. From the Acquisitions parameters tab, open the Acquisition menu.Choose External Sync from the drop down menu. Provide file and folder names in the Folder field. Then, move to the Count field and enter the previously computed number of frames and close the Acquisition menu.Start camera data acquisition by choosing Record. At this point, move to the experiment control software. Click on Activate to active the motion controller.Next, edit the Start and End Positions in millimeters to include the defect in the scan. After that enter the speed in millimeters per second. Click on Start Measurement.Left click on the Choose Area Color field. In the color dialogue, select a color for the pattern area. Go to the drawing toolbar and choose the rectangled tool.Move to the image area and use the tool to create a rectangle consistent with the previously found projector pixel domain. Continue by clicking define Area. The dialogue box allows setting the projected pattern properties.In the Signal Type drop down menu choose Sine Wave. To define the sine wave, set the Phase Shift field to zero degrees. In addition set the Frequency in hertz.Set the Amplitude to the maximum. Next, go to the Voltage field to enter the laser voltage in units of volts. In the field, Pictures per period field, enter a previously calculated value.Click on Next. Follow analogous steps to create a second rectangle of a different color in phase shift of 180 degrees. Preview the sequence of images using them in a preview slider.Then push Start to begin the experiment. The translation stage slowly moves the sample through the chosen range to expose different regions to the projected oscillating structured illumination. The total transit time for this experiment is 200 seconds.As the sample moves, the thermal infrared camera acquires thermal images at 40 hertz. This sequence thermal images provides an example of the thermal wave fields generated by the illumination. Stop the experiment when all the frames have been acquired.To perform necessary post-processing, load the data frames in the post-processing software. After the data is converted, insert the previously found projection point coordinates. Click Transform to put the data into the projector pixel domain.To extract temperature information, define the depletion line by entering the coordinates for two points. Input the parameters for the speed in the start position of the sample during the experiment. Also enter the infrared camera's FrameRate and sine wave Frequency of the pattern.Finally, ensure the data post-processing parameters are correct. When ready, click Evaluate. The crack position is shown in the highlighted field.These data were collected from a test sample with a defect at an approximate depth of 1/4 of a millimeter. The sample was translated at 0.05 millimeters per second. The black curve represents the temperature as a function of time, which is along the top horizontal axis.Time can also be translated to a position which is along the bottom axis. The solid red curve is a fit to not-oscillatory increase in temperature. The dashed red line indicates the position of the defect.Here is the same data after additional post-processing. The blue curve is the Hilbert curve and the defect is at its minimum. These data were collected after doubling the scan speed to 0.1 millimeters per second.In comparison with the first measurement, the elongation is the same but the oscillation frequency is reduced. Note that the sample was moved to a new position which is reflected in the measurements When the protocol is used with a defect one millimeter below the surface, its location can still be determined but with greater uncertainty. Both of these plots use data collected with a scan speed of 0.1 millimeters per second.After its development, the technique paved the way for researchers in the field of nondestructive testing to explore the usage of structured illumination. Following this procedure, other and more complex illumination patterns can be used in order to find other defect types. Thus far only steel has been tested, but the method is very promising, especially for plastic, compound materials, and other very sensitive materials due to the low thermal stress that is applied.The bottleneck of the current experimental setup is the thermal stress limit of the spatial light modulator. That's why we have to pay attention to the measurement time, which should be not more than two to three minutes. Up to now only two integral heat sources have been generated.But in principle, using this setup it is possible to generate and control up to one million heat sources, which opens up another field of arbitrary normal wave shaping. After watching this video, you should have a good understanding of how to locate subsurface defects using laser projected, photothermal thermography. Don't forget that working with a class four high-power infrared laser can be extremely dangerous and that precautions such as wearing laser protection goggles should always be taken.

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11.0K Views.  Bundesanstalt für Materialforschung und -prüfung (BAM). The overall goal of this method is to use structured heating and highly resolved thermal imaging in a non-destructive and non-contact manner to locate subsurface defects perpendicularly oriented to a steel sample surface. This method can help answer key questions in the thermal imaging field. For instance, how small and how deep a defect can be in order to be detected.The main advantage of this technique is that we can generate thermal wave fields that propagate in the plane of observa...