Wavelength and temperaturedependence of continuous-wave laserabsorptance in Kapton® thin films

William J. PalmMichael A. MarciniakGlen P. PerramKevin C. GrossWilliam F. BaileyCraig T. Walters

Wavelength and temperature dependenceof continuous-wave laser absorptancein Kapton® thin films

William J. PalmAir Force Research LaboratoryKirtland AFB, New Mexico 87117

Michael A. MarciniakGlen P. PerramKevin C. GrossWilliam F. BaileyAir Force Institute of TechnologyDepartment of Engineering PhysicsWright-Patterson AFB, Ohio 45433-7765E-mail: michael.marciniak@afit.edu

Craig T. WaltersCraig Walters AssociatesPowell, Ohio 43065

Abstract. Optical properties and laser damage characteristics of thin-filmaluminized Kapton® were investigated. Spectral absorptance of virgin andirradiated samples was measured from the Kapton side of multilayeredinsulation over 0.2 to 15 μm wavelengths at both room temperatureand 150°C. The laser-damage parameters of penetration time and max-imum temperature were then measured in a vacuum environment at laserwavelengths of 1.07 and 10.6 μm. Differences in damage behavior atthese two wavelengths were observed due to differences in startingabsorption properties at these wavelengths. During laser irradiation, theKapton thin film was observed with a calibrated FLIR® thermal imagerin the 8 to 9.2 μm band to determine its temperature evolution. Spectralradiance throughout the mid- and long-wave infrared was also observedwith a Fourier transform spectrometer, allowing temperature-dependentspectral emittance to be determined. Kapton emittance increased afterthe material heated past approximately 500°C, and continued to increaseas it cooled posttest. This evolving temperature-dependent spectralemittance successfully predicts the increasing absorptance that led toshortened penetration times and increased heating rates for the 1.07 μmlaser. For tests with constant absorptance and no material breakdown, asimplified one-dimensional thermal conduction and radiation modelsuccessfully predicts the temporally evolving temperature. © 2012 Societyof Photo-Optical Instrumentation Engineers (SPIE). [DOI: 10.1117/1.OE.51.12.121802]

Subject terms: laser heating; Kapton®; polyimide; spectral emittance.

Paper 120400SSP received Mar. 15, 2012; revised manuscript received Apr. 25,2012; accepted for publication Apr. 25, 2012; published online Jul. 10, 2012.

1 IntroductionPrediction of laser-target thermal interaction is complicatedby the transient nature of the target material’s properties as itundergoes laser irradiation. This research characterized thewavelength-dependent correlation between laser damageand initial laser absorptance by considering the primarymechanisms for damage. Remote-sensing equipment wasused before, during, and after laser irradiance testing toanalyze the transient temperature and absorption propertiesof thin-film aluminized Kapton® in a vacuum environment.In this fashion, spectral data could be used to confirm evol-ving temperature results from calibrated FLIR® thermalimages and compare them to evolving absorption trends dur-ing material decomposition. As expected, laser absorptancewas determined to be the greatest contributing factor to mate-rial heating, but it was also observed to vary greatly withlaser wavelength and irradiance and depend on the extentof Kapton decomposition.1 Absorptance measurementswere made for the aluminized Kapton film before, during,and after irradiation at several laser irradiance levels. Thiswork reports an independent spectral characteristic for eachphase in the life of the irradiated material, yielding tempera-ture- and wavelength-dependent absorptance. Along with asimple thermal model, this data may be useful for scalingbulk material thermal interactions between various laser

wavelengths in other materials if the transient changes inabsorptance in those materials are properly characterized.Remote-sensing instruments like the Fourier-transformspectrometer (FTS) serve as valuable tools in determininghow laser coupling evolves and affects the prime damagemechanisms.

2 ExperimentThin-film samples of aluminized Kapton measuring7.62 cm2 were irradiated at wavelengths in the near infrared(NIR, 1.07 μm) (see Fig. 1) and long-wave IR (LWIR,10.6 μm) (see Fig. 2). The film used for this experimentwas constructed of a 50 μm-thick Kapton polyimidewith a thin aluminized back surface less than or equalto 300 Å.

The tests were fully diagnosed with a full listing ofrelevant diagnostic equipment.2 A small fraction of themain beam was split off to form a diagnostic beam trainfor each laser. This diagnostic beam train is a known constantfraction of the beam power and is collected in an integratingsphere with a thermopile detector that is linear over sixorders of magnitude with a 10-ms response time (shownin Figs. 1 and 2). The thermopile detector was calibrateddaily to the power at the target plane using a Molectronpower meter. This technique ensured that the effects ofthe optics and beam dumps were factored over a range ofpowers. Irradiance was reported during the dwell timeon each sample using a constant area of 23.75 cm2 and0091-3286/2012/$25.00 © 2012 SPIE

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the calibrated power. The uncertainty is presented as thestandard deviation of power measurements collected at100 Hz during the dwell time. Spatial beam profile for theLWIR beam was determined using polymethylmethacrylate,a typical profiling material that linearly ablates. The NIRlaser followed the same setup, with an additional beam split-ter placed at the final optic to minimize errors and providea true replica of the beam profile at the target plane. Thereplica beam was imaged with a back-illuminated CMOSCCD video camera (shown in Fig. 2) and recorded to ensureprofile repeatability.3 Figure 3 depicts instruments involvedin analyzing the material interaction with reference to thetarget plane. Strict control was used to calibrate the spectraleffects of the vacuum chamber windows for the exteriorinstruments by performing a full radiometric analysis ofthe path from the sample to the detector.4

3 ResultsBefore irradiance testing, spectral reflectance for virginaluminized Kapton samples was performed from the Kaptonside using a Varian Cary 5000 spectraphotometer and aBomem MB157S FTS. The spectrum of the polyimidefilm at room temperature (∼25°C) is presented as spectralabsorptance in Fig. 4 and is identical to that measured at150°C. These results are similar to previously reportedKapton spectra.5,6

The laser irradiance test data from the FLIR thermalimager were reduced using a MATLAB® curve fit developedfrom calibration elements produced within the FLIRThermaCAM® RCal™ program in the 8 to 9.2 μm region,a relatively flat, high-absorptance/emittance region as shownin Fig. 4. The normalized interferograms from the FTS wereimported into a MATLAB code that converted them into

Fig. 1 Fiber laser beam train with beam splitters at the beginning and end of the beam train to measure power and spatial beam profile.

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uncalibrated spectra. A linear relationship to known black-body calibration curves was then developed to yieldcalibrated apparent spectra with real units of radiance.After taking into account the vacuum chamber windowattenuation, true source radiance for both the spectral mea-surement and the thermal image was achieved. Figure 5shows example spectra of aluminized Kapton irradiated at9.6� 1.4 W∕cm2 in the NIR. The midwave infrared(MWIR) radiance, collected by the FTS’s InSb detector,is shown as a dashed curve, and the LWIR radiance,collected by the FTS’s HgCdTe (MCT) detector, as a dottedcurve. The Planckian envelope was generated using theFLIR-imager-inferred temperature (∼500°C here). Thisdemonstrates the continuity achieved between the FTSand FLIR thermal imager when viewing the same samplespot. Note that at 500°C, the spectral features observed atroom temperature and the relatively flat, high-emittanceregion of the LWIR remain.

4 DiscussionThe Kapton film responded as anticipated based on the pret-est absorptance measurements at the two laser wavelengths.At 10.6 μm, Kapton is highly absorptive, as are most organicmaterials, and the heating rate was high. At 1.07 μm, Kaptonis fairly transparent, and heating rates were much lower forthe same irradiances. Temperature was recorded as a functionof time for each sample tested, and before major damage ordecomposition, showed good correlation to a simple one-dimensional (1-D) conduction and radiation model. Thismodel involved a balance between estimated absorptionusing derived values, 1-D conduction into the material,and radiation from the Kapton side only.

Figure 6 presents the calibrated FLIR data for a 10.6 μmlaser irradiance of 2.98 W∕cm2 on the Kapton side of thealuminized Kapton film. Two curves correspond to twoframe integration times that are acquired alternately duringFLIR measurements. “FLIR Data 0” presents the FLIR data

Fig. 2 CO2 laser beam train with two beam dumps and diagnostic beam splitting.

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for a 3-ms integration time, which is valid for low tempera-tures before the detector saturates; the validity range for thiscurve is 40°C to 250°C. “FLIR Data 1” presents the FLIRdata for a 1-ms integration time and is valid for temperaturesof 350°C to 700°C. To get a continuous curve for temperatureas a function of time, it was assumed that the transitionbetween 250°C and 350°C was smooth and continuousand could be represented by a parabolic splice,

T ¼ −84.667t2 þ 383.12t − 4.044; (1)

with T in °C and t in seconds. The resulting continuous curvefor temperature was then compared to a simple laser heat-ing model.

The irradiance tests were conducted in a vacuum testenvironment which negated convective heat transfer. Inthis environment, the dominant heat transfer mechanism isradiation, with very little conduction into the sample stand

due to the material’s low thermal diffusivity. Whereas thetest environment complicated the experimental setup, thetheory was greatly simplified by removing convection andonly treating 1-D conduction within the material and radia-tion from it. Based on the thermal diffusivity of Kapton, thecharacteristic distance for heat diffusion during a 60-s run isabout 2 mm. This and the appearance of the samples aftertesting confirmed that there was no significant radial heatconduction. However, thermal radiation from the front sur-face played a dominant role in the temperature history of theKapton film at long run times with little Kapton decomposi-tion. This is illustrated by the FLIR thermal imager data com-pared to the 1-D thermal model, with radiation loss shown inFig. 7(a) for 1.07 μm laser irradiance of 7.31 W∕cm2. AKapton thermal emittance of 0.41 in the 1-D thermalmodel provides an excellent fit to the FLIR test data.

The thermal conduction model is a simple 1-D explicitfinite difference numerical solver with appropriate boundary

Fig. 4 Spectral absorbance for 2-mil aluminized Kapton® at room temperature measured from Kapton side.

Fig. 3 Plan view sketch inside the vacuum chamber showing beam entrance and various diagnostic equipment locations.

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conditions. The front surface (Kapton side) was assumed toabsorb heat equal to the irradiance multiplied by the absorp-tance that was determined from the room-temperature mea-surements. Thermal radiation at the front surface was simplyassumed to follow the Stefan–Boltzmann law with a constantemittance. This emittance was adjusted to achieve a reason-ably good agreement with the data at long exposure times(near radiative equilibrium). No heat loss was assumed tooccur from the aluminized back surface, which has low emit-tance. As can be seen by the straight dashed line in Fig. 7(a),the early heating rate was empirically determined to be72°C∕s. This fit works well when there is no damage tothe material. The absorptance value of 0.076 used wasderived from a fit to the early heating rate, where radiationwas not significant. This derived absorptance agrees towithin 16% of the room-temperature steady-state value for1.07 μm found pretest (0.09, Fig. 4). The emittance fitvalue of 0.41 was lower than the manufacturer’s

specification for Kapton (0.71),7 which is probably validfor room temperature or colder. The initial material proper-ties for Kapton used in this analysis are presented in Table 1.

A similar comparison is made in Fig. 7(b) for 10.6 μmlaser irradiance of 2.98 W∕cm2. Here, the agreementbetween the FLIR data and the 1-D model is not quite asgood. Although irradiance is lower than in Fig. 7(a), higherabsorptance at 10.6 μm drives the temperature higher. Theearly heating rate was empirically determined to be approxi-mately 257°C∕s [straight dashed line in Fig. 7(b)], and theabsorptance derived from the early heating rate of 0.67agrees to within 27% of the low-temperature steady-statevalue at 10.6 μm (0.916, Fig. 4). The difference betweenthe FLIR data and the 1-D model seen between 2 and 6 smay be an indication of an endothermic chemical reactionwithin the material that slows the heating. Kapton has areported decomposition temperature of 525°C, a featurethat was observed in several different test cases.5 The perma-nent discoloration of Kapton may begin slightly below thistemperature after prolonged exposure above around 400°C.(Note that this is not pyrolysis, or char formation whichoccurs above 650°C). The effective thermal emittanceincreased to 0.48 in this example, a faster heating scenario,based on the radiation-balance, conduction, and absorptionmodel. This modeled emittance value for the Kapton layer at650°C is not significantly different than that at 430°C [0.41,Fig. 7(a)] and will mostly depend on the level of discolora-tion in the material. The nature of the discoloration is notaddressed here.

The increased effective thermal emittance at highertemperatures used in the heating model is consistent withan increase in spectral emittance at temperatures above roomtemperature measured by the FTS. FTS measurementsduring laser irradiation captured the change in spectral emit-tance with temperature and extent of surface discoloration.Throughout a test, nearly constant spectral absorptancewas observed (as in Fig. 4) until the point where the Kaptonsurface starts to change color and decompose. Then, theFTS-measured emittance increased in the MWIR andremained nearly constant in the LWIR. Owing to equipment

Fig. 6 Typical calibrated FLIR® data for two integration times. Curve “FLIR Data 1” represents the 1-ms integration, whereas “FLIR Data 0” the 3-msintegration. The splice function between the two curves is represented with the data fit equation.

Fig. 5 Calibrated source spectra of aluminized Kapton® undergoing1.07-μm laser irradiation at 9.6� 1.4 W∕cm2. Blackbody envelopes ofeach detector created for comparison using a Planckian curve fit at aFLIR®-inferred temperature of approximately 500°C.

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limitations, spectra in the NIR were not captured, but asignificant rise in emittance can be discerned by comparingthe material/laser interaction in terms of increasing heatingrates. Figure 8 shows pretest, real-time, and posttest spectralemittance in the MWIR. The real-time (FTS InSb-detector–based) emittance flattened out in the MWIR region and thengreatly increased after the sample was allowed to cool.Figure 9 shows the pre- and posttest room-temperatureKapton spectral emittances from the NIR through theLWIR. (Real-time data over this broader wavelength rangewas not available.) Although the data in Fig. 9 do not includethe remote-sensing emittance data, an increase in MWIRabsorptance was observed before cooling in the cases wherematerial discoloration or decomposition began. By extrapo-lating heating rate data between the two laser wavelengthstested and normalizing the effects of absorption in the regimebefore radiation becomes significant, we can infer a rise inemittance for the 1.07 μm laser wavelength region of about400%; that is, a starting material absorptance of 0.09 would

Fig. 7 Comparison of FLIR® temperature data in a 1-D thermal model with front surface radiation loss for 1.07-μm (a) and 10.6-μm (b) laser tests.Kapton® absorptance derived at 1.07 μmwas 0.076 and that at 10.6 μmwas 0.67. Kapton thermal emittance required for the fit was calculated to be0.41 at 1.07 μm and 0.48 at 10.6 μm.

Table 1 Initial properties of Kapton® film.5,8

Material property Value for Kapton

Mass density (ρ) 1.42 g∕cm3

Heat capacity (Cp) 1.09 J∕gC

Thermal conductivity (k ) 1.2 × 10−3 W∕cmC

Thermal diffusivity (κ) 7.75 × 10−4 cm2∕s

Absorptance at 10.6 μm from reflectance 0.916

Absorptance at 1.07 μm from reflectance 0.09

Absorptance at 10.6 μm from initial heating data 0.67

Absorptance at 1.07 μm from initial heating data 0.076

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increase to ∼0.4 after the material reached ∼500°C anddecomposed. After the Kapton is allowed to cool, the“decomposed” absorptance at 1.07 μm has increased byroughly 10 times its virgin value.

An empirical trend for aluminized Kapton absorptance atincreased temperatures due to laser irradiation can be estab-lished using the real-time FTS data in the range of 3 to 4 μm,where instrument gain is high and significant emittancechanges were observed. Figure 10 plots 3.8 μm absorptanceas a function of temperature and shows a slightly increasingabsorptance during testing, with a marked increase as thedecomposed material cools back to room temperature after-ward. The absorptance variability during testing stems fromthe uncertainties in calibration and temperature extraction.Although decomposition did occur near 500°C, the materialexperienced a significant absorptance rise at 3.8 μm aftercooling. A fully decomposed (blackened) sample at roomtemperature had an absorptance of 0.63 at 3.8 μm.

5 ConclusionThese results demonstrate the unique wavelength- and tem-perature-dependent material response that can be capturedwith remote sensing techniques such as those used here.This more-accurate characterization of the sample’s spectralabsorptance response then allows the scaling of laser testresults between laser wavelengths by using simple modifiedheating models that better capture the true laser damage.

By using only initial heating rates from a calibrated FLIRthermal imager and a simple 1-D thermal conduction model,a reasonable measure of material absorptance for aluminizedKapton was achieved at two laser wavelengths, 1.07 and10.6 μm. FTS measurements demonstrated that trends in amaterial’s spectral emittance may be captured for nondes-tructive laser/material interactions using remote sensingtechniques. Here, emittance was most accurately capturedin the MWIR, so absorptance trends for aluminized Kaptonwere charted at 3.8 μm. These results were consistent duringnondestructive testing and could be duplicated at any wave-length in the MWIR. After a laser test, when the sampleswere allowed to cool back to room temperature, aluminizedKapton absorptance increased dramatically.

The goal now is to use this varying-absorptance dataalong with the thermal model to produce more-accurate pre-dictions of real-world transient conditions. Based on the dataacquired here, it should be possible to estimate the thermalresponse of the Kapton to laser irradiation at wavelengthsanywhere between 1 and 13 μm with greater accuracythan was possible prior to this investigation.

AcknowledgmentsThis work was supported by the High Energy Laser JointTechnology Office (HEL JTO) under grant AFOSR-BAA-2010-2 and was performed at Wright-Patterson AFB, OH.

References

1. D. L. Decker, “Temperature and wavelength dependence of the reflec-tance of multilayer dielectric mirrors for infrared laser applications,”Laser Induced Damage in Optical Materials Proceedings Sponsoredby the National Bureau of Standards, NBS-SP-435, 230–235 (1975).

2. W. J. Palm et al., “Laser induced damage of Kapton thin films demon-strating temperature and wavelength dependent absorptance: a case

Fig. 8 Comparison of pretest (room-temperature virgin) spectralemittance, spectral emittance measured during experimental decom-position (FTS InSb detector), and posttest (room-temperature decom-posed) spectral emittance for a decomposed Kapton® sample.

Fig. 9 Comparison of aluminized Kapton® spectral absorptancebefore and after laser irradiation to the point of decomposition.

Fig. 10 Empirical 3.8-μm absorptance of aluminized Kapton® derivedfrom FTS spectra captured during laser irradiation. Absorptanceincreases during the heating process between approximately 0.2and 0.4, and then increases to 0.63 in the decomposed (blackened)sample after it is cooled back to room temperature.

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study in remote sensing material analysis,” Proc. SPIE 8190, 819009(2011).

3. C. T. Walters et al., High Energy Laser (HEL) Lethality Data CollectionStandards, Directed Energy Professional Society, Albuquerque, NM(2007).

4. W. J. Palm, “Multilayer insulation laser damage characterization forwavelength scaling,” MS thesis, Air Force Institute of Technology(2011).

5. W. N. Pollard and B. Hannas, “Non-contact temperature measurementof aluminized polymer for space applications,” James MadisonUniversity, Infrared Development and Thermal Testing Lab, VA(2002).

6. M. J. Rowley, “Fiber-optic infrared measurement system for thermalmeasurement of a Kapton® HN sample,” James Madison University,Infrared Development and Thermal Testing Lab, VA (2003).

7. Dupont Kapton®, Polyimide Film General Specifications, www2.dupont.com/Kapton/en_US/assets/downloads/pdf/Gen_Specs.pdf.

8. Product Data Specifications Sheet: Item No. MO09176, DE330,DE332, Dunmore Corporation (2007).

William J. Palm currently leads research anddevelopment of high-brightness fiber-basedlaser systems and components for militaryapplications on behalf of the Air ForceResearch Laboratory’s Directed Energydirectorate. He earned a BS in mechanicalengineering from the University of Illinoisand a MS in applied physics from the AirForce Institute of Technology. He haspublished scientific work related to bothlasers and spacecraft testing through

the Directed Energy Professional Society, SPIE, AIAA, and theAerospace Corporation.

Michael A. Marciniak received a BS degreein mathematics-physics from St. Joseph’sCollege, IN, in 1981, a BSEE degree fromthe University of Missouri-Columbia in 1983,and an MSEE (electro-optics) and PhD(semiconductor physics) degrees from theAir Force Institute of Technology (AFIT) in1987 and 1995, respectively. He is an associ-ate professor in the Department of Engineer-ing Physics at AFIT, with research interests invarious aspects of light-matter interaction,

including polarimetric scatterometry and thermal radiation of nanos-tructured materials, optical signatures, and high-energy-laser damageassessment.

Glen P. Perram received his BS degree fromCornell University in 1980 and his MS andPhD degrees from the Air Force Institute ofTechnology (AFIT) in 1981 and 1986, respec-tively. As professor of physics at AFIT, hisresearch interests include high-power gaslasers, remote sensing, and laser-materialinteractions.

Kevin C. Gross graduated from the Air ForceInstitute of Technology (AFIT) with a PhDdegree in physics in 2007. He joined theAFIT faculty in 2008 and is currently is anassistant professor. He runs the AFITRemote Sensing Group and has beeninvolved in the collection of high-speedradiometric, imagery, and spectroscopicmeasurement of battle space combustionsignatures including high-explosive detona-tions, muzzle flashes, rocket engines, and

jet engine exhaust plumes.

William F. Bailey received a BS degree fromthe United States Military Academy in 1964.He received an MS degree in nuclear physicsfrom the Ohio State University in 1966 and aPhD from the Air Force Institute of Technol-ogy (AFIT) in 1978. As a member of the AFITfaculty since 1978, his research interestsinclude high-power lasers and microwavesystems, plasma dynamics and diagnostics,and characterization of hypersonic aerody-namic flows.

Craig T. Walters holds a PhD in physics from the Ohio State Univer-sity and has performed extensive research in the area of laser effectson materials. He currently has his own consulting firm, Craig WaltersAssociates, a small business devoted to providing R&D services andconsultation to industry and government in the areas of laser technol-ogy and electro-optics. He has performed contract research for othersmall businesses as well as multibillion-dollar corporations in laserapplication areas as diverse as laser cleaning and coating removal,laser beam diagnostics, optical system design, laser shock proces-sing, laser-based inspection of adhesive bonds, laser-welding moni-tors, and high-power optical beam delivery systems. Prior to forminghis own company, he had a distinguished thirty-year career at BattelleColumbus Laboratories, culminating in eight years of service as aResearch Leader.

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