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Thermal imaging technique to characterize laser light reflection from thermoplastics
Optics & Laser Technology 44 (2012) 1456–1462
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Thermal imaging technique to characterize laser light reflection from thermoplastics Elizabeth Azhikannickal a,n, Philip J. Bates a,1, Gene Zak b,2 a b
Department of Chemistry and Chemical Engineering, Royal Military College of Canada, P.O. Box 17000, Station Forces, Kingston, Ontario, Canada K7K 7B4 Department of Mechanical and Materials Engineering, McLaughlin Hall, Queen’s University, Kingston, Ontario, Canada K7L 3N6
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 June 2011 Received in revised form 9 December 2011 Accepted 12 December 2011 Available online 10 January 2012
Characterization of laser light reflection during the laser transmission welding (LTW)3 of thermoplastics is especially important for applications in which non-zero laser incidence angles are used. At higher laser incidence angles, reflection increases and has the potential to burn surrounding features of the part to be welded. This study presents and validates a technique for laser reflection measurement. Reflected energy is absorbed by a black plastic plate (containing carbon black, which is the absorber of the reflected energy). The surface temperature of the plate is measured by an infrared (IR) camera. The distribution of reflected power required to generate this temperature profile is estimated using a simple heat transfer model. The technique was validated by irradiating the black plate by the laser directly, while observing the time-varying temperature distribution of the plate by the IR camera. In this case, good agreement was observed between the estimated total power and the actual laser input power. Good agreement also existed between the estimated power distribution and that determined experimentally via a knife edge based beam profiling technique. The thermal imaging technique was subsequently used to measure the magnitude and distribution of laser light reflection from unreinforced nylon 6. & 2012 Elsevier Ltd. All rights reserved.
Keywords: Laser reflection Thermal imaging Nylon 6
1. Introduction Laser transmission welding (LTW) is increasingly becoming an attractive process for joining thermoplastic parts. LTW involves joining a laser-transparent and a laser-absorbent part together. A laser beam passes through the laser-transparent part and is absorbed by the laser-absorbent part near the interface between the two parts. The laser energy absorbed by the laser-absorbent part is transferred back to the laser-transparent part by heat conduction. Both parts become molten and a solid joint is formed by diffusion of the polymer molecules. The primary advantage of LTW compared to other welding processes is the precise and controllable energy deposition resulting in localized melting over a small region within the contact zone of the two parts. The process produces a weld seam of high optical quality. It also provides flexibility for a high degree of automation due to the
n
Corresponding author. Tel.: þ1 613 967 6542, fax: þ 1 613 542 9489. E-mail addresses: [email protected], [email protected] (E. Azhikannickal), [email protected] (P.J. Bates), [email protected] (G. Zak). 1 Tel.: 613 541 6000 ext 6371 fax: 613 542 8612. 2 Tel.: 613 533 2567, fax: 613 533 6489. 3 Abbreviations: LTW—laser transmission welding, CB—carbon black, IR—infrared, NPFD—normalized power flux distribution 0030-3992/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlastec.2011.12.018
various beam delivery options available. Laser transmission welding is being used for the joining of plastic housings and containers [1–3]. It is also being examined for its use in the medical device industry [4]. When laser light strikes the transparent part, all of this light is not transmitted to the laser- absorbent part. Reflection at the surface of the transparent part and back reflection due to scattering of the light through the bulk (of the transparent part) attenuate the light reaching the interface between the two parts. In addition, absorption of some of the light as it travels through the transparent part also decreases the amount of energy available for creating a weld at the interface. Semi-crystalline polymers tend to scatter light to a larger extent than amorphous polymers due to the co-existence of the amorphous and crystalline phases. Furthermore, if reinforcing agents such as glass fibers are incorporated into the transparent material, significantly more scattering takes place as observed by Kagan et al. [5]. This increased scattering may also result in greater back reflection. The law of reflection neglects any surface roughness associated with the incident surface. The Fresnel relation can be used to determine the magnitude of this specular reflected light. For rough surfaces, the reflected light from the surface will be diffuse in nature and may emerge from the transparent part along various directions. For the specific case of laser welding, laser light may also be back-reflected from the bulk transparent
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material. The magnitude and distribution of reflected light will depend on the laser angle of incidence, the optical properties and thickness of the transparent part, its surface quality as well as the magnitude of the incident laser power [6,7]. A variety of industrial applications may involve the laser striking the transparent part at a non-zero angle of incidence due to the geometry of the part. The joining of tubes to plates in the manufacture of plastic heat exchangers is one example. Welding space restrictions may be another reason for use of a non-zero laser incidence angle. The Fresnel equation can be used to predict the specular surface reflection as a function of angle of incidence and shows that it increases with angle. This indicates that reflection characterization, both magnitude and distribution becomes more important at non-zero laser incidence angles where reflected energy has the potential to unnecessarily heat and/or damage features of a part in the vicinity of the area to be welded. Rhew et. al [8] used a power meter attached to a circular rotating rail to measure the transmission and reflection of laser light striking polycarbonate and high density polyethylene as a function of incidence angle and material thickness. The position of the power meter was changed with rotation of the optical rail to measure the transmittance and specular reflectance at various incident angles. It was found that for both materials, reflectance did not depend on thickness, but did increase with increasing laser incidence angle. The effect of laser incidence angle on the power distribution of the reflected light was not reported. A few studies have focussed on the characterization of the magnitude and distribution of reflected power. A device designed to integrate the total light reflection from a surface was reported by Mehmetli et al. [9]. The device was used to measure the reflection from a CO2 laser beam striking an aluminum alloy. The system consisted of a semicircular arc, pivoted at its ends, allowing it to sweep through the surface of a hemisphere. An infrared detector, which could be positioned at incremental locations along the arc, allowed the detector to reach any position on the surface of the hemisphere for subsequent reflection measurements. Van de Ven and Erdman [7] used a similar technique to simultaneously measure the light transmission and reflection from polyvinyl chloride having various surface finishes using a diode laser source. In this case, reflection measurements were taken at specified spatial increments by a photodiode that rotated around the sample along a semi-circle. The main drawback to this set of techniques is that they can involve somewhat more complicated set-ups. Use of infrared (IR) thermography appears to be another potential option for characterizing reflected light. Haberstroh et al. [10] used an IR camera to examine the laser entrance and exit surfaces of various polyamide plate specimens exposed to a short laser pulse. The authors examined the temperature distribution of the areas heated by the laser as well as the dimensions of these areas. They used this data as a measure of the laser light scattering behavior of the polyamide material. Determination of the magnitude and/or distribution of laser power incident on the top surface or transmitted at the exit surface from the thermal images were not considered. Haferkamp et al. [11] utilized IR and visible light cameras to characterize various polymers suitable for use in the LTW process. A CCD camera was used to observe the exit surface of laser-transparent plates made from PBT and corresponding to thicknesses ranging from 0.5 to 2.0 mm. Using a 2 mm incoming beam spot size, a linear relationship was observed between the exiting beam radius and the material thickness. Mayboudi [12] used thermal imaging observations during the welding of a transparent and absorbent part using a stationary laser beam. A laser power of 1 W was applied for 10 seconds at a
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distance of 3 mm from the front surface of the sample. The front surface, being viewed by the camera, was also coated with soot particles allowing energy from the stationary laser beam to be absorbed at this surface. The resulting temperature rise was captured by the infrared camera. The power flux at this surface was extracted from the temperature distribution (assuming the body to be a semi-infinite solid) and used as an indication of how the laser beam was scattered over this surface. Currently, there are no studies using infrared thermal imaging for characterizing the magnitude and distribution of reflected light. The present study extends the work of Mayboudi. Carbon black filled thermoplastic plates (in this case nylon 6) were used as ‘‘absorbers’’ of reflected laser energy. Due to the high carbon black content of the plates, the reflected energy was assumed to be absorbed very close to the surface after which it would be conducted through the thickness. Based on these heat transfer characteristics, the plate was modeled as a semi-infinite solid. The temperature rise on the surface of the plate due to the reflected energy was be captured by an infrared camera. The thermal imaging data combined with the semi-infinite solid model was used to calculate the magnitude and distribution of reflected power. In order to validate this technique, a series of experiments in which a carbon black plate was exposed to a direct laser beam were conducted. The thermal imaging data from these tests along with the semi-infinite solid model were used to estimate the magnitude and distribution of the input laser beam power. This data was then compared to the input laser power and the experimentally determined power distribution of the beam. The validated technique enabled its use in reflection characterization of unreinforced nylon 6.
2. Technique validation 2.1. Experimental set-up, materials and equipment A nylon 6 plate containing carbon black (further referred to as the CB plate) was exposed to a stationary laser beam with the laser incidence angle being 501. A non-zero laser incidence angle was required to view the area of the CB plate exposed to the laser beam with the IR camera, while maintaining the camera perpendicular to the viewing area. Due to the relatively high CB level, it was assumed that the laser power reaching the surface of the CB plate would be absorbed very close to its surface and result in a temperature rise over the exposed area [13]. The temperature distribution was measured via an IR camera positioned normal to the CB plate at a distance of approximately 6.6 cm (Fig. 1). This distance resulted in optimal focus of the viewing area. The CB plates used were made from unreinforced nylon 6 (provided by DSM) with 0.2% and 0.05% by mass carbon black, which acted as the laser-absorbent. The materials were injection molded on a 55-ton Engel injection molder into square plaques with dimensions 100 mm 100 mm and with a thickness of 3.2 mm. The same sized plates were also molded from unreinforced nylon 6 (provided by DSM) for the reflection experiments outlined in Section 3.0. The laser used in this study was a Rofin Sinar 160 W continuous-wave diode laser having a nominal focal spot size of 1.4 0.7 mm and a wavelength of 940 nm [14]. Powers were varied from 1.3 to 2.9 W and the working distances were varied from 82.5 mm to 98.5 mm. The working distance is defined as the distance between the CB plate and the laser lens (Fig. 1). A Thermovision A40 IR camera from FLIR SYSTEMS was used to obtain the temperature distribution across the surface of the CB plate as it was heated by the laser beam. The camera had a spectral range of 7.5 mm to 13 mm and images were captured at a
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Laser beam
3.2 mm thick nylon 6 plate with carbon black (CB plate)
50° laser incidence angle
Infrared camera
Working distance
j
i
Fig. 2. Thermal image resulting from laser striking a 0.2% CB plate with an input power of 1.3 W, at a laser incidence angle of 501, at a working distance of 86.5 mm and at a time of 8 seconds.
j
6.6 cm
z i Fig. 1. Schematic of experiment involving thermal imaging of laser beam.
frequency of 30 Hz. The field of view of the camera was a 25 mm (i direction) 33 mm (j direction) rectangle (refer to Fig. 1). There were 240 pixels in the field of view along the i direction and 320 pixels in the field of view along the j direction. Dividing the field of view dimensions by the number of pixels gave the spatial resolution of the image as 0.103 mm/pixel along the j direction and 0.104 mm/pixel along the i direction. The camera was connected to a portable computer using a FireWire interface. The thermal images were captured and the temperatures extracted from these images using ThermaCAMTM Researcher Pro 2.9 software by FLIR SYSTEMS. The temperature distribution is the result of measurements of the infrared radiation emitted by the object of interest, the radiation from the surroundings reflected via the object and the radiation contribution from the atmosphere itself. Therefore, the radiation measured by the camera depends on the temperature and surface emissivity of the object as well as the temperature of the surroundings. The reader is directed to [15] for further details regarding the determination of object temperature from measurement of the total radiation received by the camera. Note here that an emissivity of 0.95 was used for the CB plates used in this study [12]. Fig. 2 shows a typical thermal image, in this case, resulting from the laser striking a 0.2% by mass CB plate with an input power of 1.3 W, at a working distance of 86.5 mm and after an exposure time of 8 s. Temperature readings were extracted from each thermal image (corresponding to a given time) every 1 pixel along the i and j directions. Therefore, the temperature corresponding to the i, jth pixel position at time t is given by T (i, j, t). This is the temperature at the center of an area, A, (equal to 0.103 mm along i direction by 0.104 mm along j direction) and assumed to be constant over this area. The extracted temperature readings from each image at a given time step were saved into separate MATLAB data files. 2.2. Estimation of laser input power In order to predict the laser input power at a given time from the temperature distribution corresponding to that time, the CB plate was treated as a semi-infinite solid. The semi-infinite solid model assumes that, for a constant surface power flux,
q0 (Watt/mm2), all of the power is absorbed at the surface followed by transient, one-dimensional conduction. A closed form solution for this surface condition is given by [16]: 2 2q0 ðat=pÞ1=2 z q z z ð1Þ exp 0 erf c pffiffiffiffiffi Tðz,tÞT 0 ¼ k k 4at 2 at where T(z, t) is the temperature (1C) at a distance z from the surface (Fig. 1) at time t, T0 is the initial temperature of the solid (1C), a is the thermal diffusivity (mm2/s) and k is the thermal conductivity (Watt/mm 1C). It is important to note that the semiinfinite solid model does not account for the effects of lateral heat conduction, natural convection from the surface, absorption of energy over a depth or latent heat. Eq. (1) can be re-arranged and used to predict the laser input power striking the surface (z ¼0) of the CB plate. Firstly, q0 is calculated for each pixel area A of a thermal image at time, t, (knowing T(i, j, t)) and subsequently given by q(i, j, t) such that: qði,j,tÞ ¼
k½Tði,j,tÞT 0 2ðat=pÞ0:5
ð2Þ
The temperature dependent heat transfer parameters (i.e. k and a) in Eq. (2) were determined previously [13] and used in this study. The temperature at which these parameters were evaluated was taken as an average between the surface temperature over a given area A and the initial temperature of the plate prior to laser exposure. The estimated laser input power at a given time, Pt, is then given by: Pt ¼
240 X 320 X
qði,j,tÞA
ð3Þ
i¼1j¼1
This estimated power, Pt, can be compared to the actual laser input power.
2.3. Estimation of laser input power distribution In addition to estimation of the laser input power, laser power distribution can also be estimated using the thermal imaging data. The power distribution estimation was made along the i direction of the thermal image (refer to Fig. 1). The estimated power distribution can be compared with that measured experimentally using a knife-edge based beam profile characterization of the laser beam itself along the same path.
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Theoretically, having calculated q (i, j, t), the normalized power flux distribution along the i direction NPFD (i) can be defined as: NPFDðiÞ ¼
Li
P320
j¼1
Pt
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3. Technique validation results 3.1. Comparison of estimated and actual laser input power
qði,j,tÞ
for i ¼ 1 to 240
ð4Þ
Note here that Li in Eq. (4) refers to the length of the pixel area A (along j) and is given by 0.104 mm. The actual laser beam power distribution can be determined using a number of different scanning techniques. These involve moving a small opening such as a pinhole or a knife edge under the beam while measuring the total beam power passing through the opening using an optical power meter [18]. The knife edge approach can be used to characterize the 1-D NPFD of the laser beam cross-section along a given direction. This approach involves incrementally moving a sharp linear edge underneath a laser beam and recording the power using a power meter. For example, the knife edge can be moved along the x direction (Fig. 3) in Dx steps from the point where the laser beam is completely covered to the point where it is completely exposed. The power reading increases from zero to full power as the knife edge is translated along the x direction. Let Px be the power reading increase between knife-edge positions xa and xa þ Dx such that the normalized power flux distribution along x, NPFD (x), is given by [17]:
Figs. 4 and 5 show the estimated laser input power as a function of time compared with the actual laser input powers of 1.3 and 1.75 W, respectively at a working distance of 86.5 mm. In spite of the model limitations (no lateral conduction, no latent heat affect and no laser energy absorption in the bulk), the estimations are in relatively good agreement with the actual laser input powers (i.e. less than 10% error). The slight underestimation of the estimated power may be partially attributed to the fact that the power is absorbed over a depth as opposed to purely at the surface as assumed by the semi-infinite solid model. With depth absorption occurring, a lower temperature is registered on the top surface of the CB plate, thereby resulting in a lower estimated power. The underestimation of the predicted power can be also be attributed to losses at the surface due to natural convection, reflection of a portion of the laser light after it strikes the CB plate
1.4 1.2
ð5Þ
where Pt is the laser input power. For the purposes of this study, the NPFD for the laser beam along the i direction and at a given working distance was needed to compare to the corresponding estimated distribution at the same working distance. A slight modification to the standard knife edge approach described above was required since the profile was needed along an incline. Fig. 3 shows the experimental set-up for the adapted test. In this case the power meter was moved incrementally along the positive x direction (Fig. 3) and the laser beam moved incrementally downward along the negative y direction (Fig. 3) so that power measurements along the i direction were obtained. A Coherent LD30 30 W power sensor was used for the power measurements. The knife edge test was conducted at a working distance of 82.5 mm and 86.5 mm.
Power (Watt)
1 Px NPFDðxÞ ¼ DxPt
0.8 0.6
Actual laser power - 1.3 Watt IR estimation test 1 IR estimation test 2 (repeat)
0.4 0.2 0 0
1
2
3
4
5
6
7
8
9
Time (seconds) Fig. 4. Estimated laser input power for laser striking 0.2% CB plate with an input power of 1.3 W, at a laser incidence angle of 501 and at a working distance of 86.5 mm.
Laser beam
1.8 1.6 1.4
Working distance
x 130 Knife edge
Power (Watt)
1.2
y
1 0.8 0.6 Actual laser power - 1.75 Watt IR estimation test 1 IR estimation test 2 (repeat)
0.4
i 0.2 0 0
Power meter Fig. 3. Experimental set-up for characterizing power distribution of laser beam along the i direction using a knife edge based beam profiling technique.
1
2 Time (seconds)
3
4
Fig. 5. Estimated laser input power for laser striking 0.2% CB plate with an input power of 1.75 W, at a laser incidence angle of 501 and at a working distance of 86.5 mm.
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surface as well as lateral conduction effects. As mentioned previously, these effects are not accounted for by the semi-infinite solid model. Fig. 6 shows the estimated laser input power with time for an actual laser input power of 2.9 W at a working distance of 98.5 mm. A larger working distance was used in this case to avoid melting of the CB plate resulting from the higher input power. Fig. 6 indicates that the estimated curve stabilizes to a power that is within 10% of the actual input power of 2.9 W. Some variability of the actual laser input power from test to test may be one reason for the slightly higher estimated power compared with the actual. Fig. 7 shows the effect of carbon black level on the estimated laser input power for an actual input power of 1.3 W and a working distance of 86.5 mm. The estimated input power using the 0.05% CB plate is lower than that determined using the 0.2% CB plate. Since the 0.05% CB plate absorbs power over a larger depth compared with the 0.2% CB plate, a lower temperature is registered on the top surface of the former plate. This results in a lower
3.6
predicted power via the semi-infinite solid model, which assumes that all of the power is absorbed at the surface. Therefore, better predictions will be achieved with plates containing higher carbon black levels. It is important to note, however, that a balance must be struck between carbon black content and the onset of melting of the CB plates. For a given input power, higher carbon black levels will cause the polymer to melt earlier. Although this was an issue with these calibration experiments, it was not a problem for the reflection experiments, in which the temperatures resulting from reflected light were significantly lower. 3.2. Comparison of estimated and actual laser input power distribution Figs. 8 and 9 compare the estimated and knife-edge based NPFD along i for an input laser power of 1.3 W and for working distances of 82.5 mm and 86.5 mm, respectively. Note here that the IR estimation was obtained using the thermal image resulting from the laser striking a 0.2% CB plate at a time of 0.034 s. This corresponds to the first image captured once the laser struck the
0.6
3.2 2.8
IR estimation Knife-edge result
0.5 NPFD along i (1 / mm)
Power (Watt)
2.4 2 1.6 1.2 Actual laser power - 2.9 Watt IR estimation test 1 IR estimation test 2 (repeat)
0.8 0.4
0.4
0.3
0.2
0.1
0 0
1
2
3
4 5 Time (seconds)
6
7
8
9 0 -4
Fig. 6. Estimated laser input power for laser striking 0.2% CB plate with an input power of 2.9 W, at a laser incidence angle of 501 and at a working distance of 98.5 mm.
-3
-2
-1 0 1 Position along i (mm)
2
3
4
Fig. 8. Comparison of IR and knife edge based NPFD along i corresponding to an input laser power of 1.3 W and a working distance of 82.5 mm. The IR estimation was obtained using a 0.2% CB plate exposed to the laser at a time of 0.034 s.
1.4 0.45
IR estimation Knife-edge result
1.2 0.4 0.35 NPFD along i (1/mm)
Power (Watt)
1 0.8 0.6 Actual laser power - 1.3 Watt IR estimation (0.05% CB plate) test 1 IR estimation (0.05% CB plate) test 2 (repeat) IR estimation (0.05% CB plate) test 3 (repeat) IR estimation (0.2% CB plate) test 1 IR estimation (0.2% CB plate) test 2 (repeat)
0.4 0.2
0.3 0.25 0.2 0.15 0.1 0.05 0
0 0
1
2
3
4 5 Time (seconds)
6
7
8
9
Fig. 7. Effect of carbon black level of nylon 6 plate on estimation of laser input power for laser striking plate with an actual input power of 1.3 W, at a laser incidence angle of 501 and at a working distance of 86.5 mm.
-6
-5
-4
-3
-2 -1 0 1 2 Position along i (mm)
3
4
5
6
Fig. 9. Comparison of IR and knife edge based NPFD along i corresponding to an input laser power of 1.3 W and a working distance of 86.5 mm. The IR estimation was obtained using a 0.2% CB plate exposed to the laser at a time of 0.034 s.
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CB plate. Figs. 8 and 9 show relatively good agreement between the IR estimation and that obtained using the knife edge method.
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1.6
The use of thermal imaging to extract power from temperature data was then used to characterize laser reflection. The experimental set-up for the laser reflection studies is shown in Fig. 10. As indicated in this figure, the laser beam strikes a 3.2 mm thick unreinforced and unpigmented nylon 6 plate at a laser incidence angle of 301. The reflected light is absorbed by the 0.2% CB plate located vertically behind the laser beam as shown in Fig. 10. The IR camera is positioned perpendicular to the CB plate at a distance of 6.6 cm from its surface (Fig. 10). A laser power of 15, 20 and 30 W was applied for 3 s at a working distance of 82.5 mm. Fig. 11 shows the estimated reflected power for laser input laser powers of 11, 15 and 23 W based on the methodology described in Section 2.2. For all three cases, the estimated reflected power appears to stabilize to a given value. This is consistent with the fact that reflected power should not vary significantly with time when the input power is constant. However, it is important to point out that the reflected power stabilizes to a nearly constant value after approximately 1 s. This ‘‘stabilization’’ time required for the reflected light can be related to the time needed for the actual laser input power to stabilize. Dividing the stabilized value of the reflection (taken at 1 s) by the corresponding input power, one would obtain the reflectance, which is expected to be independent of input power. The reflectance obtained at the 11, 15 and 23 input powers were 0.06, 0.05 and 0.05, respectively. One can conclude that the average and standard deviation based on these three reflectance values is 0.053 and 0.0058, respectively. These values also provide some indication of the error, which can be applied to the overall technique. As a baseline reference, the estimated reflectance can also be compared to the specular reflectance calculated via the
1.2 1 0.8 0.6 0.4 IR estimation for laser input power of 11 Watt IR estimation for laser input power of 15 Watt IR estimation for laser input power of 23 Watt
0.2 0 0
0.5
1
1.5 Time (seconds)
2
2.5
3
Fig. 11. Estimation of reflected power for laser striking nylon 6 plate with an input power of 11, 15 and 23 W, at a laser incidence angle of 301 and at a working distance of 82.5 mm. The reflected light was absorbed by a 0.2% CB plate.
0.06 IR estimation 0.05 NPFD alongy (1/mm)
4. Reflection characterization results
Reflected Power (Watt)
1.4
0.04
0.03 0.02 0.01
Laser beam
0 -15
y
-10
-5 0 5 Position along y (mm)
10
15
Fig. 12. IR estimation of NPFD along y of reflected light resulting from laser striking nylon 6 plate with an input power of 23 W, at a laser incidence angle of 301, a working distance of 82.5 mmand at a time of 0.47 s. The IR estimation was obtained using a 0.2% CB plate, which was the absorber of the reflected light.
x j 30° IR camera 0.2% CB plate
Nylon 6 plate
Fig. 10. Experimental set-up for reflection studies.
Fresnel relation. Assuming an index of refraction for nylon 6 of 1.53 and taking into account the p-polarization state of the laser, the Fresnel reflectance is approximately 0.028. The higher reflectance estimated via the thermal imaging technique can be attributed to a number of factors including added reflection originating from the bulk and/or bottom surface of the nylon 6 plate as well as increases in reflection resulting from surface roughness effects. The Fresnel reflectance only accounts for ideal specular surface reflectance and, therefore, is expected to be lower than that obtained with plates having a finite thickness and some degree of surface roughness. Fig. 12 shows the estimated NPFD along the y direction (refer to Fig. 10) of the reflected light resulting from the laser striking the nylon 6 plate with an input power of 23 W and corresponding to a time of 0.47 s. Fig. 12 shows that the reflected power is distributed over a minimum 25 mm distance based on the tail ends of the estimated curve. This value can be compared with the approximately 6 mm distance over, which the input power is
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distributed along the i direction (refer to Fig. 8) at the greater incidence angle of 501. Since the reflected light diverges after it strikes the nylon 6 sample, the reflected power is distributed over a significantly larger distance compared with the input beam. In addition, if a portion of the reflected light is diffuse in nature, increased divergence is expected compared with the pure specular reflection case.
5. Conclusions This study presented and validated a technique for the use of thermal imaging combined with heat transfer modeling to estimate the magnitude and distribution of laser light reflection from nylon 6. The technique validation involved using infrared thermal imaging to characterize the temperature distribution resulting from a laser striking a carbon black (CB) filled nylon 6 plate. Assuming the CB plate could be modeled as a semi-infinite solid, all of the laser energy was assumed to be absorbed at its surface and subsequently conducted through its thickness. A methodology for extraction of the magnitude and distribution of the laser input power from the temperature distribution was developed. Good correlation was evident between the estimated laser input power and the actual laser input power. In addition, good agreement existed between the estimated normalized power flux distribution and that determined from a knife edge based beam profiling test of the laser beam. The technique was successfully adapted for use in characterizing the magnitude and distribution of reflected power from 3.2 mm thick nylon 6 plates. The technique can now be used to examine laser light reflection from a variety of materials including glass fiber reinforced composites.
Acknowledgments The authors would like to thank the AUTO21 Network of Centers of Excellence for financial support for this work and MAHLE Canada for their technical input.
References [1] Kagan V., Innovations in laser welding of thermoplastics. This advanced technology is ready to be commercialized. In: Proceedings of the SAE 2003 World Congress Welding and Joining. Detroit; 2003. p. 1–20. [2] McGrath G, Cawley B. Review of market for laser welding of plastics. Joining Plastics 2007;2:175–9. [3] Chen JW, Zybko J., Laser assembly technology for planar microfluidic devices. In: Proceedings of the 60th ANTEC—Annual Technical Conference, San Francisco; 2002. p. 1153–1158. [4] Amanat N, Chaminade C, Grace J, McKenzie DR, James NL. Transmission laser welding of amorphous and semi-crystalline poly-ether-ether-ketone for applications in the medical device industry. Materials & design 2010;31(10): 4823–30. [5] Kagan V, Bray RG, Kuhn WP. Laser transmission welding of semi-crystalline thermoplastics. Part I. Optical characterization of nylon-based plastics. Journal of Reinforced Plastics and Composites 2002;21(12):1101–22. [6] Aden M, Roesner A, Olowinsky A. Optical characterization of polycarbonate: Influence of additives on optical properties. Journal of Polymer Science Polymer Physics 2010;48(4):451–5. [7] Van de Ven JD, Erdman AG. Simultaneous measurement of laser reflection and transmission of poly(vinyl chloride). Optical Engineering 2006;45(9). 94301-1-6. [8] Rhew M, Mokhtarzadeh A, Benatar A., Diode laser characterization and measurement of optical properties of polycarbonate and high-density polyethylene. In: Proceedings of the 61st ANTEC—Annual Technical Conference, Nashville; 2003 p. 1056–1060. [9] Mehmetli BA, Takahashi K, Sato S. Direct measurement of reflectance from aluminum alloys during CO2 laser welding. Applied optics 1996;35(18): 3237–42. [10] Haberstroh E, Schulz J, Luetzeler R., Thermographic characterization of polymers for the laser transmission welding, In: Proceedings of the 60th ANTEC—Annual Technical Conference, San Francisco; 2002. p. 550–555. [11] Haferkamp H, Von Busse A, Hustedt M. Utilisation of a thermographic process in order to determine the laser weldability of plastics at different wavelengths. Welding and Cutting 2004;3(1):43–9. [12] Mayboudi LS., Heat transfer modelling and thermal imaging experiments in laser transmission welding of thermoplastics. PhD thesis. Queen’s University, Kingston, Ontario, Canada; 2008. [13] Chen M., Gap bridging in laser transmission welding of thermoplastics. PhD thesis. Queen’s University, Kingston, Ontario, Canada; 2009. [14] Operating manual for the Rofin Sinar DLx16 industrial diode laser; 2003. [15] Operating manual for the FLIR Systems Thermovision A40 camera. [16] Incropera FP. Fundamentals of heat and mass transfer.3rd ed. New York: John Wiley and Sons; 1990. [17] Mayboudi LS, Chen M, Zak G, Birk AM, Bates PJ, Characterization of beam profile for high power diode lasers with application to laser welding of polymers. In: Proceedings of the 64th ANTEC—Annual Technical Conference, Charlotte; 2006. p. 2274–2278. [18] Plass W, Maestle R. K. Witting, A. Voss, A. Giesen, High-resolution knife-edge laser beam profiling. Optics communications 1997;134:21–4.
Contents lists available at SciVerse ScienceDirect
Optics & Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Thermal imaging technique to characterize laser light reflection from thermoplastics Elizabeth Azhikannickal a,n, Philip J. Bates a,1, Gene Zak b,2 a b
Department of Chemistry and Chemical Engineering, Royal Military College of Canada, P.O. Box 17000, Station Forces, Kingston, Ontario, Canada K7K 7B4 Department of Mechanical and Materials Engineering, McLaughlin Hall, Queen’s University, Kingston, Ontario, Canada K7L 3N6
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 June 2011 Received in revised form 9 December 2011 Accepted 12 December 2011 Available online 10 January 2012
Characterization of laser light reflection during the laser transmission welding (LTW)3 of thermoplastics is especially important for applications in which non-zero laser incidence angles are used. At higher laser incidence angles, reflection increases and has the potential to burn surrounding features of the part to be welded. This study presents and validates a technique for laser reflection measurement. Reflected energy is absorbed by a black plastic plate (containing carbon black, which is the absorber of the reflected energy). The surface temperature of the plate is measured by an infrared (IR) camera. The distribution of reflected power required to generate this temperature profile is estimated using a simple heat transfer model. The technique was validated by irradiating the black plate by the laser directly, while observing the time-varying temperature distribution of the plate by the IR camera. In this case, good agreement was observed between the estimated total power and the actual laser input power. Good agreement also existed between the estimated power distribution and that determined experimentally via a knife edge based beam profiling technique. The thermal imaging technique was subsequently used to measure the magnitude and distribution of laser light reflection from unreinforced nylon 6. & 2012 Elsevier Ltd. All rights reserved.
Keywords: Laser reflection Thermal imaging Nylon 6
1. Introduction Laser transmission welding (LTW) is increasingly becoming an attractive process for joining thermoplastic parts. LTW involves joining a laser-transparent and a laser-absorbent part together. A laser beam passes through the laser-transparent part and is absorbed by the laser-absorbent part near the interface between the two parts. The laser energy absorbed by the laser-absorbent part is transferred back to the laser-transparent part by heat conduction. Both parts become molten and a solid joint is formed by diffusion of the polymer molecules. The primary advantage of LTW compared to other welding processes is the precise and controllable energy deposition resulting in localized melting over a small region within the contact zone of the two parts. The process produces a weld seam of high optical quality. It also provides flexibility for a high degree of automation due to the
n
Corresponding author. Tel.: þ1 613 967 6542, fax: þ 1 613 542 9489. E-mail addresses: [email protected], [email protected] (E. Azhikannickal), [email protected] (P.J. Bates), [email protected] (G. Zak). 1 Tel.: 613 541 6000 ext 6371 fax: 613 542 8612. 2 Tel.: 613 533 2567, fax: 613 533 6489. 3 Abbreviations: LTW—laser transmission welding, CB—carbon black, IR—infrared, NPFD—normalized power flux distribution 0030-3992/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlastec.2011.12.018
various beam delivery options available. Laser transmission welding is being used for the joining of plastic housings and containers [1–3]. It is also being examined for its use in the medical device industry [4]. When laser light strikes the transparent part, all of this light is not transmitted to the laser- absorbent part. Reflection at the surface of the transparent part and back reflection due to scattering of the light through the bulk (of the transparent part) attenuate the light reaching the interface between the two parts. In addition, absorption of some of the light as it travels through the transparent part also decreases the amount of energy available for creating a weld at the interface. Semi-crystalline polymers tend to scatter light to a larger extent than amorphous polymers due to the co-existence of the amorphous and crystalline phases. Furthermore, if reinforcing agents such as glass fibers are incorporated into the transparent material, significantly more scattering takes place as observed by Kagan et al. [5]. This increased scattering may also result in greater back reflection. The law of reflection neglects any surface roughness associated with the incident surface. The Fresnel relation can be used to determine the magnitude of this specular reflected light. For rough surfaces, the reflected light from the surface will be diffuse in nature and may emerge from the transparent part along various directions. For the specific case of laser welding, laser light may also be back-reflected from the bulk transparent
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material. The magnitude and distribution of reflected light will depend on the laser angle of incidence, the optical properties and thickness of the transparent part, its surface quality as well as the magnitude of the incident laser power [6,7]. A variety of industrial applications may involve the laser striking the transparent part at a non-zero angle of incidence due to the geometry of the part. The joining of tubes to plates in the manufacture of plastic heat exchangers is one example. Welding space restrictions may be another reason for use of a non-zero laser incidence angle. The Fresnel equation can be used to predict the specular surface reflection as a function of angle of incidence and shows that it increases with angle. This indicates that reflection characterization, both magnitude and distribution becomes more important at non-zero laser incidence angles where reflected energy has the potential to unnecessarily heat and/or damage features of a part in the vicinity of the area to be welded. Rhew et. al [8] used a power meter attached to a circular rotating rail to measure the transmission and reflection of laser light striking polycarbonate and high density polyethylene as a function of incidence angle and material thickness. The position of the power meter was changed with rotation of the optical rail to measure the transmittance and specular reflectance at various incident angles. It was found that for both materials, reflectance did not depend on thickness, but did increase with increasing laser incidence angle. The effect of laser incidence angle on the power distribution of the reflected light was not reported. A few studies have focussed on the characterization of the magnitude and distribution of reflected power. A device designed to integrate the total light reflection from a surface was reported by Mehmetli et al. [9]. The device was used to measure the reflection from a CO2 laser beam striking an aluminum alloy. The system consisted of a semicircular arc, pivoted at its ends, allowing it to sweep through the surface of a hemisphere. An infrared detector, which could be positioned at incremental locations along the arc, allowed the detector to reach any position on the surface of the hemisphere for subsequent reflection measurements. Van de Ven and Erdman [7] used a similar technique to simultaneously measure the light transmission and reflection from polyvinyl chloride having various surface finishes using a diode laser source. In this case, reflection measurements were taken at specified spatial increments by a photodiode that rotated around the sample along a semi-circle. The main drawback to this set of techniques is that they can involve somewhat more complicated set-ups. Use of infrared (IR) thermography appears to be another potential option for characterizing reflected light. Haberstroh et al. [10] used an IR camera to examine the laser entrance and exit surfaces of various polyamide plate specimens exposed to a short laser pulse. The authors examined the temperature distribution of the areas heated by the laser as well as the dimensions of these areas. They used this data as a measure of the laser light scattering behavior of the polyamide material. Determination of the magnitude and/or distribution of laser power incident on the top surface or transmitted at the exit surface from the thermal images were not considered. Haferkamp et al. [11] utilized IR and visible light cameras to characterize various polymers suitable for use in the LTW process. A CCD camera was used to observe the exit surface of laser-transparent plates made from PBT and corresponding to thicknesses ranging from 0.5 to 2.0 mm. Using a 2 mm incoming beam spot size, a linear relationship was observed between the exiting beam radius and the material thickness. Mayboudi [12] used thermal imaging observations during the welding of a transparent and absorbent part using a stationary laser beam. A laser power of 1 W was applied for 10 seconds at a
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distance of 3 mm from the front surface of the sample. The front surface, being viewed by the camera, was also coated with soot particles allowing energy from the stationary laser beam to be absorbed at this surface. The resulting temperature rise was captured by the infrared camera. The power flux at this surface was extracted from the temperature distribution (assuming the body to be a semi-infinite solid) and used as an indication of how the laser beam was scattered over this surface. Currently, there are no studies using infrared thermal imaging for characterizing the magnitude and distribution of reflected light. The present study extends the work of Mayboudi. Carbon black filled thermoplastic plates (in this case nylon 6) were used as ‘‘absorbers’’ of reflected laser energy. Due to the high carbon black content of the plates, the reflected energy was assumed to be absorbed very close to the surface after which it would be conducted through the thickness. Based on these heat transfer characteristics, the plate was modeled as a semi-infinite solid. The temperature rise on the surface of the plate due to the reflected energy was be captured by an infrared camera. The thermal imaging data combined with the semi-infinite solid model was used to calculate the magnitude and distribution of reflected power. In order to validate this technique, a series of experiments in which a carbon black plate was exposed to a direct laser beam were conducted. The thermal imaging data from these tests along with the semi-infinite solid model were used to estimate the magnitude and distribution of the input laser beam power. This data was then compared to the input laser power and the experimentally determined power distribution of the beam. The validated technique enabled its use in reflection characterization of unreinforced nylon 6.
2. Technique validation 2.1. Experimental set-up, materials and equipment A nylon 6 plate containing carbon black (further referred to as the CB plate) was exposed to a stationary laser beam with the laser incidence angle being 501. A non-zero laser incidence angle was required to view the area of the CB plate exposed to the laser beam with the IR camera, while maintaining the camera perpendicular to the viewing area. Due to the relatively high CB level, it was assumed that the laser power reaching the surface of the CB plate would be absorbed very close to its surface and result in a temperature rise over the exposed area [13]. The temperature distribution was measured via an IR camera positioned normal to the CB plate at a distance of approximately 6.6 cm (Fig. 1). This distance resulted in optimal focus of the viewing area. The CB plates used were made from unreinforced nylon 6 (provided by DSM) with 0.2% and 0.05% by mass carbon black, which acted as the laser-absorbent. The materials were injection molded on a 55-ton Engel injection molder into square plaques with dimensions 100 mm 100 mm and with a thickness of 3.2 mm. The same sized plates were also molded from unreinforced nylon 6 (provided by DSM) for the reflection experiments outlined in Section 3.0. The laser used in this study was a Rofin Sinar 160 W continuous-wave diode laser having a nominal focal spot size of 1.4 0.7 mm and a wavelength of 940 nm [14]. Powers were varied from 1.3 to 2.9 W and the working distances were varied from 82.5 mm to 98.5 mm. The working distance is defined as the distance between the CB plate and the laser lens (Fig. 1). A Thermovision A40 IR camera from FLIR SYSTEMS was used to obtain the temperature distribution across the surface of the CB plate as it was heated by the laser beam. The camera had a spectral range of 7.5 mm to 13 mm and images were captured at a
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Laser beam
3.2 mm thick nylon 6 plate with carbon black (CB plate)
50° laser incidence angle
Infrared camera
Working distance
j
i
Fig. 2. Thermal image resulting from laser striking a 0.2% CB plate with an input power of 1.3 W, at a laser incidence angle of 501, at a working distance of 86.5 mm and at a time of 8 seconds.
j
6.6 cm
z i Fig. 1. Schematic of experiment involving thermal imaging of laser beam.
frequency of 30 Hz. The field of view of the camera was a 25 mm (i direction) 33 mm (j direction) rectangle (refer to Fig. 1). There were 240 pixels in the field of view along the i direction and 320 pixels in the field of view along the j direction. Dividing the field of view dimensions by the number of pixels gave the spatial resolution of the image as 0.103 mm/pixel along the j direction and 0.104 mm/pixel along the i direction. The camera was connected to a portable computer using a FireWire interface. The thermal images were captured and the temperatures extracted from these images using ThermaCAMTM Researcher Pro 2.9 software by FLIR SYSTEMS. The temperature distribution is the result of measurements of the infrared radiation emitted by the object of interest, the radiation from the surroundings reflected via the object and the radiation contribution from the atmosphere itself. Therefore, the radiation measured by the camera depends on the temperature and surface emissivity of the object as well as the temperature of the surroundings. The reader is directed to [15] for further details regarding the determination of object temperature from measurement of the total radiation received by the camera. Note here that an emissivity of 0.95 was used for the CB plates used in this study [12]. Fig. 2 shows a typical thermal image, in this case, resulting from the laser striking a 0.2% by mass CB plate with an input power of 1.3 W, at a working distance of 86.5 mm and after an exposure time of 8 s. Temperature readings were extracted from each thermal image (corresponding to a given time) every 1 pixel along the i and j directions. Therefore, the temperature corresponding to the i, jth pixel position at time t is given by T (i, j, t). This is the temperature at the center of an area, A, (equal to 0.103 mm along i direction by 0.104 mm along j direction) and assumed to be constant over this area. The extracted temperature readings from each image at a given time step were saved into separate MATLAB data files. 2.2. Estimation of laser input power In order to predict the laser input power at a given time from the temperature distribution corresponding to that time, the CB plate was treated as a semi-infinite solid. The semi-infinite solid model assumes that, for a constant surface power flux,
q0 (Watt/mm2), all of the power is absorbed at the surface followed by transient, one-dimensional conduction. A closed form solution for this surface condition is given by [16]: 2 2q0 ðat=pÞ1=2 z q z z ð1Þ exp 0 erf c pffiffiffiffiffi Tðz,tÞT 0 ¼ k k 4at 2 at where T(z, t) is the temperature (1C) at a distance z from the surface (Fig. 1) at time t, T0 is the initial temperature of the solid (1C), a is the thermal diffusivity (mm2/s) and k is the thermal conductivity (Watt/mm 1C). It is important to note that the semiinfinite solid model does not account for the effects of lateral heat conduction, natural convection from the surface, absorption of energy over a depth or latent heat. Eq. (1) can be re-arranged and used to predict the laser input power striking the surface (z ¼0) of the CB plate. Firstly, q0 is calculated for each pixel area A of a thermal image at time, t, (knowing T(i, j, t)) and subsequently given by q(i, j, t) such that: qði,j,tÞ ¼
k½Tði,j,tÞT 0 2ðat=pÞ0:5
ð2Þ
The temperature dependent heat transfer parameters (i.e. k and a) in Eq. (2) were determined previously [13] and used in this study. The temperature at which these parameters were evaluated was taken as an average between the surface temperature over a given area A and the initial temperature of the plate prior to laser exposure. The estimated laser input power at a given time, Pt, is then given by: Pt ¼
240 X 320 X
qði,j,tÞA
ð3Þ
i¼1j¼1
This estimated power, Pt, can be compared to the actual laser input power.
2.3. Estimation of laser input power distribution In addition to estimation of the laser input power, laser power distribution can also be estimated using the thermal imaging data. The power distribution estimation was made along the i direction of the thermal image (refer to Fig. 1). The estimated power distribution can be compared with that measured experimentally using a knife-edge based beam profile characterization of the laser beam itself along the same path.
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Theoretically, having calculated q (i, j, t), the normalized power flux distribution along the i direction NPFD (i) can be defined as: NPFDðiÞ ¼
Li
P320
j¼1
Pt
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3. Technique validation results 3.1. Comparison of estimated and actual laser input power
qði,j,tÞ
for i ¼ 1 to 240
ð4Þ
Note here that Li in Eq. (4) refers to the length of the pixel area A (along j) and is given by 0.104 mm. The actual laser beam power distribution can be determined using a number of different scanning techniques. These involve moving a small opening such as a pinhole or a knife edge under the beam while measuring the total beam power passing through the opening using an optical power meter [18]. The knife edge approach can be used to characterize the 1-D NPFD of the laser beam cross-section along a given direction. This approach involves incrementally moving a sharp linear edge underneath a laser beam and recording the power using a power meter. For example, the knife edge can be moved along the x direction (Fig. 3) in Dx steps from the point where the laser beam is completely covered to the point where it is completely exposed. The power reading increases from zero to full power as the knife edge is translated along the x direction. Let Px be the power reading increase between knife-edge positions xa and xa þ Dx such that the normalized power flux distribution along x, NPFD (x), is given by [17]:
Figs. 4 and 5 show the estimated laser input power as a function of time compared with the actual laser input powers of 1.3 and 1.75 W, respectively at a working distance of 86.5 mm. In spite of the model limitations (no lateral conduction, no latent heat affect and no laser energy absorption in the bulk), the estimations are in relatively good agreement with the actual laser input powers (i.e. less than 10% error). The slight underestimation of the estimated power may be partially attributed to the fact that the power is absorbed over a depth as opposed to purely at the surface as assumed by the semi-infinite solid model. With depth absorption occurring, a lower temperature is registered on the top surface of the CB plate, thereby resulting in a lower estimated power. The underestimation of the predicted power can be also be attributed to losses at the surface due to natural convection, reflection of a portion of the laser light after it strikes the CB plate
1.4 1.2
ð5Þ
where Pt is the laser input power. For the purposes of this study, the NPFD for the laser beam along the i direction and at a given working distance was needed to compare to the corresponding estimated distribution at the same working distance. A slight modification to the standard knife edge approach described above was required since the profile was needed along an incline. Fig. 3 shows the experimental set-up for the adapted test. In this case the power meter was moved incrementally along the positive x direction (Fig. 3) and the laser beam moved incrementally downward along the negative y direction (Fig. 3) so that power measurements along the i direction were obtained. A Coherent LD30 30 W power sensor was used for the power measurements. The knife edge test was conducted at a working distance of 82.5 mm and 86.5 mm.
Power (Watt)
1 Px NPFDðxÞ ¼ DxPt
0.8 0.6
Actual laser power - 1.3 Watt IR estimation test 1 IR estimation test 2 (repeat)
0.4 0.2 0 0
1
2
3
4
5
6
7
8
9
Time (seconds) Fig. 4. Estimated laser input power for laser striking 0.2% CB plate with an input power of 1.3 W, at a laser incidence angle of 501 and at a working distance of 86.5 mm.
Laser beam
1.8 1.6 1.4
Working distance
x 130 Knife edge
Power (Watt)
1.2
y
1 0.8 0.6 Actual laser power - 1.75 Watt IR estimation test 1 IR estimation test 2 (repeat)
0.4
i 0.2 0 0
Power meter Fig. 3. Experimental set-up for characterizing power distribution of laser beam along the i direction using a knife edge based beam profiling technique.
1
2 Time (seconds)
3
4
Fig. 5. Estimated laser input power for laser striking 0.2% CB plate with an input power of 1.75 W, at a laser incidence angle of 501 and at a working distance of 86.5 mm.
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surface as well as lateral conduction effects. As mentioned previously, these effects are not accounted for by the semi-infinite solid model. Fig. 6 shows the estimated laser input power with time for an actual laser input power of 2.9 W at a working distance of 98.5 mm. A larger working distance was used in this case to avoid melting of the CB plate resulting from the higher input power. Fig. 6 indicates that the estimated curve stabilizes to a power that is within 10% of the actual input power of 2.9 W. Some variability of the actual laser input power from test to test may be one reason for the slightly higher estimated power compared with the actual. Fig. 7 shows the effect of carbon black level on the estimated laser input power for an actual input power of 1.3 W and a working distance of 86.5 mm. The estimated input power using the 0.05% CB plate is lower than that determined using the 0.2% CB plate. Since the 0.05% CB plate absorbs power over a larger depth compared with the 0.2% CB plate, a lower temperature is registered on the top surface of the former plate. This results in a lower
3.6
predicted power via the semi-infinite solid model, which assumes that all of the power is absorbed at the surface. Therefore, better predictions will be achieved with plates containing higher carbon black levels. It is important to note, however, that a balance must be struck between carbon black content and the onset of melting of the CB plates. For a given input power, higher carbon black levels will cause the polymer to melt earlier. Although this was an issue with these calibration experiments, it was not a problem for the reflection experiments, in which the temperatures resulting from reflected light were significantly lower. 3.2. Comparison of estimated and actual laser input power distribution Figs. 8 and 9 compare the estimated and knife-edge based NPFD along i for an input laser power of 1.3 W and for working distances of 82.5 mm and 86.5 mm, respectively. Note here that the IR estimation was obtained using the thermal image resulting from the laser striking a 0.2% CB plate at a time of 0.034 s. This corresponds to the first image captured once the laser struck the
0.6
3.2 2.8
IR estimation Knife-edge result
0.5 NPFD along i (1 / mm)
Power (Watt)
2.4 2 1.6 1.2 Actual laser power - 2.9 Watt IR estimation test 1 IR estimation test 2 (repeat)
0.8 0.4
0.4
0.3
0.2
0.1
0 0
1
2
3
4 5 Time (seconds)
6
7
8
9 0 -4
Fig. 6. Estimated laser input power for laser striking 0.2% CB plate with an input power of 2.9 W, at a laser incidence angle of 501 and at a working distance of 98.5 mm.
-3
-2
-1 0 1 Position along i (mm)
2
3
4
Fig. 8. Comparison of IR and knife edge based NPFD along i corresponding to an input laser power of 1.3 W and a working distance of 82.5 mm. The IR estimation was obtained using a 0.2% CB plate exposed to the laser at a time of 0.034 s.
1.4 0.45
IR estimation Knife-edge result
1.2 0.4 0.35 NPFD along i (1/mm)
Power (Watt)
1 0.8 0.6 Actual laser power - 1.3 Watt IR estimation (0.05% CB plate) test 1 IR estimation (0.05% CB plate) test 2 (repeat) IR estimation (0.05% CB plate) test 3 (repeat) IR estimation (0.2% CB plate) test 1 IR estimation (0.2% CB plate) test 2 (repeat)
0.4 0.2
0.3 0.25 0.2 0.15 0.1 0.05 0
0 0
1
2
3
4 5 Time (seconds)
6
7
8
9
Fig. 7. Effect of carbon black level of nylon 6 plate on estimation of laser input power for laser striking plate with an actual input power of 1.3 W, at a laser incidence angle of 501 and at a working distance of 86.5 mm.
-6
-5
-4
-3
-2 -1 0 1 2 Position along i (mm)
3
4
5
6
Fig. 9. Comparison of IR and knife edge based NPFD along i corresponding to an input laser power of 1.3 W and a working distance of 86.5 mm. The IR estimation was obtained using a 0.2% CB plate exposed to the laser at a time of 0.034 s.
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CB plate. Figs. 8 and 9 show relatively good agreement between the IR estimation and that obtained using the knife edge method.
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1.6
The use of thermal imaging to extract power from temperature data was then used to characterize laser reflection. The experimental set-up for the laser reflection studies is shown in Fig. 10. As indicated in this figure, the laser beam strikes a 3.2 mm thick unreinforced and unpigmented nylon 6 plate at a laser incidence angle of 301. The reflected light is absorbed by the 0.2% CB plate located vertically behind the laser beam as shown in Fig. 10. The IR camera is positioned perpendicular to the CB plate at a distance of 6.6 cm from its surface (Fig. 10). A laser power of 15, 20 and 30 W was applied for 3 s at a working distance of 82.5 mm. Fig. 11 shows the estimated reflected power for laser input laser powers of 11, 15 and 23 W based on the methodology described in Section 2.2. For all three cases, the estimated reflected power appears to stabilize to a given value. This is consistent with the fact that reflected power should not vary significantly with time when the input power is constant. However, it is important to point out that the reflected power stabilizes to a nearly constant value after approximately 1 s. This ‘‘stabilization’’ time required for the reflected light can be related to the time needed for the actual laser input power to stabilize. Dividing the stabilized value of the reflection (taken at 1 s) by the corresponding input power, one would obtain the reflectance, which is expected to be independent of input power. The reflectance obtained at the 11, 15 and 23 input powers were 0.06, 0.05 and 0.05, respectively. One can conclude that the average and standard deviation based on these three reflectance values is 0.053 and 0.0058, respectively. These values also provide some indication of the error, which can be applied to the overall technique. As a baseline reference, the estimated reflectance can also be compared to the specular reflectance calculated via the
1.2 1 0.8 0.6 0.4 IR estimation for laser input power of 11 Watt IR estimation for laser input power of 15 Watt IR estimation for laser input power of 23 Watt
0.2 0 0
0.5
1
1.5 Time (seconds)
2
2.5
3
Fig. 11. Estimation of reflected power for laser striking nylon 6 plate with an input power of 11, 15 and 23 W, at a laser incidence angle of 301 and at a working distance of 82.5 mm. The reflected light was absorbed by a 0.2% CB plate.
0.06 IR estimation 0.05 NPFD alongy (1/mm)
4. Reflection characterization results
Reflected Power (Watt)
1.4
0.04
0.03 0.02 0.01
Laser beam
0 -15
y
-10
-5 0 5 Position along y (mm)
10
15
Fig. 12. IR estimation of NPFD along y of reflected light resulting from laser striking nylon 6 plate with an input power of 23 W, at a laser incidence angle of 301, a working distance of 82.5 mmand at a time of 0.47 s. The IR estimation was obtained using a 0.2% CB plate, which was the absorber of the reflected light.
x j 30° IR camera 0.2% CB plate
Nylon 6 plate
Fig. 10. Experimental set-up for reflection studies.
Fresnel relation. Assuming an index of refraction for nylon 6 of 1.53 and taking into account the p-polarization state of the laser, the Fresnel reflectance is approximately 0.028. The higher reflectance estimated via the thermal imaging technique can be attributed to a number of factors including added reflection originating from the bulk and/or bottom surface of the nylon 6 plate as well as increases in reflection resulting from surface roughness effects. The Fresnel reflectance only accounts for ideal specular surface reflectance and, therefore, is expected to be lower than that obtained with plates having a finite thickness and some degree of surface roughness. Fig. 12 shows the estimated NPFD along the y direction (refer to Fig. 10) of the reflected light resulting from the laser striking the nylon 6 plate with an input power of 23 W and corresponding to a time of 0.47 s. Fig. 12 shows that the reflected power is distributed over a minimum 25 mm distance based on the tail ends of the estimated curve. This value can be compared with the approximately 6 mm distance over, which the input power is
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distributed along the i direction (refer to Fig. 8) at the greater incidence angle of 501. Since the reflected light diverges after it strikes the nylon 6 sample, the reflected power is distributed over a significantly larger distance compared with the input beam. In addition, if a portion of the reflected light is diffuse in nature, increased divergence is expected compared with the pure specular reflection case.
5. Conclusions This study presented and validated a technique for the use of thermal imaging combined with heat transfer modeling to estimate the magnitude and distribution of laser light reflection from nylon 6. The technique validation involved using infrared thermal imaging to characterize the temperature distribution resulting from a laser striking a carbon black (CB) filled nylon 6 plate. Assuming the CB plate could be modeled as a semi-infinite solid, all of the laser energy was assumed to be absorbed at its surface and subsequently conducted through its thickness. A methodology for extraction of the magnitude and distribution of the laser input power from the temperature distribution was developed. Good correlation was evident between the estimated laser input power and the actual laser input power. In addition, good agreement existed between the estimated normalized power flux distribution and that determined from a knife edge based beam profiling test of the laser beam. The technique was successfully adapted for use in characterizing the magnitude and distribution of reflected power from 3.2 mm thick nylon 6 plates. The technique can now be used to examine laser light reflection from a variety of materials including glass fiber reinforced composites.
Acknowledgments The authors would like to thank the AUTO21 Network of Centers of Excellence for financial support for this work and MAHLE Canada for their technical input.
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