Showing papers on "Lamb waves published in 1996"
TL;DR: In this paper, the authors discuss the selection of an appropriate mode and its excitation and reception for the use of Lamb waves for the detection of delaminations in composite materials and corrosion in pipes.
323 citations
TL;DR: In this paper, the authors extended the boundary element method to study the mode conversion phenomena of Lamb waves from a free edge and formulated the elastodynamic interior boundary value problem as a hybrid boundary integral equation in conjunction with the normal mode expansion technique based on the Lamb wave dispersion equation.
Abstract: The boundary element method, well known for bulk wave scattering, is extended to study the mode conversion phenomena of Lamb waves from a free edge. The elastodynamic interior boundary value problem is formulated as a hybrid boundary integral equation in conjunction with the normal mode expansion technique based on the Lamb wave dispersion equation. The present approach has the potential of easily handling the geometrical complexity of general guided wave scattering with improved computational efficiency due to the advantage of the boundary‐type integral method. To check the accuracy of the boundary element program, vertical shear wave diffraction, due to a circular hole, is solved and compared with previous analytical solutions. Edge reflection factors for the multibackscattered modes in a steel plate are satisfied quite well with the principle of energy conservation. In the cases of A0, A1, and S0 incidence, the variations of the multireflection factors show similar tendencies to the existing results fo...
236 citations
TL;DR: In this article, the development of a dry-coupled piezoelectric transducer system for the detection of corrosion in chemical plant pipework using cylindrical Lamb waves is described.
Abstract: The development of a dry-coupled piezoelectric transducer system for the detection of corrosion in chemical plant pipework using cylindrical Lamb waves is described. It is shown that the axisymmetricL(0,2) mode at a frequency of about 70 kHz is an attractive mode to use for longdistance propagation. The results show that a ring of piezoelectric length-expander elements can be used to excite theL(0,2) mode and to suppress all the nonaxisymmetric modes. Tests have been carried out both with the piezoelectric elements bonded to the pipe and fabricated into a simple transducer which was clamped against the pipe. The performance of the dry-coupled system was very similar to that of the bonded elements. In pulse echo tests, the noise floor obtained with the dry-coupled system was less than 1% of the amplitude of the propagatingL(0,2) mode. The drycoupled transducers provide a simple, light, readily detachable system for the long-range inspection of pipework.
235 citations
TL;DR: A finite element study of the generation of Lamb waves in plates from a finite air coupled transducer, the interaction of these waves with defects, and their detection using an air coupled receiver is described in this article, where the use of an ideal collimated beam in the model, instead of using the real pressure field generated by the transducers, is demonstrated to have negligible effect on the predicted Lamb waves.
Abstract: Air‐coupled nondestructive testing has become feasible following recent improvements in air‐coupled transducer design. However, the large acoustic impedance mismatch between air and solid materials does not allow normal incidence pulse‐echo inspection. Nevertheless, air‐coupled transducers can be used for the generation and detection of Lamb waves, the receiver being outside the field of the specular reflection. A finite element study of the generation of Lamb waves in plates from a finite air‐coupled transducer, the interaction of these waves with defects, and their detection using an air‐coupled receiver is described. These predictions are compared with experimental results obtained on a variety of specimens using a pair of 1‐3 composite, air‐coupled transducers. The use of an ideal collimated beam in the model, instead of using the real pressure field generated by the transducers, is demonstrated to have a negligible effect on the predicted Lamb waves. It is shown both theoretically and experimentally ...
217 citations
TL;DR: Martinez-Sala et al. as discussed by the authors used the plane-wave method in order to get the band structure of waves propagating in an infinite periodic system and employed, for the first time for acoustic waves, the transfer matrix method to find the transmission coefficient of waves along a system with a finite thickness.
Abstract: We study acoustic waves propagating in two-dimensional (2D) systems consisting of steel cylinders surrounded by air. We use the plane-wave method in order to get the band structure of waves propagating in an infinite periodic system. In addition, we employed, for the first time for acoustic waves, the transfer matrix method in order to find the transmission coefficient of waves propagating along a system with a finite thickness. Both methods are in good agreement with each other and their results agree with recent measurements in a similar system (R. Martinez-Sala et al., Nature, 378 (1995) 241). We also find the optimum conditions for the appearance of spectral gaps.
121 citations
TL;DR: Numerical calculations show that MT-water system supports interface elastic waves with maximal frequencies in a gigahertz range, and models MTs by thin elastic cylindrical shells derive the eigenfrequencies and eigenmodes of confined elastic vibrations in a shell-fluid system.
Abstract: We study thoreticaly vibrational properties of microtubules (MTs), which are long hollow cylindrical macromolecules with a diameter of the order of 25 nm and serve as a major component of cytoskeleton in eukariotic cells. Modeling MTs by thin elastic cylindrical shells, we derive the eigenfrequencies and eigenmodes of confined elastic vibrations in a shell-fluid system. Numerical calculations, based on recently obtained experimental data for Young's modulus of MT, show that MT-water system supports interface elastic waves with maximal frequencies in a gigahertz range. In a long-wavelength limit, there exist three axisymmetric acoustic waves with velocities of about 200 to 600 m/s, and an infinite set of helical waves with a parabolic dispersion law. \textcopyright{} 1996 The American Physical Society.
113 citations
01 Apr 1996
TL;DR: The measurement of stiffness using seismic methods relies on the propagation of elastic waves through the ground as mentioned in this paper, and weak rocks display a more linear stress-strain behaviour and hence the loss of stiffness with increasing strain is much less than for most soils.
Abstract: Weak rocks display a more linear stress-strain behaviour and hence the loss of stiffness with increasing strain is much less than for most soils. 3. The measurement of stiffness using seismic methods relies on the propagation of elastic waves through the ground. When a hammer strikes the ground, two types of elastic wave are generated body waves and surface waves. Body waves comprise compressional (or P) waves and slower shear (or S) waves, both of which propagate into the ground...
109 citations
TL;DR: In this paper, the authors derived mathematical expressions for the far-field backscattering amplitude spectrum resulting from oblique insonification of an infinite, transversely isotropic elastic cylinder by a plane acoustic wave.
Abstract: Mathematical expressions are derived for the far‐field backscattering amplitude spectrum resulting from oblique insonification of an infinite, transversely isotropic elastic cylinder by a plane acoustic wave. The normal‐mode solution is based on decoupling of the scalar potential representing the horizontally polarized shear wave from those of the compressional and vertically polarized waves. The solution degenerates to the well‐known simple model for isotropic cylinders in the case of very weak anisotropy. The solution is used to study the influence of each element of the stiffness matrix on the various resonant modes of vibration. Perturbations of the elements c33 and c44, which characterize the cylinder along the axis, significantly affect resonant frequencies corresponding to axially guided waves. Perturbations of c11 and c12, which characterize the material on the transverse plane, predominantly affect the Rayleigh and Whispering Gallery resonance frequencies. Perturbations of c13 affect all three ty...
90 citations
TL;DR: In this paper, the authors derived closed-form expressions for the transmission and reflection coefficients on a fracture by replacing the boundary conditions for a welded interface by those for a fracture into the standard procedure for plane wave analysis.
Abstract: This paper investigates the details of reflection, transmission, and conversion of plane waves incident upon a fracture at arbitrary angles. The elastic compliance of fractures that is produced by the presence of a planar collection of void spaces and asperities of contact is modeled as a displacement-discontinuity boundary condition between two elastic half-spaces. Closed-form expressions for the transmission and reflection coefficients on a fracture are derived by replacing the boundary conditions for a welded interface by those for a fracture into the standard procedure for plane wave analysis. The closed-form expressions reveal that a single fracture can produce a variety of potentially diagnostic waves such as transmitted waves, reflected waves, converted waves, head waves, and P interface waves and introduce a finite group time delay to all these waves with respect to the incident wave. The amplitude and group time delay of the fracture-induced waves are controlled by the fracture stiffness, wave frequency, and the Poisson's ratio of the medium. The head wave and inhomogeneous P interface waves are generated when an SV wave is incident upon a fracture, at and beyond a critical angle, respectively, which is determined by Snell's law. For some combinations of the fracture stiffness and the Poisson's ratio of the half-spaces, no reflection or transmission of a P wave or an SV wave occurs.
89 citations
TL;DR: In this paper, a single fracture in an elastic solid is investigated theoretically and numerically using plane wave analysis and a boundary element method, where the finite mechanical stiffness of a fracture is modeled as a displacement discontinuity.
Abstract: Interface waves on a single fracture in an elastic solid are investigated theoretically and numerically using plane wave analysis and a boundary element method. The finite mechanical stiffness of a fracture is modeled as a displacement discontinuity. Analysis for inhomogeneous plane wave propagation along a fracture yields two dispersive equations for symmetric and antisymmetric interface waves. The basic form of these equations are similar to the classic Rayleigh equation for a surface wave on a half-space, except that the displacements and velocities of the symmetric and antisymmetric fracture interface waves are each controlled by a normalized fracture stiffness. For low values of the normalized fracture stiffness, the symmetric and antisymmetric interface waves degenerate to the classic Rayleigh wave on a traction-free surface. For large values of the normalized fracture stiffness, the antisymmetric and symmetric interface waves become a body S wave and a body P wave, respectively, which propagate parallel to the fracture. For intermediate values of the normalized fracture stiffness, both interface waves are dispersive. Numerical modeling performed using a boundary element method demonstrates that a line source generates a P-type interface wave, in addition to the two Rayleigh-type interface waves. The magnitude of the normalized fracture stiffness is observed to control the velocities of the interface waves and the partitioning of seismic energy among the various waves near the fracture.
73 citations
TL;DR: In this paper, a method to construct solutions for elastic waves generated in a half-space has been developed, based on representing the wave field by a superposition of time-transient plane waves.
TL;DR: In this article, a pair of pin transducers are used to excite and detect the A 0 mode Lamb wave in the pipe wall, and the wave velocity is obtained by time-of-flight measurement.
Abstract: Ultrasonic Lamb wave techniques are widely used in a number of NDE applications. To excite Lamb waves, mode conversion of bulk waves or photoacoustic excitation often are used. Both of these approaches suffer from the need for liquid couplant or ablation of materials to reach a good signal-to-noise ratio. In this paper, we propose a novel technique that utilizes point source excitation and detection of Lamb waves through dry, elastic contacts to monitor velocity changes. A pair of pin transducers is used to excite and detect the A 0 mode Lamb wave in the pipe wall, and the wave velocity is obtained by time-of-flight measurement. Any change in the pipe wall thickness can be detected by the change in the Lamb wave velocity due to the dispersive nature of the A 0 mode. We demonstrate the power of this approach in ultrasonic pipe erosion/corrosion monitoring and its potential application in aircraft skin defect imaging. We present results of measurements of plate thickness and erosion/corrosion in a ...
TL;DR: A novel integrating ultrasonic sensor was achieved using the signal arm of an actively stabilized 633-nm homodyne Mach-Zehnder fiber interferometer which was either bonded directly to the plate surface or spliced to single-mode fibers embedded within a composite plate during manufacture.
Abstract: The effectiveness of surface-bonded and embedded optical fibers for the detection of ultrasonic Lamb waves in 2–3-mm-thick steel, carbon-fiber-reinforced plastic (CFRP) and glass-reinforced plastic (GRP) plates are compared. A novel integrating ultrasonic sensor was achieved using the signal arm of an actively stabilized 633-nm homodyne Mach–Zehnder fiber interferometer which was either bonded directly to the plate surface or spliced to single-mode fibers embedded within a composite plate during manufacture. An embedded fiber is shown to be about 20 times more sensitive to Lamb wave motions than a surface-bonded fiber. However, the latter may be more practical.
TL;DR: In this paper, surface-bonded singlemode optical fiber sensors have been used to monitor the interaction of ultrasonic Lamb waves with defects in carbon fiber composite plates using Perspex-coupled piezoelectric transducers.
Abstract: Surface-bonded single-mode optical fibre sensors have been used to monitor the interaction of ultrasonic Lamb waves with defects in carbon fibre composite plates. Lamb waves were initiated using Perspex-coupled piezoelectric transducers. The defects investigated comprised holes, regions of impact damage and delaminations. Holes could be identified by analysing direct reflections and impact damage by back-wall echo amplitude. Large delaminations gave a poor direct reflection. Evidence was found for mode conversion at centre plane delaminations.
TL;DR: In this paper, a new scanning technique using leaky Lamb waves is presented to detect internal defects in a multilayered fiber-reinforced composite plate specimen (SCS-6 fibers in Ti-6Al-4V matrix).
TL;DR: In this paper, an ultrasonic inspection technique using Lamb waves was evaluated to detect and determine the exact location of flaws present in long steel pipes, and the experimental results showed that the A 0 mode is the most effective for detection and exact determination of the location flaws.
Abstract: An ultrasonic inspection technique using Lamb waves was evaluated to detect and determine the exact location of flaws present in long steel pipes. Since multiple modes of Lamb waves are generated due to their dispersive characteristics in the inspected pipes, the selection of a specific Lamb wave mode is very important for inspection of flaws. Experimental studies of flaw detectability with the use of each Lamb wave mode, namely, the A 0 S 0 , A 1 , and S 1 modes and their ultrasonic attenuation characteristics were conducted. The experimental results showed that the A 0 mode is the most effective for detection and exact determination of the location flaws. A lucite wedge containing a water column that generates the A 0 Lamb wave mode was developed and used in the present inspection study. It was found that the ultrasonic beam divergence after wrapping around the inspected pipe once interferes with exact determination of the location of flaws and that the maximum reflection signals are obtained when the transducer is offset axially from the straight line with the position of the flaw. The present study showed feasibilities of ultrasonic inspection with the use of Lamb waves for detection of flaws in insulated or inaccessible steel pipes.
TL;DR: In this article, the authors present novel techniques to selectively excite the lowest order symmetric (So) and antisymmetric (Ao) Lamb wave modes in thin solid plates by applying shear and/or longitudinal traction at the edge or surface of the plates according to the results of a two-dimensional normal mode theory.
Abstract: We present novel techniques to selectively excite the lowest order symmetric (So) and antisymmetric (Ao) Lamb wave modes in thin solid plates. Hertzian contacts are formed between the plates and the end of specially designed quartz rods which guide extensional waves generated by PZT‐5H transducers bonded at their other end. Mode selectivity is achieved by applying shear and/or longitudinal traction at the edge or the surface of the plates according to the results of a two‐dimensional normal mode theory. In aluminum plates, mode selectivity is measured as a function of frequency for different traction forces. With normal forces, Ao mode selectivity of more than 46 dB is obtained for fd<0.4 MHz mm. With antisymmetric shear traction at the edge of the plate, a selectivity exceeding 55 dB is achieved for single mode So operation.
03 Nov 1996
TL;DR: In this paper, a time reversal mirror, made of a 32 element transducer array coupled to a Duralumin plate with a Plexiglas wedge, is applied to compensate for the effect of dispersive Lamb waves.
Abstract: In this paper, the time reversal processing is applied to compensate for the effect of dispersive Lamb waves. A time reversal mirror, made of a 32 element transducer array is coupled to a Duralumin plate with a Plexiglas wedge. In the first experiment, a Nd-YAG laser is used to create a point source of Lamb waves. The transmitted pulse is then recorded and time-reversed by the array and both time recompression and spatial refocusing are observed at the source point. In the second part of this paper, flaw detection in plate is performed with a pulse echo time reversal technique and we show the effectiveness of this technique to enhance the signal to noise ratio.
TL;DR: In this paper, the elastic wave field and the excitation mechanism of the surface waves in multilayered elastic solid media are studied in the B, P, C coordinate system and the so-called new type of propagator matrix introduced by Menke to avoid loss of the precision problem is improved.
Abstract: The elastic wave field and the excitation mechanism of the surface waves in multilayered elastic solid media are studied in this paper On the basis of Abo‐zena [Geophys J R Astron Soc 58, 91–105 (1979)] and Menke [Geophys J R Astron Soc 59, 315–323 (1979)], the elastic wave field is further investigated in the B, P, C coordinate system The so‐called new type of propagator matrix introduced by Menke to avoid loss of the precision problem is improved It presented an important result and some new properties The dispersion characteristics and excitation mechanisms of the surface waves (Rayleigh and Love waves) are also investigated via numerical simulation The excitation intensities of the surface waves strongly depend on the frequency range of the source The source frequency should be controlled in a proper range to effectively excite the surface waves Two quantities, β1 (the ratio of B to P components of displacement) and β2 (the ratio of B to P components of stress), are defined for the Rayleigh wave It is found that β1 and β2 are sensitive to the material property of the medium and the layered geometry, and they are two important physical quantities for exploring the structures of the interfaces and the velocity distributions of layers under the free surface The relative error in estimating the thickness of each medium by β1 and β2 is less than 10% The effects of the thickness of each layer of media and other factors on the dispersion characteristics of Rayleigh and Love waves and the values of β1 and β2 are also analyzed
TL;DR: In this article, the amplitude and phase of the S0 mode Lamb wave and SH0 mode plate wave generated by the EMATs were measured while changing the static field strength to the sheets.
Journal Article•
TL;DR: In this paper, the relationship between the surface response and microfracture modes in composite laminates is studied to establish the theoretical background for waveform analysis of acoustic emission signals.
Abstract: This paper is concerned with the development of acoustic emission (AE) waveform analysis in advanced structural composites. The relationship between the surface response and microfracture modes in composite laminates is studied to establish the theoretical background for waveform analysis of AE signals. Lamb waves produced by arbitrary internal sources in unidirectional and cross-ply composite laminates are investigated. Laboratory experiments are performed to validate the theoretical models. The results of this research should be useful in developing practical nondestructive testing tools to monitor damage initiation and evolution in composite structures.
TL;DR: In this paper, the authors present a theoretical study for modeling the thermoelastic excitation of transient Lamb wave propagating along the principal directions in an orthotropic plate, and provide a quantitative analysis for noncontact and non-destructive detection of the elastic stiffness properties of the machine-made paper by the laser-generated Lamb wave technique.
Abstract: This paper presents a theoretical study for modelling the thermoelastic excitation of transient Lamb waves propagating along the principal directions in an orthotropic plate. The normal mode expansion method is employed to express the transient displacement field by a summation of the antisymmetric and symmetric Rayleigh - Lamb wave modes in the surface stress-free orthotropic plate. This method is particularly suitable for waveform analyses of transient Lamb waves in thin sheet materials because one needs only to calculate contributions of the lowest few antisymmetric and symmetric modes. The dispersion characteristics and the transient Lamb waveforms excited by a pulsed laser in machine-made paper are analysed numerically and discussed in detail and attention is focused on the influence of the elastic stiffness constants. This work provides a quantitative analysis for noncontact and nondestructive detection of the elastic stiffness properties of the machine-made paper by the laser-generated Lamb wave technique.
Journal Article•
TL;DR: In this article, the S0 Lamb mode can propagate over distances of the order of 1 m in composite laminates and so has the potential to be used in long-range nondestructive inspection.
Abstract: The S0 Lamb mode can propagate over distances of the order of 1 m in composite laminates and so has the potential to be used in long‐range nondestructive inspection. This paper discusses the interaction of the S0 Lamb mode with delaminations. The dispersion curves and the corresponding stress and displacement mode shapes of the lower order Lamb modes are obtained analytically and the interaction of the S0 mode with delaminations at different interfaces in a composite laminate is then studied both by finite element analysis and by experiment. It is shown that the amplitude of the reflection of the S0 mode from a delamination is strongly dependent on the position of the delamination through the thickness of the laminate and that the delamination locations corresponding to the maximum and minimum reflectivity correspond to the locations of maximum and minimum shear stress across the interface in the S0 mode.
TL;DR: In this article, surface acoustic waves (SAW) were generated by a Q-switched YAG laser and monitored by a heterodyne laser interferometer to evaluate subsurface lateral defects, noncontact and non-destructive laser ultrasonics was employed.
Abstract: To evaluate subsurface lateral defects, non-contact and non-destructive laser ultrasonics was employed. Surface acoustic waves (SAW) were generated by a Q-switched YAG laser and monitored by a heterodyne laser interferometer. Both the source and monitoring lasers were scanned over the samples to monitor the location of defects and bond quality. Based on the fundamental works of the thin metallic foil and model specimen with an artificial subsurface slit, bond quality of the stainless foil brazed on the carbon steel was evaluated by the quantitative analysis of velocity dispersion of the Rayleigh and lowest order Lamb waves. Velocity dispersion of SAW over the well bond area well agreed with theoretical dispersion of the Rayleigh wave for multi-layer laminate, while that over the unbond area agreed with the lowest order anti-symmetric Lamb wave. Velocity dispersion of the SAW over the weak bond was calculated by inserting a slip layer which allowed displacement discontinuity across the interface. Analysis of the generalized Lamb wave was demonstrated to be promising in evaluating the bond quality as well as identifying the location of defects.
TL;DR: In this article, a simple finite-difference method is described to simulate and simulate Lamb wave modes in plane metal sheets. But the simulation is used to predict time-domain histories of field displacements vertical to the sheet at groups of points on the sheet.
Abstract: This paper describes a simple finite-difference method to simulate and Lamb wave modes in plane metal sheets. The simulation is used to predict time-domain histories of field displacements vertical to the sheet at groups of points on the sheet. The time - space data so obtained are transformed by two-dimensional FFT to yield frequency - wavenumber representations for comparison with the Lamb mode dispersion waves. An experimental rig is developed for transmission and reception of Lamb waves in sheet metal, with adjustable transducer positions on the Lamb wave axis. The rig enables two-dimensional time - space, and by transformation, frequency - wavenumber, data to be obtained experimentally. An electronic system to provide appropriate band-limited signals for single-mode Lamb wave excitation is outlined and used in conjunction with the physical rig. Experimental results show good agreement with simulation.
TL;DR: In this paper, the effect of the generation laser spot size, energy density, and total energy on the transfer of energy from the generator to the two lowest Lamb wave modes, a0 and s0, was investigated.
TL;DR: In this article, structural wave propagation along an elastic and sagging cable has been analyzed and the controlling role played by equilibrium cable curvature has been highlighted, a role not captured in previous studies of cable wave propagation that employ a simple taut string model.
01 Oct 1996
TL;DR: In this paper, the L(0, 2) mode is used for long-range (> 15 m) propagation in 2-12 inch (51-305 mm) nominal bore pipes and the behaviour of this mode has been studied using a bonded piezoelectric transducer system.
Abstract: Laboratory tests and supporting finite element predictions of the interaction of Lamb waves with typical features found in chemical plant pipework are reported. The L(0, 2) mode is an attractive mode to use for long-range (> 15 m) propagation in 2–12 inch (51–305 mm) nominal bore pipes and the behaviour of this mode has been studied using a bonded piezoelectric transducer system. The amplitude of the reflection from butt welds and welded supports was found to be between 5 and 20 per cent of the incident amplitude, while flanges gave almost 100 per cent reflection. It is therefore not possible to test past a flange. It has been shown that wet or dry insulation has little effect on the propagation of the L(0, 2) mode, so inspection under insulation is feasible. Initial work on the reflections from defects is also reported. It has been shown that when the axial extent of the defect is significantly less than a wavelength of the L(0, 2) mode (about 80 mm at 70 kHz), the reflectivity is largely a function of t...
TL;DR: In this article, the spatial and frequency dependencies of the transmitted leaky waves have been measured for a variety of composite plates including aluminum-aramid and graphite-epoxy, and the results have been employed to deduce, with high precision, the transmission coefficient of the plate over the frequency and angular bandwidths covered by the measurements.
TL;DR: In this article, an acoustic temperature sensor (ATS) is used to monitor the temperature of silicon wafers in rapid thermal processing environments from room temperature to 1000/spl deg/C with /spl plusmn/5/spl/C accuracy at 20 Hz data rate.
Abstract: Acoustic techniques are used to monitor the temperature of silicon wafers in rapid thermal processing environments from room temperature to 1000/spl deg/C with /spl plusmn/5/spl deg/C accuracy. Acoustic transducers are mounted at the bases of the quartz pins that support the silicon wafer during processing. An electrical pulse applied across the transducer generates an extensional mode acoustic wave which is guided by the quartz pins. The extensional mode is converted into Lamb waves (a guided plate mode) in the silicon wafer which acts as a plate waveguide. The Lamb wave propagates across the length of the silicon wafer and is converted back into an extensional mode at the other pin. The extensional mode acoustic wave is detected and the total time of flight is obtained. The time of flight of the extensional mode in the quartz pin is measured using pulse echo techniques and is subtracted from the total time of flight. Because the velocity of Lamb waves in the silicon wafer is systematically affected by temperature, the measurement of the time of flight of the Lamb wave provides the accurate temperature of the silicon wafer. The current implementation provides a /spl plusmn/5/spl deg/C accuracy at 20 Hz data rate. Further improvements in electronics and acoustics should enable /spl plusmn/1/spl deg/C measurements. The acoustic temperature sensor (ATS) has several advantages over conventional temperature measurement techniques. Unlike pyrometric measurements, ATS measurements are independent of emissivity of the silicon wafer and will operate down to room temperature. ATS also does not have the contact and contamination problems associated with thermocouples.