Lock-in Thermography System Research Articles

Lock-In Thermography with Cooling for the Inspection of Composite Materials.

This paper presents the development of the lock-in thermography system with an additional cooling system. System feasibility is tested by investigating a square-shaped glass fiber-reinforced polymer (GFRP) with artificially made outer flaws. The influence of heating mode and sinusoidal excitation period on the defect detectability is considered. Thus, the experiment is split into two modes: the sample is solely heated in the first mode or simultaneously heated and cooled in the second. In each mode, the temperature measurement is performed first with a shorter excitation signal period and second with a longer one. The signal-to-noise ratio (SNR) is used to assess defect detection quantitatively. The comparative analysis shows that employing a mixed heating-cooling mode improves the SNR compared to the conventional heating mode. The further enhancement of the SNR is obtained by extending the excitation period. The combination of simultaneous heating and cooling with longer periods of the excitation signal allows for the best SNR values for the most detected defects.

56 Monitoring Diesel Exhaust Particles by Lock-in Thermography

Abstract Diesel exhaust particles (DEPs) can deposit onto the respiratory epithelial surface upon inhalation. In vitro exposures of various lung models to DEPs have been performed under the air-liquid interface (ALI) conditions in previous studies, however, methods to quantify DEPs onto/into cells during and after exposure are still challenging and not well established. In our study, a new particle detection system, i.e. lock-in thermography (LIT), was explored. LIT is a heat- sensitive imaging method that applies the light illumination to induce the heat of carbon-based particles for detection. By testing a standard DEPs sample (SRM2975), a positive linear relationship (R2 = 0.98) was established between the thermal emission signals and DEPs levels ranging from 0 to 30 µg/mL, with limit of detection (LOD) at 1.00 µg/mL. Moreover, phototoxicity tests showed that no cytotoxic effects were observed on human lung epithelial (A549) cells upon excitation light wavelength of 525 and 660 nm for 1 – 2 mins. Taken together, it suggests that the LIT can be used for detecting DEPs exposed with A549 cells at the relatively low concentration. Further improvement and optimization of the LIT system are ongoing to evaluate DEPs exposed lung cells.

Spatio-Temporal 3-D Residual Networks for Simultaneous Detection and Depth Estimation of CFRP Subsurface Defects in Lock-In Thermography

Nondestructive thermography is a high-speed, low-cost, and safe solution for subsurface defects detection of carbon fiber reinforced polymer (CFRP) materials, providing essential quality control in aerospace, automobile, and sports industries. In this article, we build a reflective lock-in thermography system and construct a dataset that contains real-captured thermal image sequences of CFRP samples with various simulated internal defects under different excitation frequencies. Then, we present a novel 3-D convolutional neural network (CNN) model incorporating a combination of spatial and temporal convolutional filters and batch-size independent group normalization (GN) as a unified framework to process thermal image sequences captured by lock-in thermography for simultaneous subsurface defect detection and depth estimation. Finally, we define a multitask loss function to perform end-to-end training of both defect detection and depth estimation tasks based on the real-captured infrared sequences. Comparative experiments are carried out on CFRP specimens with artificial defects of various sizes/shapes and at different depths. Qualitative and quantitative results illustrate that our 3-D CNN model is capable of predicting accurate locations and depths of subsurface defects and performs favorably against the hand-crafted and CNN-based methods in lock-in thermography for individual defect detection and depth estimation tasks. The captured dataset and the source codes will be made publicly available.

Thermal phase lag heterodyne infrared imaging for current tracking in radio frequency integrated circuits

With thermal phase lag measurements, current paths are tracked in a Class A radio frequency (RF) power amplifier at 2 GHz. The amplifier is heterodynally driven at 440 MHz and 2 GHz, and its resulting thermal field was inspected, respectively, at 1013 and 113 Hz with an infrared lock-in thermography system. The phase lag maps evidence with a higher sensitivity than thermal amplitude measurements an input-output loop due to a substrate capacitive coupling. This limits the amplifier's performance, raising the power consumption in certain components. Other information relative to local power consumption and amplifier operation is also inferred. This approach allows the local non-invasive testing of integrated systems regardless of their operating frequency.

Autonomous mobile lock-in thermography system for detecting and quantifying voids in liquefied natural gas cargo tank second barrier

In this study, an autonomous mobile inspection system that can detect and quantify hidden voids in a secondary barrier (triplex layer) of a liquefied natural gas cargo tank is developed using lock-in thermography. The triplex layer is the secondary barrier to prevent gas leaks from a Mark III–type membrane liquefied natural gas carrier cargo tank and consists of three sub-layers: a flexible secondary barrier, a bonding layer, and a rigid secondary barrier. The proposed mobile inspection system consists of a lock-in thermography measurement unit, a mobile inspection unit, and image processing algorithms. First, thermal images are obtained using the lock-in thermography unit as the mobile inspection system maneuvers over triplex layers. Second, the raw thermal images are processed by several image processing techniques to compensate for non-uniform heating, eliminate noise components, and disregard trivial voids in accordance with the current inspection guideline. Third, the void size is more precisely quantified using an empirical mapping function that relates the void size estimated in the previous step to that measured by an independent X-ray test. The contributions of this study include the following: (1) an autonomous mobile inspection system is developed for real-time inspection of the triplex during its installation, significantly saving the inspection cost and time; (2) a suite of image processing techniques is developed, overcoming shortcomings of the existing thermography non-destructive testing techniques; and (3) the sizes as well as the locations of the hidden voids are quantified with high accuracy, reliability, and fast inspection speed.

위상잠금 열영상 현미경의 온도분해능 분석

Abstract In this study, we analyzed and showed the enhanced thermal resolution of a lock-in infraredthermography system by employing a blackbody system and micro-register sample. The noise level or thermalresolution of an infrared camera system is usually expressed by a noise equivalent temperature difference (NETD),which is the mean square of the deviation of the different values measured for one pixel from its mean valuesobtained in successive measurements. However, for lock-in thermography, a more convenient quantity in thephase-independent temperature modulation amplitude can be acquired. On the basis of results, it was observed thatthe NETD or thermal resolution of the lock-in thermography system was significantly enhanced, which we considerto have been caused by the averaging and filtering effects of the lock-in technique. Keywords: Lock-in Thermography, Noise Equivalent Temperature Difference, Infrared Microscope, Thermal Resolution[Received: September 23, 2014, Revised: October 20, 2014, Accepted: October 29, 2014] *한국기초과학지원연구원첨단장비개발사업단, ** IT , Corresponding Author: Center for Analytical한국과학기술원스마트 융합시스템연구단 ✝Instrumentation Development, Korea Basic Science Institute, 169-148 Gwahak-ro, Yuseong-gu, Daejeon 305-806, Korea(E-mail: ksc@kbsi.re.kr)

Laser lock-in thermography for thermal contact characterisation of surface layer

Lock-in thermography was applied to determine the thermal contact conductance of a W-layer (140μm) on a CFC-substrate. A lock-in thermography system together with a pulse repetition rate Nd:YAG laser (1064nm, 1–500Hz pulse repetition rate) for layer heating was applied for phase shift measurements on the W-layer. A numerical model for direct phase shift calculations was developed and applied to rapid determination of the Fourier amplitudes and phases of the temperature. Thermal conductance coefficients were obtained by comparing the experimental and simulation phase shifts.

Dark I-V Characteristics and Lock-in Thermography (LIT) Techniques as a Diagnostic Tools for Silicon Solar Cell After 4000 hours of Thermal Stress at 400K

In this paper, a highly sensitive lock-in thermography system has been used; enabling the detection of periodic surface temperature oscillations below 10 μK (r.m.s) has been obtained. Spatially resolved power loss images obtained by Lock-In Thermography (LIT) for a single crystalline silicon solar cell carried out. A significant difference is shown for the solar cell with shunts, while series resistance and charge carrier recombination cause only minor differences in the images. This system has been used to investigate edge leakage shunts currents in silicon solar cells of the construction n + pp ++ PESC Passivatted Emitter Solar Cell (silicon wafers doped with Boron) after 4000 hrs of thermal stress at 400 K. The dark I-V characteristics of the solar cell, as a diagnostic too, are studied and analysed. A decrease of the electrical parameters of the solar cell has been obtained after thermal stress.

Lock-in IR thermography for functional testing of solar cells and electronic devices

Lock-in thermography (LIT), which is a standard tool in nondestructive testing (NDT), is also very advantageous for electronic device testing (EDT). In this contribution the special points of view of LIT applied in EDT are reviewed. In EDT it may be useful to display not only the amplitude or the phase image but also images of the magnitude of special phase components. The “0°/-90°” image as a new kind of representation is especially advantageous to display microscopic lock-in thermograms of integrated circuits (ICs). Just as the phase image, the 0°/-90° image is inherently emissivity-corrected, but it provides a considerably better spatial resolution. The quantitative interpretation of EDT thermograms in terms of a lateral power distribution is reviewed with special emphasis to the measurement of local power sources and the numerical inversion of lock-in thermograms enabling a higher spatial resolution. Experimental details of a lock-in thermography system specialized to electronic device testing are introduced with a number of representative application examples demonstrating the universal applicability of this technique in EDT.

Thermophysical analysis of thin films by lock-in thermography

A method for the determination of the thermophysical properties of thin films is introduced. The procedure is based on the contact-free thermal analysis of free standing thin films by means of a lock-in thermography system. The thermophysical properties are deduced from the thermal diffusion length and the temperature amplitude of a thermal wave propagating in the sample excited by a laser heat source. An infrared camera images the wave and the thermal diffusion length Λa of the amplitude and the thermal diffusion length Λp of the phase are measured. Thermal losses influence both Λa and Λp. Their geometric mean ΛaΛp is, however, not effected by thermal losses. In turn 1∕Λa2−1∕Λp2 is determined by the thermal losses and does not depend on the lock-in frequency, as Λa and Λp do. These theoretical predictions are experimentally verified in this work. The measured values of Λa and Λp yield the in-plane thermal diffusivity and a damping factor. The latter quantifies the thermal losses to the ambient. The use of a vacuum chamber and a temperature calibration are not required. If, however, the camera is calibrated, the in-plane thermal conductivity and the volumetric heat capacity are obtained from the temperature amplitude of the thermal wave. The measurement accuracy is 10% for the thermal diffusivity, thermal conductivity, and volumetric specific heat. The thermophysical properties measured for thin films of copper, nickel silver, and polyimide agree with literature values.

Microscopic lock-in thermography investigation of leakage sites in integrated circuits

The detection limit of infrared thermographic investigations can be improved down to 10 μK by using a highly sensitive high-speed infrared camera in an on-line averaging lock-in thermography system. Together with a microscope objective, this allows lock-in thermography to be used as a simple and sensitive technique to localize the sites of leakage currents and other heat sources in electronic components. The practical realization of a novel lock-in thermography system is described and both test measurements and practical applications are introduced. The detection limit for surface-near local heat sources in silicon is a few microwatts with a spatial resolution down to 5 μm. Leakage sites in several microelectronic structures are imaged and assigned to the layout of the integrated circuit by comparing direct images with lock-in ones. The direct comparison of an averaged and background-subtracted stationary thermogram with a lock-in one, both measured under similar conditions at the same sample, clearly demonstrates the gain in information obtained by using lock-in thermography.

Shunts due to laser scribing of solar cells evaluated by highly sensitive lock-in thermography

Using a 217 Hz focal plane array (FPA) thermocamera attached to two parallel-running DSP frame grabber boards in a PC, a highly sensitive lock-in thermography system has been built, enabling the detection of periodic surface temperature oscillations below 10 μK (r.m.s). This system has been used to investigate edge leakage currents in silicon solar cells after laser scribing and cleavage.