In this paper, a method of material category identification using the step heating thermography technique is proposed. The proposed method is simple, fast, implementable for in situ measurement and also reliable. The pertinency of the method has been demonstrated on different types of materials.
KEYWORDS: Digital micromirror devices, Solar cells, Tomography, Radon, Crystals, Time metrology, Point spread functions, Image quality, Absorption, Sensors
Light Beam-Induced Current (LBIC) imaging is a well-known characterization technique for solar cells, which allows to
detect regions of low crystal quality. In this paper a fast, robust and reliable LBIC system is proposed by the use of
digital micromirror device (DMD). The LBIC technique is usually performed by point-by-point mechanical sample
scanning under a laser spot or by laser scanning, which leads to a measurement time of at least several minutes. In this
proposed system with DMD, a new technique is introduced, in which a solar cell is scanned from different angles by a
light-line instead of a light-spot. The obtained photocurrent data from these scans are used to reconstruct an LBIC image
by using tomography principles. This leads to a lower number of measurements compared to any point scan method.
This method helps in reducing measurement time and makes LBIC a fast characterization tool capable for inline
investigations. Light-line scans over the cell from different angles are realized by a digital micromirror device (DMD)
and its parallel interface controller. The DMD provides a fast solution for line-scanning the cell at speed up to 4 kHz,
leading to a measure time of a few tens of seconds for a 256x256 pixel image. Since there are no moving parts involved
in this setup, it is a robust and compact system, which will be ideal for the field environment and inline characterization.
This paper presents modeling of transient thermography in terms of equivalent electrical parameters, its simulation using a popular circuit simulator SPICE (Simulation Program with Integrated Circuit Emphasis), followed by experimental verification. A novel current source based electro-thermal modeling of radiative heat sources is introduced. Analytic comparison of thermal and electrical circuits forms the basis for modeling and simulation of transient thermography experiments, in which the current source (modeling rate of incident radiative energy) drives a 3-dimensional (3-D) Resistance-Capacitance (RC) network (modeling heat conduction in the material). The current source value was derived from pyranometer-based measurements of the heat flux from the source. A mild-steel sample with a blind hole below the front surface, irradiated by a heat pulse, has been modeled by the proposed technique. SPICE then simulates the absolute thermal contrast of the surface as a function of time, in moderate computing time (seconds). The simulations compare well with experimental observations and similar to generally reported results. Current source approach, allows estimation of radiative heat flux necessary, to view sub-surface defects in a given material, at different depths, in general, and to predict time and magnitude of surface temperature over the defect and non-defect region in particular.
Silicon, apart from conventional integrated circuits, is also the basis for fabricating miniaturized 3-dimensional (3-D) mechanical structures. This paper presents a technique for the optimization of time duration of heat pulse required for transient thermography in silicon wafers. In the present work, a silicon diaphragm fabricated on one surface of a silicon wafer has been electro-thermally modeled as a 3-D Resistance Capacitance (RC) network. The region below the diaphragm was treated as a defect. Heat transfer by all three modes: conduction, convection and radiation has been taken into account. A C++ program generates the equivalent electrical circuit of the given sample, which was then directly simulated by SPICE (Simulation Program with Integrated Circuit Emphasis), a popular electrical circuit simulator. Experimental verification was performed on the silicon diaphragm sample. Prediction of a time duration in which temperature contrast of the sample reaches its maximum (saturation) value with minimum rise of sample temperature, is experimentally verified. This could be very useful in thermography situations where temperature rise should be no more than necessary to avoid potentially dangerous thermal stresses. Another possible use of the technique is for finding the heat flux of very short-pulsed heat sources.
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