Dual-reference photothermal imaging for the estimation of thermal effusivity of solids

Abstract

A photothermal imaging method for the fast measurement of thermal effusivity of solids is suggested. On the basis of the principles of thermal wave interference in a layered medium, we have derived the conditions for a linear variation of the phase difference with effusivity for a sample–reference substrate assembly coated with an optical absorber. Exploiting this linearity, a dual-reference scheme has been introduced for simultaneously normalizing the instrumental offset and coating parameters out of effusivity estimation. As a means of improving accuracy and reducing sample size, three ranges of measurement within 5000–40 000 W s1/2 m−2 K−1 are used. For the absorber, we have used spin or spray coated carbon layers whose reliability has been verified using common-mode-rejection-demodulation-assisted photothermal imaging. Once set for an effusivity range and coating parameters, a threshold frequency can be found, below which the method offers the assumed accuracy. Typical error is 2.8–3.8% in most measurements and the technique turns out to be an easy and fast scheme, which can be implemented with minimum instrumentation and computation. The method has been demonstrated for the effusivity evaluation of hardened steel as a function of the quenching time.