Comment on Photothermal radiometry parametric identifiability theory for reliable and unique nondestructive coating thickness and thermophysical measurements, J. Appl. Phys. 121(9), 095101 (2017)

TL;DR

This paper critiques a previous study claiming to identify coating parameters via photothermal radiometry, showing that the parameters are correlated and the original identifiability analysis was flawed, making the thickness estimates unreliable.

Contribution

It provides a critical analysis demonstrating the correlation of parameters and errors in the original identifiability theory for photothermal measurements.

Findings

Parameters are correlated in the experimental setup.

Original identifiability criterion is incorrect.

Inferred coating thickness is unreliable.

Abstract

A recent paper [X. Guo, A. Mandelis, J. Tolev and K. Tang, J. Appl. Phys., 121, 095101 (2017)] intends to demonstrate that from the photothermal radiometry signal obtained on a coated opaque sample in 1D transfer, one should be able to identify separately the following three parameters of the coating: thermal diffusivity, thermal conductivity and thickness. In this comment, it is shown that the three parameters are correlated in the considered experimental arrangement, the identifiability criterion is in error and the thickness inferred therefrom is not trustable.