INSTITUTE OF PHYSICS PUBLISHING PHYSICS IN MEDICINE AND BIOLOGY Phys. Med. Biol. 52 (2007) 1089–1101 doi:10.1088/0031-9155/52/4/015 Re-evaluation of pulsed photothermal radiometric profiling in samples with spectrally varied infrared absorption coefficient Boris Majaron and Matija Milaniˇ c Jožef Stefan Institute, Jamova 39, SI 1000 Ljubljana, Slovenia E-mail: boris.majaron@ijs.si and matija.milanic@ijs.si Received 23 August 2006, in final form 10 November 2006 Published 25 January 2007 Online at stacks.iop.org/PMB/52/1089 Abstract Spectral variation of the sample absorption coefficient in mid-infrared (µ IR ) demands caution in photothermal radiometric measurements, because a constant µ IR is regularly assumed in inverse analysis of the acquired signals. Adverse effects of such approximation were recently demonstrated in numerical simulations of pulsed photothermal radiometric (PPTR) temperature profiling in soft biological tissues, utilizing a general-purpose optimization code in the reconstruction process. We present here an original reconstruction code, which combines a conjugate gradient minimization algorithm with non-negativity constraint to the sought temperature vector. For the same test examples as in the former report (hyper-Gaussian temperature profiles, InSb detector with 3−5 µm acquisition band, signal-to-noise ratio SNR = 300) we obtain markedly improved reconstruction results, both when using a constant value µ eff and when the spectral variation µ IR (λ) is accounted for in the analysis. By comparing the results, we find that the former approach introduces observable artefacts, especially in the superficial part of the profile (z< 100 µm). However, the artefacts are much less severe than previously reported and are almost absent in the case of a deeper, single-lobed test profile. We demonstrate that the observed artefacts do not result from sub-optimal selection of µ eff , and that they vary with specific realizations of white noise added to the simulated signals. The same holds also for a two-lobed test profile. 1. Introduction Pulsed photothermal radiometry (PPTR) is based on measurement of transient change in mid-infrared (IR) emission following pulsed laser exposure of a sample. When thermal properties of the sample are known, the laser-induced temperature profile can be reconstructed from acquired radiometric signals. Such PPTR temperature profiling was recognized as a 0031-9155/07/041089+13$30.00 © 2007 IOP Publishing Ltd Printed in the UK 1089