Figure 3.

Estimate of the maximum deviation of the PTIR signal from perfect linearity with respect to the absorption coefficient. For the estimate, we considered an artificial weakly absorbing material with constant a₀= 0.02 μm⁻¹ and n = 1.4 and calculated $H_{\mathit{opt}}\left(a\right)-a\left(\frac{H_{\mathit{opt}}\left(a_0\right)}{a_0}\right)$, normalized to $a\left(\frac{H_{\mathit{opt}}\left(a_0\right)}{a_0}\right)$ Sample parameters were n₀=2.4, n₁ = 1.4 + iκ, n₂=1. Calculations were carried out for 100 nm thick (1^st column), 500 nm thick (2^nd column),1000 nm thick (3^rd column) sample and for a semi-infinite sample (4^th column, labelled ATR). The horizontal scales above 1900 cm⁻¹ and below 1900 cm⁻¹ are different to highlight changes in the mid-IR fingerprint region.