In FTIR spectroscopy, a typical spectrometer output is a *transmittance* spectrum (*k*, *T*). In order to match the spectrum data against a table of *absorbance* peaks, the original data set is converted to an absorption spectrum (*k*, *α*):

$$\alpha(k) = - \frac{1}{2 w \sqrt{2}} \log\left(\frac{T(k)}{1 - r^2}\right).$$

The width of the sample film *w* is a result of direct measurement, i.e. microscopy. The reflectance *r* is deduced from the mean maximum transmittance level *Tₒ* (dashed line) disregarding signal oscillations as *r = 1 - Tₒ*. Much of this data processing can be entrusted to a machine.

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a transmittance spectrum (*k*, *T*) or an absorption spectrum (*k*, *α*) data set.

Units: wavenumber, *k* — cm⁻¹, transmittance, *T* — %.

For a *transmittance* spectrum (*k*, *T*), take the following steps to convert it
to an absorbance spectrum (*k*, *α*) first.
In case of an *absorption* spectrum (*k*, *α*), skip the following conversion and
proceed directly to the peak detection.

— Thickness of the film, μm

— Reflectance of the film, autodetection

There is a default built-in reference data set of absorption peaks for Si films. If it is applicable, proceed directly to the peak identification. To apply a different reference data set, edit it.

The default demo spectrum was taken from a porous 53-micron thin silicon film previously exposed to electrochemical etching in a fluoric acid solution.