Validity limits of Kubelka–Munk theory for DRIFT spectra of photodegraded solid wood

The validity of the Kubelka–Munk (K-M) theory was investigated to determine the IR absorption spectra of wood based on diffuse reflection infrared Fourier transform (DRIFT) measurements taken on photodegraded samples. After analysing plenty of DRIFT spectra of wood and examining the shape of the K-M equation, it can be concluded that the measured K-M function can be used as an absorption spectrum if the values of the function are below 14 K-M units. Above this limit, the K-M theory, which was created for poorly absorbing materials, does not give the absorption of wood properly. If a matt aluminium plate is used as a background material and the values are between 14 and 40, absorption changes can be calculated after normalisation of the spectra. This normalising manipulation is only successful if there is an absorption peak close to the examined one which does not change its absorption during the photodegradation.