Effect of irradiation with ultraviolet and visible (VIS) light on the VIS spectral properties of sugi heartwood and sapwood

Abstract Discoloration trends in sugi sapwood and heartwood exposed to various irradiation wavelengths, ranging from ultraviolet to visible light, were independently investigated to obtain insights into elucidating the discoloration mechanisms. The tested samples were prepared in a way that minimized factors contributing to discoloration as much as possible, and the analysis was conducted using visible light spectroscopy, which reflects the absorption patterns specific to individual wood constituents. The comparison of the differential reflectance of samples irradiated at various wavelengths revealed three peaks (A, B, and C) related to irradiation wavelength. At peak A, a clear and straightforward trend was observed in which the irradiation wavelength coincided with the wavelength of the constituent affected by discoloration. At peak B, changes were observed at a measured wavelength of approximately 430 nm, and a wavelength-dependent response was confirmed, with irradiation at 310 nm producing the maximum effect. These peaks appeared in the Δρ spectra of both sapwood and heartwood samples. In contrast, peak C was observed only in heartwood. This effect was attributed to extractive constituents unique to this wood type that absorb light at a wavelength of around 520 nm. It was also confirmed that these constituents exert a suppressive effect on discoloration associated with changes in peaks A and B.