Wood discoloration patterns depending on the light source

The wood photodegradation, including discolouration caused by exposure to UV and solar radiation, has been intensively studied, while the effect of artificial lighting on wood has been little investigated. In the present study, the effect of three types of artificial light sources (LED, incandescent, and fluorescent lamps) on the colour changes of wood was evaluated. LEDs with high (6500 K) and low (3000 K) correlated colour temperature were employed in the experiments. Wood colour was assessed by spectrophotometric measurements of reflectance spectra, which were converted into colour parameters of the CIELAB colour system. The total discolouration as well as the changes in colour lightness, chroma (saturation), and hue were evaluated for two hardwood species (birch, oak) and two softwood species (spruce, pine - sapwood and heartwood) depending on the irradiation dose. Visually perceivable changes in colour of all woods were observed already at relatively low irradiation doses, indicating a high sensitivity of the wood to radiation emitted by artificial light sources. Comparing the softwoods and hardwoods included in the study, the latter proved to be more resistant to discolouration caused by the tested light sources. Overall, greater colour changes in long-term exposure were caused by incandescent and fluorescent lamps, although more rapid discolouration developed in the early stage irradiation with LEDs. A substantial difference between the effect of the tested LEDs was only observed in the initial phase, when the cool LED (6500 K) caused more discolouration. The changes in the colour parameters were complex and varying in directions, including a reversal with the accumulation of the irradiation dose, indicating that the exposure to artificial light sources resulted in continuous alteration in the shade of the wood colour.