This book describes all optical properties of wooden materials, including definitions and measurement methods of optical parameters such as absorbance, diffuse and specular reflectance, colour and gloss. Basic knowledge regarding the reflectance measurement in the ultraviolet, visible, near- and middle infrared radiation ranges is also discussed. It examines conducting correct optical measurements, as well as introduces the validity limits of the individual methods. Steaming as an environmental-friendly colour modification process is introduced by the description of the steaming properties of eight species. Steaming schedules for wood-working industry are suggested to create optimum colour modification. Natural and artificial aspects of photodegradation are monitored by studying the colour change and using infrared reflectance spectrum measurement. The effect of influencing parameters such as temperature, relative air humidity and leaching effect of rain is also discussed. Combinedeffects of heat and light on the optical properties of wood in all possible combinations are presented. The book helps wood researchers and Ph.D. students perform correct and repeatable optical measurements and evaluations in order to draw the right conclusions.