This procedure provides a convenient method for evaluating the light fastness of art materials in indoor lighting. By exposing the sample to sunlight through a window and comparing it to a blue wool reference sample, artists and technicians can visually judge the light Stability of the material under actual use conditions. This method is suitable for assessing light fastness when testing with more precise instruments is not possible, but is not suitable for evaluating oil-based Coatings or Long-Term Stability.