Refers to the resistance of inks to the spectrum of natural light. According to the German standard DI 54003/4, the “wool scale” defines eight levels from very low to excellent. Level three moderately light-fast indicates that the ink can be exposed to sunlight for 4 to 8 days in summer and 2 to 4 weeks in winter without any noticeable fading. At the highest level of light-fastness, the ink can withstand exposure to summer sun for more than 18 months.