This pen glows beautifully in a dark and semi-dark with a blue green color after an hour exposure to light. Please accept variations in duration of glowing depending on exposure time to light, and the intensity of the light source. There are different light sources you can use for “charging” the pen to glow—natural sunlight, regular desk lamps, or LED lamps. Usually, LED lights produce a weaker glow compared to sunlight and traditional incandescent bulbs. However, avoid overheating especially when using incandescent lights and direct sunlight during “charging.” Long exposure to warm sources with strong overheating (over 50°C / 122°F) can degrade the quality of the resin.