Radium in luminescent paints that were used to paint watch faces was eventually replaced with promethium-147 by the second half of the 20th century. It is a low-energy beta-emitter, unlike radium which is an alpha-emitter. Additionally, unlike radium, it doesn't emit penetrating gamma rays. The half-life of promethium-147 is only 2.62 years which makes them easier to dispose of. Assuming that the light given off is directly proportional to the amount of promethium, determine how much will the face of a 15-year-old watch be dimmed.