The green pen was an audiophile tweak from Audio Prism that came out about 25 years ago. The green pen was used around the outer edge of CDs to improve the sound. The green pen appears to work by absorbing stray scattered CD laser light that might other wise be detected as “real signal” by the photodetector. The green pen is a very good CD tweak as far as it goes. The reason I say that is because the green pen can only absorb visible red light which accounts for only a fraction of the scattered laser light inside the CD transport. Most of the scattered light is actually invisible, in the range 700-850 nanometers. Only the light just below 700 nm (visible red) can be absorbed by the color green or green blue (cyan). Thus the problem of scattered laser light is only partially solved by the Green Pen. Around 2/3 of the scattered light still has the potential to enter the photodetector even with the Green Pen. Oh, dear!

Addendum: the CD laser appears red to the naked eye only as a safety precaution. Most of the laser bandwidth is invisible. The nominal wavelength of the CD laser is 780 nm, which is near infrared.

Yes, I know what you’re thinking - “I always thought the laser was monochromatic.”