I knew we are not done with that subject.
Originally Posted by Ken Nadvornick
A CD does not(!) work like a prism, the colours are made by interference. It's physically impossible that a red LED emits shorter wavelengths. A LED can emit longer wavelengths = less energy, this is f.e. done with blue LEDs by targeted contamination of the light emitting substrate so that they can act as a white LED. You can't get green light = more energy from a red LED. LEDs with the basic blue green yellow orange or red colours are practically monochromatic with an emission spectrum that is typically only 25 nm wide.
I verified this with my selfmade orange and red LED safe lights. With reflection from a CD you can see all colours our eye can see from deep blue to dark red. With a real prism there is only orange or red and absolutely nothing else.
If an additional filter makes your red safe light "safer", there's something wrong and you don't have a red LED but maybe a white one with a colour layer to imitate a red LED.
and my darkroom