Why is the jacket or sheath of fibre optic endoscopes always black? Is it just a convention, or is there a more logical explanation for this?
I really have no idea, but I did come up with the following possibilities.
To minimize reflections from the instrument itself. Light reflecting off the endoscope could interfere with the doctor's view.
Because little (if any) body tissue inside the body is black, thus making it easier to distinguish the instrument from body parts.
Because that's the natural color of the plastic including additives. Adding dyes could be undesirable if they leech over time, and since it's not a typical consumer item there's no need to make it colorful and flashy.
To shield the fibers from outside light, thus minimizing interference.
To prevent ambient light from "leaking" into the fiber optic cables. It's mainly a redundancy (extra layer of protection)
Industrial polymers are often black by default because carbon black pigment is added to stabilise them against environmental degradation, especially the effects of UV light. This is certainly the reason why tyres are almost always black.
And black goes with everything :P