I agree that one would not be able to see people or things inside a room using one of these skylights. Never said that I thought this was possible, don't know where you're getting the idea I did.

What I'm saying is that if the skylight is not blocked and a light is turned on in the room, light will be conducted backward up the skylight. You will see the skylight glow from the roof port end unless room interior lighting is prevented from entering the tube. This will be noticeable from aircraft- and more importantly, by neighbours who can see the skylight port on your roof, especially after dark.

Like I said, my neighbours certainly can see the lights come on in my frosted skylight-equipped bathroom. Of course, I know they can't see anyone in the bathroom, but they would certainly be able to see the glow when my bathroom light is turned on.

Very few people use HPS lighting inside their homes. HPS lighting has a distinct colour. I don't think I want to alert the neighbourhood that I'm using HPS lighting indoors.