When I read in a dark room in my iPad Kindle app, I tend to get afterimages of the page, even though I have the brightness turned down. This doesn’t happen if I read a paper book or my e-ink Kindle with a book light. It doesn’t seem that the iPad is any brighter, even though it is a light source.
Why do I get the afterimages with the iPad?
As I think you know, the answer is that you are staring at a light source. It probably is brighter than your book under a book light, or, at least, contrasts more with the dark surroundings, and in such a way that the edge between light and darkness is much sharper. An afterimage with a relatively sharp edge is going to be more noticeable than a more diffuse one.
Are these negative afterimages, or positive ones?
Negative. The black text creates an afterimage of white text on a dark background that kind of floats around. It’s not so disruptive that I can’t read, but is occasionally distracting.