Misha A hot mirror is a little piece of colored glass that goes in front of the "film" plane, inside the camera, that basically filters out red light. I believe its goal is to improve focus, since all wavelengths focus at slightly different lengths. But if you have a hot mirror which takes out (at least) long red wavelengths, and then you add to the front of your lens a filter which takes out everything but red light, then there is really not much of the spectrum left to expose the film or sensor, (potentially none, actually :-D). So you get long exposures, and all the problems that go with long exposures.