Light pollution is another important side aspect of this. What if, as light becomes cheaper, people/ cities just use more of it? From other comments, it sounds like this is already happening with LED.
LED lighting is much easier to make directional. With LED lighting, the difficulty is making it more isotropic.
LEDs + phosphor, not so much, due to their wider spectrum. That less is emitted directly skyward doesn't help if what's reflected off pavement can't be filtered out. (Pavement is surprisingly reflective even though it's "black".)