It gets darker, and then it goes, so both. You obviously can swap out the bulbs if the imaging is noticeably dimming before it goes. I had a bulb in a Toshiba 52hm84 that I let dim too long and it ended up blowing. Of course this occurred during a MLB playoff game, top of the 9th inning, during a comeback. At least I was running my audio through a receiver, so I could listen while I let the TV cool and replaced the bulb.
Two things are infinite: the universe and human stupidity. And I'm not sure about the universe. -- Albert Einstein