To cause dim lights to become brighter. We knew that the band wasn't going to play another encore when the lights in the arena dimmed up. I dimmed up the lights so that he didn't get the wrong idea that this was some kind of romantic dinner. The lights dimmed up as the play ended and everyone began to applaud.

to use a dimmer to make the lights brighter. (Theatrical. A dimmer is a rheostat, variable transformer, or something similar. The expression, a seeming contradiction, is the opposite of dim something down.) As the curtain rose, the electrician dimmed the lights up on a beautiful scene. You dimmed up the lights too fast.