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.
dim something up
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.