object instead of particle doesn't work


i haven’t worked with blender for a very long time, but i tried the fire-tut of blender magazine(http://blenderart.org/issues/ #16), and decided to do the lightning with radiosity. when i insert keys for emit, to get different light for different heights, blender uses the emit-curve for the entire animation, so it becomes pretty dark after 30 frames.
Anybody knowing a solution?