This may be a very basic question or a bug in blender.
I used two spotlights with the halo button enabled and a low haloint value of 0.200
The result is not good. I can see gradual steps on the volumetric light. How can this be changed to make the volumetric halo smoth?
Here you can see the problem (you may have to look from a specific angle if you are using a LCD monitor):
It’s not a volumetric light, it is a hack someone put in Blender to fake it.
Make sure your spotlight is set to BufShadow, this give you extra parameters. Increase Halo Step to 16 and samples to 12. That is the best quality you can get out of this fake light. Also try using sample buffers of 4 or 9 to smooth it out, the default is 1.
Actually, I think you’re going to have to add some noise to that image to remove the banding. The problem you’re seeing is because there are a limited number of brightness steps on a plain black background like that. If you have other elements in your scene that show through the halo area, the odds are that the bad effect will be nearly as pronounced.
