Volumetric Spot Light with Cycles I'm using blender 2.76 rc1 with an AMD Raedon HD 7900 series, I'd like to create a volumetric spot light with cycles render. I set the volume scatter (density 0.1) in volume and mist pass in world, and put the spot light slightly below the street light, but, there isn't a volumetric effect.

UPDATE 1: I tried to set volumetric light in a pc with an Nvidia graphic card and works, I think it's an incompatibility problem

UPDATE 2: I read the wiki blender of openCL status, and talks about the AMD limitations, including the volume:


  AMD
  
  Limitations
  
  Initial split kernel patch was done by AMD and was aimed to make Cycles render on Windows platform. This kernel works reasonably well with simple scenes, but has much harder time to work on a production files, it also lacks following features:
  
  
  Transparent shadows (they're disabled due to AMD compiler issues)
  Correlated multi jitter pattern, SSS and Volumes, which could be
  added to the split kernel, but prefer to focus on making existing
  feature set rock-solid first.
  Branched path tracing which is quite tricky to integrate into current
  implementation of split kernel and supporting it will mean we'll do
  split kernel once again
  


.
For volumetric lights you need to control a few things: 

The emission values of the lamps need to be a bit higher than normal to compensate for loss due to the volume Scatter.



Rendering will be slower, a lot noisier, and with lots of Fireflies, so be ready to bring up the samples and clamp indirect samples. The best solution time/quality I've found is using branched Path Tracing and increasing the volume samples and a value for clamp indirect around 3.00



Work with the lowest volume scatter value that still yields the desired result. Higher values will be even noisier and the lights will need to be even brighter.



Keep in mind that volumetric light effects are better seen when the background is dark. How you combine your mist pass will make a huge difference as mist will tend to make things brighter and make volumetric lights less visible.



In my opinion a more realistic approach would be not to use mist, but to use volume scatter and volume absortion together.



sample file to play with: