I haven’t been able to get volumetrics to work properly in Blender for some time now. However, I have developed an awesome node setup that creates the effect I think many of us artists want. It may not be entirely realistic and it’s slightly unorthodox, but I’ll say it definitely did the trick.
Essentially I wanted to created a volumetric lighting effect like you’d see from a stage spotlight at a concert or theater event. Simulating this correctly in cycles is really difficult and eats up render time, requires super wonky setups, and overall is just a big pain. So I decided to fake it.
The trick is to use a cone mesh as your volume/emitter.
Check out the hack here: --> http://www.blendswap.com/blends/view/78862
It is a simple node group, with most of the calculations done inside the group itself. You can pop it open and take a look to make further tweaks on your own. However, it should work for your needs just fine right out of the box. All of the relevant settings are tweakable outside of the node tree, so you can treat it as a custom shader for use however you see fit.
Settings are pretty self explanatory, default values are a good starting point to start to see the effect.
I did ALOT of random plugging and playing around with the Light Paths node as well as used transparent and volume absorption/scatter shaders. I’d love to see how people use this and any further iterations to improve the shader.
*NOTE: Transparency Color I left white as default. This changes the transparency color of the emission shader. There is a second transparency node used in conjunction with a reflection ray light path node and mixed in at the end so that the cone does not cast light onto anything. I left this mix connected by default, but if you want the cone to shine light on other objects in your scene, you’ll want to disconnect it.
*Also be sure to turn off all Ray Visibility except for Volume Scattering and Camera or else the transparency hack won’t work