In this recipe, we will create a material to fake the typical effect of a cone of light visible when passing through the dust suspended in the air, or falling from the sky on a cloudy day (the so-called God's rays) different from the real volumetric effect described in the There's more section of the Using Cycles volume materials recipe. This is a fake—just a mesh and not a real light to be used for the scene. Therefore, a matching Lamp must be set for the real lighting, as shown in the already made blend file.
Start Blender and open the 9931OS_09_volumelight_start.blend
file, where there is a scene set with a ground Plane, a Cube, the volumetric cone mesh (volume_light), a spot mesh object (spot_mesh), and an effective Spot lamp parented to the volume_light. The Spot lamp cone follows the shape of the volume_light, and its purpose is to light the Cube leaning on the ground...