Rollover image This shader simulates light scattered by a thin, uniform atmosphere. It produces shafts of light and volumetric shadows cast from geometric objects. It works with point, spot, and area lights, but not with distant or skylights. This is a scene-wide volume shader (or an atmosphere shader in Arnold's terms). Currently, atmosphere_volume does not compose well against volumes. This is because atmospheres return a single flat result that is opacity mapped on top of whatever is in the background of the pixel. atmosphere_volume cannot 'penetrate' through the cloud volume atmosphere_volume should be composited using an 'additive' mode such as 'screen' because volumetric scattering is the light that cannot be represented in the alpha channel.
enable_matte enabled for standard_surface shader assigned to wall geometry (left). atmosphere_volume render composited using 'screen' (right). Rollover images. The example below demonstrates the effect of atmosphere_volume through a medium. It consists of a polygon plane with a circular ramp texture connected to the opacity of a standard_surface shader. The spotlight is pointing at the plane and atmosphere_volume is enabled.
Polygon plane with circular ramp texture -> opacity of a standard_surface shader
See the Environment page for more information on creating and attaching Atmosphere Volume nodes.