Rendering with the Volume Shader
This shader simulates light scattered by an atmosphere. It produces light rays and volumetric shadows from geometric objects. It works with point, spot and area lights, but not with distant or sky lights.
We start by creating a Polygon Sphere as a container for our volume. In the attribute Editor at the shape tab under Arnold > Volume Attributes we increase the Step Size to 0.1. Lower values take longer to render, it can be set higher until artifacts occur.
Assign an aiStandard material to the sphere and click on the output connection icon.
A shading group will be crated. Click on the checkerboard icon at the Volume material slot and from the Create Render Node popup select the aiVolumeCollector.
Open the Hypershade Window and MMB drag the aiVolumeCollector node to the work area. From the create panel bring in a SolidFractal node. You can also try different 3D Textures like Cloud, Volume Noise, Brownian, etc. Connect the Out Color port of the solidFractal node with the Attenuation Color port of the aiVolumeCollector.
Create an Arnold > Lights > Arnold Point Light increase the Exposure to 5 and Use Color Temperature to change the light color.
The light decay behaves like in reality, so to see an effect zoom in really close to the point light or increase the light intensity. The appearance of the noise can be tweaked with the 3D Texture.
The IPR below is rendered with a Camera (AA) sampling rate of 1. That low value produces a lot of noise but renders quite fast.