Light linking in Maya, include/exclude objects for lights in CINEMA 4D, light masks in Houdini, inclusive/exclusive lights in Softimage, they all are translated to light_groups and shadow_groups in Arnold.
Here’s a sphere on a plane, with two area lights (cyan and green).
If we add a light_group to the sphere polymesh:
use_light_group on light_group "CyanAreaLight"
then we get this (see below). The sphere is lit only by the cyan area light: the light_group is the lights that affect the object.
Note that the sphere still casts shadows from the green light, even the sphere isn’t lit by the green light. That’s because there’s no shadow_group, so all lights cast shadows from the object.
If we add a shadow group, so that the light_group and shadow_group are both “CyanAreaLight”, then the sphere is lit by the cyan light only, and casts shadows from the cyan light only.
use_light_group on light_group "CyanAreaLight" use_shadow_group on shadow_group "CyanAreaLight"
And finally, here’s the sphere when light groups are on, but there’s no lights in the light group:
use_light_group on use_shadow_group on shadow_group "CyanAreaLight"