How to use light linking feature

Sycles addon with the version 2.2 and above supports the light linking feature of the Cycles render engine. This feature allows to exclude specific scene elements from the lighting (as illumination and shadows).

As an example, consider the simple scene: plane and several balls

This is a render result

We would like to highlight the central sphere by red light. So, create the spot light with red color

Render it

It's not what we would like.

Create two groups. The first group contains all geometry except the central sphere. Call this group geometry

Also create the second group, which contains all objects from the scene. Call it scene

The first group geometry contains objects which should be excluded from the lighting of the spot light. The second group contains all objects, which should be excluded from the shadowing of the spot light.

Add to the spot light the property CyclesLightLinking

This property contains two fields: Light and Shadow. Select for the light group the group geometry, and for the shadow group - scene. Render it

So, no additional shadows from the red spot light, and no additional light on spheres, except the center one. As it should be.