How to use light linking feature

The Sycles addon, version 2.2 and above, supports the light linking feature of the Cycles render engine. This feature allows you to exclude certain scene elements from the lighting).

As an example, consider the following simple scene: plane and several spheres.

This is a render result.

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

Render it.

It's not what we would like.

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

Create a second group containing all the objects in the scene. Name it scene.

The first group, geometry, contains objects that should be excluded from the spotlight's illumination. The second group scene contains all objects that should be excluded from the shadow of the spotlight.

Add the CyclesLightLinking property to the spotlight.

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

So no extra shadows from the red spotlight and no extra light on the spheres, except for the centre one. As it should be.