Light linking fog

As you can link a light to an object or a shader, you can also link a light with another light. Thsi particularly works well (*1) when you have a fog light effect.

    Create a plane and a sphere, then create a spotlight with a fog light effect. Render your scene and store the resulting image. Now add to your scene another light source (say an ambient light), select the spotlight and 'make light links' with the hotbox. Render once again and compare.
By linking the 2 lights (link light tool), your ambient light is affecting your spotlight's fog intensity (in fact the coneShape is lit). So if you animate your 2nd light intensity, the fog intensity will react according to the other.

(*1): It is clear that it enhances the image look, but it also slows down the rendering, so be careful with your light links.

I'm actually using this technique in a production I'm working on. This is pretty slow, but gives uncompared results with a normal light fog effect.

    Here is an example. Simple scene, a spotlight (with a fog effect) on the left, and a point light. Both lights have a quadratic decay, and casts shadows.