Global illumination is a method of lighting a 3-D scene by the use of light-emitting scene elements rather than traditional lights. It's also known as environmental mapping or environmental lighting.

It's primary function is to simulate daylight lighting, both for entire scenes and to facilitate composition of rendered elements into real life scenes.

I'll illustrate a painfully simple version below.

First we need a sky sphere.

This is mapped to a sphere which encloses your scene and camera. as shown below
this is a painfully simple puts a white 'sun' when mapped to a sphere in UV fashion. We use this as the color and glow map. Used in the Stencil channel, this could make the rest of the light source invisible.
Rendering with radiosity yields the following...which is illuminated only by the glowing part of the sohere
Using the same principle with a sky image will accomplish the same thing.