Why are lamps and mesh lights not illuminating anything?

Question

There are several techniques to do a 3 point lighting system.

How do you properly light a scene while using mesh planes as emission light sources?

I'm following this tutorial, however despite setting up planes to act a lighting sources, the scene is still very dark.

Actual Blender file here: https://dl.dropboxusercontent.com/u/460997/clock%20copy.blend

Answer 1

In your file, Clamp direct and Clamp indirect have both been set to .03 in Render settings > Sampling.

This is forcing all the samples to return a value less than or equal to .03, which results in the apparent lack of light.

Set them both to zero to disable clamping:

Then it should work as expected: