Use a spotlight
Source file: SpotLightDemo
This example show how to add a spotlight to your scene.
How to use the demo: When the cursor is over the red cylinder representing the spotlight, dragging with left mouse button depressed will rotate the spotlight. If the cursor isn't over the red cylinder, such dragging will rotate the whole scene.
Look in the code to see how the default lights are first removed from the scene graph so that only the spotlight effects the shading.