In previous versions of After Effects, getting the light intensity to “fall off” (decrease with distance from the light source) was difficult, requiring third-party plug-ins or hacks using expressions.
After Effects CS5.5 provides built-in light falloff as a feature of light layers.
Chris & Trish Meyer provide a video on Adobe TV that shows how light falloff works.
For a demonstration of the new light fall-off feature, see this video on the Video2Brain website.
For more about what’s new and changed in After Effects CS5.5, including details on how to order, see this post.