The idea is to be able to count how many people are currently in the room, so that you can keep the light on. If you get bored you can also add on a simple AI to predict the number of people in the room based on hour of day and day of week.

Currently I control plenty of lights at home that are LED by using a big transistor and an arduino connected to a server running a java server program.

I would recommend doing something simple like gluing a servo to a switch to begin with.

For facial recognition, go with something off the shelf to get things up and running. And if you want to expand the scope, try to write your own simple engine instead. https://www.openshift.com/blogs/day-12-opencv-face-detection...