This was an excellent contest topic- I had to think very carefully about what I wanted to do and how to communicate the idea. My first thoughts tended towards 'product shots' or photographs of the light source itself; this is not what I wanted.
I realized that a 'light fixture' has a single essential function- to control where the light goes. Without a fixture, the light source has no focus (pun somewhat intended), so I wanted to photograph this function- a relationship between source and target. I kept thinking about the view from an airplane, approaching an airport at night, when I can see the illumination pattern from streetlights. The lights themselves are not visible, and the street is not much more than a uniform surface- what is interesting is the pattern of illumination, which is due to the fixtures.
It took a while and lots of test shots, but this is what I came up with. Nothing fancy- straight off the camera, a basic 50/1.8, opened up all the way and at close focus to minimize the depth of field. Shutter speed 1/1000s, ISO 100. The light as about a foot away from a blackboard, and the room lights are on, allowing you to see the heat sink elements on the near side. The angle of view was chosen to let you 'see though' the back side of the lamp.