Doesn't anyone else remember that kids used magnifying glasses to cook ants on the sidewalk?
It would likely be the same with plants.
Almost no light comes out of the top of a bulb. If you're looking for greater efficiency then turn your bulb horizontally and get a nice reflector made of super shiny aluminum. People call it german aluminum. In that configuration 50% of a bulb's light is reflected so shinyness is crucial. Or you could make some sort of light tube, but I doubt that's very efficient.
You best bet is to get a higher powered bulb. If you're looking for energy savings go with a metal halide or high pressure sodium. Fluorescent is fairly efficient as well.
I have a 150w high pressure sodium fixture that produces 16,000 lumens. It was only 80 bucks.
Or you could get a fresnel lens from american science and surplus to catch stuff on fire.
actually you dont even have to order a fresnel lense from online. you can find them in the auto section of fred myers or walmart. theyre the big flat weird looking magnifying things you see stuck on the back windows of RVs and stuff
Indigo: i think its a cool idea... you should try it