Diffuse lighting/Alpha control

I was just wondering if there was a way to control the alpha of an object based on where light is getting to it. Like if I set a lamp straight above a large plane, the middle (where the light is hitting) will be the color of the lamp, but the outside (where there is no light) will be completely transparent. I know you can do this with secularity, under the Transparency tab by setting the alpha to 0 and leave the secularity at 1. I just want to know if you can do the same with diffuse lighting.

Thanks