Doom3 like attenuation ?

[sqrt_1]

Have alook at http://www.ronfrazier.net/apparition/index.asp?appmain=research/per_pixel_lighting.html
and
http://www.ronfrazier.net/apparition/index.asp?appmain=research/advanced_per_pixel_lighting.html

for low end per pixel attenuation using 1-D^2