dirty lens effect over light help

i need some help with this effect I'm trying to achieve, you see the gui texture pops up and everything but the problem I'm having is the fact it doesn't show up on light its just always there. I'm trying to achieve an effect where you look at a light source and it'll show up the gui texture where it is. i have my texture that i made with photoshop but i don't know how to do this type of script, i've done AI and inventory etc but what would this be? i don't know how to do this script so some help will be greatly appreciated thanks.