Homebrew PCBs

Hi There,

I've been creating my own pcb's for a while now using photosensitive
boards and a uv light. My current uv light consists of a old scanner
with three uv tubes in it. I'm currently looking to rebuild it and i
found this website :

http://www.prototrains.com/etch2/uv1.html

This looks like a nice unit, but I don't understand what the grid
between the light and the boards are for? Won't it break the light
going to the board ?

Any other ideas on possible improvements for a uv exposure unit while
I'm redesigning?

Thanks,
Tom