Homebrew PCBs

I've been experimenting with some UV LEDs for building an exposure unit. 
I'm using the negative dry film photoresist and discovered to my 
surprise that there is a limitation on minimum light intensity. If the 
intensity its too low then no amount of time will cause photo 
polymerisation of the resist.
For example, one LED I tested at 15mA, 120mm distance did not cause any 
polymerisation in 40 minutes. The same LED at 30mm distance, 20mA, took 
only 30 second for polymerisation.

I found if the light cannot expose the resist within about 2 or 3 
minutes then the photoresist do not seem to react at all. At boarder 
line UV intensity, it partly polymerises leaving behind a soft matted 
looking resist after development.

This imposes a design constraint on anyone who wants to make a LED 
exposure unit must because it there will be a minimum density of LEDs 
needed in the array. So far I've only experimented with single LED 
exposures. I'll make a 9 x 9 array for testing.