When to give germinated seed light, and transfer?

This is my first attempt growing hydro, and decent seeds are VERY had to come by around here, so I want everything to be rite.

I started by germinating 4 seeds on rockwool cubes on a plate covered by a bowl. All 4 craked and have little bits of root sticking out. Not much at all, just like a little white bulge coming out.
What I need to know is when do I want to start giving them light?
Obvoiusly when they have a leaf they need light. Do I keep them in the dark though till a stem and leaf start to show? (Like if they were outside and starting to poke outa the ground?) Or should I give them light now?
I have had little bits or root showing for about 3 days now and havent seen any progress. Being that I only have 2 seeds left after this I wannt make sure these grow up happy. Any guidance for dummies would be helpful.