how dark for flowering?

ok guys i am starting to flower and i have 2 seperate rooms next to each other .1 room is for veg and 1 for flower. now my problem is this i have the lights connected with ducting when the flowering room goes off the light from the ducting is coming thru the hood but not much.so how dark does it really have to be ?and how would i fix this ? and also would it hurt the plants if they get this glare while in flowering?

Yes, light coming through the duct is bad. If you can make a U turn with the duct somewhere along the line it will prevent the light from getting into the flowering room.