i'd say that if you haven't had them under a lamp up until this point, you're probably fine. otherwise they would have died, which, happened to some friends of mine who figured, eh, it's hawaii, they will be fine. sadly, they were not.
anyway, i got my chicks during the 3rd week of july. they were under a heat lamp as soon as i got them home. using a thermometer to judge, i kept one end of their brooder at 90ish degrees the first week. 85 degrees the second week. during the 3rd week, we turned the lamp off during the day as it was in the 80s. at night it came back on.
during the 3rd week they also started spending all day outside, coming in around nightfall. when they turned 4 weeks old, and the night time lows were around 75, out they went permanently. no heat lamp at all. and they don't have a closed in coop. it has a roof and a couple walls but they are very much outside.
makes sense to me to get them outside as soon as possible. more space to run and flap and scratch and build their strength.
as of about 5 and a half weeks they started sleeping on a 3.5' high roost. there is a ladder to help them get up there, though some could probably make it that high without.
it's a little hard to tell from your picture, but i bet that, what with yours just now experiencing the heat lamp, they are pretty well feathered out by now. when they are fully feathered, they can definitely go out. when not, it's all about what the night time lows are.