How far away must a 1kW HPS be to prevent bleaching?

Topic says it all. I have air cooled lights that I have been putting as close to my plants as temp allows but I can see that it's damaging my plants.

No replies :*( I can say that 4" is too close, I had highly cooled hoods and did not suffer any temperature burn but I did see a lot of bleaching. I've reduced my setup to 1', ripped all the effected leaves off and I'll look again next week.

new bulbs-14-18''
old bulbs 8-16 ''
I cant say I know, just a good guess.

Having light bleaching issues as well. Bump.

i never seen bleaching before,