Don't know but that's my guess on why there's bleaching when too close sometimes as well. 18-24" is a good safe zone have definitely had plants that had to be higher... mostly the ones without much or any super soil... i think there is a nutritional component to it, ie over stimulation combined with lack of the right nutrients to handle that stimulation. if the nutes aren't there it's rapid problems in the leaves... if they are it's fantastic bud growth

I have seen the same kind of bleaching with HIDs if you have an air cooled glass hood you can definitely get close enough to bleach the leaves in the exact same way without burning them with heat. Better growth and fewer issues in my last grow correlated well with the amount of super soil in each pot... the ones with the most had huge buds and almost no problems with the LEDs... the ones that had no super soil there was always some kind of issue... the ones with some but not as much as the others had minor symptoms but nothing serious. Then again it was just one grow could be coincidence... but going to find out next flower