Even with 30" of separation, I was still having bleaching problems (albeit much, much slower developing) with my 240W Blackstar UV (bloom) light. Thinking to swap the 240W panel for the 180W light, I called Lighthouse Hydro. The gentleman who answered the phone was quite helpful, and spent about 1/2 an hour trying to figure it out the problem. Ultimately, he was at a loss as to the specific cause of these symptoms, but he was insistent that:
--The symptoms I was describing were not light bleaching, but were more likely some kind of deficiency. With their lights, light bleaching takes the form of a slight lightening of the leaves when they come within 6" of the LEDs.
--30" was way too much separation. For the 240W UV, they recommend 12" above the canopy.
--Plants are able to actually touch the light and keep on growing. The only symptoms they've encountered is a slight lightening of the leaves when they grow within 6".
--There's no way my light could be defective.
--The problem could be some weird plant genetics.
--My dealer (Gotham) is usually pretty good about swapping the 240W for the 180W if that's what I really want (he recommended against the swap).
--I should keep trying various combinations until I figured it out. Once I found the solution to the problem, I'd really like the 240W UV. It's their best selling light by far, and it has completed thousands and thousands of successful grows.

After I hung up, I was all set to call Gotham and make the swap, but my mind started chewing on bits and pieces of the conversation. To make a long story short, after days of research and much trial and error (involving the sacrifice of 4 prime clones), I finally zeroed in on iron deficiency as the likely culprit. The exclusivity to new growth only was the tip-off symptom. Once I started adding chelated iron to my nutes, the symptoms began abating, and with one plant, the symptoms have disappeared altogether. The two photos below show a branch of this plant merrily growing within 6" of the light. The burned tips you see is left-over damage from earlier severe iron deficiency. Notice the slight lightening of the leaves in the foreground. I gather this is the real Blackstar light bleaching. Incidentally, those leaves are still growing quite robustly.

Attachment 283525Attachment 283526

So what in the heck is going on here?

After piecing together all my observations and all the results of my experiments, here's my working theory (it's not set in stone so if you have a better explanation I'm all ears). These iron deficiencies appear to be directly related to growth rates. I'm using SensiGrow and SensiBloom which have 0.12% and 0.13% iron respectively. With the growth rates of most all lights out there, this amount of iron is adequate. The 240W Blackstar, however, is so efficient at hitting plants at their chlorophyll-producing wavelengths, that all available iron is quickly drawn into growth, and the plant is unable to suck up enough new iron to prevent deficiency. The insidious aspect of this problem is it takes only a tiny and brief shortage of iron in the growing tips to effectively shut down all growth, and if left untreated, the shutdown will eventually kill the plant (I found this out the hard way on 3 fantastic plants). Making matters worse, recovery is slooow and growth rates never seem to return to their former glory. Although I've never heard of too-fast growth rates causing deficiencies, I have personally experienced too-slow growth rates causing toxicities, so it's not beyond the stretch of plausibility.

Thus, my end goal in all this is to come up with some method of preventing 240W Blackstar "iron shock" in the first place. Right now I'm experimenting with adding iron well before switching lights, and although the results of my experiments are a ways off, it's looking like if this approach works at all it will take between 2% and 5% added iron.

If any of you know of a proven method for preventing Blackstar iron shock, I'd greatly appreciate hearing about it in a post to this thread. If I come up with something, I'll do the same. Thanks in advance. Hataman.