1000w HPS 20" away - still have light burn

So, my 1000w HPS bulb is currently 20" from the nearest plant and its top leaves are curling up and there is light burn in various places.

I was going to put it up higher, but then I saw that the lumen output becomes significantly lower the further away from the plants the light is.

What does it mean and what do I do?

I would higher for now and then lower as time goes on to be fair they are probably only small to be even getting burned at 20 inch away i know peeps have lights feet away from top of plant depending on size I would try say 25 inch then move out further as needed its either that or u could get a fan like me bud and have it blowing directly above the canopy but below the light basically it clears the heat before it even touches the tops of plant GL bud

Nah, man. They're full size. Maybe it's a heat issue? It's 30C in there.

I am finding out from my own experience the idea of the top of the hand over the top buds theory to feel the heat doesnt always work. I've scorched a couple top buds and I'm only using 400 watt lights.
I was comfortably able to keep my hand under the light and it wasn't much more than warm, and they scorched anyway.
Its a tough one because you want to be as close as possible without burning them.

if its 30 degrees C in there you should try and lower your temps because it going to be even hotter near the canopy. I have my cool tube about 7 inches away from the tops and they seem to be doing ok (1000watthps). maybe you could get get a cool tube???however now that cobnobuler said that even if your hand is warm it can still burn them. i may raise them up a bit higher... but defintely try and cool your grow room down if u can..