distance from plants to my Sealed hood

Just wondering what distance i should keep my tops from my sealed hood. First time indoor grower and i started with 18 inches but the plants dont seem to be getting much negative effect from being only 7-8 inches from the glass. When im measuring the distance should i measure from the glass or from the bulb? I know these are dumb questions but i havent found any answers really. just seeking knowledge like the rest of us!

oh yea im running 1000 watt hoods with hps bulbs 8 inch inline fan and carbon filter all inline

Bump!!! Please help

If your running a 1000 I would say 24 inches I run my 600 at 18 inches.

You want to get the lights as close to the plants as possible without heat-stressing them. If your sealed hood is air-cooled, then that could be extremely close. I think 7-8" should be fine as long as the temperature at the top of the plants is ok. A 1000w light will have good penetration, so ~12" might be optimal. At that distance even if you have a day with a big growth spurt they won't grow into the light.

A good rule of thumb that's constantly mentioned here is to see if you can comfortably hold your hand under the light at the distance. If so, then your plants will be happy there. If it's too hot for your hand, it's too hot for your plants.

I have a 400, no air cooling, and I have to keep the light about 18" from the tops, with an oscillating fan blowing cool air between them. Closer than that and they'll get too hot.

Hope this helps.

I can practically get my 600 6 inches away from my plant with an ac hood. I can touch the glass without any discomfort whatsoever.