What height HPS over 2x3 sq feet to get good coverage?

[Happyjoy]

hey everyone
i have a quesiton which may sound simple, but i just wanted to make sure im understnading the research ive been doing correctly. as u can see from the qeustion, im a newb in growing, and a complete newb in hps

im lighting a 2x3 foot room, with a 400w hps. (no cooltube, temps run around 82F lights on, 61F lights off)
how high above the canopy should the hps be?

ive read opinions from 'as close as they can be', to 'average of 18"' etc.

my qeustions is, for my 2x3 foot space, whats the best height that still allows the light to reach the whole canopy?

my plants are scrogged pretty tightly, so the canopy is pretty even. so do i just measure up from the plants directly under the light? i currently have them at about 20" from the plants directly underneath, im afraid if i put the light closer, the plants on the edges wont be getting as good light....?

 

[Emilya Green]

It all depends on your airflow and type of reflector you are using. With a bare 400w bulb I was able to easily get within a foot of my canopy without burning, and with an AC hood or a cool tube, you could get within 6 or 8 inches. The problem is footprint... the closer you get, the smaller your footprint gets. A 400w HPS will penetrate about 8" into your canopy and you can get as far away as 21" and still have good usable light at 5000 lumens.

This chart should help a lot... stay in the yellow area and you will be good. Use your light meter to see where in your tent the light intensity drops off. I like to use those little solar powered wigglers in my tent to see where the light has enough energy to get things done.

 

[Happyjoy]

It all depends on your airflow and type of reflector you are using. With a bare 400w bulb I was able to easily get within a foot of my canopy without burning, and with an AC hood or a cool tube, you could get within 6 or 8 inches. The problem is footprint... the closer you get, the smaller your footprint gets. A 400w HPS will penetrate about 8" into your canopy and you can get as far away as 21" and still have good usable light at 5000 lumens.

This chart should help a lot... stay in the yellow area and you will be good. Use your light meter to see where in your tent the light intensity drops off. I like to use those little solar powered wigglers in my tent to see where the light has enough energy to get things done.

thnks for that chart. i found that chart online and found it very helpfull also.
the question though is about footprint.

being that i dont have a lightmeter, i was hoping to find out from peoples experience whats the best distance with my 400w hps to cover a 2x3 footprint evenly
things are scrogged low, so i wont be needing more than 8" penetration probably at all.

when i put it at 18" from the center, if i measure the buds on the edges, to the center of the hps light, theyre about 21" from the light, which is still in the yellow zone. i just wasnt sure if thats the proper way to measure things....

 

[Emilya Green]

it is exactly how to measure things... it sounds like your plants are in good hands.