400 HPS Optimal light pattern

If one is doing a 3"x3" canopy grow, what is the optimal height of the light? For instance, let's say the floor is my canopy and the canopy size is 3'x3' (scrog). Does anyone know the exact height of the light to cover the 3'x3' area with minimal light waste?

I understand you want the light as close as possible without the burn. Right now I can get my light down to about 8" above the plants, but 8" above the plants will not cover the final 3'x3' area.



View attachment 1868877

This is the hood I have.


Input anyone? Thanks.

not sure of the penetration specs but i don't seem to get much from 400 W HPS below 2 feet down, so keep them close without burning. keep your canopy up there and full , if light hits the floor it is wasted good luck

rolaand said:

not sure of the penetration specs but i don't seem to get much from 400 W HPS below 2 feet down, so keep them close without burning. keep your canopy up there and full , if light hits the floor it is wasted good luck

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Thanks rolaand - This all makes sense and that's exactly what I would do. I was thinking there would be a calculation say:

For every vertical foot you raise the light, it covers xxx horizontally. Maybe there is no such calculation due to the different hood and reflector sizes.

try about 18 in dude...
or get a light mover and put it close

My grow area is a bit less than 3x3. I keep the light a foot above the top of foliage and raise the light a little at a time. I measure temp at the top of the canopy under the light and keep it 82 or lower. I do that with lots of circulation and not growing in warm months.

The best thing you can do though, is to experiment with your own setup and find your own benchmarks.

the amount of area your 400watt HPS will cover is dependant on the lip of the hood in relation to the height of the bulb in it...a more narrow "beam" helps direct more penetration, the wider the path the less penetration it will have. So if you can't cover the entire 3'x3' area without going beyond 12-18" above your canopy with it I'd wonder if that hood is not what you are looking for

10-12 inches is optimal without a air aircooled hood or cooltube.

Thanks all. And massah thanks for the chart. 12"-18" seems to be in agreement.

Now that I think about it, my question really can't be answered the way I asked.

Just for the hell of it, when the light comes on, I'm going to take two yard sticks and place them on the angle of each side of the hood pointing down (following the light pattern). When the sticks are 3' apart I will measure the height of the light. I would think if the height of the light is within 12"-18" my 3'x3' scrog should work fine.

Is this a fair gauge?

HighLowGrow said:

Thanks all. And massah thanks for the chart. 12"-18" seems to be in agreement.

Now that I think about it, my question really can't be answered the way I asked.

Just for the hell of it, when the light comes on, I'm going to take two yard sticks and place them on the angle of each side of the hood pointing down (following the light pattern). When the sticks are 3' apart I will measure the height of the light. I would think if the height of the light is within 12"-18" my 3'x3' scrog should work fine.

Is this a fair gauge?

Click to expand...

Bingo...now yer thinking!

Ok. So I did the yard stick test. I placed a yard stick on each side of the angled hood (to follow the light pattern). I measured across the yard sticks at 3'. The height of the light was exactly 18". Now that I know that is my max, I will need to tighten up my scrog to about 2.5'x2.5' to be the most efficient.

Here is a link to what I have currently going if anyone is interested.

https://www.rollitup.org/grow-room-design-setup/472399-entrance-door-new-6x6x6-grow.html

that chart has been online for years and is totaly wrong. how does light gain lumens at a further distance form a bulb. it diminishes not gets brighter. that shows all get brighter the further away
it shows for a 400 watt we get 53k lumens and is about right. then at 4 inch i assume or is that foot..then it jumps to 151k...the output of a 1k bulb. how do they beat inverse square law? you cant.
1 foot from a light source and we only have 25% of the original lumens.
basicaly a 400 is only good for 18 inch total. 600 is about 22 inch and a 1k is about 24 inches to keep good light penitrations. this isnt to the canopy...its total light to the floor.
at 1 foot from a 400 watter at 55k lumens is now only about 13k lumens..pretty lame. should be running a 400 about 4 inches from a hood. mine touch the glass now n then i keep so close. no it wont burn if you have correct venting. this is why othersw need to be so far away. most go cheep on room venbting or conditions.

it shows for a 400 watt we get 53k lumens and is about right. then at 4 inch i assume or is that foot..then it jumps to 151k...the output of a 1k bulb

I didn't understand that either.

cause its wrong. thats been floating around for years. there may be more to his testing than we know as its just a chart that explains nothing to be honest. ballast?, bulb type? brand? hood? all those play a huge roll in this. inver square law is the only factual numbers as its law...for those that know science, there is no variations from a law unless things or set up has been changed.

lumens are not calculated based on a distance of 0 on the bulb...and those distances it says are without a cooltube or glass covered hood...its a combination of measurements that give us lumens that lands that mark lumen calculation at about 7"...Lumens isn't an accurate representation of strength anyway since it only takes into account what our eye can process...its just a rough guide to follow...so yes the closer you are to the bulb the "higher" the perceved "lumens" will get...the same as how it decreases with distance...this also depends on factors like reflection...so that "chart" you are calling bullshit is just an approximate guide for somone without a cooltube/hood

i never said anything about a lumens being the best at anything. its what the chart shows so is what i used as a reff. and yes it all relates lumens and distance.
ok so tell me how a 400 goes from the bulb at 53k or so. to an output of a 1k at a distance. man id better sell my 1k and buy 400 for less cost but 10x more light. makes no sence? the hood may have made it last a further distance but you say no hood or any type of refector. was this oyur test or know the dood? wondering how you knew what they did to get these numbers. its not what i get with my light meters

aproximate is right...400 watts from 53k to over 150k is wild aproximates

Cannabisworks said:

i never said anything about a lumens being the best at anything. its what the chart shows so is what i used as a reff. and yes it all relates lumens and distance.
ok so tell me how a 400 goes from the bulb at 53k or so. to an output of a 1k at a distance. man id better sell my 1k and buy 400 for less cost but 10x more light. makes no sence? the hood may have made it last a further distance but you say no hood or any type of refector. was this oyur test or know the dood? wondering how you knew what they did to get these numbers. its not what i get with my light meters

aproximate is right...400 watts from 53k to over 150k is wild aproximates

Click to expand...

so you have a light meter...and an HPS...go stick at at 0 on the bulb and tell me what it says...wait...nvm that...because the light meter can't "see" the entire bulb at that distance...

wont read as lumens are to hi..been there.

Also here is how he measured it:

http://forums.cannabisculture.com/forums/ubbthreads.php?ubb=showflat&Number=1225062

I have the same hood with a 600w in it in a 3x4' tent and keep it ~6" during veg and ~18" during flower. This is definitely not the best hood for dispersion over a large area - it drastically focuses the light in the center. I generally go by the cast pattern my light makes on the back of the tent (see the V shape coming down from it?) - i try to keep the line where the light hits the wall at least 6" above the canopy so that the buds on the sides aren't just receiving horizontal lighting.

The higher it is the more even the dispersion - the lower you have it the brighter it is. Tricky decision.

This pic might be more helpful -

well not sure on his info but i do as he says not to do and dont get those issues. and those pics cant be realy used as a ref here as there is side lighting so all this is out the window. i also run a 600 and tops are about 6 inch all the time. and a 400 i let them touch the glass. no burns no bleaching non of the issues other say. maybe its them and not the light distances