How far to keep HPS light from plants

My light is 400 watts. Switched the lighting to 12/12 two weeks ago. Can anyone tell me what the difference is between having the light 16in away from the plants verses 3 feet? Right now I've got the light raised about 3 feet. Someone told me I need to lower it to about 16 inches from the plants. Also the humidity in the grow room is about 50 and they tend to start drying out after a day and a half. I'm trying to get that under control. I s 50 normal?

I have my 400W HPS about 10-12inches from the tops of my plants.. just do the hand test - place your hand under the HPS light & when it becomes too uncomfortable then it will also be for your plants, so raise it until it becomes comfortable. If the light is too far away the plants will stretch more & become lanky with bigger gaps between the internodes. 50 is a little high for flowering mine is at 23% humidity.. I recommend over 40% humidity for veg & under 40% for flowering - that's how I gauge it

lighthouse said:

My light is 400 watts. Switched the lighting to 12/12 two weeks ago. Can anyone tell me what the difference is between having the light 16in away from the plants verses 3 feet? Right now I've got the light raised about 3 feet. Someone told me I need to lower it to about 16 inches from the plants. Also the humidity in the grow room is about 50 and they tend to start drying out after a day and a half. I'm trying to get that under control. I s 50 normal?

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3ft is way to high , especially for a 400w.Bring it down to about 16 to 18 and hold your hand on top of plants and see how it feels.
The 50rh is fine this early in flower.
What size containers are they in?
Is your hood aircooled? How much ventilation do you have?
When you water, how much run off do you allow.
All these things have an affect on how close you can put your lights.

get one of those thermometers with the probe on a wire at home depot or somewhere. place the probe directly at the level of the canopy so you always know the exact temp coming down on your plants; eliminates the guesswork. I like to keep mine around 75F since I have no co2 at the moment.

the hand test is key. if your hand can be there without burning you're good. honestly my plants are fine less than 5 inches away from my 400w hps. it's all about venting that light and the heat under the light. the better you vent the light the less heat there will be directly underneath. my camera is broken so this pic is a little old but you can see how close they are to the glass. if you do not have an air cooled hood i would NOT put them this close. i also have a fan blowing right under the hood now that further rids that space of concentrated heat and pushes it out of the closet.

Ok gotcha. My problem is I don't have anything to vent the heat from the light. What kind of stuff do i need to get? And can i get the supplies from Lowes where it's cheaper? I got them in 2 1/2 gallon pots. The temps get up to 90 at times in there.

lighthouse said:

Ok gotcha. My problem is I don't have anything to vent the heat from the light. What kind of stuff do i need to get? And can i get the supplies from Lowes where it's cheaper? I got them in 2 1/2 gallon pots. The temps get up to 90 at times in there.

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If you can't afford an air cooled lamp/hood direct a fan onto the bulb so it dissipates the heat from one spot, that's what I do.. I serioulsy recommend a out-take fan too to suck all that built up heat out..There are some DIY air cooled lamp & hood set ups about like here: https://www.rollitup.org/do-yourself/8505-improve-air-flow-air-cooled.html
I guess your from the US? so wouldn't know where to get all the fixtures etc, but I've heard places like Lowes & Home Depot seem popular with people on RIU for DIY home grow set-ups.

The intensity of radiation from the sun is 9140 wastts per square meter at the distance of Mercury; but only 1370 watts per square meter at the distance of Earth —a threefold increase in distance results in a ninefold decrease in intensity of radiation.
i.e the further away the canopy from the source the less intense the light(i.e less lumens reaching the canopy less bud

so every inch counts 10 inch from canopy and an oscillating fan aimed at the canopy\lamp sounds like a good idea(in the middle

mines a 600w hps @ 10-12 inch from canopy with a 6inch oscillating fan aimed smack ban in the middle of them both.
I couldnt afford an air cooled hood this time round as iv just changed over to hydro but i will be investing in one next time round.

Easy answer!! Keep any light as close to the plants as you can with out burning them. I do not know the exact #'s but I have read that for every certain distance the light is away from the plants the lumens are reduced exponentially. For those of you that don't understand I will break it down like this, x= distance and y= lumens so if x=12" y may equal 50,000 lumens so commons sense would say if you up x to 24" there should only be a small reduction in lumenation right? As I stated I don't know specific #'s but that extra 12" will give you a far greater reduction in effective light than one would think.

I have my 600W about 12 inches or so and its an open hood not cooled, I just use a box fan blowing onto the light as suggested by some others in this thread. I do the same with my 1000w

There ya have it!! + Rep to Robert

I've been growing for years, and never have I used a vented HPS hood, I'm doing 2 600 watts in an 8x4 tent, my plants are already 5.5ft tall and I'm maxed with the top of the tent being 6.5ft ,almost through stretch! My question is ,is how close can I get them? I'm about 6- 8 inches away from the hid now

lighthouse said:

My light is 400 watts. Switched the lighting to 12/12 two weeks ago. Can anyone tell me what the difference is between having the light 16in away from the plants verses 3 feet? Right now I've got the light raised about 3 feet. Someone told me I need to lower it to about 16 inches from the plants. Also the humidity in the grow room is about 50 and they tend to start drying out after a day and a half. I'm trying to get that under control. I s 50 normal?

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check the above, with a 400* you can get down to 6 inches to get 63662Lu

just keep a fan and gauge close by

good luck

vostok said:

check the above, with a 400* you can get down to 6 inches to get 63662Lu

just keep a fan and gauge close by

good luck

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Thanks! This chart will be a life safer!

I run vented hoods without glass.

Johnny Lawrence said:

I run vented hoods without glass.

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How close are you getting your plants?

GangusKush said:

How close are you getting your plants?

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I don't actually measure - I kinda eyeball it and rely on the hand test under the light. If I had to guess, I would say the tops are about 12 to 16 inches from the bottom of the hood. But that's on the edges of the reflector, not right under the bulb.

The general rule of thumb is 18 to 24 inches above the canopy, or the top most part of your plants.

Aegean said:

The general rule of thumb is 18 to 24 inches above the canopy, or the top most part of your plants.

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Johnny Lawrence said:

I don't actually measure - I kinda eyeball it and rely on the hand test under the light. If I had to guess, I would say the tops are about 12 to 16 inches from the bottom of the hood. But that's on the edges of the reflector, not right under the bulb.

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Had to cover my hoods with paper haha