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What distance should my light be from my plants?

SmokeyMacPot

New Member
For (artificial) light, there is a law that always applies known as the Inverse Square Law. It states that light diminishes exponentially in energy as the distance is increased from the source.

A good example is that you might be getting 1000 PAR Watts at 4" from your light source, but that would change to 250 PAR Watts at double the distance (8"). This law makes it EXTREMELY important for indoor plant growers to get their light source as close as possible to their plants. The amount of light your plant receives is directly related to it's yield/flower density.

The problem: Indoor lamps used for plant cultivation (HID - High Intensity Discharge) give off large amounts of heat, to such a degree that they could cause damage to the plant if put too close.

Indoors, there is an optimum distance/height between the plants and the light source. This distance fully illuminates the whole canopy with direct light from the source, but is as close as possible to the plants for maximum lumen intensity. This will be called the OLH, for Optimum Lamp Height.

Ultimately, to get the best light efficiency from your lamp, you want it at the OLH at all costs. But how can you get it there without causing harm to your plants with the abundant heat?

First, try moving your light to the OLH and see what it does to your plants. If they have no problem, then you're fine. If you have a high output HID, this probably won't be enough.

Next, try actively exhausting your light hood by hooking up a direct exhaust system to the hood, and then move your lamp to the OLH and see if the plants are O.K. If the plants still seemed affected by the heat, then you must add glass to your actively exhausted lamp hood. Glass will absorb/reflect/filter some of the light energy being emitted by the lamp.

The number would seem relatively low, around 2-3% of PAR wattage, but it will effectively filter out almost all of what little UV-B is emmitted by the lamp. UV-B is believed, and has been shown, to have a positive influence on the potency of Cannabis.

Overall, it would be beneficial for one to add glass if needed to keep their lamp at the OLH, due to the all-powerful Inverse Square Law; moving light farther away will greatly reduce the amount of energy being emitted and is reaching your plants (Light intensity is directly related to yield and flower density).

Almost all glass offered today for insertion in air-cooled lamp hoods is tempered glass, which is regular glass with low amounts of impurities. If one was looking for the most efficient glass for their hood, quartz glass will allow the transmission of UV-B, but is not made specifically for light hoods.

And also remember, that if you have a rectangular garden, it is important to position the longest side of the reflector parallel to the shortest side of your garden. (from FAQ by Head Rush)

Additional note: you should periodically inspect and clean your light hood and bulbs, especially after foliar feeding or underleaf spraying for insects. The dust and dirt that collects will definitely decrease reflectivity. Isopropanol alcohol, glass cleaner or water (and a soft cloth) can all be used to remove streaks, dust and spots.

Contributed by: MedMan

There are a number of factors which play a part in the temperature radiated from your bulb, watts, hood design and air circulation for example.

A simple method of testing for temperature is to use the back of your hand; if its too hot for your hand, its too hot for your plants. Good ventilation is the key to getting your light closer to the garden.

Editor's note:

Recommended typical OLH distances:

Flourescentsroximimty
400w HPS: 1 foot
600w HPS: 1.5 foot
1000w HPS: 2 foot

Author: BobbyDigital
 

Leary

New Member
How far away should my 250 hps light be from my plants? They are quite young.. about 25 days old.
 

Be Irie

Nug of the Year: 2009 - Plant of the Month: Aug & Sept 2008, Dec 2008, Jan 2009 - Member of the Month: Feb 2009 - Nug of the Month: May 2008, Jan & Feb 2009, Nov 2012
A simple method of testing for temperature is to use the back of your hand; if its too hot for your hand, its too hot for your plants. Good ventilation is the key to getting your light closer to the garden.
:smokin2:
 

bud-wizer

New Member
i would like to be using a MH and HPS dual system 600 W each, should i keep the OLH aroud 1.5 feet?
im sorry this might sound abit of a dumb question but wat is HPS? im guessing high powered sodium?
 

bud-wizer

New Member
thanks mate .
 

bud-wizer

New Member
Is it true that the LED grow lights are much cooler, but also much less effective for plants?
yes i believe so , ive grown with LED and they do work but it takes twice as long ... the best thing about them is that they r cheep to run and dnt cause much heat. i now grow with hps and LED to get the best of both worlds .
 

dogstick

New Member
"Flourescentsroximimty
400w HPS: 1 foot
600w HPS: 1.5 foot
1000w HPS: 2 foot

Author: BobbyDigital "


do these distances change with an air cooled hood? I can leave my hand an inch from the glass all day long but i assume that getting much closer then the 18" with a 600 in not a great idea?

thanks


Yea Vapor that is just a rule of thumb, Air cooled will allow you to get much closer. Best bet would be just to hang a thermo guage at the distance you want the light at and measure it from that. I can keep my 600hps at about 12in and I dont even have a air cooled hood. You should be able to get to 12 or 10 I would say at least. well GL broham.
 

420goat

New Member
Optimum temp @ top of plant closest to light is 70 degrees. Therefore, your room must be much cooler than 70 degrees in order for the temp to be 70 degrees at the top of your plants. Air cool your lights with turbo exhaust fans w/glass. Make sure your air cool system is sealed so you don't loose CO2 if you're using it. Have wall-mounted fans blowing across the plants inbetween your light and plants and lastly and most important, GET AN AIR CONDITIONING UNIT! lol There are also water cooled lights as well that emit no heat too!

BOTTOM LINE:
Get the lights as close to the plants as possible w/o the temp at the top of the plant exceeding an optimum 70 degrees. (max 80 degrees)

Good Luck
aka
420guru
 

Hypo Hippy

New Member
Do you know if the MH numbers are similar? I am trying to get my light as close as possible and right now am at 18 inches for a 400W MH in a 4 x 8 closet. Temps are running at around 80 degrees. Thanks for the chart. I will be switching to a HPS bulb for flower cycle.
 

bizz

New Member
Do you know if the MH numbers are similar? I am trying to get my light as close as possible and right now am at 18 inches for a 400W MH in a 4 x 8 closet. Temps are running at around 80 degrees. Thanks for the chart. I will be switching to a HPS bulb for flower cycle.
MH puts out slightly less lumens than HPS, that chart is really for lumens not heat as it tells you how close you can get without doing damage from too many lumens and how far you can get while still getting enough lumens to grow proper

as for heat, if you use air cooling then heat would not really be an issue and you could get as close as this chart states

if you do not have an active fan air cooling system then back of the hand test will probably be good enough to tell you how close you can get, my friend has a 400w hps in a 3x2 space and his plants are growing up into the reflector hood and almost to the bulb without any damage but his bulb is old, i had a 400w in same size space years ago and got some slight heat damage on plants centered under light, mostly just leaves...

in my opinion, a 400w MH or HPS is good for like 3x3 maybe 4x4 or a little more if done vertical, 4x8 is 32sq ft... you could do 400w for veg but for flower I would think you'd be better off doing 4x4... like half the plants and maybe more yield, def bigger nugs...
 
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