Page 1 of 8 12345 ... LastLast
Results 1 to 15 of 114
Like Tree23Likes

Thread: What distance should my light be from my plants?

  1. #1
    420 Member SmokeyMacPot's Avatar
    Join Date
    Jan 2004
    Posts
    3,175

    What distance should my light be from my plants?

    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

  2. #2
    New Member
    Join Date
    Oct 2009
    Posts
    6

    Re: What distance should my light be from my plants?

    i would like to be using a MH and HPS dual system 600 W each, should i keep the OLH aroud 1.5 feet?

  3. #3
    420 Member
    Join Date
    Sep 2009
    Posts
    188

    Re: What distance should my light be from my plants?

    I thought marijuana was supposed to destroy brain cells, after reading this, I doubt that it is true.
    Last edited by littlelil; 11-10-2009 at 05:20 PM. Reason: spellng error

  4. #4
    420 Member Baja Big Dog's Avatar
    Join Date
    Oct 2009
    Location
    Cali
    Posts
    219

    Re: What distance should my light be from my plants?

    Quote Originally Posted by littlelil View Post
    I thought marijuana was supposed to destroy brain cells, after reading this, I doubt that it is true.
    I have destroyed may cells trying to read all this great info!!

    Gotta keep reminding myself, its just a weed!!!!
    THE OFFICIAL "SHORT BUS" DRIVER...

    And part time window licker!!!

  5. #5
    420 Member Leary's Avatar
    Join Date
    Aug 2008
    Posts
    81

    Re: What distance should my light be from my plants?

    How far away should my 250 hps light be from my plants? They are quite young.. about 25 days old.
    "If we could see the miracle of a single flower clearly, our whole life would change."

    "When you realize how perfect everything is you will tilt your head back and laugh at the sky"

    - Siddhartha Gautama


  6. #6
    Cannabis Connoisseur - Nug of the Month Winner - November 2012 Be Irie's Avatar
    Join Date
    Dec 2007
    Location
    Colorado
    Posts
    6,902
    Blog Entries
    2

    Re: What distance should my light be from my plants?

    Quote Originally Posted by SmokeyMacPot View Post
    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.

  7. #7
    420 Member bud-wizer's Avatar
    Join Date
    Oct 2008
    Location
    england
    Posts
    49

    Question Re: What distance should my light be from my plants?

    Quote Originally Posted by gazan114 View Post
    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?

  8. #8
    420 Member dogstick's Avatar
    Join Date
    Sep 2009
    Posts
    392

    Re: What distance should my light be from my plants?

    Quote Originally Posted by bud-wizer View Post
    im sorry this might sound abit of a dumb question but wat is HPS? im guessing high powered sodium?
    high pressure sodium

  9. #9
    420 Member bud-wizer's Avatar
    Join Date
    Oct 2008
    Location
    england
    Posts
    49

    Re: What distance should my light be from my plants?

    thanks mate .
    its a new world 2 me . im taking so much in and learning more than my brain can handle. keep it comin though as im hooked!!!!!!!!!!

  10. #10
    420 Member
    Join Date
    Dec 2009
    Posts
    49

    Re: What distance should my light be from my plants?

    Is it true that the LED grow lights are much cooler, but also much less effective for plants?

  11. #11
    420 Member bud-wizer's Avatar
    Join Date
    Oct 2008
    Location
    england
    Posts
    49

    Re: What distance should my light be from my plants?

    Quote Originally Posted by P8riot View Post
    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 .
    its a new world 2 me . im taking so much in and learning more than my brain can handle. keep it comin though as im hooked!!!!!!!!!!

  12. #12
    420 Member
    Join Date
    Dec 2009
    Posts
    49

    Re: What distance should my light be from my plants?

    Thanks for responding. It's always good to hear from experienced people.

  13. #13
    420 Member
    Join Date
    Sep 2009
    Posts
    537

    Re: What distance should my light be from my plants?

    "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

  14. #14
    420 Member dogstick's Avatar
    Join Date
    Sep 2009
    Posts
    392

    Re: What distance should my light be from my plants?

    Quote Originally Posted by Vaporeyes View Post
    "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.

  15. #15
    420 Member
    Join Date
    Sep 2009
    Posts
    537

    Re: What distance should my light be from my plants?

    thanks Dog, ill have to drop it a few....

Page 1 of 8 12345 ... LastLast