1. We are currently experiencing issues with viewing and uploading images, our team is working on the issue.
    Dismiss Notice

how far should i keep my light from the plants?

Discussion in 'General Marijuana Growing' started by GlassFreak, Dec 29, 2008.


    GlassFreak Well-Known Member

    i have a 400w hps with a mh convertion bulb too. i need to know how far away to keep my light from the top of my plants for the best result.

    autotek500 Well-Known Member

    no closer than 24 inches.....set you're light at this hight and hold you're hand at the top of your plants...if it is not too hot after about 30 seconds you're fine .......

    GlassFreak Well-Known Member

    k thanks man!!
    ThE sAtIvA hIgH

    ThE sAtIvA hIgH Well-Known Member

    its a case of trial and error really , im going through the same thing at the moment ,im used to growing with cfl ,but i recently bought a hps light and im trying to get the distant right now , im using a big desk fan blowing straight on the bulb , and keeping the light about 10inches from the tops , but lastnight i somehow forgot to turn the desk fan on , thismorning i woke to some burnt leaves ,lol ive not burnt leaves for ages , oh well ....they will recover .
    i kind of forgot how hot hps lights get, i used to use a 600w years ago then i went into using cfl , and now i just got a 250w hps , the 250 gets hotter than i thought it would , but im gonna add a couple of intake fans to the cab wich should help lower the temps a bit .
    the answer to your question is as close as you dare , the closer the better , light diminishes the further away it is , so its a case of keeping it as close as possible without burning your plants .id recomend a poerfull fan blowing at the bulb or straight over the tops of your plants it will really help ,otherwise you could buy a cool tube .

    gangjababy Well-Known Member

    That's way too far, you are supposed to keep a 1000 watter 36 inches away from the tops. I keep my air cooled 400HPS 4 inches from the tops of my plants, if it's not covered then maybe keep the light 8 inches away.

    autotek500 Well-Known Member


    sarah22 Well-Known Member

    yea 24 inches is waaaay too far...i have a 400w hps and its almost never more than 6 inches away from the plants :D but yea...just put ur hand on top of the plants to find out if its too hot just like autotek said!

    GlassFreak Well-Known Member

    k well i dont really have any venting in my room. even if i bought a cool tube i would have no venting to hook it up with... i have a few small 6 inch fans i use but i just bought a bigger 8 inch fan. i dont know whether i should use the bigger one and clamp it at the top of my room and make at least an outlet for airflow with the fan pointing toward it or if i should just clamp the fan near the light and have it right on it all the time... i am going to put some inlet holes at the bottom too... any advice on that?

    TRICKKY Well-Known Member

    Depends what sort of room your in really. The fan pointing towards an outlet is not going to be the most efficient or effective way.

    An inline fan, that will pull air out of your room would be a good option, even if you only get a small one.

    Air cooled hoods are really good for small cabs.

    TRICKKY Well-Known Member

    Yes closer to the bottom the better, it should be nice cool fresh air. And hot air out of the top.

    I try to make sure the area of inlet (that cool air comes through) is at least 2x the size of the outlet. Doesn't have to be one hole, can be numerous smaller holes.

    GlassFreak Well-Known Member

    sounds great man

    madmax420 Member

    I use a 150 watt hps light when off it goes down 10 dregrees when on 12 to 13 dregrees higher I would use a 120 mm intake computer fan and then a 80 mm out take fan if you can. if you ever get to hot in your room with no vent take a frozen water bottle and put it in a plastic bag and set it in there. the vapor off the water well stay in the bag. there for cooling your room I think it depends on the type of plants you have and your suroundings in your grow room. just a thought. If it didn't make since. I don't care I'm stoned and I don't care. Thank god for prop 215 in california.

    CLOSETGROWTH Well-Known 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:

    Flourescents: proximimty
    400w HPS: 1 foot
    600w HPS: 1.5 foot
    1000w HPS: 2 foot

    KaleoXxX Well-Known Member

    YES! im glad someone mentioned the hand technique!

    but yes 24 in is way too far, do the hand test yourself and see how far away the light gets really warm

    and you really DO need ventalation in your grow space. that should controll temps and make it easier to have your lights and your plants closer, or as closet said maximize your OLH

Share This Page