What distance should my light be from my plants?
Added by: MedMan edited by: snoofer
Contributed by: BobbyDigital
Submitted: April 24th, 2004
For (artificial) light, there is a law that always applies known asthe Inverse Square Law. It states that light diminishes exponentiallyin 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 atdouble the distance (8"). This law makes it EXTREMELY important forindoor plant growers to get their light source as close as possible totheir plants. The amount of light your plant receives is directlyrelated to it's yield/flower density.
The problem: Indoor lamps used for plant cultivation (HID - HighIntensity Discharge) give off large amounts of heat, to such a degreethat they could cause damage to the plant if put too close.
Indoors, there is an optimum distance/height between the plants andthe light source. This distance fully illuminates the whole canopy withdirect light from the source, but is as close as possible to the plantsfor maximum lumen intensity. This will be called the OLH, for Optimum Lamp Height.
Ultimately, to get the best light efficiency from your lamp, youwant it at the OLH at all costs. But how can you get it there withoutcausing harm to your plants with the abundant heat?
First, try moving your light to the OLH and see what it does toyour plants. If they have no problem, then you're fine. If you have ahigh output HID, this probably won't be enough.
Next, try actively exhausting your light hood by hooking up adirect exhaust system to the hood, and then move your lamp to the OLHand see if the plants are O.K. If the plants still seemed affected bythe heat, then you must add glass to your actively exhausted lamp hood.Glass will absorb/reflect/filter some of the light energy being emittedby 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 isemmitted by the lamp. UV-B is believed, and has been shown, to have apositive influence on the potency of Cannabis.
Overall, it would be beneficial for one to add glass if needed tokeep their lamp at the OLH, due to the all-powerful Inverse Square Law;moving light farther away will greatly reduce the amount of energybeing emitted and is reaching your plants (Light intensity is directlyrelated to yield and flower density).
Almost all glass offered today for insertion in air-cooled lamphoods is tempered glass, which is regular glass with low amounts ofimpurities. If one was looking for the most efficient glass for theirhood, quartz glass will allow the transmission of UV-B, but is not madespecifically for light hoods.
And also remember, that if you have a rectangular garden, it isimportant to position the longest side of the reflector parallel to theshortest side of your garden. (from FAQ by Head Rush)
Additional note: you should periodically inspect and clean yourlight hood and bulbs, especially after foliar feeding or underleafspraying for insects. The dust and dirt that collects will definitelydecrease reflectivity. Isopropanol alcohol, glass cleaner or water (anda 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 temperatureradiated from your bulb, watts, hood design and air circulation forexample.
A simple method of testing for temperature is to use the back ofyour 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<
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