What distance should my light be from my plants?

[gr865]

One of the best vertical grows I have seen -----excellent set-up !! thanks (be kind of small for 5 plants ? maybe 1 at the most 2 ?

Hey dude, that is a very old post, and that grow failed due too irrigation issues, will never use Blumats again. LOL
My last couple of vert grows were 1.18 and 1.1 G/W, 26.5 and 24.5 zips per grow. Have had a couple of horizontals in between but only one came close to matching the G/W @ 0.89. don't remember the weight right now, somewhere in the 20 zip range. Did have a two plant 15 zip grow that was nice.

Check out my current grow in my signature. Big Buddha Cheese.
Very overgrown because I missed my planned flower date by 2 weeks and they are out of control now, but I will be working on them big time this next week at day 21 of 12/12 to get them back under control.

 

[The70’s]

So if heat isnt an issue, as with using LED, it seems more about MicroMol’s (usable plant light) than distance from the canopy, i do hear bleaching my be an issue though.......

 

[Edwina420]

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

Hi. Does the directions apply to LED'S?

 

[safeman]

they are out of wack -for a 300 led 48 inches veg and 34 inches flower now i reality 18-24 veg and 12-14 flower - never no heat issues

 

[volksball]

Hey everyone, its nice to be back around such a great community.

About to start my second grow after quite a successful one in here a year ago. I've literally been high for 5 months. Have a look at my first journal in my signature line, if you like. Alot of smart people weighing in.

Looking to have a better light rig for my seedlings and looking to do it in the tent where I can better control temp and RH, rather than outside the tent for the seed stage, then in under the big lights when they're old enough.

I bought the following products in the pics to rig up overhead CFL lighting for 3-5 seedlings. The bulbs are 6500k and 23w CFLs. Seems like a good rig for what I'll be doing, but let me know if you see issue. More lights? Seems like enough, but always open for more advice.

I'll be starting another journal once the seeds pop.

Thanks.
V

 

[safeman]

Hey everyone, its nice to be back around such a great community.

About to start my second grow after quite a successful one in here a year ago. I've literally been high for 5 months. Have a look at my first journal in my signature line, if you like. Alot of smart people weighing in.

Looking to have a better light rig for my seedlings and looking to do it in the tent where I can better control temp and RH, rather than outside the tent for the seed stage, then in under the big lights when they're old enough.

I bought the following products in the pics to rig up overhead CFL lighting for 3-5 seedlings. The bulbs are 6500k and 23w CFLs. Seems like a good rig for what I'll be doing, but let me know if you see issue. More lights? Seems like enough, but always open for more advice.

I'll be starting another journal once the seeds pop.

Thanks.
V

go to "1,000 bulbs.com" for you cfl's - big watters 500W both spectrums 65.00K to 2400K all are self ballast -

 

[volksball]

go to "1,000 bulbs.com" for you cfl's - big watters 500W both spectrums 65.00K to 2400K all are self ballast -

Thanks. I'm a bit late to that party. You think for seeds this will do? I have a 1000w HIDs to take over after the seeding stage.

 

[safeman]

Thanks. I'm a bit late to that party. You think for seeds this will do? I have a 1000w HIDs to take over after the seeding stage.

Yea ! cfl's do good for germination and seedlings - like you said the 6500K

 

[volksball]

Yea ! cfl's do good for germination and seedlings - like you said the 6500K

Perfect! Thanks man!

 

[fanleaf]

The inverse Square Law can be much more complicated than saying

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").

I have shown many times that there are more factors than just distance. Many of my setups do not show the same ratio of PAR to Distance ratio and disagreeing with the "inverse square law".

 

[Whitesmoke13]

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.

You will be fine ,flowering should drop temps with less hours of light.I use same light.

 

[Maddie Young]

a single HPS 600-watt light is enough right? hmmm for vertigo strain?

 

[Weedman11]

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

Yes they run much much cooler. It depends on the light you buy. There are led’s that completely replace the need of mh/hps with more wattage output to reach the same level, but using only about 1/3-1/2 the electricity. I’d say just do your research on PAR to understand fully how it all works along with full band spectrums on leds. I recommend vespectra or bestva lights if your on a budget for pretty solid quality, but definitely Advanced LED’s if you’ve got some cash to play with.

 

[Weedman11]

a single HPS 600-watt light is enough right? hmmm for vertigo strain?

Depends on the coverage area, how big is your tent/space?

 

[safeman]

wit a 300 watt Led 12 inches from top (no burn) RANGE 18-12 INCHES when flower and for veg 28 inches (28- 24 inches_ depends on strain and plant growth

 

[Victorytoker]

Its worth mentioning that getting a led light "as close as possiblle" will totally destroy your plants and is bad bad advice. Give a plant in veg 1000par and it will grow wacky and show all kinds of deficiency that are not actual deficiency, the plant can only move nutrients so fast, but it can photosynthesis all the light. Enter fox tailing buds and bad growth. There absolutely IS a thing as too much light