Quantifying Light for the Average Grower

I've spent quite a lot of time studying and stressing over light. After all, light is one of the most important factors. I think that once I get going here, people are going to say...It's PAR that matters, not LUX! Well, this thread is dedicated to people who don't want to splurge on a PAR meter, but can stomach a $20 lux meter. I think we could stay true to the science of we compare lux of HPS light, lux of T5, lux of MH.

I think there is probably a range of optimum lux and light type for each stage of plant, seedling, clone, veg(maybe early/late even) and flower (early/late).

Some will say, there is no such thing as too much light. Common sense says a 1000w HPS, cooled, will vaporize seedlings. If you have a light 5 feet away to use it, you are wasting electricity! In my opinion, if it's 2-3' it is kind of a waste, but within the realm of, who wants to micro-manage light height.

The best thing I've read is, set your light and let the plant grow into the space. When we study light we hear alot about how much light penetration we need. That's really a spectrum. Are you growing 30" plants? You need penetration. Are you growing 12" above a scrog screen?, maybe you don't need that super penetration power. So we need to look at, how far from the light will our plant be when we start a phase of growth? Does that satisfactorily provide light for good growth, while leaving enough room for the plant to grow 12" without exceeding maximum light levels at full height.

Hopefully we can get some good contributions of lux vs distance from reflector. 2" increments. Name the reflector used, that is an important variable. Temps and light venting could be pertinent.

I'm not at home, but I have some figures already.

The best we can get here is seasoned growers getting good yields, providing lux data and factors to develop a good lux spectrum. For those not familiar, 1 lux = 10.76 lumens.

My initial thoughts are that 25,000-30,000 lux are the low end of intensity and 80,000 is the high end of HID bulbs. I just doubt people who say the plant can handle full unobstructed sunlight at the equator all day long, which is over 100,000 lux. Also, the sun and HID bulbs probably have different PAR values.

Here is a few of what I have found geared toward Veg, with some opinions:

23 Watt 6500K CFL in 10" Bayco Brooder Reflector

Distance Lux Lumens

2" 35,000 3,253
4" 32,000 2,974
6" 27,000 2,510
8" 22,000 2,045
10" 17,000 1,580
12" 12,000 1,115

*Basically useless 10" plus away, maybe 8-12" could be good for cloning
*Could be good with close placement getting seeds from germination through seedling, maybe early veg.

96 Watt T5 by HTG Supply (2'x4 Bulb) in a small diamond foil space

Distance Lux Lumens
2" 33,000 3067
4" 25,000 2323
6" 20,000 1859
8" 18,000 1673
10" 15,000 1394
12" 12,000 1115

*Really only useful for clones and seedlings

250 Watt MH in a Cool Tube Reflector

Distance Lux Lumens

2" -
4" 84,000 7807
6" 58,000 5390
8" 42,000 3903
10" 33,000 3067
12" 26,000 2416
14" 23,000 2138
16" 20,000 1859
18" -

*Coming into some powerful values if heat can be controlled. This could be used at a distance for seedlings and clones, vegetatively maybe young into mid to late veg depending on the height you wish to veg to. Short Scrogs it could work completely, larger, tall, may need supplemental or a bigger light.

400 Watt MH in a Cool Tube Reflector

Distance Lux Lumens/sf
2" -
4" -
6" 85,000 7900
8" 60,000 5576
10" 44,000 4089
12" 40,000 3717
14" 33,000 3067
16" 30,000 2788
18" 26,000 2416
20" 24,000 2230
22" 22,000 2045
24" 20,000 1859


*This lamp would allow veg of a pretty decent sized plant, and also a wider band of higher intensity light for optimizing growth of shorter plants.

The part I struggle with is the stages of life.

What are we talking about here? Clones 10,000 to 20,000 lux? Seedlings 15,000 to 30,000 lux? Standard Veg 25,000 - 50,000+ lux?

"Some will say, there is no such thing as too much light. Common sense says a 1000w HPS, cooled, will vaporize seedlings. If you have a light 5 feet away to use it, you are wasting electricity! In my opinion, if it's 2-3' it is kind of a waste, but within the realm of, who wants to micro-manage light height."

On that, i wanted to put it out there that ive sprouted seeds, and taken clones under 1kw hps/mh. Just putting the on the edge of the area. Here my logic and it kinda fits with the last bit you said in the quote: the plants are going to finish under that lamp, whats the point changing it?

The answer is nearly always to use the most light possible, so i have seeds/clones on the edge of the footprint, flowering plants in the middle. Then i just move the seeds/clones closer as they can handle it. My area has "plant booster seats" also so i can adjust the height of each quater of the area.

I dont have a light meter so i dont have any figures, but the meathod of moving them closer and father from the light (simularly to how i feed nutrients) seems to work well for me.

Any stressed plant gets moved away from the light just like if a plant is nute burned you use plain water.

I think the higher watt blubs have a "higher density of light" least thats how i think of it. Helps the whole plant grow not just the top near a weak bulb.

growingforfun said:

"Some will say, there is no such thing as too much light. Common sense says a 1000w HPS, cooled, will vaporize seedlings. If you have a light 5 feet away to use it, you are wasting electricity! In my opinion, if it's 2-3' it is kind of a waste, but within the realm of, who wants to micro-manage light height."

On that, i wanted to put it out there that ive sprouted seeds, and taken clones under 1kw hps/mh. Just putting the on the edge of the area. Here my logic and it kinda fits with the last bit you said in the quote: the plants are going to finish under that lamp, whats the point changing it?

The answer is nearly always to use the most light possible, so i have seeds/clones on the edge of the footprint, flowering plants in the middle. Then i just move the seeds/clones closer as they can handle it. My area has "plant booster seats" also so i can adjust the height of each quater of the area.

I dont have a light meter so i dont have any figures, but the meathod of moving them closer and father from the light (simularly to how i feed nutrients) seems to work well for me.

Any stressed plant gets moved away from the light just like if a plant is nute burned you use plain water.

I think the higher watt blubs have a "higher density of light" least thats how i think of it. Helps the whole plant grow not just the top near a weak bulb.

Click to expand...

My SCROG leanings may be showing through in my logic. I like even canopy.

Underground Scientist said:

My SCROG leanings may be showing through in my logic. I like even canopy.

Click to expand...

And Perpetual Harvest...

Maybe someone out there has done some research on Umols and PAR and optimum levels at varying stages of the Cannabis life cycle. Umols for clone/seedlings, veg (early/late), flower (early/late).

I found one thread somewhere where someone suggested 200 Umol for seedlings/cuttings, 650-800 Umols for Veg, 1,000-1,200 Umols for flower. I'm sure many have read the research that 1500 Umols is peak, but the temp and co2 has to be peak as well.

Some might be saying, who the frig even has something to measure Umols?? or how to convert it? That's what I thought because I don't want to buy an expensive meter at this point. I found a link in Read Spear's book, Marijuana Cultivation Reconsidered...

PPFD to lux is 82 for HPS

The full chart is:

HPS 82
DEHPS 77
Flouro 74
Sun 54
CMH 4200k 65
CMH 3100k 59

So 100 Umol is 8200 HPS lux, 7700 DEHPS lux, 7400 Flouro lux, etc.

the link is to Apogee Instruments website

So if these guys had a credible source (which I can't find)

200 Umols for Seedlings or Cuttings

My CFL or T-5 (74 Conversion Factor x 200) = 14,800 lux

Hmm, for clones, maybe. For seedlings, maybe a tad more. That's about 10" on those fixtures.

650 Umols for Early Veg

My MH Veg Bulbs (71 Conversion Factor x 650)(whoops, forgot the regular MH factor in last post) = 46,150 lux

800 Umols for Late Veg

My MH Veg Bulbs (71 Conversion Factor x 800) = 56,800 lux

Interesting...pretty powerful. Thats 8-10" from the light with a 250 or 400, not unrealistic with air cooling

1,000 Umols for Flower

My HPS Flower Bulb (82 Conversion Factor x 1,000) = 82,000 lux

That's some fire power right there!

That would be about 123,000 lux for 1500 Umols

That's why those with full climate control use a 140,000 lumen, 1,000 watt hps.


It's funny the recommendation jumped from clone/seedling so quick, three times the intensity. Probably makes more sense that a seedling could handle 400 Umols, at about 30,000 lux. and progress it's way to early veg at 46,000 lux. There's one growth spectrum to aim for right there, and expand that to 57,000. Let's say 30,000 - 55,000 lux for a vegetative plant. Then switch the light and set the top of the plant at 55,000 lux and let it stretch to 82,000. I can imagine going higher than 80,000 lux is going to take very active monitoring of deficiencies created by huge amounts photosynthesis, and blur the lines between light/heat stress and nute burn/deficiency. These could be dialed in with experience.

That's my OCD synopsis of light intensity and fixture distance, which I can expand on my HPS fixtures. Waiting for my Dominator XXXL Hood to come in.

I adjusted my light for my veg plants. They are very short, like 6-7" with about 8 nodes at least, and a big ass stem. I did some LST, but still, and branching is pretty well underway. However, I did have a CalMag deficiency that has been treated with an increase in feeding and foliar spray. Even a while after, which it was like this much of the veg grow, the leaves are dry to the touch, not quite crispy where they would break, or yellow on the tips or edges, but troubling. Humidity was low, and I've gotten that up to about 25%. Still issues. The leaves are drooping. I wait plenty of time between waterings, but I don't dry them out completely. The new growth on tops have been thin and narrow, angry looking. I have had the plants under 50-56,000 lux MH at roughly a foot below a 400 watt mh, as many say, the more light the better as long as heat is under control, which it is. I now have it about 18" above, giving it 35,000-40,000 lux of MH light. If I see the leaves downward pointing stops and recovery in the new leaf growth, it should be clear these plants don't want the light I've been giving them. Unfortunately, I would have to consider the deficiency I had causing stress, warranting less light, and may make this a poor baseline for comparison for future grows.

Very interesting read, makes me wonder about my lux and what I should be doing throughout the grow to adjust for the diffrent stages of a plants life. I have a 1000w hid with 140000 hps output and 100000 mh output, didn't realize how much I had until comparing it to what you have written here. I haven't used the light to grow anything yet but fortunately I have a dimmer on my ballast so I can start my seedlings under a 200 w cfl or 400 w of cfl then move to the veg tent and use my hid dimmed down to 600 for the first few weeks and have plenty of light. How long into growing do you think i would be able to safely switch to the 750w? 1000w? I would think I would want to keep it 16- 24 inches from the top of the canopy to evenly disperse light across the 4x4 tent. Does that sound about right for height for my size tent?

Disclaimer: I'm not an expert...lol. I love research though. I do find measuring the intensity interesting and using it as a measurement of what my plants actually want. When I switch from one light to another, or one area to another, I can have some tangible data to know I'm not stressing the snot out of them. I'm a fan of the Grow Boss. He puts it pretty simple...When to increase the light?...When the plant matter has increased to handle it. So you start at 600, when you are adding all that material from the stretch, you could increase it. Now, would they stretch less if you increased wattage at shorter intervals? Interesting experiment. I'm sure lots would say, go ahead and just use the 1000. My theory is, if you have to put the light higher because of intensity, you may be wasting electricity. As far as footprint goes, my take on it is this...If your hood gives you too narrow of a focus so you have to position it too high, maybe you need a mover, or maybe you need a XXXL hood. 16-24 I think is pretty good though. I'm set up to use a screen, with a XXXL Hood 24" from the screen, allowing for 12" of my vegged plants to stretch up through. That leaves the top 12" away, which I might have to raise, but I have about 6"+ to raise it if need be.

A lux meter is 20 bucks...I like having it in my grab bag of toys...PAR meters are expensive, and you really won't need this stuff unless you are testing new lights or new spaces, or modding your space. It's pretty interesting that when I have the door off my grow box, the lux meter will drop almost 5,000 lux. Enclosed spaces and reflectivity do make a big difference.

A 4x4 - 5x5 is perfect for 1,000w. I have a 3x5, I might only go to 750w.

I use one of those cheapo meters from Homedepot to compare lighting at diffrent spots in the tent. I'm not sure how accurate it's readings are but it's helpful to compare.
Using that it seems like the majority of the light reflected can reach the sides of the tent about 18" from the bottom of the hood

I agree that it feels like a waste of electricity to have a 1000w running but having to keep it 4 feet up to not kill the plants. That's why I'm going to start my plants in a smaller box with 2 200w cfls, probaby even start with one for the first couple weeks. After that I will move them under the mh dimmed to 600

Where are you placing seedlings/clones? You might look at 250w MH versus 400w cfl. I bet it will work fine though. My veg is about 2x2

Underground Scientist said:

Where are you placing seedlings/clones? You might look at 250w MH versus 400w cfl. I bet it will work fine though. My veg is about 2x2

Click to expand...

I have a 1×2 cab that's works pretty good, I have a 150w hps I actually flowered with in there but it's hot, that's why I like the CFLs, less work the exhaust has to put in

If you do a little searching, you can find a hack to turn that $20 lux meter into a par meter for a few dollars.

ҖҗlegilizeitҗҖ said:

I have a 1×2 cab that's works pretty good, I have a 150w hps I actually flowered with in there but it's hot, that's why I like the CFLs, less work the exhaust has to put in

Click to expand...

Nice, I use 100watt T-5 in my 1x2. Small space can heat up quick.

Underground Scientist said:

Nice, I use 100watt T-5 in my 1x2. Small space can heat up quick.

Click to expand...

Got a 6" inline Fan at the top, it helps a lot. I just bought a reducer for it because it's actually quite a bit more than I need for that small of an area

So I reduced the intensity in veg from 40-50000 lux to about 20,000. The leaves started uncurling and lifting. I increased the intensity and they started lowering. I really think there is an optimum here. I'm a scrogger, so a tall veg is not important. I thinking gonna pull the halide and toss in my 10.5" brooder reflectors with CFLs. 4 in a 2x2. I can safely run 30,000 lux with a 23w 6500k. Interestingly, a splitter with 2 23w had a weaker output, which I think is due to how the bulbs reflect in the reflector. Gotta grab some 42 or 45 watt so I can shift gears in growth. This is gonna save me some significant electricity and I can dial back my ventilation and make co2 and humidity control (I'm adding humidity at the moment)more effective.

Underground Scientist said:

So I reduced the intensity in veg from 40-50000 lux to about 20,000. The leaves started uncurling and lifting. I increased the intensity and they started lowering. I really think there is an optimum here. I'm a scrogger, so a tall veg is not important. I thinking gonna pull the halide and toss in my 10.5" brooder reflectors with CFLs. 4 in a 2x2. I can safely run 30,000 lux with a 23w 6500k. Interestingly, a splitter with 2 23w had a weaker output, which I think is due to how the bulbs reflect in the reflector. Gotta grab some 42 or 45 watt so I can shift gears in growth. This is gonna save me some significant electricity and I can dial back my ventilation and make co2 and humidity control (I'm adding humidity at the moment)more effective.

Click to expand...

Damn, I set up the brooders and was unsatisfied with the light coverage. There were some areas shaded a little, without light reflecting and hitting lots of areas. The brooders look like they would be great for seedlings and early veg, but not much more without significantly higher wattage. I tossed in a 150w hps. At 9" I'm getting around 33,000 lux (3,000 lumens per square foot) and good side reflectivity. I had to put the ventilation back on full time, but I wish I could dial it down just a touch more. Piggybacking fan speed adjusters did not work.