Here are some lux readings for a 300W LED

[rupert]

Forgive me if this info is out there already, but I want to post while it's fresh in my mind. I just received a lux meter in the mail, and am a little surprised but what I've learned about my LED light. I ordered the lux meter based on this recommendation in a thread where I'm fretting about having plants that are somewhat short (but not too bad!) at 4 wks old.

I have a Platinum LED 300p, which is of course a very popular light. This is my first grow and I'm growing 6 plants in a 2.5' deep, 4.5' wide, 5.5' tall cabinet.

I'm surprised by just how rapidly the lux drops off when the meter is slightly outside the width of the light. Here's what I'm seeing...

90,000 -- Outdoor reading. ~93 million miles from the sun, on a hot sunny June day in the early afternoon.
80,000 -- Indoors, 12" below my LED light, dead center beneath the light. Not bad!
19,000 -- 12" below the LED light, right at the corner of the light. If you hung a plumb line from the corner, this is the area right where that line would hang.
8,000 -- 12" below the LED light, an inch or two outside the zone directly below the light. If you hung a plumb line from the corner, this is the approx reading an inch or two inches further outside of that boundary.

So the lux 12" below the center of the light is 10x more intense than the light just an inch or two outside of the "drip line" of the light. Outside this core zone, the reading will actually go up a bit if you raise the light, since there is some light coming down at an angle outside the core zone.

I knew the lenses were engineered to direct light straight down, but I didn't guess they'd be quite so effective.

On the other hand, there seems to be very little correlation between growth of the plants in veg and whether they are positioned directly below the light or somewhat outside the core zone. In fact, I think I now see why my tallest, most proactive plant bends toward the light when I lower it... because the lower the light, the less light falls outside the core zone. Raise the light up a bit, and the plants outside the core just grow straight up... with significant variations from plant to plant that I'm sure have much more to do with genetics than light.

I have moved my plants around at times, so this is all quite unscientific, but I do know that my tallest plant has never lived in the core zone... tho it does reach a hand or two toward that zone.

I'm also now somewhat more confident that some burning I see on a few leaf tips is due to periods of time when I lowered the light down too much, rather than my nutes. A plant will reach toward the light even if this results in burning.

My room has orca film on the walls, FYI, but given how much light drops off outside the core zone, I'd say fretting over the reflectivity of one's walls is overrated. No doubt it's worth adding some reflectivity with paint, plastic or whatever. But I doubt it matters much whether walls are 85% reflective or 95%, etc. I do like my orca film though.

I suspect a 450p would work quite well in my space. The 300p will probably get me some reasonably good results, I suppose. For this grow I'll either add a couple CFLs for flower or I'll order the 450p and sell the 300p... or use the 300p for a separate smaller cabinet. I might also make a small triangular reflector (cardboard + orca film) to hang ~6" below the light and bounce some more the light to the perimeter.

Despite all of the above, I'm not at all sure that position of the plants relative to the light has been a major factor in the growth rate to this point, compared with other factors. Like I said, the tallest plant, by far, has been living outside the core zone and has not bent aggressively toward the core zone except when I lower the light too much. All my plants tend to angle their solar panels roughly perpendicular to the falling light, most of the time.

 

[rupert]

PS -- I'm not endorsing a 12" height from plants. I suspect manufacturer's recommendation of 18" is probably more like it, but I would start higher for seedlings and work down to 18". This based on my very limited experience.

 

[iwltfum]

Pricless information. Thank you for taking the time to take the readings and and type it up in such a useful way!

 

[madscientist9]

interesting for sure

 

[rupert]

I've learned a bit more about light and lux. Lux does not tell you a lot about how powerful an LED light is for photosynthesis purposes. However, it does give some sense of relative intensity within a light's coverage. It likely also gives some sense of the relative strength of two lights, but only if those two lights have very similar bands of light in similar proportions.

A lux meter is intended for use in photography, so the readings are weighted toward bands that appear brighter to the human eye, but those bands are in some cases not very useful for photosynthesis. A lux meter will show a much higher reading for green light, for example, since humans see green light well. But a plant doesn't absorb green light... which is why leaves appear green.

As a rough guide, my lux meter is most sensitive from around 500nm to 600nm. So the meter heavily discounts light less than 500nm and more than 600nm where most photosynthesis occurs.

If you look at the "photosynthesis rate" graph at Photosynthetically active radiation - Wikipedia you will see that photosynthesis is weakest at exactly the wavelenths my lux meter is most sensitive -- 500 to 600. Like I said, these lux meters are for photography.

Among light makers, one point of contention seems to be whether several different bands, including white light, is a good thing or a waste.
Some think that lights with a lot of bands misallocate the light's total output.
While the folks at Platinum LED think the additional bands are a good use of the light's total output, as will be apparent from watching their videos, you can see from the spectrum reading that they have given more weight to the middle of the PAR range than the competing lights they test... though they also seem to get more output in the more important bands as well.

I wonder if the PAR meter being used in the Platinum LED video gives even weight to all PAR wavelengths, or does it weight them according to what wavelengths are most useful to the plant.

 

[iwltfum]

Nailed it. Not enough people talk about par, although generally the stronger your light is, in lumens, the better chance you have of getting a good par spectrum out of it.

 

[MassMedMan]

Great info, rupert. Really appreciate the work you put in. I've got two p600s and I'm waiting on a third due this week. I'm covering a space, or intend to, that is 5x7. I've been testing my lights with a four pack of autos that were previously outside in a greenhouse. I have my lights about 20" away,, but, I brought these plants under the Platinum LED's after having been flowering for a while,, I'm just going by Platinums recommendation. Next plants in that space/room will start and finish flowering in that room so it'll be interesting to see... Your research helps,, thanks. A lot!

 

[rupert]

I said in the orig post that my floor dimensions are 2.5' x 4.5'. Not sure where I got that idea. I re-measured and the floor is 2'x4'.

I'm thinking about adding a second P300. It's a bit more than I intended to spend on light, originally.

The manufacturer says one P300 could handle a 2x4 space, but there is definitely a significant area around the P300 where I'm not getting very good coverage, at least if I go by lux... on the other hand, if the white LEDs are toward the center of the light, that could partly explain why the lux readings are so high toward the center of the light, since the lux meter could be giving heavy weighting to the white light.

Even with just two plants in the space, I have some upper bud sites outside the core zone where I'm seeing only 10,000 lux in my meter, vs the 50,000 - 65,000 readings nearer the center of the light.

As far as I know, a reading of 10,000 lux might indicate that there is enough light to grow good buds outside the core zone... hard to say, since I lack experience. And if I buy another P300, I'll never know.

 

[rupert]

Hmm. I saw in another post that 37 - 50 watts (actual draw, not rated watts) per sq ft is a good amount for LED lighting. If that's true then the P300 should be great for 2x2. Which means it would not be excessive, at all, for me to add another P300 to my 4x2 space.

 

[Jazzhands]

I used two p300's in my 2.5 x 2 x 7 foot space. At first I thought it might be overkill, but after seeing your readings, it makes me feel better about the purchase. With good ventilation, you should be able to keep the temps around 66 in dark and 80 when the lights are on full blast.

 

[rupert]

I used two p300's in my 2.5 x 2 x 7 foot space. At first I thought it might be overkill

I suspect anything more would be overkill, but what you have now could be termed ample.

I ended up adding another P300 to my 2x4 space and I'm quite satisfied with it. I won't add more. I make the variations work to my advantage by bending the tops so they are nearer to the drip line of the light. That way they can grow a little closer to the light without getting burnt, which is key because I have just about 5'4" of height in my space.

My temps rose to 81 in the shade when I added the second light, but I've knocked it down to 79 by running a small duct directly from my HVAC to my cabinet. Also knocks the humidity down a little. As it gets hotter outside and my A/C cycles more often, temps in my cab will probably drop down a few more degrees.

 

[rupert]

To be honest, I'm now coming to the conclusion that my two P300s may be overkill. My plants are getting burnt a little and I'm pretty sure it's the lights. My healthiest leaves are the ones farthest to the sides of my cab, out of the direct light. Due to space constraints my lights are about 14" from the tops of my plants... higher would be better for my PPPs. They don't like quite this much light. Three P150s would probably be perfect in this space, and two P150s might be adequate. On the other hand my buds on my lower branches are looking pretty good to me, not much worse than what's up top, so maybe the penetration helps with that. Overall the plants are pretty healthy.

Yeah, my healthiest leaves are those to the far ends of the cab. Leaves between the two lights look a tiny bit bleached to me. A photo would be useless, as my camera doesn't show colors well.

My leaf burn looks much like calcium deficiency in places, but that's not what it is. I have even younger leaves that show no nutrition problem at all. It's the middle-aged leaves closest to the light that got zapped, before I finagled a way to get my lights up higher.

I have 68" of vertical space floor to ceiling of the cab, and 14" of that is floor to the top of the medium. So that leaves 54" for the plants, the lights, the light hangers, and distance between the light and plant.