# Thread: Cree says XM-L (T6) is 40% efficient - how many lumens per watt is that?

1. ## Cree says XM-L (T6) is 40% efficient - how many lumens per watt is that?

Cree's XLamp/XM-L High-Bay reference design document is a facinating thing to read... Indeed; the statement that I found most profound therein was: "The XLamp XM-L LED operates at up to 10 watts of electrical power, depending on the drive current, and requires a heat sink to dissipate this thermal load. About 40% of that energy is converted to radiant flux and the rest to heat..." (ref: here - under Thermal Requirements)

Whoa! Can it be that simple? Bear with me, this really got me thinking..

Imho, they're probably alluding to a T6 bin emitter w/o lens running at 700 mA and 3.337 Volts, but that is *still* amazing to me because too many people get hung-up trying to convert luminous flux (Lumens) into radiant flux (Watts) using the widely misunderstood 683 Lu/W value from Wikipedia..

Keep-it-simple: 40% of 700 mA * 3.337 Volts is 0.93436 Watts of radiant power emitting from said LED, give or take a little..

At this point I thought; don't get too excited; Flashlight-Wiki states here that that the XM-L (T6) emits between 280 & 300 Lumens when driven at 700 mA; So, we're nowhere near the maximum output yet. Let's just do some math:

290 Lu / 0.93436 W ==> 310 "XM-L Cool White" Lumens per emitted Watt!

Let's consider that number in comparison to Wikipedia's vaulted 683 Lu/W for pure 555 nm "green" light. Countless people have said that our eyes are most sensitive to green light, but I have nearly gone blind reading stuff that never addresses Lu/W for XM-L lights or any white LED that I would recognize in a flashlight... But wait; we just calculated that ourselves! And, for CPF technical purists; Cree documents XM-L's relative spectral power distribution here. Further, if the spectral power distribution between led's is fairly similar, then my 310 Lu/W value shouldn't vary much between LED's either.

Of course I don't know the accuracy or details behind the 40% efficiency that someone slipped into one of Cree's application notes, but I don't think I've ever seen it so concisely published before and I hope that people reading this will embrace an approximately 310 Lumens-per-Watt value for XM-L emitters. The main thing to note is that it's only 45% of Wikipedia's 683 value for pure green.

Last, but not least, I assume that radiant efficiency is proportional to luminous efficacy and both obviously degrade as power levels increase from 700 mA. For example, the radiant efficiency of said XM-L (T6) emitter drops to 28% at 3A corresponding with its efficacy drop from 124 to 94 Lu/W - i.e. 72% of said emitter's power has to be dissipated as heat at 3A - Btw, didn't MikeAusC already try to demonstrate 75% dissipation for XM-L here?

==> Does anybody see any glaring flaws in my logic or care to post refinements upon the 310 Lu/W value that I derived for "XM-L Cool White"?

Looking forward to your posts, William

2. ## Re: Cree says XM-L (T6) is 40% efficient - how many lumens per watt is that?

LER (luminous Efficacy of Radiation), as is called the figure of lm/emitted watt, varies with spectrum, so actually the exact figure depends of the color bin.

All your logic is perfect, based on average values, Cree coolwhite is about 310lm/W. I have measured from 306lm/W (6650K) to 317lm/W (6200K).

I cant find right now the value for the typical coolwhite spectrum on Cree datasheet, but for Neutral White LER is 335lm/W and for Warm White, 315lm/W.

The absolute max LER for white light is below 500lm/W, and that for low color rendering. With current phosphor technology, getting about 400lm/W is the best manufacturers can get, way less for high CRI spectrum.

3. ## Re: Cree says XM-L (T6) is 40% efficient - how many lumens per watt is that?

Thanks for weighing-in, @Kinnza! You are obviously an expert regarding LER & CRI!!!

I always hope to see LER published along with nice spectrum graphics because it's a single number and relatively constant, but LER is unduly hard to find.

Respectfully Yours, William

4. ## Re: Cree says XM-L (T6) is 40% efficient - how many lumens per watt is that?

I recall AVAGO used to publish it on datasheets, but the big ones do not.

5. ## Re: Cree says XM-L (T6) is 40% efficient - how many lumens per watt is that?

Originally Posted by WmArnold1
Cree's XLamp/XM-L High-Bay reference design document is a facinating thing to read... Indeed; the statement that I found most profound therein was: "The XLamp XM-L LED operates at up to 10 watts of electrical power, depending on the drive current, and requires a heat sink to dissipate this thermal load. About 40% of that energy is converted to radiant flux and the rest to heat..." (ref: here - under Thermal Requirements)

==> Does anybody see any glaring flaws in my logic or care to post refinements upon the 310 Lu/W value that I derived for "XM-L Cool White"?

Looking forward to your posts, William
There is no need to estimate as the specified document contains a subset of the LM-79 data. Scroll down to the bottom of page 19 and the top of page 20. Radiant flux is the key value as you can now derive both LER and Wall Plug Efficiency for each of the three luminaire configurations.
I'm on my way out the door but will post the derived values when I return later this morning.

Stephen Lebans

6. ## Re: Cree says XM-L (T6) is 40% efficient - how many lumens per watt is that?

Originally Posted by slebans
There is no need to estimate as the specified document contains a subset of the LM-79 data. Scroll down to the bottom of page 19 and the top of page 20. Radiant flux is the key value as you can now derive both LER and Wall Plug Efficiency for each of the three luminaire configurations.
I'm on my way out the door but will post the derived values when I return later this morning.

Stephen Lebans
The problem with those figures is they refer to whole luminaries measured into IS. So secondary optics affect the result, by having differential transmission at each waveband. The luminary without lens should correspond the closest to the LED's spectra. LER=17110lm/60.85W=281.18lm/W.

I have measured CW with this LER (and lower), but I dont think it is representative of the range, as it is outside ANSI bins, as CCT is over 7000K. Color bin is not cited, but x,y coordinates correspond to a 0C. WPE of the whole luminary very good, at 38.5% (60.85W out vs 158W input) for XML running at 1.4A. I think Cree choose this color bin in order to get the best radiant efficacy, but I dont think it is representative of CW any of us would like to use (the idea that using that tone to match MH is tricky as currently MHs are available on a full rage of CCTs). Indeed, I wouldnt call that tone "white".

LER with optics raises to 303lm/W (Carclo) and 310.8lm/W (Ledil), probably due to reduced transmittance at wavelengths on the extremes (blue mainly) and different current level (1.8A with Carclo).

This report allows to know with accuracy the efficiency of XM-L T6 bin on typical operating conditions (1.4 and 1.8A, good heatsinking and mounting) at 38,5% (1.4A) and 33.2% (1.8A)

7. ## Re: Cree says XM-L (T6) is 40% efficient - how many lumens per watt is that?

Originally Posted by Kinnza
The problem with those figures is they refer to whole luminaries measured into IS. So secondary optics affect the result, by having differential transmission at each waveband. The luminary without lens should correspond the closest to the LED's spectra. LER=17110lm/60.85W=281.18lm/W.

I have measured CW with this LER (and lower), but I dont think it is representative of the range, as it is outside ANSI bins, as CCT is over 7000K. Color bin is not cited, but x,y coordinates correspond to a 0C. WPE of the whole luminary very good, at 38.5% (60.85W out vs 158W input) for XML running at 1.4A. I think Cree choose this color bin in order to get the best radiant efficacy, but I dont think it is representative of CW any of us would like to use (the idea that using that tone to match MH is tricky as currently MHs are available on a full rage of CCTs). Indeed, I wouldnt call that tone "white".

LER with optics raises to 303lm/W (Carclo) and 310.8lm/W (Ledil), probably due to reduced transmittance at wavelengths on the extremes (blue mainly) and different current level (1.8A with Carclo).

This report allows to know with accuracy the efficiency of XM-L T6 bin on typical operating conditions (1.4 and 1.8A, good heatsinking and mounting) at 38,5% (1.4A) and 33.2% (1.8A)
Hi Kinnza,
I was really only interested in the luminaire spec without secondary optics in regards to LER and WPE calculations. It is interesting to see how the addition of a secondary optic affects not only efficiency but LER as well. I always thought about the optic in terms of loss without realizing that not all wavelengths are effected equally.

I spent an hour trying to make the public Cree PCT match the values in the document but without success. The Cree engineers obviously have a more powerful/flexible tool not available to us. There are several issues surrounding power supply efficiencies and drive currents that do not add up. Specifically, I don't see how they state a drive current of 1.4 amps unless they are using ultra low voltage XM-Ls.

My last issue has to do with the reference design for the luminaire without any secondary optics. You could not place this exact design in a warehouse without protecting the LEDs in some fashion. Bugs and dust would soon lower the efficiency of the luminaire, leaving the user without an easy method to remove the accumulated grime. I realize it is simply a reference design, and as you said--Cree needed to produce a design exceeding the efficiency of the comparative MH fixture--but I feel like they are cheating a bit by not including the real world losses from using a protective cover of some type.

Stephen Lebans

8. ## Re: Cree says XM-L (T6) is 40% efficient - how many lumens per watt is that?

Originally Posted by Kinnza
This report allows to know with accuracy the efficiency of XM-L T6 bin on typical operating conditions (1.4 and 1.8A, good heatsinking and mounting) at 38,5% (1.4A) and 33.2% (1.8A)
Kinnza, do you remember last year when we looked at a reference design, driving the LEDs at 700ma or less, and we only ended up with a 30% Wall Plug Efficiency? This Cree spec is a huge leap in one year. With the OSRAM reds in the low 40% range, XM-Ls at lower drive currents coming in at or just above 50% and power supplies at greater than 90% efficiencies, it would be possible to produce a grow light fixture with close to a 45% WPE. My design spec for the lettuce requires 135 PAR watts per square meter therefore I would only need a 300 watt fixture(s) to cover an entire square meter of bench space. This would have a huge impact on the budget for the annual electricity costs!

Stephen Lebans

9. ## Re: Cree says XM-L (T6) is 40% efficient - how many lumens per watt is that?

Yeah, this last year has been plenty of efficiency improvements, but just after the large improvement on past years, we noticed it less, as the gain is each times smaller in reference to the previous performance.

As soon as the new improved reds reach the market 40-45% WPE efficiency of grow lights are on sight.

135 PAR W/sq m? Thats a lot. Lettuce dont like more than 400micromols of photons per second and sq meter (uE), if what I read is true (not direct experience growing it). At 5uE/PAR W (reddish spectrum), it does a max of 80 PAR W/m2. I believe lettuce is usually grown under 150-250 uE/m2 irradiances. At 40% of WPE, you would need just about 100W/m2. With stacked stands, multiplying grow area, growth rate is not as important, so still less electricity would do the job, only way to do it profitable (about 5\$ on electricity bill per sq meter harvest)

10. ## Re: Cree says XM-L (T6) is 40% efficient - how many lumens per watt is that?

Originally Posted by Kinnza
135 PAR W/sq m? Thats a lot. Lettuce dont like more than 400micromols of photons per second and sq meter (uE), if what I read is true (not direct experience growing it). At 5uE/PAR W (reddish spectrum), it does a max of 80 PAR W/m2. I believe lettuce is usually grown under 150-250 uE/m2 irradiances. At 40% of WPE, you would need just about 100W/m2. With stacked stands, multiplying grow area, growth rate is not as important, so still less electricity would do the job, only way to do it profitable (about 5\$ on electricity bill per sq meter harvest)
The growth parameters you quoted are based on two issues with growing lettuce:
1) Lettuce leaves develop tip burn with high light and low humidity levels.
2) Lettuce will bolt(go to seed) at a specific time period.

Based on peer reviewed research, we have been growing lettuce at 600PPF for 22 hours per day at a constant 30 degrees celsius and 70% relative humidity.
For lighting, the target is a Daily Light Integral of 50 moles per meter squared per day. We are currently using just over 650 watts per square meter(Philips WW and LSG Cool White BR30s at around 20% WPE) in dumb purpose built fixtures that are DMX controlled).
Fans blow air directly onto the meristems to eliminate tipburn. The plants are harvested, as baby lettuce leaves, at just under 20 days from planting. CO2 levels become the growth limiting factor and are kept at a target of 1000ppm.

We are using multiple levels(multi shelved production carts). Growth rate, and therefore turnover, are the two keys to profitability.
Electrical is the single largest cost, mainly in the form of lighting but cooling is a significant expense as well. Luckily, this cost will continue to decline each year as LEDs reach new levels of efficiency.

Stephen Lebans

11. ## Re: Cree says XM-L (T6) is 40% efficient - how many lumens per watt is that?

XM-Ls at lower drive currents coming in at or just above 50% and power supplies at greater than 90% efficiencies, it would be possible to produce a grow light fixture with close to a 45% WPE
...And would cost a fortune. While the XM-L has remarkable efficiency and I don't dispute the math here what commercial fixture is going to use XM-L's at 700mA and remain cost competitive? Also, high current / low voltage emitters might make for practical driving in battery driven applications but not anything powered by 120/220 VAC.

As for plant growth, I would guess that using XTEs and newer bin 630nm rebels or Crees in a purpose built fixture would increase efficiency by 50-100% and decrease the price over standard BR30s.

12. ## Re: Cree says XM-L (T6) is 40% efficient - how many lumens per watt is that?

Originally Posted by blasterman
...And would cost a fortune. While the XM-L has remarkable efficiency and I don't dispute the math here what commercial fixture is going to use XM-L's at 700mA and remain cost competitive?
You are absolutely correct, Blasterman. But with the price of LEDs dropping annually by 30% or more, and their efficiencies increasing anually as well, your statement will not be accurate at some point in the next few years. Further, from my specific business perspective, the capital cost of the fixture is secondary to the annual operating costs. As long as the fixture continues to operate during its depreciation schedule, the annual savings in electrical costs will more than make up for the depreciation amount on my income statements.

Also, high current / low voltage emitters might make for practical driving in battery driven applications but not anything powered by 120/220 VAC.
You have forgotten more than I know about building purpose built LED fixtures. But I do not understand your statement as power supplies are readily available with efficiencies exceeding 90%. Finally, I think it was you that had previously mentioned that whole building AC/DC conversion at the mains would be a viable alternative to individual fixture based power conversion.

As for plant growth, I would guess that using XTEs and newer bin 630nm rebels or Crees in a purpose built fixture would increase efficiency by 50-100% and decrease the price over standard BR30s.
Again, in terms of current costs, a purpose built fixture would be both cheaper and more efficient than the BR30s I am currently using. The BR30s currently have a WPE of around 20%. But we are only experimenting at this point and have not reached the stage where I would even contemplate having a fixture designed and built. We would need to purchase hundreds of purpose built fixtures in order for the entire process to be cost effective. Also, since our work is considered R&D we are able to claim roughly 50% of the costs.

Finally, I like the idea of using off the shelf technology. If a bulb dies a quick trip to Home Depot provides a solution. And due to the economies of scale, once LED replacement bulbs become more efficient and mainstream - I am not sure if a purpose built fixture will be able to compete from a first cost basis enough to cover the difference in WPE. But it is a simple accounting decision and I will go whatever way yields the most bottom line profit.

Stephen Lebans

13. Well, guys, all I know is that these things are bright.

14. ## Re: Cree says XM-L (T6) is 40% efficient - how many lumens per watt is that?

Originally Posted by slebans
The growth parameters you quoted are based on two issues with growing lettuce:
1) Lettuce leaves develop tip burn with high light and low humidity levels.
2) Lettuce will bolt(go to seed) at a specific time period.

Based on peer reviewed research, we have been growing lettuce at 600PPF for 22 hours per day at a constant 30 degrees celsius and 70% relative humidity.
For lighting, the target is a Daily Light Integral of 50 moles per meter squared per day. We are currently using just over 650 watts per square meter(Philips WW and LSG Cool White BR30s at around 20% WPE) in dumb purpose built fixtures that are DMX controlled).
Fans blow air directly onto the meristems to eliminate tipburn. The plants are harvested, as baby lettuce leaves, at just under 20 days from planting. CO2 levels become the growth limiting factor and are kept at a target of 1000ppm.
All references I have found about lettuce uses way lower irradiation levels, with daily integrals between 15 and 20 mols of photons. With long photoperiod as 22/2, it means a PPF below 300uE/m2 (about 275uE/m2)for getting 20 mols/day.

Check this reference of NASA, at 1000ppm CO2 aswell: http://docs.lib.purdue.edu/cgi/viewc...context=nasatr, tipburn observed at 300uE/m2 (23şC air and roots all day, 16/8 photoperiod, 17 mols photon/day), density of 40 plants/m2 (initial) to 20-24 (final, harvest at 28d from seed, 20-30cm heads). Productivity in this conditions of 7g/m2*day from seed and 10g/m2*day from nursery (10-12 initial days).

Tipburn is due to low Vapor Pressure Deficit, which produces a reduced uptake of Ca. 30şC seems too much for lettuce. If you couple it with high PPFD (thus, higher leaves' internal temperature and faster growth rate), risk of tipburn is strongly enhanced. For that NASA study and some others, I got the impression PPF over 400uE/m2 only adds marginal benefits. In other study I read photosynthetic saturation point of lettuce is about 470 uE/m2, and max photosynthesis rate, at 145uE/m2.

I understand that profit key is on shortening the harvest time, and that paying 2-3\$ more for Kg may worth. But I think you should try to grow under lower irradiance and check the differential growth rate vs electricity cost and initial lamps cost. Maybe lowering PPF to 400uE/m2 dont get any noticeable lengthening of harvest time, or cost reduction of lowering PPF still more result profitable (if lengthening is just 1-2 days, for example). Reducing PPF would help lowering the cost of air flow aswell (as using LEDs instead of BCR, due reduced IR and photochemical quenching)

This other study found optimal conditions for lettuce (although harvest time parameter was not analyzed, just environmental conditions) as 22şC (air), 20şC (root), 1578ppm CO2, 75% RH, 1.3 m/s air speed and 216uE/m2.

The ideal spectrum for lettuce has been studied in detail. Just red+blue works perfect, with blue fraction minimum at 6% PPF, optimal at 8% and up to 12%. Although not studied so much, I believe far red suplementation could be useful on nursery stage to increase ground cover as fast as possible aswell as heads size.

15. ## Re: Cree says XM-L (T6) is 40% efficient - how many lumens per watt is that?

Originally Posted by Kinnza
In other study I read photosynthetic saturation point of lettuce is about 470 uE/m2, and max photosynthesis rate, at 145uE/m2.
Hi Kinnza - thanks for your interest.
I have caused this thread to move off topic but to address your point above, the yield saturation in response to PPF is due to tipburn, which reduces leaf expansion in the latter stages of growth(after canopy closure) when growth rates are maximal. The primary study we used showed a straight line increase in PAR versus yield up to 1000 PPF.

I will EMail you the research paper where we have been able to duplicate their yields, which are significantly above those of the papers referenced in your post.

Stephen Lebans

16. ## Re: Cree says XM-L (T6) is 40% efficient - how many lumens per watt is that?

But I do not understand your statement as power supplies are readily available with efficiencies exceeding 90%.
The XM-L is expensive per lumen compared to other packages even though it's near the front in terms of efficiency. You need to drive the things at 2amps to be cost competitive, and that makes your drivers \$\$\$. -1

Most 'on the grid' power circuits are 120/220 volts. Converting from 120volts AC to ~3volts DC at several amps to run XM-Ls just isn't practical in terms of mainstream lighting because you lose efficiency the more you steer away from native voltage (or increase circuit costs). Sure, you can do this with a variety of buck type drivers, but then you need a DC source feeding them. It adds costs and costs efficiency. This is why Cree and Bridgelux are bending over to produce higher voltage packages - makes power supply design simplier and cheaper at higher voltages and lower currents - typically around 350mA or 700mA - both seem to be an unofficial standard. Somewhere Tesla is rolling in his grave with laughter.

As for plants, I've noted on many agricultural sites that to grow the more light hungry crops like tomatoes, 25watts per ^foot in terms of white halide is a common number and has been for years. That's around 250watts per square meter by my math. That may seem like a lot, but we're getting 2:1 efficiency advantages over halides in reefing forums, and that's with chinese based fixtures. Dedicated Cree or high bin Rebel fixtures are prolly closer to 3:1. 300watts of XT-E or 448nm top bin Rebels concentrating in a ^meter area would likely render the ground sterile.

Probably wouldn't take that many XT-Es to provide maximum vegetative growth in a ^ meter area. Usually it's red where you need such high energy levels (??) In any respect, they are easy to build and royal blue Crees are very cheap right now.

17. ## Re: Cree says XM-L (T6) is 40% efficient - how many lumens per watt is that?

Originally Posted by slebans
Hi Kinnza,
I spent an hour trying to make the public Cree PCT match the values in the document but without success. The Cree engineers obviously have a more powerful/flexible tool not available to us. There are several issues surrounding power supply efficiencies and drive currents that do not add up. Specifically, I don't see how they state a drive current of 1.4 amps unless they are using ultra low voltage XM-Ls.
Coming back to topic, document dont state at all the current of each lamp. I just made a gross estimation. The no optics lamp emits 17110lm with 40LEDs, so it gives 428lm/LED, which is roughly the emission of a T6 at 1.4A. After your post, I wanted to check it in deep. XML at 1.4A emits 1.8x the flux of binning at 700mA, and thermal loss is about 12% at the temps specified. So 280lm(T6)*1.8*0.88=443lm at 1.4A. I would say that maybe actual temps on the final lamp were a little higher, so it explains the difference, but maybe the lamp were actually running a little below 1.4A.

Power drawn will clear the question: 158W at a driver efficiency of ~95%, so 150W, for 40LEDs, 3.75W/LED, which indeed means the no optic lamp was running below 1.4A. For the average Vf of the XML, about 1.25A. Checking again theoretical lm emission at 1.25A (~1.63x), 280*1.63*0.88=402lm. For average emission T6s (290lm@700mA), 290*1.63*0.88=416lm. This reverse engineering means that lamp was running at about 1.3A on low Vf XM-Ls.

For the carclo lamp, 17800lm net at an optical efficiency of 92% correspond to 19348 gross lm. Between 32 LEDs, 605lm/LED. I thought it correspond to 1,8A just by eye, but a look to the datasheet shows actual current was higher. At 1.9A (~2.37x), 280lm*2.37*0,88=584lm- For an average T6 (290lm), 290*2.37*0.88=605lm. Good match, so first bet is a current of 1.9A in this version. It match with low Vf XM-L aswell, for a power of about 5.3W/LED.

18. ## Re: Cree says XM-L (T6) is 40% efficient - how many lumens per watt is that?

Thanks for the great detective work, Kinnza.
:-)

Stephen Lebans

19. ## Re: Cree says XM-L (T6) is 40% efficient - how many lumens per watt is that?

Originally Posted by blasterman
300watts of XT-E or 448nm top bin Rebels concentrating in a ^meter area would likely render the ground sterile.
That is an interesting thought.
Royal Blue is nominally 50% efficient. Let's forget about power supply losses and say that 300 watts of input would yield 150 watts of output.

For Solar irradiation, generally an average of 1000 watts per square meter is an accepted value. If we restrict the irradiation to the wavelengths between 400nm and 500nm we would derive a total value of 140 watts per square meter.

If I was a betting man, and I am, I would bet that 300 watts of Royal Blue XT-E would not render the ground sterile.
;-)

Stephen Lebans

20. ## Re: Cree says XM-L (T6) is 40% efficient - how many lumens per watt is that?

I started this thread saying that Cree's XLamp/XM-L High-Bay reference design document is a facinating thing to read...

But everyone's posts here have been absolutely awesome and a great thing to study in their own right! Kudos to Kinnza, Slebans, and Blasterman

The thing I hope to remember from this thread is that Luminous Efficacy of Radiation (LER) is relatively constant for a particular emitter and family of emitters because it is a function of the output power spectrum. And, using LER, we can convert lumens into luminious Watts for all kinds of thermal considerations.

Hopefully, LER values will be published more often...

Thanks again for posting, William

21. ## Re: Cree says XM-L (T6) is 40% efficient - how many lumens per watt is that?

If I was a betting man, and I am, I would bet that 300 watts of Royal Blue XT-E would not render the ground sterile.
Unlike the sun, which has this habit of moving across the sky, a fixed LED source tends to stay in one spot which tends to produce a far different effect on light absorption in plants. That's one reason using the sun as a reference source doesn't work real well. You would think that matching the spectral out-put intensity of specific colors ranges in the equatorial sun would be a good reference point for crops that prefer southern lattitudes, but it doesn't work real well when transposed to artifical light sources. No need for a spectrometer either given the blue cutoff in digital cameras tends to be right about 455nm.

The issues get's compounded when LED are combined with optics. Something about a point light source collimated with a narrow optic produces a photosynthetic reaction greater than if the LED is bare and lowered closer to the source as per law of squares. A single 3watt blue Cree with 15 degree optic will kill Acropora, house plants etc. in a couple of days Been there - done this. Not sure what the ergs per ^ inch threshold is, but it's certainly less than 300watts per ^ meter of blue light.

22. ## Re: Cree says XM-L (T6) is 40% efficient - how many lumens per watt is that?

Just to clarify, it was you that stated:
"300watts of XT-E or 448nm top bin Rebels concentrating in a ^meter area would likely render the ground sterile."

I responded, showing you that a square meter of plants exposed to sunlight receive roughly the same amount of light from the range of 400nm-500nm and therefore I did not believe your statement to be true. I still don't.

Unlike the sun, which has this habit of moving across the sky, a fixed LED source tends to stay in one spot which tends to produce a far different effect on light absorption in plants.
Plants have several mechanisms allowing them respond to changing lighting conditions. Many plants utilize some form of Heliotropism that allow the leaves to follow the path of the sun. Plants can cause Chloroplasts to migrate to specific areas within the leaf. Plants can create higher/lower densities of Chloroplasts at specific locations. Plants will manufacture individual pigments according to the spectra of the source. In summary, plants have the ability to respond to current lighting conditions. A fixed -non variable- lighting source just makes it easier for the plants to respond to the current lighting conditions.

The issues get's compounded when LED are combined with optics. Something about a point light source collimated with a narrow optic produces a photosynthetic reaction greater than if the LED is bare and lowered closer to the source as per law of squares.
I completely disagree with that statement. Almost all of the incident radiation is absorbed at the leaf surface. But the majority of the photons never make it to start the photosynthetic process. They are lost to various mechanisms. If you concentrate the output of your fixture to relatively small areas then you are simply reducing effectiveness of your source. As a point of fact, suppliers of LED Grow lights commonly make the same statement as you did. From my point of view, they promote the use of concentrating lenses simply so that can then produce site specific hyper inflated PPF readings that they then mistakenly apply to the entire output of the fixture. If you want the most efficient use of your light source, it should be spread out over as much of the leaf surface as possible. Further, you should be lighting not just the top of the leaf surface but the bottom and sides as well. Hell, you should even be allocating a small portion to any green areas on the stems.

A single 3watt blue Cree with 15 degree optic will kill Acropora, house plants etc. in a couple of days Been there - done this. Not sure what the ergs per ^ inch threshold is, but it's certainly less than 300watts per ^ meter of blue light.
It is difficult to accurately respond to that statement without knowing the distance your source with the 15 degree optic is from the plant(s). I'm not sure what this has to do with my original statement as we were referencing 300 source watts of Royal Blue XT-E per square meter-not concentrated to an area less than a square meter.

300 source watts Royal Blue XT-E at 50% efficiency = 150 optical watts of output
1 square meter = 1550 square inches
150 watts / 1550 square inches = .096774 watts per square inch

In summary, I do not believe that less than a tenth of one watt of Royal Blue per square inch would render the soil sterile or even kill a plant. Sure, if you want to use a lens to concentrate the output to less than a square meter then your statement would be accurate depending on the total area exposed to the source.

Stephen Lebans

23. ## Re: Cree says XM-L (T6) is 40% efficient - how many lumens per watt is that?

Missed this one Stephen :-)

I responded, showing you that a square meter of plants exposed to sunlight receive roughly the same amount of light from the range of 400nm-500nm and therefore I did not believe your statement to be true. I still don't.
First, I use light demanding corals as an analogy towards your lettuce scenario because both the symbiotic algae in shallow water SPS corals and vegetative plants have a similiar demand for blue light.

Next, I really don't care about matching the spectral energy the sun emits because you simply cannot match the broad spectrum response of the sun with a solid state light source and expect the same results on plants by only focusing on specific spectral weighting. It's billiard ball physics and far too simplistic given the additional spectral characteristics of the sun -vs- any number of LEDs. It's even problematic with artificial light sources given that a metal halide with a measured peak spectral weighting doesn't translate into a LED light source with the same measurement. The halide source typically has a much narrower emission range in the 440-460range, and if you use the same peak spectral energy flux measurement with any given LEDs the results will be way too much energy. At present plasmas are the only known artificial light source I'm aware of that can kind of mimmick the broad spectral weighting of the sun. For that matter the energy of the equatorial sun on plants is likely mostly wasted, but it's other factors including infra-red for leaf heating, and possibly blocking pigments having a much greater factor in this that are totally skewed with solid state light sources.

Plants have several mechanisms allowing them respond to changing lighting conditions. Many plants utilize some form of Heliotropism that allow the leaves to follow the path of the sun.
See above. This same 'mechanism also robs the plant of energy. One thing I've noted with artificial light sources, and possibly you have as well, is that high efficiency T5/T8 do a much better job with vegetative growth than corresponding halides or mass market high density LED fixtures likely because the light source is distributed along a far larger area. It's a guess, but that same issue holds true on salt water tanks.

They are lost to various mechanisms. If you concentrate the output of your fixture to relatively small areas then you are simply reducing effectiveness of your source
Never said I disagree. I also recall that every fixture I've advised people to build utilizes long bars of aluminum and not high density clusters aka Chinese Ebay lights so I'm not sure why you accuse me of being in the Ebay crowd. However, LED optics aren't true point sources like the sun is, nor do they behave like typical halide reflectors.

From my point of view, they promote the use of concentrating lenses simply so that can then produce site specific hyper inflated PPF readings
Exactly, and to promote their claims they use non-standard PAR meters which have no basis for either fixed calibration, nor relevant spectral weighting other than a purple filter over a cheap photo sensor. They spit a number out, and the higher better, right?

However, the collimating effect I noticed with plants had more to do than simply law of squares. Plants have not adopted biologically to highly intense blue light sources a few feet away emitting a narrow band of 455nm energy. Spread out fluorescent tubes have a closer analogy to a lightly overcast sky, which may be why certain plants respond so well to this type of light source more efficiently that halides (with blue light).

Further, you should be lighting not just the top of the leaf surface but the bottom and sides as well. Hell, you should even be allocating a small portion to any green areas on the stems.
Agree as well. I recall back in the early 90's large scale tests done on various crops with reflective materials placed underneath the plants to produce different growth patterns, and the results ranged from minor to major depending on the plant and color. This was done over acres of land and not single plants, so the results were quite conclusive. As I recall plants the required a higher weighting of red light responded better than vegetative, but there were mixed results on both sides.

I'm not sure what this has to do with my original statement as we were referencing 300 source watts of Royal Blue XT-E per square meter
Since it's not the same type of plant the analogy will be nothing but prone to arguement, however I was working with a surface area that translated into far less than 300 watts per sq meter of LED light and an organism that thrives under 1200watt halides. However, I stand my my statement that 300watts per^2 meter of XT-E blue light focused into that area exceeds typical published requirement for horticulture, so your numbers are up for speculation and not mine. I've killed plants with far less. We also need to stop focusing on a single band of blue energy and possibly discuss broadening this. Just because LED makers want to sell us LED's based on specific bins doesn't mean lettuce only responds to a specific one.

Red light...maybe. Different physics and efficiency factors apply to all. The really irritating thing is usually I'm the one being jumped about being way to over-zealous with blue light requirements and now I'm getting it from the other direction :-) Trying to light a greenhouse with halides that produce the equivelant of 300watts of XT-E emitted light though would likely require your own power plant.

As for Cree in general, I'd appreciate that if any metric they use be applied to all manufacturers on an equal footing, and right now we know it's not. Nothing, and I mean NOTHING published by Cree (Or Bridgelux or Phillips, etc) is going to be presented without being filtered via their marketing dept first. These companies are governed by shareholders, not scientists. I noticed with the spectrometer you sent me that published numbers from Cree and Bridgelux for example did not translate into their actual products in relative testing.Cree of course will say 'test it this way' while Bridgelux will say 'test it this way' according to their benefit.

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