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

[blasterman]

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.

 

[slebans]

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.

To address your latest post:

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

 

[blasterman]

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.