Self-made LED panel general discussion

When it comes to comparing light when it comes to plants it's a bit tricky; Neither PAR nor Lumen are going to give you the whole picture.

Say you have 100w of 450nm light. All of the 100w of light will be PAR but that doesn't mean all 100w of the light will be used by the plants. You see plants have different chlorophyll and caratenoids for different spectrum and they have a cap - a maximum limit of how much light in a particular spectrum they can absorb at any particular time. With a narrow narrow spectrum you very quickly hit the maximum absorption levels of those few matching photosynthetic pigments, leaving the rest of the pigments with nothing to do meaning the surplus light is wasted.

White light is not narrow spectrum and does not have these problems. Additionally white LEDs are much more efficient when it comes to flux/w which helps offset that they are a slightly lower % PAR compared to total radiation. And although white LEDs have a broad spectrum the majority of it is still in the PAR range unlike MH and HPS. Aside from the small bit of green (which is still technically PAR light according to a meter) the majority of light emitted by white LEDs is usable.

Well, going over the plant's absorption cap can easily be avoided by distributing the light more evenly (instead of using a small fixture, we should use a large one; about the size of the growing surface). Moreover, while it was true a few years ago, I don't think that white LEDs are much more efficient than reds or blues anymore, as I stated before. If you want detail about my calculations, I can give you my full procedure in a latter post. Basically, I use the two info available on white LEDs specsheets: total luminous flux (in lumen) and relative spectral irradiance curve. I also have the ISO-10527-2007 specification at my disposal in order to have accurate radiant flux@wl => luminous flux conversions.
The results I found make sense but it is true that I cannot be sure. I wish the manufacturers provided the total radiant output of their LEDs because I believe it is a primordial value when it comes to electronic efficiency.

I think you should probably go by the numbers released by the manufactures which do state a pretty significant difference in efficiency. I'll upload the tables direct from CREE and Bridgelux once they fix this new forum bug. I can't post any pictures till they fix that.

I would indeed be very interested to see that, can you tell me where I can find this info ? Or maybe you could upload the images to http://imgur.com/ and paste the links here, it might work.

Won't let me attach a linked picture... lame. Most of the info can be found direclty from CREEs website as well as the BridgeLux website. Looking at the new XRE they are deffinately getting closer. Much much better than their older diodes.

Here's the specs for CREEs XREs. http://www.cree.com/~/media/Files/Cr...amp7090XRE.pdf

Here are the XPEs which contain the red - http://www.cree.com/~/media/Files/Cree/LED Components and Modules/XLamp/Data and Binning/XLampXPE.pdf

100-122 for the white and 45-73 for the red.

There is no mention of radiant efficiency and the figures you are quoting are those of the luminous flux, which is not relevant in the grow lighting context. As I mentioned before, the luminous flux is the intensity of light as perceived by the human eye. Basically it is the radiant flux (power output, in watts) multiplied by a constant of 683 lm/w and then multiplied by luminous efficiency of the emitted wavelength which is given by the following black curve:


What that curve means is if we have two light emitters with the same power output -let's say 500 milliwatts- and the same efficiency, but one emitting at 555nm and the other one emitting at 625nm, the first one would be rated at 541.5 lumens and the other one at only 219.2 lumens. What I'm trying to say is that more lumens doesn't mean a higher efficiency.
So, with white LEDs, the lumen output is high because there is a lot of radiance in the 525-600nm range but that does not mean that they are more efficient; in terms of PAR (watt per m²) or in terms of photons emitted (cf an earlier post about photons).

I swear those one of those pages is in mW flux - but even that isn't going to help you much more than lumen rating when most all light energy for both colors is PAR. You're looking for total umol? Still won't help you compare them in the context of plants... We need someonr to compare PAR and lumen readings for all of these so we can't just adjust the provided tables by the appropriate ratio - but even then we're still no closer to mathimatically settling the argument. lol