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Other Analysis Of Cree Cxa Cob's


Staff member
Global Moderator
Hi guys,

While browsing the specs for the Cree CXA family (the new COBs) I happened by these promising looking spectra:
cree spectrum.jpg

Look at the beautiful purple curve of the high CRI part. It has extra power in the red and blue region, just where we want it.
Surely it looks much better than the red curve, which is the normal CRI counterpart. But is the purple curve really as sexy as it looks?

Well lets see. The main problem here is that the spectra are all relative; Each curve is scaled to have its top at 100%. This makes it impossible to compare the absolute power that the different parts put out. So the first step would be to create absolute spectra. After writing a Python script to do just that, this is the result:
spectral power.png

See what happened to the purple line. In the blue region it now has only a very slight edge over the normal CRI part. In the red region it gets more powerful after only 650nm, which is basically the deep and far end.

Instead of plotting the power we can also plot the number of photons. Since photosynthesis is driven by the number of photons this would tell the story a little bit better:
spectral flux.png

This is for the CXA1304 part btw, the smallest member of Cree COB family. The spectra of the other members are all identical and the absolute power will scale accordingly.

The Python script also calculates some properties that can not be found in the specs directly but are useful for comparing the different options. This is for the CXA1304, higher bin, 25C at nominal current:

3000K, 93 CRI
Power in : 3.8 W
Luminous flux : 317 lumen
Efficacy : 83 lumen/W
LER : 291 lumen/W
Radiometric efficiency: 28.7%
PAR efficiency : 82.3%
Combined efficiency : 23.6%
Radiant flux : 1.09 W
Photon flux : 5.36 uMol/s

3000K, 80 CRI
Power in : 3.8 W
Luminous flux : 423 lumen
Efficacy : 111 lumen/W
LER : 344 lumen/W
Radiometric efficiency: 32.3%
PAR efficiency : 86.5%
Combined efficiency : 28.0%
Radiant flux : 1.23 W
Photon flux : 5.98 uMol/s

4000K, 70 CRI
Power in : 3.8 W
Luminous flux : 490 lumen
Efficacy : 129 lumen/W
LER : 339 lumen/W
Radiometric efficiency: 38.1%
PAR efficiency : 84.3%
Combined efficiency : 32.1%
Radiant flux : 1.45 W
Photon flux : 6.77 uMol/s

5000K, 70 CRI
Power in : 3.8 W
Luminous flux : 527 lumen
Efficacy : 139 lumen/W
LER : 339 lumen/W
Radiometric efficiency: 40.9%
PAR efficiency : 84.1%
Combined efficiency : 34.4%
Radiant flux : 1.55 W
Photon flux : 7.16 uMol/s

This is all a bit technical but the most useful parts are the various efficiencies. The radiometric efficiency tells what fraction of electricity is turned into light energy. The PAR efficiency is a property of just the spectrum, namely how well it can be utilized for photosynthesis. The high CRI part turns out to have the lowest efficiencies, both radiometric and PAR. So it would not be the best choice for an efficient grow light. It could be used for some added deep and far red though.

Also take a look at the photon flux. It turns out a lot of photons get lost in the phosphor...

It is a bit of a bummer that the 2700K spectrum is not in the specs, if you know where to find it let me know.