nimbulan, with availability, KingBrite seems to be the easiest source at this moment. I think aerogrower contacted him.
Also, generally, you lose the total PAR output with higher CRI. This may not be an accurate description, but it is easy to understand in this way. Phosphor-based white LEDs are basically blue LEDs + phosphors. When phosphors are hit by blue, they fluoresce at a longer wavelength (yellow-red). So different CRI and K are generally done by changing mostly the phosphor layers. So to get warmer color (lower K) or higher CRI, they generally apply more phosphors. The light can be lost at the phosphor layers, so some photons get lost with more phosphors.
Info below here is probably too much details for most people, but I'm just showing that the above statement isn't based on "myth".
If you want to see more exact number, here is the calculation done by alesh of RIUp forum (well, I added a bit more calculation and re-implemented it in R by myself). Basically, you digitize the Spectral Power Distribution (SPD) curve, and then you can calculate these conversion factors with R (or a spreadsheet).
Explanation of columns:
## ler qer par.qer par.per.lum ppfd.per.fc ypf.per.lum
## Cree.3000K.80CRI 327.26551 4.868716 4.662723 0.01424752 0.1533591 0.01288196
## Cree.3000K.90CRI 276.12267 4.915020 4.511533 0.01633887 0.1758702 0.01458771
## Cree.4000K.80CRI 322.10079 4.678840 4.533168 0.01407376 0.1514886 0.01230506
## Cree.5000K.70CRI 323.42007 4.605715 4.471141 0.01382456 0.1488064 0.01204868
## Vero.3000K.80CRI.v1.2 323.10113 4.879973 4.658829 0.01441910 0.1552059 0.01305418
## Vero.3000K.80CRI.v2 319.73025 4.944016 4.704169 0.01471293 0.1583687 0.01346317
## Vero.4000K.80CRI.v1.2 321.96749 4.730732 4.564988 0.01417841 0.1526152 0.01252857
## Vero.4000K.80CRI.v2 323.62228 4.823261 4.619960 0.01427578 0.1536632 0.01281032
## Vero.5000K.70CRI.v1.2.2 331.47526 4.614143 4.489774 0.01354482 0.1457953 0.01179274
## Vero.Decor 271.76787 4.977243 4.558739 0.01677439 0.1805580 0.01510056
- ler: Luminous efficacy of radiation. Basically how much lumen is given per 1 watt of emitted light (not 1 watt of electric power went into the diode). The unit is lumen/W
- qer: quantum efficacy of radiation. I'm not sure if this is the correct term, but instead of lumen in LER, we are using number of photon flux (micromol/s). The unit is micromol/s/W = micromol/J. In this, I'm not using PAR, and using all emitted light.
- par.qer: Similar to QER, but now we are counting the light between 400-700nm (PAR). unit is micromol/J
- par.per.lum: For a given lumen, how much PAR PPF is there. Companies only gives the lumen output for a given current and temperature. You can multiply the lumen number with this factor to get the PAR PPF. The unit is micromol/s/lumen
- ppfd.per.fc: If you have only footcandle meter, you can use this factor to get the PPFD. The unit is micromol/m^2/s/footcandle
- ypf.per.lum: Yield photon flux per lumen. This quantity is similar to par.per.lum. Blue light is about 20-30% less efficient than red light in terms of photosynthesis. While each photon between 400-700nm is counted equally in PPF, each photon is weighted by the photosynthetic efficiency in YPF without limiting the wave lengths to 400-700nm. This difference in photosynthetic efficiency is expressed by Relative Quantum Efficiency (RQE) of photosynthesis. I used the RQE obtained by McCree (1972). See the figure in this wikipedia. A lot of people are familiar with the chloroplast absorption spectrum (top of the same wikipedia page), but many people are not aware that absorption is just a part of the story; some of the light is absorbed by inactive pigments, which don't contribute to the photosynthesis, or released as heat before it initiate the photosynthetic reactions. McCree's RQE is a more relevant curve than the absorption spectrums of chlorophylls. The unit is micromol/s/lumen.
For the discussion of different CRI, we can focus on par.per.lum column. Then we get the lumen from the datasheet of CXB3070. The value is for 1.9A (higher than I would use) and junction temperature of 85C (higher than what I would target). Forward voltage is 36V, so 68.4W of electric input (not including the loss of power due to AC/DC conversion at the driver). Here are the minimum flux value for the top bin for each.
3000K 80CRI AD bin: 9000lm = 128.2 micromol/s (PPF) = 115.9 micromol/s (YPF)
3000K 90CRI Z2 bin: 7390lm = 120.7 micromol/s (PPF) = 107.8 micromol/s (YPF)
4000K 80CRI BB bin: 9500lm = 133.7 micromol/s (PPF) = 116.9 micromol/s (YPF)
5000K 70CRI BD bin: 10000lm = 138.2 micromol/s (PPF) = 120.5 micromol/s (YPF)
So with 3000K where I could compare the different CRI, lower CRI is better as expected (in both PPF and YPF). Basically if you go up in CRI, the bin number goes down a couple steps.
Now, if you look at PPF values, you may think that 5000K is better than 3000K 80CRI (about 7.8% advantage). From the action spectrum (RQE) of photosynthesis (by McCree 1972 and Inada 1976), one photon of blue light is 20-30% less efficient in terms of photosynthesis than one photon of red light. Although LEDs with lower K have lower PPF, it contains more red light. So the difference in terms of YPF, which incorporates the difference in photosynthetic efficiency, become smaller among different K (4.0% advantage).
Another point is that the PAR efficiencies of these COBs are pretty high. Assuming that we lose about 10% during the AC/DC conversion, so the actual input electricity in this example would be 68.4/0.9 = 76W. With 4000K, 133.7 micromol/s / 76W = 1.76 micromol/J. The top commercial grow light is around 1.6-1.7 micromol/J for both LED and HPS. See table 3 of this paper. To make the output similar to the commercial grow light, you can use 4-5 of these. So the total cost of <$400 can beat the $1000 top LED fixture.
If we use lower current and better heatsinks (to lower the junction temperature), the efficiency improves. Also if we go with CXB3590, the efficiency goes beyond whatever available grow lights. My newest one is CXB3590 at 50W each, which I think gives >2 micromol/J.