CRI90 does produce less light even over the whole range. That's the inevitable loss you get from the stokes shift. Converting light to other frequencies always comes at a loss.
Thanks for pointing that out.
IT IS ALL ABOUT THE AMOUNT OF PHOTONS! Check the reaction equation of photosynthesis and read up on both photosystems.
660 nm does nothing magical except driving photosynthesis.
A 660 nm photon just has the
minimal amount of energy needed for the photo reaction and therefore it makes sense to use photons with this wavelength
IF your light source can produce exactly these photons.
Example: Out of one watt of energy, you can produce 5.6 µmol of photons with an wavelength of 660 nm. If you would split this watt in photons with an wavelength of say... 605 nm, you just get 5.09 µmol out of it. So, which one do you use?
Probably the 660 nm light source.
The thing is, a phosphor converted LED produces blue photons (1 W = 3.7 umol @ 450 nm). These photons then hit the phosphor molecules and get radiated from there with longer wavelength (=less energy). The difference of these two energy levels is the stokes shift and is lost as heat! Surely there are cheaper ways to create heat then using LEDs.
So if you had an 100% efficient blue LED and convert 100% of these photons to 660 nm, the LED would put out 3.7 µmol. (Compare that with an 100% efficient hyper red -> 5.6 µmol!)
So from a energetic point of view you can say: The less conversion you do, the better it is. The bigger the difference of the wavelengths are, the worse it gets.
In an hyper red (660 nm) LED there is NO conversion loss as it produces the most usable photons right from the beginning!
Why usable?
Because both photo systems can use them! They need AT LEAST 680 nm and 700 nm to work (COMMON MISCONCEPTION OF THE EMERSON EFFECT*!!). Photons with shorter wavelength (more energy) work just fine - the surplus of energy (again) gets radiated as heat (this time) via the leafs.
So in regard of CRI90 and photosynthesis: The plant gives a fuck whether it gets 660 or 610 nm photons (look up RQE and absorption charts!)!
In regard of far red: If you want far red, then buy far red LEDs and don't waste all his energy in the phosphor layer.
*
Deliver plants just photons with 700 nm -> one of two photo systems are active.
Deliver photons with 680 nm -> both work, and they work together which boosts efficiency further (again, look up the Wikipedia article of photosynthesis)
100 dollar question: How many photo systems work with a standard CRI80 spectrum?