Different types of white lights

I suppose this should start by pointing out that in non-black body emitters, like fluorescents and LEDs, the claimed color temperature is a correlated color temperature, not a true color temperature. So, rather than being an indicator of the spectrum, it is an indicator of what spectrum it mimics to the human eye.

That said, if your fluorescent is a true plant light, then 6500-degree K lamps are commonly supplied in horticultural fixtures, and are quite good.

With white LEDs, on the other hand, even though they use similar technology to fluorescents (the light is emitted by a phosphor that is excited by the "guts" of the lamp), nobody seems to be using a combination of phosphors that match plant lights, hence the needs of the plants. Consequently, white LEDs tend to be very strong in the blue and green parts of the spectrum, with green being the vicinity to which the eye is most sensitive (hence the reason they look so bright), but lacking in red. That is why you'll see white chips interspersed with red (as in my lamps). However, if you are going with 100% white chips, I'd recommend something in the 3000- to 4000-degrees K vicinity.

 

That has to do with the size and shape of the lamp. PAR refers to parabolic aluminized reflector - i.e., spot or flood lights. The number is the nominal diameter in 8ths of an inch, so your PAR 38 is 38/8" or about 4.5" in diameter.

 

For LEDs, my calculation with Cree and Bridgelux LEDs (3000-5000K range) says that higher K (e.g. 5000K) has the higher number of photons (photosynthetic photon flux, PPF) per given energy consumption. However, once we take into account that red light is more efficient for photosynthesis (using something called McCree's relative quantum efficiency, RQE), then the difference become smaller. This is called yield photon flux (YPF).

Sorry to introduce too much jargons here, but the punch line is from photosynthesis, it doesn't matter much which one you use (within 3000-5000K). But the different spectra can influence other aspects. For example, there are some data showing that in some orchids, more blue light (higher K) causes production of certain chemicals, which may make the plant more resistant to leaf bleaching (and potentially disease resistance, too).

I use whatever available LEDs between 3000-5000K, but if I have to choose, I would like 4000K (for no particular scientific reason).

I haven't done the analyses of fluorescent light emission spectra (it can be done, but I'm not that interested in), but from casually measuring PPFD, there isn't much difference between 3000-6500K. Those purple plant bulbs did have quite a bit higher PPFD (per energy consumption).

 

Here is the measured emission spectrum of Cree 4000K 70CRI CXB3590 from Cutter Electronics. Note that Y-axis is corrected to be photon count (instead of power in watt like typical SPD curve). The black line is the typical photosynthetic relatively quantum efficiency (RQE) of McCree.

 

You'll see a common thread with me (cheap), but I usually purchase what is in sale. I have a total of 6 PAR38 LED floods in use over an @ 4'x4' area where I'm wintering phrags, catts, dendrochillums and vanda types and lower light stuff on the periphery. I think there is a mix of 3000K/4000K and maybe one 6500K.

I don't believe my thinking is accurate about this, but: I always pick the one with the highest Lumen count when all else (price, size, etc...) is the same.

I've used the red/blue/orange spectrum LED lights in the past (still do in one 'out of the way' location), excellent light but then it is pretty ugly (shocking pink!) to the human eye and looks strange glowing out the window.


Good luck,


Bob