when 6500k is not 6500k...

[Plantcrazed101]

On one of the forums, Ray mentioned (if I'm understanding this right? Please correct if I'm wrong as I'm eager to learn!!!) that the color temperature specified for black body radiator type lights are not necessarily the exact same wavelengths for LEDs or CFLs with correlated color temperature names. In other words, a 4100k HPS and a 4100k LED are not the same light wavelengths, they just appear similar to the human eye and so are called the same. That means there is room for error on what our plants are actually getting.

I think I saw that this answer from Ray was from 2005, so I'm wondering if y'all (or Ray if you see this) have any thoughts or comments as to what would be the best color temperature for LEDs or CFLs?

Originally Posted by Ray

Color temperature is a way of describing the spectrum, and yes, something in the neighborhood of a 6000°K color temperature is ideal for plants, as it comes close to replicating the light provided by the noontime sun plus the blue back-scatter from a clear sky - the light all plants get.

However, as neither LEDs nor fluorescents are black body radiators (incandescents and HID lights are), the color temperature specified for them is a correlated, or corrected color temperature (CCT). That is, it is the correlation of how that light looks to the human eye, when compared to a black body radiator, and NOT the same spectrum as one.

I really have no idea what the "ideal" red/blue mix is, but for what it's worth, the 35W Philips "Production modules" I am using in my "basement incubator" have 68 reds and 24 blues per strip. Their "flowering lamps" on the other hand, have a much higher red:blue ratio.

[naoki]

You are correct that the 4000K LED emission spectra are very different from the jagged 4000K fluorescent light emission spectra.

I measured the plant relevant light quantity (called photosynthetic photon flux density). You can think it to be similar to foot-candle, lux, but it is not adjusted to the green sensitivity of human eyes. There are not much difference in the PPFD between 3000K-5000K in both fluorescent and LEDs. Higher K LEDs do have slightly more PPFD, but this could be negated by the fact that red light is more efficient for photosynthesis. With LED, once you start to go above 5000K, the blue peak becomes too much. But I haven't seen a convincing evidence showing that the color temp makes a big difference for orchids (there are lots of scientific evidences in other plants, though). With LED, I would choose 4000K-3500K as my first choice, but I'm ok with 3000-5000K.

[Ray]

I am uncomfortable with using color temperature as a selector for LED lamps, as it is - as was described well by the OP - only a descriptor of how it looks to us, and tells little to nothing about the actual spectrum. PPFD may be better, but it doesn't tell us much about spectrum either - "X" micromoles/square meter at 400 nm and at 700 nm are likely to give entirely different growth to plants. If we know the spectrum emitted, AND the PPFD, we're a lot better off.

[Plantcrazed101]

So Ray, how do you find out the spectrum emitted and the PPFD? haha I'm guessing a cheap light meter won't do that

[naoki]

Emission spectrum is usually published by the manufacturer of LEDs. To measure it, it requires a pricey equipment. There are some variations, but most white LEDs which use Phosphor+Blue emitter have similar spectra for a given K value.



Note this kind of SPD can be misleading because Y-axis is in watt of light (not micro mol/s), and the height is "relative" to the peak power for a given K.

Yield photon flux (YPF) may be a better representation than PPFD. See the figure close to "yield photon flux" section of this wikipedia page, not the absorption spectrum of the chlorophyl (top of the page). If we believe that this McCree action spectrum of photosynthesis, then 1 micro mol/m^2/s of 400nm light is about 70% efficiency of 1 micromol/m^2/s of 640nm (I don't think you meant 700nm, because the PS efficiency at 700nm is pretty low, and I don't think there is 700nm LED). So I agree that PPFD is not perfect in this sense. With the known emission spectra, we can calculate the YPF, which incorporates the mentioned difference in the efficiency within the PAR region. So YPF may be better, but we do not know if McCree curve, which was obtained from full-sun crop plants, is applicable to shade plants. There are some meters which can measure in YPF (from Skye), but most of scientists report PPFD, which we consider to be a relatively good quantity to report the growing environments of plants.

So I do agree that it is better to know the spectra and PPF, but if we know what kinds of LEDs are used, it isn't difficult to find/guess the shape of the emission spectrum.

This effect of light on photosynthesis part is easy, but going back to Ray's example. Different spectra can have different influence on the shape of plants. So 100% blue light may results in quite different shape/physiology of plants from plants under 100% red. This part is not so well known for orchids, though.