LED grow light technology is moving away from using specific bands and instead the industry is focusing on providing the broadest possible spectrum. The most reputable LED companies are moving away from pink/purple lighting and replacing their LEDs with “white” chips.
These white chips are produced by a phosphor-coating method, where the coating is deposited on the LED die. The exact shade or colour temperature of white light produced is determined by the dominant wavelength of the blue LED and the composition of the phosphor. And the thickness of the phosphor coating produces the variations in the colour temperature of the diode.
The perfect grow light would be one that replicates the spectrum of our sun, while allowing us to adjust the light intensity to our exact needs. This would be the pinnacle of “Full Spectrum.”
For our intents and purposes, the Sun’s radiation spectrum is very evenly spread and peaks in wavelengths around the PAR spectrum.
· Spectrum of Solar Radiation. Note how the Irradiance peaks within the PAR/Visible spectrum.
While plants certainly can use some of the light wavelengths outside the PAR spectrum, the light that falls outside of this range is usually either too powerful or too weak to be of primary use for photosynthesis.
As an example, with certain exceptions UV light is too destructive to be used to synthesize large molecules, and infrared on the other hand is relatively weak, and produces a lot of heat. By comparison, within the PAR range each photon contains just enough energy to excite the electrons of molecules without causing damage to the cell.
So, how should the perfect spectrum be? How much of every color do plants need?
Luckily, science has the answer. It turns out that a publication by McCree (1972) figured all this out for us and published a chart similar to the following one:
To absorb light, plants use a somewhat primitive but highly effective version of our eyes, which we call pigments. The most abundant plant pigment is chlorophyll and it is most efficiently used to capture red and blue light. Other than those, there are many other pigments, including carotenes and xanthophylls which harvest light in other wavelengths and pass it on to the photosynthetic process.
It should be pointed out that green light actually penetrates deeper into the leaf interior than red light and can drive photosynthesis more efficiently. This is because the top layer of the chloroplasts that contains chlorophyll becomes saturated while green and yellow can penetrate deeper into leaf tissue and be reflected around until absorbed by another chloroplast containing chlorophyll or by an accessory pigment.