We want the consumer to be able to make an educated decision on the right light for their application, whether that light is ours or from another company.
What is a Photon?
For simplicity, we'll call a photon a "packet" of light. Photons are one of the ingredients plants use to make glucose during photosynthesis. (the other ingredients being water and carbon dioxide)
You might be asking "what is a packet of light" - well lets say you had a laser that only emits 660nm red light.....each 660nm photon is a packet of the same amount of energy. 660nm photons have an energy of 1.878eV (electron Volts) - so the if you shine the laser at a detector for any period of time, the total amount of energy given off by the laser will always be a multiple of the 1.878eV because each photon carries exactly that amount of energy. This quantified behavior of subatomic and atomic particles is where the field of Quantum Physics gets its name.
The wavelength of a photon determines its energy. A blue photon has inherently more energy than a red photon because it has a shorter wavelength.
Photosynthesis and the McCree Action Spectrum
For the purpose of growing plants we mainly focus on light in the visible range from 400nm (violet) to 700nm (red). This range encapsulates the light spectrum that horticulturists refer to as Photosynthetically Active Radiation, or PAR. The most widely accepted (but still highly debated) photosynthetic efficiency of each wavelength in the PAR spectrum is known as the “McCree Action
Spectrum” and was developed by studying the the photosynthesis rate of 22 different plant species over wavelengths from 300nm-800nm.( K.J. McCree, 1971)
Why white LEDs?
White LEDs are often a great approximation of the McCree action spectrum when compared to MH, CMH, T5, and HPS lighting. Many companies will claim that their LEDs were developed for hortuculture application but the truth is: growers got lucky. Most white LEDs were developed for applications like streetlights, tunnel lighting, or warehouse lighting. A white LED is really a blue diode with a phosphorus coating that fluoresces when it's hit with blue light. This scatters the high energy blue photons across the visible spectrum into lower energy green, yellow, and red photons. Different manufacturers (Cree, Bridgelux, Samsung) have proprietary phosphors and these different phosphors are what make a 3000K vs a 5000K or 6500K. Generally, a given color temp white LED will have roughly the same spectral output as any other white LED of the same color temp.
Recent studies have also called the McCree Action spectrum into question by showing that plants utilize green light more efficiently than red or blue light when the total light intensity is high enough (above about 600 PPFD) (Terashima, et al., 2009). This photosynthesis happens inside the leaf as opposed to on the surface, which is why leaves appear to reflect green light even though they use green light very efficiently. There is speculation that this utilization of green light is why white LED's and also HPS bulbs work so well for growing plants.