Wavelength is a measurement of the physical size of the light “wave”. Longer wavelengths carry less energy, but are also able to pass through more material before being deflected or interrupted. Shorter wavelengths carry more energy, but also bounce into more objects in their way. This is why microwaves cannot exit the microwave machine, but radio signals can pass through nearly everything.
Image Courtesy of Wikipedia
The wavelengths of light that we can see (390nm – 700nm) make up in the visible light spectrum. Conveniently, this also the majority of the spectrum that plants use, in what’s known as “Photosynthetically Active Radiation,” abbreviated as PAR. Plants rarely use energy from wavelengths of light that are outside of the visible spectrum, although it does happen.
Image Courtesy of Wikipedia
The chart above shows the portions of the visible light spectrum that plants can use. As you can see, the most efficient wavelengths are between 400 to 500 nanometers (blue light), and 650 to 700 nanometers (red light). While there is usable light in the middle (green light), it is not as efficiently used. There is still much debate in the growing community over the value of green light in a growing scenario, but red and blue light is always ideal.
LOOKING FOR WAVELENGTH WHEN BUYING LIGHTS
Almost every horticultural lighting product will provide a spectral chart illustrating what wavelengths of light their lights should be able to hit. The gold standard for horticultural lighting is none other than the sun — go figure! So let’s get a look at what the sun’s spectrum looks like in the visible light range:
Image Courtesy of ResearchGate. Red Lines added to indicate visible portion of spectrum.
As you can see, the sun’s output in the visible spectrum is roughly uniform. It peaks around the 450 nm range and gradually descends from there. This is the spectrum that many horticultural lighting producers are attempting to emulate, to varying degrees of success. Your ideal horticultural light will have a spectrum that looks something like this.
However, there isn’t one horticultural light that fits all needs. We’ll cover why in the next How-to-Grow Cannabis article, but essentially it stems from the fact that Cannabis is an annual plant.