The Meaning of Full Spectrum, PAR and How Plants Use Light​
Column:Industry News Time:2018-03-02

Full spectrum is a common term that many lighting companies use to promote their particular model of LED fixture. Usually they reference the Chlorophyll A and B absorption spectrum chart, which you may be familiar with. I should probably point out that they probably didn’t know that such a chart is only valid for extracted chlorophylls and not for the living leaf itself. But that's another story.

The plain truth is this: as of right now, there is no spectrum available that will allow a 100w LED to replace a 1000w HID. This is because it focuses on chlorophyll absorption. And although plants certainly do have multiple pigments and photoreceptors across the PAR range, nothing will beat delivering just the right light quality and quantity to your plants.

Lumens & Par

As a grower, I’ve been comparing how powerful my grow lights were by using a lux meter. A lux meter is a device that measures the density of luminous flux (lumens) at a certain point from the fixture. The problem is that when it comes to measuring grow lights for plants, I should have been using PAR all along. And thus a PAR meter.

Just in case you don’t remember, PAR stands for “Photosynthetically active radiation." It refers to the spectral range of light from 400 to 700 nanometers that most plants use for the process of photosynthesis. For our purposes the difference is that a PAR meter measures intensity of light within the whole spectrum. While a lux meter (lm) is usually calibrated only for the brightest light wavelengths that we humans can see, where white and yellow light are seen as the most intense, while ignoring other wavelengths like blue and red which are also very useful for plants.

For more information, read my article on how light affects plant growth.
Lumens vs. Par
The way we perceive light is naturally much higher biased for green-yellowish light with a sensitivity peak around 555 nanometers. Our eyes have a combined sensitivity curve where the peak of our sensitivity is also where the peak reflectivity is going to be for a plant.

So then, what does "Full Spectrum" really mean?

Should the term even be used? Well, when a company decides to call their products a Full Spectrum Grow Light, they usually mean that their product outputs a broad, continuous and significant light across most (if not all) of the PAR range. That’s it. In fact, remember this: “Full Spectrum” as a term, is only as reliable as the Grow Light manufacturer. It is by no means a certification standard; whether legal, industrial or otherwise.

The fact is: as of right now, LED grow light technology is moving away from using specific bands and instead the industry is focusing on providing the broadest possible spectrum. You can see this if you noticed that 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.

Alright! Now that we know how today’s top notch LED grow lights are made, we can talk about the “best spectrum”.

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.