Light Spectrum Charts: Why They Are Important And How To Read Them

One of the great things about LED grow lights is that they consist of a vast number of diodes. Each diode can be manufactured to emit any light color. There are blue diodes, green, yellow, red, UV, FR, and of course, white. White diodes have different “Kelvin” (K) rating, which could simply be described as “shades of white”. Most LED lamp fixtures (not COBs or QBs) in the lower price ranges typically use different colored light emitting diodes to create a customized spectrum. Each light color has its own wavelength as light, or photons, are both a particle but also a wavelength. Wavelengths are measured in nanometers (nm) and the shorter they are, the more energy each photon contains. Blue photons have the shortest wavelength of ~450-500 nm, then green 500-670 nm, yellow 570-590 nm , orange 590-620 nm, red 620-750 nm, then above that far red.

Understanding that all light consists of photons and that the colors we see are determined by the wavelength of the photons is the fundamental theory we need to be aware of when continuing to LED grow light spectrum charts.

When browsing LED grow lights, practically every brand out there claims their spectrum is the “ideal” spectrum. You’ll come across a whole lot of different and funny looking spectrums. Let’s see which ones are good, which ones are bad, and which ones have their own specific use purpose. What’s important to keep in mind is that the goal with the light we project onto our plans is to stimulate photosynthesis inside the plant’s cells. Photosynthesis is what makes plants grow. Correct light + CO2 + water + nutrients = growing and thriving plant.

Photosynthesis is best stimulated with certain wavelengths, or light colors. If we are to get a bit technical, during cannabis plants’ early stage, the vegetative stage, they prefer a spectrum that has more blue in it. During flowering stage, weed plants want more red light. 

Here we see that the highest stimulation comes from blue 400-475 nm and red 630-670 nm. Plants also make use of green, yellow, and orange light (475-630 nm) but to a lesser extent.

There’s also a nice “middle ground” that works well for the entire grow cycle. This “middle ground” spectrum has both a fair amount of blue and green in it but has especially a lot of red, which is what we want to grow big buds and get plenty of yield. The middle ground spectrum is white light with 3500K.

K, or Kelvin. Kelvin describes the so called “color temperature” of white light. You may have noticed both in your home but also in office buildings, schools, hospitals, etc, that some lamps have a more sharp white whereas other lamps have a more mild, yellow/red-ish white. This is because they are manufactured to emit a certain Kelvin.

Here we see four different white lights but they all have different Kelvin. Low Kelvin below 3500K is considered “warm white” whereas above 3500K, is “cold white”. 3500K is a neutral middle ground.

Here’s the spectrum graph of a 3500K white light:

We see that it has a blue spike, then not a whole lot of cyan and green, but plenty of orange and, especially red. Similar to what stimulates photosynthesis well.

There’s some discussion in the science community that this much green, yellow, and orange may be a little excessive. Plants do use these light colors to some extent but a little bit less of these colors and more blue and red would be the most efficient spectrum. After all, all light that lamps emit costs electricity, and in turn money. As we want don’t want to be wasteful and also keep electricity use at a minimum, this spectrum could potentially be tweaked a little bit so that we get the “best light” for every consumed watt of electricity.

Generally speaking, the more light we give our plants, the less difference the spectrum makes. I.e. light quantity over light quality. But as light manufacturers want to keep costs down, which isn’t a bad thing, they sometimes make custom colored spectrums with a combination of colored diodes instead of going all white. Let’s take a look at that.

The first example I want to start with is the trending “BLURPLE” spectrum that I’d like to recommend everyone to stay away from. As far as I know, this blue-red-purple (= BLURPLE) spectrum came about when Chinese brands wanted to remove all diodes they deemed were unnecessary, greens and yellows in this case, and only go with diodes that stimulated photosynthesis. However, research shows that green light benefits plants’ health, form, and appearance. There’s also a debate that the yield, the buds from the marijuana plants, will have different characteristic when subjecting plants to a very unnatural light, which BLURPLE for sure is. So, point being, don’t fall for the BLURPLE trap. Just because a manufacturer claims their lamp is “full spectrum” doesn’t mean that it’s really full spectrum if it only has minimal green.

Example 1:

Here’s a random light from the first page result of “LED grow light” on Amazon. At this moment it has a 4.3 star average rating but I suspect it will fall pretty quickly once customers realize that this lamp barely puts out any light and that it’s spectrum entirely BLURPLE.

This manufacturer barely show a spectrum graph, which seems to be consistent with most other Chinese brands. But if you look closely and zoom in on one of the images, you’ll see this:

I don’t know about you when but when I think “full spectrum” I want to see more than a microscopic amount of green. 

Example 2:

Then we have this guy:

Once again, this listing makes no sense. In one of the images we see this:

There’s a tiny spectrum graph in their spectrum meter (which could be a generic stock image?) in the top image which shows a fullon BLURPLE spectrum. Then below we see a balanced, possibly 3500K or sunlight spectrum below. So which is the lamp’s spectrum? Confusing to say the least. If we take a look at the diodes in the lamp:

… we see that there are like 6 white diodes out of the 120 diodes in total. Rest seem to be blue and red. Definitely a BLURPLE spectrum and definitely false to claim “full spectrum”.

Example 3:

We saw this brand in Maximum Yield and based on the information they have on their site and Amazon listing, they seem to be different than many of the generic China-sellers.

Right away we see that they clearly show their lamp’s spectrum and we also see that it has a pretty good balance of blue, green and red. About 18% of both blue and green then 6x% of red, which is a nice distribution.

We can also confirm that the spectrum we see on this chart should be correct based on the diodes we see on the lamp in one of the other pictures. There’s a nice mix of blue, red, and white. White diodes emit, of course, all colors of the spectrum, which is why we’re getting a fairly even spectrum. The spectrum report this brand published also gives us other useful info like efficacy, PPF, and draw power, to name a few. 

Example 4:
We’re going off Amazon to one of the biggest brands in the high-end category, HLG. They sell so called “Quantum Boards”,QBs, more on that HERE. QBs are essentially just a LED panel with a driver and sometimes a heat sink.

We see that this company uses only white diodes and they sell their boards both with 3000K (recommended for flowering) and 4000K (recommended for veg). QBs are generally speaking aimed towards experienced growers and there are usually loose wires and not fully covered connections to make a minimal lamp with maximum efficiency. Good for pros but not the best light to start with for beginners.

There you have it folks. I hope I covered most of your questions about light spectrum and that the examples I used showed what to consider when you are grow light shopping.

Drop a comment if you have any questions.