Why Are Grow Lights Purple? (And Are “Blurple” LEDs Effective?)
Last updated February 29, 2020 By Steven 12 Comments
Only LED grow lights are purple. And only some.
Why are they purple?
Well, technically they aren’t.
The light appears purple due to the differently colored diodes in use: mainly blue and red. This pinkish/purplish glow has also been dubbed “blurple”, a term that is often used in a somewhat derogatory way these days.
Because many of the earliest LED grow lights, especially the low quality ones from China, emitted a “blurple” light. And people who bought those lights got burned.
They ended up with a light that couldn’t grow a blade of grass and understandably developed a negative impressions of purple lights.
Is that impressions justified? Yes and no.
We’ll get into that a bit more below. First we’ll go into more detail on why these grow lights look purple.
Why Are LED Grow Lights Purple?
LED grow lights have multiple diodes that each emit a single color of light. Some have only white diodes, while others have diodes in various different colors. It is the combination of those colors that creates the light we see.
Fixtures that emit a purple light contain a large amount of red and blue diodes. The smaller the ratio of red to blue, the more purple the light looks. The larger the ratio, the more pink it appears.
Since red wavelengths are the most important for plant growth and plants like a ratio of around 5 to 1 of red to blue light, most LED grow lights actually contain more red than blue diodes. This means that their light is actually more pinkish than purplish. It is the “blurple” light I mentioned above.
Why Do LED Grow Lights Use Mainly Red And Blue Diodes?
Plants use mostly red and blue light for photosynthesis. They use about 5 times as much red light as blue, and far more of either than any other color.
Many LED manufacturers took this knowledge and constructed grow lights featuring nothing but red and blue diodes.
They said that using white diodes would mean lots of light in other wavelengths like green and yellow, which plants don’t use. You would essentially be paying for electricity to create light that just goes to waste.
With only red and blue diodes, all of the electricity is used to create light that plants absorb and turn into energy. That is why their LED fixtures had only blue and red (and sometimes a few white) diodes.
Or so they claimed.
In actuality, they used mainly red and blue diodes, because they are easier and cheaper to make.
But consumers bought into the “red and blue light only is best” myth. At one point, it was very hard to find lights that were not mostly red and blue.
So why do I call it a myth? Is red and blue light bad?
No, it isn’t. It works just fine. But it is not ideal.
Are Purple LED Grow Lights Effective?
Purple LED lights work. They veg and flower plants well.
But more and more, we are coming to the realization that full spectrum white light is better for plants. It turns out that, while plants don’t use green or yellow light as much during photosynthesis, they do make use of some.
Besides, sunlight is white light that contains all colors of the spectrum. And sunlight has been growing plants since long before humans ever even walked the earth.
HID lighting hasn’t been used for growing quite that long, but it has been in use for decades. And it is also white light.
Metal halide bulbs emit cooler white light, with higher amounts of blue, while HPS bulbs emit a warmer white light, with much more yellow, orange and red.
Both do not contain all colors in similar amounts, like sunlight, but both are still white light. Newer ceramic metal halide bulbs come much closer to natural sunlight, containing a very similar spectrum.
The light from white LED diodes also contains a spectrum similar to sunlight and CMH bulbs. Both white LEDs and CMH bulbs are much more effective at growing and flowering plants than LED fixtures with only red and blue diodes.
But there is an even better spectrum for plants than this.
What Color Spectrum Is Better Than Purple?
As mentioned, plants want light in every color of the spectrum. But they want more blue and even more red, than any of the other colors. Thus, it stands to reason that the best light for plants contains all colors, but contains an extra heavy dose of red, and more blue than other colors, but less than red.
And that is the ideal spectrum. Almost.
Adding in a bit of UV and IR light has been shown to improve yield qualities, so having some light in the UV and IR wavelengths is even better. Learn more about the best color lights for plants here and about the effect UV light has on them here.
In short, what we want is a spectrum like the one shown in the chart above. One with light in every color, but a large peak on red and a smaller peak on blue. In addition, small tails that extend from the blue side into the UV wavelengths and from the red side into the IR wavelengths.
How Do We Get This Ideal Spectrum?
These days, there are a lot of LED grow lights that provide the ideal spectrum. But there are only a handful that do it efficiently and also sell for a reasonable price.
Black Dog lights, for example, have a great spectrum. But have you seen their pricing? The Phytomax-2 1000 costs over $2000!
It is possible to get the same great spectrum while spending far less. Let’s look at some of those lights.
Many people swear by HLG grow lights. Most of their fixtures have all-white diodes. Two of their fixtures add in some deep red.
They are much cheaper than Black Dog, but still cost quite a bit more than Chinese brands (HLG lights are made in China, but the company is American). Read my review of HLG grow lights.
Mars Hydro’s newest lights also use all white diodes and some of their new fixtures also combine those with deep red diodes. They are very similar to HLG lights, but with a much lower cost. Read my review of the Mars TS and SP fixtures.
The main difference is that HLG uses Samsung chips, while Mars uses lower quality Epistar chips. For budget lights with the higher quality Samsung LED diodes, check out my Spider Farm quantum LED review.
Neither of those options have UV light and only the ones with far red diodes have IR light, but only a small amount. To get both UV and IR, along with all colors and peaks on the red and blue, your best bet are COB LEDs.
I have a rundown of the best COB LED grow lights here and another one of the best full-spectrum white LED grow lights. Both feature the Phlizon COB series of lights as the number one pick.
The Phlizon lights have full-spectrum white COB LEDs, plus additional diodes that provide red, blue, UV and IR light. In short: they have the perfect spectrum for all stages of plant growth and are especially great at flowering.
COB LEDs have taken the growing world by storm. Not only have a great white light spectrum, but they are also incredibly intense and achieve a much deeper canopy penetration than other LED diodes. I’ve written a whole article on why COBs are great for plants.
For years, Amare were the only good provider of COB LEDs. They still are a good provider, but they are expensive. Optic LED then came along and sold very similar lights for less. But they are also still expensive.
Phlizon sells fixtures that are virtually identical to the Optic lights, but cost half the price. That is why I recommend them in both of the article mentioned above. Read my review of the Phlizon COB fixtures here for more.
Do you know why many LED grow lights give off a purple light? I explain the reason and also tell you why these purple lights are not as good as you were led to believe.
7 Myths LED Grow Light Companies Tell You
So you’re interested in buying the best LED grow light for indoor plants? You have come to the right place.
To make an informed decision, you should understand how plants use light, especially from artificial light sources. We are going to deconstruct a few myths in the industry to help you identify the best grow lights on the market.
Myth #1: “HPS puts out over 50% wasted light”
“This is why our purple 12-band LEDs are more efficient! Our perfect 12-band spectrum puts out the only light plants really need.”
If you see this quote, run! 🙂 Some LED companies say that the reason why LEDs are more efficient than High-Pressure Sodium bulbs is because plants don’t use yellow or green light. In other words, the reigning champion of grow lights for several decades, the mighty High-Pressure Sodium, is putting out a lot of “wasted light”. What?
Here’s what the LED companies haven’t been telling you. There’s a phenomenon known as the green gap for narrowband LEDs — they can’t create green, yellow or infrared wavelengths very efficiently. So when you look at the graph below, you’ll suddenly understand why we have purple grow lights. It’s because red and blue are the only colors narrowband LEDs are good at making! Also notice how narrowband LEDs only create very small “points” of color, which is why you might see 8, 10, or 12 bands being advertised as a full spectrum led grow light – it’s a company’s way of trying to deal with LED technology developed in the 1990’s.
The efficiency percentage of narrowband LEDs at different colors (wavelengths)
Truth: LEDs are more efficient than HID bulbs because they create light directly from electricity, and do not have to heat up a bulb.
But really, where are they getting the idea that purple light is the best for plants? Read on to Myth #2…
Myth #2: Plants Only Need Red and Blue (Purple) Light
Here is where the LED company will pull out their favorite chart – the Chlorophyll A/B absorption graph (see “Pigment Extract” in the below image). They’ll show their spectrum on top of this chart and say that they’re giving plants only the colors of light that they can absorb, or 100% usable light. Sounds great right? It’s more like a marketing trick.
Remember the colors that narrowband LEDs could create from Myth #1? Notice how red (
450nm) conveniently coincide with the colors most absorbed by the “Pigment Extract” which is Chlorophyll. In actuality, the leaf itself is absorbing more than just red and blue. Green and yellow light (500-600nm) are getting absorbed by the leaf, too. In fact, these colors are very important for the full development of the plant since that is what they experience when growing under the sun.
Absorptance spectra of spinach leafs
If you look closely at the graph, notice how different parts of the plant leaf absorb varying portions of light. The “Chloroplasts” are the centers of photosynthesis — notice how much green, yellow, and orange they absorb. Green doesn’t just “bounce off” of the leaves after all!
If you can imagine green and yellow light as the “free agents” that can slip by Chlorophyll A/B to other parts of the leaf, think about how a red and blue spectrum could fail. Red and blue light cannot easily penetrate past the first few layers of plant cells, into buds, or to the leaves below because Chlorophyll A/B is blocking those colors! See the illustration below where the narrowband Red/Blue LED having most of its light blocked by the leaf (e.g. Chlorophyll A/B) whereas HPS or broadband light penetrates past the canopy through to the leaves below.
Myth #3: Blue is for Veg, Red is for Flower
This myth originates from the ritual of switching between Metal Halide (MH) and High-Pressure Sodium (HPS) bulbs when transitioning from the vegetative stage to the flowering stage of growth. Growers use MH because it has blue light which is needed to prevent stretching. HPS bulbs have more yellow/orange light, which is important for rapid growth during flowering. Neither of these bulbs is designed for plant growth, but each is more suitable for a certain stage of plant growth than the other.
Since growers are in the habit of switching bulbs between veg and flower, LED companies provide two spectrum modes to increase marketability. Usually, the veg mode is almost 80 percent blue light. Studies show that giving your plants mostly blue light during veg is wasting the potential of your harvest. Your plants will grow slower, create smaller leaves, and won’t be prepared to grow big for the final yield.
Check out the image below illustrating the vegetative growth of lettuce under varying amounts of blue and green light. Watch what happens when blue light passes about 20 percent — the plants shrink! Think about the results that you might get from a grow light with more than 50 percent blue light. LED companies that “match” the chlorophyll A/B graphs will provide this much blue light, seriously decreasing yields. For curious minds, check out the study (quoted below) here.
As the fraction of blue light increased, leaf size and plant growth decreased significantly. However, while the addition of green light considerably reduced the leaf photosynthetic rate, it did not reduce plant growth.
Myth #4: UV LEDS Help Plant Growth
First things first, UVB light has shown benefits to certain plants, but only when applied at the correct intensity, for the proper length of time, and at the right time. Otherwise, UV light can be harmful to plants, let alone your eyes or skin — it’s the reason we use sunscreen and wear sunglasses when going outside.
All UV Rays can lead to eye damage. If your grow light emits UV, wear eye protection !
That being said, if used properly, could UV LED’s help plant growth? It’s a toss-up of whether it’s worth it when reviewing scientific articles. Most UV LEDs actually only produce UVA light, which can be safer and less damaging than UVB. However, the reason why UVB light is beneficial to plants is because it introduces damage and the plant responds with a defense, so UVB light is ideal.
What’s more, UV LEDs are not as efficient, more expensive, and burn out more quickly than other LEDs. With that in mind, it bears the question if using UVA LEDs is worth it, especially if that power could be redistributed to colors that are proven to increase plant growth.
See the graph below from an LED chip manufacturer to see the limited output range of UV LEDs:
Leading UVA/UVB LED supplementation bars last about 15,000 hours (vs the standard LED lifetime of 50,000) and cost hundreds of dollars, whereas a UVB bulb such as the T8 Reptisun 10.0 UVB cost about $15 and will last you an entire year if used for the flower period of your grow.
Myth #5: 1000W LED = 1000W HPS
- The HPS bulb wattage the LED fixture is intended to replace.
- An addition of the “highest rated power output” of LED chips.
- The actual power draw in watts.
Myth #6: The More Lumens, the Better
Lumens are a measure of how bright light is to the human eye. Looking at the graph below, you can see that green/yellow light is the most visible to the human eye, with blue and red light being the least visible.
The sensitivity of the human eye to color. 400-500nm is blue 500-600nm is green/yellow/orange, and 600+nm is red
You may find white LED companies advertising Lumens because they can “beat” the other fixtures that emit mostly red and blue light. A Red/Blue light will score a low amount of lumens.
Each color of light has a different efficiency and plays a different part in a plant growth, so we need a plant-centric light measurement as a benchmark. This is called Photosynthetically Active Radiation (PAR), which weights all wavelengths from 400-700nm equally.
Be critical about videos comparing one LED light to another with a PAR meter. Many PAR meters on the market incorrectly read LED lights. If you’re really interested about this, check out Apogee Instrument’s superb presentation [Scroll down to “In-depth Look at PAR-Quantum Meters” click play and skip to
15:00]. If you’re a grower, you should understand the concept of how light is measured. And hey, pick up a quality light meter!
Truth: PAR is the correct light measurement for plants, and not lumens.
Myth #7: A High Center-Point PAR Reading Means the Brightest Light!
Most grow lights are only good at shining light directly below the fixture in a “spotlight”, so the most attractive light reading an LED company can advertise is the highest reading they can measure — the one right in the center of the light.
Plants easily burn with this type of focused LED light, which is one of the reasons you may want to pay attention to this reading: if a light is putting out over 800 umol.m .2 s 1 of PAR in the center, that is in the danger zone for burning your plants. You’ll have to raise the light to the point where the center isn’t so “hot”, which actually decreases the brightness for the rest of your grow area. Counterintuitive, huh?
Look for LED grow lights that cover the entire grow area like the our lighting systems. These systems help eliminate plant burn while keeping your entire grow area at a uniform light level.
7 Myths LED Grow Light Companies Tell You So you’re interested in buying the best LED grow light for indoor plants? You have come to the right place. To make an informed decision, you should