Why is LED Grow Light Purple?

Reading Time: 7 minutes

Sure, by now we are all aware that LED grow light does wonderful things for plant growth. Plants grow quicker and healthier than under other, traditional lighting methods such as HPS. If the LEDs are good, plants can grow even better than under natural sunlight. But why are they always purple/pink?

All light sources contain a spectrum of colors within it – some light sources contain some of the colors, some contain all. Sunlight contains all spectrum colors and because of that feeds plants with all information they need. Additionally, because of the presence of all spectrum colors, it appears color-free to human eyes.

Sunlight Spectrum

Figure 1 – Spectrum of colors contained in sunlight.

Constructing an LED Grow Light

When constructing an LED lamp, we can decide which color LED chips to place inside. This depends on what kind of response we want to achieve from plants. For instance, if we want plants to grow tall, we will increase the amount of far-red, yellow, orange and green chips inside the luminaire. If we want plants to be compact, we will put more blue or UV colored chips.

Why plants respond in those ways to those particular colors is a broad topic. It is something that we will cover in a separate post. Either way, these responses are encoded in plants’ DNA. So, when designing LED lamps, we can count on plants responding the way nature has designed them to.

We could say that the two most important light colors to place in an LED lamp are: red and blue. Red is the main component that plants need for photosynthesis and stem elongation inhibition. Additionally, it signals to the plants that there are no other plants above it and that it can thus have uninhibited development. Blue stimulates stomatal opening, stem elongation inhibition, leaf expansion, curvature towards light and photoperiodic flowering.

The combination of these two sets of effects will, in simple terms, get the plant from seed to the vegetative stage and eventually to flowering. This however will be much slower than under continuous spectra a.k.a spectra that contain more than  just red and blue.

Adding some other spectrum colors such as green, could enhance the leaf expansion rate and stem elongation, which in turn results in higher biomass (yield) accumulation. By adding UV wavelenghts one can affect the accumulation of compounds such as phenolics which could boost the flavor of the end product or its health benefits to humans.

Economic Considerations

However, from the business perspective, red and blue LED chips are the cheapest to procure. And this is why most LED grow light manufacturers opt for simple red-blue combinations. Red chips have been manufactured for a long time and used as LED indicators in TV remotes, computers and other gadgets. Blue LEDs came into the market in the early 1990’s and both of these, just as they are, are suitable for plant cultivation.

Yes, that means that when you illuminate your plants with an LED light, it is as if you are pointing your TV remote towards it. It is the same light. Both red and blue LED chips are off-the-shelf products, meaning quickly accessible in countless factories in China. And with the combination of red and blue LED chips we get: purple or pink light that has become the visual synonym of the LED grow light industry. So, there is the answer to our question, but…

Watch our webinar on how LED spectra are designed.

Why is the LED Purple so Strong and Unpleasant to Human Eyes?

There is a standard of how comfortable or pleasant light is to human eyes. We call it CRI – The Color Rendering Index. The authority that determines this quantitative measure is The International Commission on Illumination (CIE) and it deals with issues of light, illumination, color and color spaces.

In simple terms, what CRI determines is: how natural do colors of objects look under different kinds of lighting. Natural in this case means, as they would appear under light that appears color free (such as sunlight or some types of incadescent light) and thus possible for the human spectator to identify all shades of color of the given object.

Low CRI Lighting

For instance, street lighting, typically High Pressure Sodium (HPS) has a CRI value of 20-40. Incadescent lights that we typically use to illuminate our homes have a CRI of 100. Typically, we consider CRI values below 50 to be difficult to work under and unable to portray objects in their true colors. Values above 50 are the opposite. This is why everything in street lighting (if it’s HPS) looks yellowish while under incandescent bulbs, even though they too are yellow-ish and warm, objects are visible in their natural state.


HPS Light – CRI Value 30

Incandescent CRI 100

Incandescent Light – CRI Value 90

LED lamps that are simply made up of red-blue LED chips, reappropriated from gadget indicators, have a CRI value of 0! What that means is that it is effectively impossible to identify any color of objects underneath it.

When looking at, for instance lettuce under this kind of light it looks entirely purple. As does the soil, and everything around it.

While not damaging to human eyes, it is extremely unpleasant to work under this kind of light (to read a detailed breakdown of how LED light affects human health, please click here). Additionally, it makes it impossible to identify details on plants such as discolorations due to diseases, various insects etc. Humans are not used to this kind of light as CRI=0 light is nowehere to be seen in nature.

Red Blue LED grow light

LED Light (Red/Blue Chips) – CRI Value 0

Does It Have To Be Purple?

Valoya is one of few LED manufacturers that make their own, custom LED chips. We optimize the LED chips for plant growth and not for other uses. Furthermore, Valoya does not build LED lamps with simple red-blue chip combinations. There are bits of green, UV, far-red etc, depending on the spectrum.

This makes Valoya’s light a closer imitation of sunlight and a better source of information to plants than red-blue LED lights. The CRI value of Valoya lights is between 60 and 95. This means it appears to human eyes either as a pleasant, soft pink, or even plain white! LED grow lights do not have to be purple.

Full Spectrum LED Grow Light

By complementing the LED chips combination with bits of other spectrum colors, we not only feed more information to the plants but also make a richer spectrum. We make one that contains more colors and is thus a closer match to sunlight.

The primary goal is making light that is beneficial for plants and not pleasant for human eyes. At the same time having a pleasant looking light is a great add-on that users of LED grow lights apreciate. We call this kind of light spectrum full, wide or continuous. Marketers of LED companies however use these terms incorrectly and we can not always rely on their statements.

A good rule of thumb is to trust your own eyes as well. If the light looks strange to human eyes, is overly sharp and looks unnatural, it has a low CRI value. It also probably means it is a result of a simple red-blue LED chip combination. The light that looks natural and pleasant to human eyes has a high CRI value and is probably made up of a combination of many differently colored LED chips. Below you can see Valoya’s NS12 and AP673L patented spectra.

Wide spectrum led grow lights

Valoya LED Light, Spectrum NS12 – CRI Value 90

Wide spectrum led grow lights

Valoya LED Light, Spectrum AP673L – CRI Value 60

To achieve these types of LED light spectra, Valoya has invested significant efforts in research. This is reflected in more than 600 large scale trials over the past 12 years. To learn more about Valoya’s patented spectra, please click here.

Would you like to try to make your own light plan using Valoya’s proprietary light planner? It is free-of-charge and no sign-up needed.

Crop Science Light Planner

Cannabis Light Planner

Vertical Farming Light Planner

The blog posts are written by Valoya’s biologists and engineers. All of the content is original and is aimed at helping growers and researchers get a better understanding of the LED grow light technology.


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