Blogs Apr 1, 2020

Full Spectrum LED Grow Lights: The Truth You Need to Know

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Many LED lighting suppliers will say that full-spectrum LED grow lights are the best option for growing plants because they mimic the natural light from the sun. The argument goes:

“Plants have grown under sunlight for millions of years. Why would we want to change what mother nature knows is best?”

Well, we want to let you know that there is no such thing as a full-spectrum LED grow light.

There, we said it.

But before we get a flood of messages from concerned growers wondering what all the confusion is about, let’s first uncover what full spectrum means. Then we’ll let you in on the truth about full-spectrum LED grow lights so you can make the best grow light choice for your facility.

What is a Full-Spectrum LED Grow Light?

A full-spectrum LED grow light is simply a marketing term that implies that your grow light closely resembles light from the sun. This marketing term comes from the concept of “full-spectrum light,” which in recent years has been used to refer to electromagnetic radiation from the UV to infrared wavebands.

Spectral range of a full spectrum LED grow light
Figure 1 – In recent years, full spectrum is a term that has been used to refer to light between the UV and infrared wavebands as seen in the graphic above.

The History of the Full-Spectrum LED Grow Light

The full-spectrum LED grow light is the newest evolution of an already confusing term. Originally, full-spectrum light described the only real full-spectrum light source, the sun.

Over time, the term began to take on other characteristics of sunlight. The commercial lighting industry began using the name “full-spectrum” to sell lights that produced a Color Rendering Index (CRI) over 90. Humans perceive colors more accurately under light sources with a CRI over 90, much like how we see colors in our natural world under daylight. This was a beneficial feature for human environments such as offices, outdoor spaces, and others.

comparison of sunlight spectrum to full spectrum
Figure 2 – One of the reasons that lighting engineers could achieve a high CRI was by creating a smoother and more continuous spectral distribution curve (SDC) reminiscent of daylight.

With the advent of horticultural lighting, companies once again began to borrow the term. Only this time, they claimed that full-spectrum LEDs could reproduce the effects of sunlight for plants.

Thus, the full-spectrum LED grow light was born. Unfortunately, lighting for plants is not quite that simple.

Problems with Full-Spectrum LED Grow Lights

There are many issues with the concept of full-spectrum LED grow lights. For starters, just because you name something, doesn’t make it true. This rhetoric may have made sense for lighting designers interested in selling lights so humans could see, but plants require light to feed, grow, and live.

There are three major problems when talking about full-spectrum grow lights:

  1. Full-Spectrum Grow Lights Aren’t Optimized for Plants
  2. Full-Spectrum Grow Lights Don’t Include the Full Solar Spectrum
  3. Full-Spectrum Grow Lights Are Not Dynamic Like the Sun

We’ll briefly look at these problems with full-spectrum grow lights one-by-one, so you can understand how deep the roots of this problem run:

1. Full-Spectrum Grow Lights Aren’t Optimized for Plants

A major problem with many full spectrum LED grow lights is that they are designed to give the appearance of daylight without being custom-tailored for rigorous plant growth.

It’s the reason why we at LumiGrow coined the phrase, “PAR is for plants and Lumens are for humans.” Not all wavelengths of light are optimal for photosynthesis. Plants photosynthesize electromagnetic radiation in the 400 to 700 nanometer range, known as Photosynthetically Active Radiation or PAR. So, plants don’t care how bright your light fixture appears to you.

Still, most full-spectrum lighting companies build fixtures with this visual appeal in mind.

When you hear that the diodes in your full spectrum grow light are 3,000k to 4,500k, or 5,000k+, this degree of Kelvin (K) refers to how “cool” or “warm” your light is in appearance.

2800k full spectrum grow light example
Figure 3 – For example, the 2800k spectrum above is used to create “warm” environments for restaurants or other places that can benefit from accentuating earthy tones.

Our understanding of plant photobiology has come a long way. We understand much more about plants than to be using human lighting metrics to design our grow lights.

Our goal as growers is to improve the lighting characteristics most important for plant growth. This means not only getting enough PAR light, but also the right mix of light spectra, which brings us to problem #2.

In Figure 3 above, we see a standard 2800k full-spectrum LED curve. This LED has been optimized for its warm visual appeal by stacking most of its energy in the orange-to-red wavebands. This spectrum is optimized for the visual spectrum between 380 to 740 nanometers. Emphasis has also been placed on light around 555-nanometers because this is where our eyes are most sensitive.

2. Full Spectrum Grow Lights Don’t Include the Full Solar Spectrum

The thinking behind many full spectrum LED grow lights on the market is that by creating a spectral distribution similar to sunlight, your plants will grow well. A decent theory, except that full spectrum grow lights are not actually similar to the sun.

We can see below that the sun’s radiation includes much more than the visible or PAR wavebands.

chart showing the full sunlight spectrum on earth
Figure 4 – The image above shows the spectrum of solar radiation at sea level (red) and out of the atmosphere (yellow). Source – File:Solar_spectrum_ita.svg. Author – Nick84

Sunlight itself is complex, and many scientists are still working to understand it today. You can see that sunlight also contains ultraviolet (UV) and infrared light (as well as x-rays, radio waves, and others, but we’ll leave those alone for now).

Although PAR is the most important light for photosynthesis, plants still respond to radiation outside of the PAR spectrum. For instance, UV light elicits protective compounds in plants similar to the way humans become tanned in the presence of UV.

Plants also use a type of infrared light called “far-red light” to induce a shade avoidance response, causing them to stretch and can induce early flowering.

To create a light source that elicits plant response the same way the sun does would be too costly and downright impossible given current grow light technology. Nor would you want to create such a grow light, which takes us to problem #3.

3. Full-Spectrum Grow Lights Are Not Dynamic Like the Sun

Not only would it be too costly to create an actual full spectrum LED grow light, but if such a thing even existed, its performance would still not accurately reflect what’s happening in nature.

graph showing variance in sunlight spectrum
Figure 5 – Spectrum of the visible wavelengths at approximately sea level; illumination by direct sunlight compared with direct sunlight scattered by cloud cover and with indirect sunlight by varying degrees of cloud cover. Source – Data is from X-Rite i1Pro. Author – Txbangert

The sun’s spectrum is in constant flux due to changes in weather or its position in the sky relative to earth. In the graphic above, you can see how sunlight spectra change throughout the day or in different weather conditions.

Because of this phenomenon, it’s best to think about the interaction between sunlight and plants as a continually changing process.

If you hang your full spectrum grow lights in a greenhouse, you will still reap the benefits (and disadvantages) of this natural process from the sun. But if you take those same full-spectrum lights and hang them indoors, they will not behave like the sun.

Photomorphogenic responses by plants are co-regulated, which means that certain expressions of the plant may turn on or off based on the amount of light within one waveband relative to another.

How Sunlight Absorption Works in Plants
chlorophyll absorption spectrum chart
Figure 6 – Chlorophyll absorption spectrum

Photosynthesis depends upon the absorption of light by photoreceptors and pigments in the leaves of plants. The most well-known of these pigments is chlorophyll-a, but there are many accessory pigments that also contribute to photosynthesis.

The relative light absorption of chlorophyll pigments as shown in the graph to the right is one of the reasons why red light has become popular among LED grow lights. Not all PAR light contributes to photosynthesis equally, though we now understand that other wavebands of light such as green, do play an important role in this process.

Since photoreceptors in plants also have their own ranges for light absorption, they co-regulate processes that create plants’ form and structure depending on the spectral mix they receive.

For instance, higher ratios of blue light can induce more robust root growth, more favorable plant biochemistry, and a hardier structure. But these effects may not be as pronounced when more red light is introduced.

Thus, the ever-changing spectrum of the sun is constantly signaling to plants to change their form and structure based on the natural conditions of the environment.

But before you rush and begin moving your grow room outdoors, let’s consider why plants don’t need the full spectrum of sunlight. For starters, plants don’t need UV or infrared light to live. Also, in a controlled environment, plants are given ideal conditions to grow in and often don’t need to compete with other species to live.

Plants only require light in the 400 to 700-nanometer range to photosynthesize. So, you’ll want to choose a grow light that produces your desired results, most often higher yields and better quality for your plants.

What is the Best Light Spectrum for Plant Growth?

By now you must be wondering:

If I can’t mimic sunlight, then what light spectrum should I use?”. The answer is both simple and quite complex.

Plants only require PAR light for photosynthesis. So, if your grow light is optimized within the PAR spectrum, you’re going to get the most bang for your buck when it comes to minimizing electrical costs while maximizing plant health.

Beyond PAR, it’s important to choose a light spectrum that’s:

  • best for the environment you’re growing in (greenhouse or indoors)
  • tailored to your plant’s growth phase (propagation, vegetative, flowering, or finishing)
  • or specific to the cultivar being grown

Full Spectrum LED Grow Lights vs. Other Grow Light Options

It should be clear by now that there are no real standards around full spectrum LED grow lights. Full-spectrum is simply a term used to sell you a simple idea.

Although you cannot mimic sunlight, you can use light spectrum to your advantage.

Luckily, there are many grow lights available with designs intended to do just that. So, let’s uncover your options so you can pick the best grow light for your cultivation.

Narrow Spectrum LED Grow Lights

Narrow-spectrum LED grow lights use a higher ratio of narrow-band LEDs. These grow lights most often have a pink or purplish hue since they are optimized for the blue and red PAR wavebands.

Targeted Spectrum Grow Lights
Figure 7 – The LumiGrow Targeted Spectrum above is a narrow-band spectrum that puts most of its energy in the blue and red wavelengths, with adequate green light included for secondary metabolic processes when used in indoor environments. Available with TopLight & BarLight light fixtures.

These types of pink grow lights have been popular since the early days of LEDs for horticulture. Though this doesn’t mean they are outdated by any means.

In greenhouse environments a narrow spectrum is almost always desired. The sun already fills out a full spectrum, so it makes sense to put most of your energy into wavelengths that are most optimal for photosynthesis.

Also, because of the added efficiency of red diodes versus other colors, you will get more bang for your buck when it comes to energy efficiency.

Broad-Spectrum LED Grow Lights

Broad-spectrum LED grow lights have a higher ratio of broad-band LEDs. These lights are white in appearance, though there are no actual white wavelengths. The white hue is a mix of blue, red, and green wavebands.

These grow lights also don’t claim to mimic the sun, but they will effectively replace the sun to drive high yields and premium quality in any environment.

Broad-Band LED Spectrum
Figure 8 – The LumiGrow Hybrid Spectrum is an enriched broad-band spectrum that’s been optimized for photosynthesis in any environment. This enriched broad-spectrum is designed to take the most important parts of HPS, Metal Halide and narrow-band LED grow lights, and create a spectral mix that’s versatile for any plant growth application. Available with TopLight & BarLight light fixtures.

Our broad-spectrum has been enriched with red and blue peaks to drive robust photosynthesis and plant structure while emphasizing the green waveband to be versatile with any crop type or cultivation environment.

Recommended for indoor environments, except in specialized cases where narrow-band lighting is preferred.

Adjustable Spectrum LED Grow Lights

These modern LED grow lights allow for precision control of your plants. By adjusting your grow light spectrum wirelessly, it’s possible to speed up flowering times, improve your plant’s biochemistry, or customize your plants’ structure to root better and be more easily managed.

adjustable spectrum LED grow lights
Figure 9 – Image of LumiGrow LED fixtures being adjusted with the smartPAR Wireless Control System to create lighting zones based on the spectral requirements of the crop.

These futuristic lights are the closest thing you’ll get to replicating sunlight’s dynamic qualities. The possibilities are endless with spectrum control. Designed for scientific or commercial applications where precision is required.

guide for hps lights versus led grow lights

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TopLightTM
Designed for Greenhouse & High Bay Applications
BarLightTM
Designed for High Performance Vertical Applications
smartPARTM
Wireless Grow Light Control