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Red & Blue LED Grow Lights
Where's the green???

LED lighting in our homes offers brighter lights with even less power consumption. But looking into grow lights, we see a whole new dynamic: Multicolored lights. Some LED grow lights claim to be full spectrum, appearing white. But others are pink or purple. Specifically, they have combinations of blues and reds, boasting a finely tuned plant spectrum. LED grow light sellers offer a partial explanation but it's not the entire truth.

Why Use Red / Blue Lights?

The story begins with chlorophyll, the key compound that drives photosynthesis in plants. We have chlorophyll-A and chlorophyll-B which are most effective at absorbing light in the blue and red sides of the visible light spectrum.


Photosynthesis Absorption Rates
This shows the wavelengths of light that are most effective in promoting plant growth.

This makes sense, right? Chlorophyll is green, just like many plant tissues. And so rather than absorbing the green wavelengths, it reflects that light. And so, it might seem that green light is wasted light -completely unused and even unusable by plants. But researchers have demonstrated this to be a false assumption1.

Is Green Light Wasted?

Early experiments with algal suspensions have highlighted the action of blue and red light2 in photosynthesis. But this research does not represent the more complex plant systems that grow on land and in our gardens. Such plants have leaves and a layered canopy. So what happens to green light shining on land plants?

REFLECTED LIGHT: Some of the green light that shines on a leaf is reflected. Thus we see the green coloration of that plant. This might seem like wasted energy. But not when we account for canopy penetration. Reflected light is scattered and passed on to leaves that are lower down on the plant3. This reduces die back (senescence) of lower leaves, allowing them to get at least a portion of light.

ABSORBED LIGHT: If we look at a cross section of a plant leaf, we find that most of the blue and red light wavelengths are absorbed in the upper layer4. Although some green light is reflected, a portion is also absorbed by accessory pigments. In addition to chlorophyll-A, plants have carotenes and xanthophylls. These are able to expand the absorbable light spectrum5, harvesting some of that green light for photosynthesis.

LIGHT PENETRATION: The upper (adaxial) layer of a leaf's surface harvests mostly blue and red wavelengths. So how do the plant tissues in the bottom portion of the leaf get any light? That is where green light comes in. It penetrates deeper, being transmitted throughout the leaf6. Travelling a long path, green light is reflected many times inside of the cellular structures. Thus green light serves a vital role in driving photosynthesis in the lower portions of a leaf.

Light: Beneficial vs Essential

Despite the many online sources that state otherwise, research has shown that green light can grow plants and that it serves a beneficial function7. However, just because something is beneficial, that doesn't mean it is essential. Many low end LED grow lights offer a spectrum consisting of a few "essential" light wavelengths.


GlowPanel 45 (2010) LED Growlight - Sample Spectrum
This depicts the wavelengths of light produced by a low watt LED grow light I purchased in 2011.

Each wavelength is referred to as a band. Depending on how the colors are generated, each band may cover a narrow range of color, perhaps 25nm. Most commonly, manufacturers will combine one or more blues with two or more reds. These have been demonstrated to support the basic development and growth of most plants.

At the same time, other bands of color have been shown to impact plant development and/or health. UV light has been linked with improved disease resistance8. And as mentioned earlier, green light does serve a function as well9.

Video: Red Light vs Blue Light:
How Spectrums Effect Plant Growth

This lettuce experiment demonstrates the effects of different colors on plant morphology.

LEDs Aren't Just Pretty Colors

LED light sellers paint a picture that colors outside of the chlorophyll reactive range are useless to plants. But that's not the case. Why are custom LED grow lights so efficient? Custom spectrum is only a portion of the equation. Much of the advantage deals with efficacy and efficiency of LEDs as a technology.

EFFICIENCY: This can encompass a broad range of factors. But one excellent example deals with the directional nature of LED light. In a fluorescent or HID system, the bulbs emit light in all directions. More than half of the light may shine upward, away from the plant canopy. To improve efficiency, reflectors are used to bounce those photons downward, back towards the plants. LEDs are more efficient in this regard as their light can be focused directly onto the target area.

EFFICACY: LED efficacy is a more specific term. It is a measure of how much light is produced per unit of energy. So we could look at the lumens per watt or the micromoles per joule. But the bottom line is, LEDs are outpacing many other lighting solutions. They are becoming extremely effective at converting electricity to light.

A Big Reason for Red / Blue LEDs:

COLOR LIMITATIONS: Here's the interesting part of the story. Not all LED colors are rendered as efficiently as others. Each color band has its own unique material used to create the desired wavelength. Some of these colors require more power than others.

Blue (455nm) light has the highest efficacy10 of all colors (1.87 mmol/J). Red (655nm) wavelengths come in at a close second (1.72 mmol/J). However, using phosphors to create white light (5650 Kelvin) causes an overall reduction in efficacy (1.52 mmol/J). And then there is green light...

THE "GREEN GAP": Green LEDs are low on the efficacy list11, requiring much more energy per unit of light. The technology is less advanced than blue and red LED colors. Fortunately, recent advancements in the LED world12 have shown some promise. But until now, the reality has been than green light sucks up more power to create it. This is a big reason for why it hasn't been included in many grow lights.

Researchers have known for decades that green light is useful for plants. But conveniently for LED makers, people have become fixated with the chlorophyll absorption spectrum. -A spectrum that has spikes in the general areas where LEDs are most efficient.

A Future for Green Light?

It's true that green light is slightly less effective than red at fuelling photosynthesis. But that hardly makes it useless. There is more to plant growth than energy production. Photomorphogenesis is also an important concern. This deals with the way plants grow and develop in response to various frequencies of light.

For example, far-red light can play a major role in plant development8. Specifically, the ratio of red to far-red can signal plants to grow in an elongated fashion. This is a morphological response to plant competition in closed canopies. Then there is UV light, which is now gaining popularity due to its potential to bolster plant immunities. Meanwhile, UV LEDs are the least efficient of the LED colors. But people recognize the benefits. As a result, many high end grow lights are now including them.

But what about green light? Much of the growing community is in the dark when it comes to the effects of green light on higher plants. As advances in LEDs continue, perhaps we might see even more interest in this neglected wavelength. We may also learn more about its effects on various plant types.

The most thorough approach is to include the fullest spectrum possible. LEDs continue to offer more and more color bands. In greenhouses or homes, we can harness a measure of full spectrum sunlight which fills the gaps between those primary photosynthetic bands.


GlowPanel 45 + GE 6500k - Combined Spectrums
Here, I have merged the light profiles of my LEDs & CFLs to try to show the overall light output.

Selecting a secondary supplemental light source can also be an option, especially in 100% artificial environments. My first LED was very low wattage, with only 3 color bands. But by pairing it up with some beefy CFLs, I got really good results! Of course, CFLs have a lower efficacy than LEDs. But for a small-scale short-term environment, it was an easy solution.


Young and Mature Healthy Pepper Seedlings Grown Under LEDs & CFLs
In my first year of LEDs these seedlings grew way faster than I expected!

These pepper seedlings reached 12" tall after 7 weeks from planting the seeds. And the total power consumption was a mere 132w (28w LED + 26w CFL x 4). It's worth noting that the plants were grown in a room with no ambient light. So all growth was from artificial light. Of course, I was only growing within a 24" x 24" area. But that space could support 64 moderately sized seedlings. In the end, I was able to squeeze 10 to 12 oversized peppers within that space.



SOURCES:
1) http://pcp.oxfordjournals.org/content/39/10/1020.full.pdf+html
2) http://pcp.oxfordjournals.org/content/50/4/684.full.pdf+html
3) http://msue.anr.msu.edu/news/green_light_is_it_important_for_plant_growth
4) http://photobiology.info/Gorton.html
5) https://www2.estrellamountain.edu/faculty/farabee/BIOBK/BioBookPS.html
6) http://hortsci.ashspublications.org/content/39/7/1617.full.pdf+html
7) http://www.greenhousegrower.com/production/plant-culture/growing-seedlings-under-leds-part-two/#Tinsel/55866/4
8) http://www.plantphysiol.org/content/160/1/145.full%EF%BB%BF
9) http://hortsci.ashspublications.org/content/43/7/1951.full.pdf+html
10) http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0099010
11) https://www.compoundsemiconductor.net/article/91299-cranking-up-the-efficacy-of-green-leds.html
12) http://cpb.iphy.ac.cn/fileup/PDF/2015-6-067804.pdf


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