In this article, I’m going to explore one of the most important criteria to consider when buying LED lighting, which is the light spectrum. If you don’t get the right light color, your plants won’t grow well no matter how much money you spend.
If you’re looking to get a new grow light, you should really consider LED lights because they are more energy efficient. It’s good for your wallet and the environment. The problem is that choosing the right type of light source is not an easy task. The market is flooded with products in all price ranges, and many manufacturers are making claims designed to confuse you.
If you just want a quick answer, go straight to the last section, The Best Spectrum for LED Grow Lights. If you want to understand what you’re doing so you can make an informed choice, read the entire article.
What is spectrum?
Plants are genetically programmed to grow using sunlight (what we think of as white or yellowish-white light). The light appears white because it contains all the colors of the rainbow, and when these colors are all mixed together, they appear white.
A spectrum is a graphical display of each color in light.
Scientists use wavelength numbers instead of color names to refer to colors, which is a more accurate way to measure color. So red might have a wavelength of 630 or 660. Both colors are red to us, but they are actually different colors.
Grow lamps that use fluorescent bulbs and call the bulbs cool white (with more blue) or warm white (with more red). This is useful for fluorescent lamps, but such a designation does not apply to LED lamps. For LEDs, it’s more accurate to talk about wavelengths and show the actual spectrum.
As you can see from the image above, the light from the sun contains all colors. It has more blue light (higher relative intensity) than red.
Which colors do plants use?
Plants mainly use light for photosynthesis, and light is done through specific chemicals in the leaves. More important chemicals include chlorophyll A and B. In the absorption spectrum (which measures the amount of light absorbed), you can clearly see peaks in the blue and red regions, which means that these colors are used for photosynthesis.
Almost no light is absorbed in the green range.
This leads to the erroneous conclusion that plants only need blue and red light.
The misunderstanding of blue and red light
The idea that plants only grow well with blue and red light is actually a misunderstanding. The spectrogram above is for chlorophyll purified in a test tube, it doesn’t tell you what’s going on in the leaves of the plant. Photosynthesis is more complex and involves other chemicals such as carotene and lutein. The spectral spectrum of light absorbed by the entire leaf shows that the plant actually uses a wider range of wavelengths, including green.
It’s true that blue and red are important and represent most of the light used by plants, but other colors, including green and yellow, are also used for photosynthesis.
Different colors do different things
NASA has done a lot of work on the light used by plants and they have identified the following.
Red light (630 -660 nm) is essential for stem growth and leaf expansion. This wavelength also regulates flowering, dormancy periods and seed germination.
Blue light (400 – 520 nm) needs to be carefully mixed with light in the rest of the spectrum, as overexposure to this wavelength of light may hinder the growth of some plant species. Light in the blue range also affects the amount of chlorophyll present in the plant as well as the thickness of the leaves.
Green light (500 – 600 nm) passes through the thick top canopy to support the leaves in the lower canopy.
Far-red light (720 – 740 nm) also passes through the dense upper canopy to support the growth of leaves located on the lower part of the plant. Additionally, exposure to infrared light reduces the time it takes for plants to bloom. Another benefit of far-red light is that plants exposed to this wavelength tend to produce larger leaves than plants not exposed to this spectrum.
The best spectrum depends on your planting purpose
As plants mature and go through the entire growth cycle from seedling to adulthood, then flowering and fruiting, they use different light spectra, so the ideal LED light is different for each growth stage.
The best spectrum also depends on the type of plants you want to grow.
This can get very complicated and really only matters to commercial growers who want to maximize results.
Generally, plants do best with all wavelengths of light, but they don’t need the same wavelengths of light.
The spectrum of LED lamp beads
It is important to distinguish between LED lamp beads and LED lights. LED grow lights are complete light fixtures that can contain one or more LED light beads. Usually more than one. LED lamp beads are small independent components that emit light.
There are LED lamp beads for specific wavelengths. The figure shows the spectrum of three light bulbs. Blue, yellow and red. Note that each bulb produces a very narrow spectrum. For example, a blue light bulb is about 60 nm wide and contains only blue light.
Since many people believe that plants only need blue and red light, many low-cost LED grow lights only offer blue and red LED beads. This seems like a perfect solution, especially since blue and red LED beads are more efficient and less expensive than other color bulbs.
Many pictures of LED grow lights on the internet show “coarse” light – the industry name for the color created by using a combination of blue and red LED beads.
LED lamp beads now come in more than a dozen different colors.
How to emit white light with LED?
As mentioned above, each LED bulb has a specific wavelength, but none emits a full white spectrum like the sun.
One solution to provide white light is to combine bulbs of different colors into a single light fixture. The base unit combines blue and red. More advanced units will include yellow and green bulbs. Since a typical light fixture contains many bulbs, it can be customized to produce different amounts of each color. Mix enough bulbs of different colors and you’ll get white light.
Another way to create white light is to coat the bulb lens with a phosphorous compound. Such bulbs typically use blue light to illuminate the phosphor and produce white light. This is similar to how fluorescent light bulbs work.
White LED bulbs seem like the best option, but there’s a catch. Whenever light is converted to another color, some intensity is lost during the conversion. This means that white bulbs produce less light than equivalent LED bulbs without a phosphor coating. White bulbs are also more expensive. Even with these limitations, they have become a popular choice for grow lights.
Is white light the best?
The sun emits white light, and plants perform best when they receive all the colors of the visible spectrum, so it is reasonable to conclude that the best LED lights are white. Many manufacturers try to convince customers, for example:
“Our LEDs provide the best full spectrum to provide plants, vegetables and flowers with everything they need for natural light at all stages of growth”
“Our LED lights replicate the solar spectrum”
The problem with this logic is that plants don’t need light that looks white to us, nor does it need to mimic the sun’s light. Plants are best with more red and blue light and less green and yellow light.
White light is not important to plants – the correct amount of each wavelength is critical.
Light intensity is also important
So far we’ve focused on the very important spectrum, but light intensity is also important. Greenhouse lights have been high-intensity sodium lamps for many years. I’ve been using it for years and it’s great for both young plants that need lower light levels and flowering orchids that need high light levels. It’s a very yellow light, with only a small amount of blue, but at 400 watts, it’s very intense. The high intensity means that even if blue is only a secondary component of light, it is still enough to grow plants.
The white LEDs mentioned above seem like a perfect solution, but they are less intense than uncoated bulbs. So, uncoated bulbs are still a good option.
Don’t care about lumens
Strength is important, but how is it measured?
A common way to do this is to measure lumens, which is a measure of the brightness of light. The problem with lumens is that it measures the brightness of light the human eye sees, and our eyes see green and yellow light much better than blue and red light.
Most of the light that produces blue and red will not appear very bright to us, so the lumens are low. Yellow-green light emits the same number of photons and appears bright to us, hence a high lumen value. But this high lumen light does not have the optimal spectrum for plant growth. Lumens are great for choosing a light source for your home, but are pretty useless for choosing LED grow lights.
You may be wondering how lumens relate to LUX and foot candles. Lux is lumens/square meter, foot candles are lumens/square foot.
PAR and PPDF
Scientists have come up with a better way to measure the light that plants grow, called PAR (photosynthetically active radiation). PAR defines the relative amount of plant light used for photosynthesis and ranges from 400 nm to 700 nm.
The term is often used incorrectly as a measure of the amount of light, for example:
“PAR is the amount of light available to the plant”
“This is a system with a high PAR output, meaning the light fixture emits 2 to 3 times the intensity of other grow lights”
These statements are meaningless because PAR defines the spectrum to be considered, not the amount of light.
In practice, the amount of light is measured in PPFD (Photosynthetic Photon Flux Density), sometimes abbreviated as PFD. Industry and gardeners tend to use the term PAR interchangeably when speaking of PPFD.
Compared to lumens, PPFD is a better way to measure the amount of light from an LED grow light.
Even this is problematic. Since it only observes the main visible spectrum (400-700 nm) and ignores the near-ultraviolet and near-infrared, it misses certain wavelengths that plants can use. But it’s the best, most versatile system we currently have for evaluating grow lights.
The Best Spectrum of LED Grow Lights
What is the optimal spectrum for an LED light? It should be close to the spectrum used by plants. Lots of blue and red, lots of green and yellow. Add some near the IR and even some near the UV for better results.
Don’t worry about matching the sun or white light.
I think it’s important to look at the output spectrum of light before buying, but most manufacturers don’t show these. The newly proposed label for LED grow lights will display PPFD (known as PFD) at various wavelengths including the PAR range.
Comparing PPFD values is the next best thing. A higher PPFD will provide more light for plant growth.
