In this post, I’m going to look at one of the most important criteria you should consider when buying an LED grow light, 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. This is good for both your wallet and the environment. The problem is that choosing the right type of light source isn’t 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, which is the best light spectrum for LED grow lights. If you want to understand what you’re doing so you can make an informed choice, read the entire post.
What is a spectrum?
Plants are genetically programmed to grow using sunlight (which we think of as white or yellow-white light). This light appears white because it contains all the colors of the rainbow, and when those colors are all mixed together, they appear white.
A spectrum is a graphical display of each color in a light.
Instead of color names, scientists refer to colors using wavelength numbers, which is a more accurate way to measure color. So, red might have a wavelength of 630 or 660. Both of these colors appear red to us, but they are actually different colors.
Plant lights that use fluorescent bulbs refer to the color of the bulb as cool white (with more blue) or warm white (with more red). This works well for fluorescent lights, but such designations do not apply to LED lights. For LEDs, it is more accurate to talk about wavelengths and display the actual spectrum.
The Sun's Spectrum
As you can see from the image above, the light emitted by the sun contains all colors. It has more blue light than red (with a higher relative intensity).
Which colors do plants use?
Colors absorbed by chlorophylls a and b
Plants use light primarily for photosynthesis, which is accomplished by specific chemicals in their leaves. The more important chemicals include chlorophylls 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 these colors are used for photosynthesis.
Almost no light is absorbed in the green range.
This leads to the incorrect conclusion that plants only need blue and red light.
Blue and Red Light Misconception
The idea that plants only grow well with blue and red light is actually a misconception. The spectrum graph above is for chlorophyll purified in a test tube, and it doesn't tell you what's going on in a plant's leaves. Photosynthesis is much more complex and involves other chemicals like carotenes and xanthophylls. A spectrum of the light spectrum of light absorbed by the entire leaf shows that plants actually use a much 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.
Wavelengths of Light Absorbed by Plants for Photosynthesis
Different Colors Do Different Things
NASA has done a lot of work on the light used by plants and they have determined the following.
Red light (630 -660 nm) is essential for the growth of stems as well as the expansion of leaves. This wavelength also regulates flowering, dormancy periods, and seed germination.
Blue light (400 -520 nm) needs to be carefully mixed with light from other spectrums because overexposure to light of this wavelength can stunt the growth of some plant species. Light in the blue range can also affect 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 in the lower part of the plant. Additionally, exposure to infrared light reduces the time it takes for a plant to flower. 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 growing purpose
As plants mature and go through the growth cycle from seedling to adult, then flowering and fruiting, they use different spectrums, so the ideal LED light is different for each growth stage.
The best spectrum also depends on the type of plant you are growing.
This can get very complicated and is really only important for commercial growers who want to maximize results.
Generally, plants do best with all wavelengths of light, but they don't need the same wavelengths.
The spectrum of LED lamp beads
Spectra of common individual LED beads; blue, yellow, and red
It is important to distinguish between LED beads and LED lamps. LED grow lights are complete fixtures that can contain one or more LED beads. Usually more than one. LED beads are small, independent components that emit light.
LED beads exist for specific wavelengths. The figure shows the spectrum of three bulbs; blue, yellow, and red. Note that the spectrum produced by each bulb is narrow. For example, the blue 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 because blue and red LED beads are more efficient and cheaper than other color bulbs.
Many LED grow light pictures on the Internet show "bold" light - the industry name for the color created by using a combination of blue and red LED beads.
LED beads now come in more than a dozen different colors.
How to make white light with LED?
LEDs with phosphor coatings produce white light that is very similar to daylight
As mentioned above, each LED bulb has a specific wavelength, but none can produce a full white spectrum like the sun.
One solution to providing white light is to combine different colored bulbs into a single fixture. Basic units combine blue and red. More advanced fixtures will include yellow and green bulbs. Since a typical fixture contains many bulbs, they can be customized to produce different amounts of each color. Mix enough different colored bulbs and you get white light.
Another way to produce white light is to coat the bulb lens with a phosphor compound. Such bulbs typically use blue light to illuminate a phosphor and produce white light. This is similar to how fluorescent bulbs work.
White LED bulbs may seem like the best option, but there is a catch. Whenever light is converted to a different color, some intensity is lost in the conversion process. This means that white bulbs produce less light than an equivalent LED bulb 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 best?
The sun emits white light, and plants do best when they receive all the colors of the visible spectrum, so it would be reasonable to conclude that the best LED light is white light. Many manufacturers try to convince customers with statements such as:
"Our LEDs offer the best full spectrum and provide everything plants, vegetables and flowers need from natural light at all stages of growth"
Or
"Our LED lights replicate the sun's spectrum"
The problem with this logic is that plants do not need light that looks white to us, nor do they need light that mimics the sun. Plants do best with light that has more red and blue, and less green and yellow.
White light is not important for plants - the correct amount of each wavelength is critical.
Light intensity is also important
So far we have focused on the spectrum, which is very important, but light intensity is also important. For many years, the go-to greenhouse light has been the high-intensity sodium lamp. I have used one for years and it has worked well for both seedlings that need lower light levels, and for flowering orchids that need high light levels. It is a very yellow light with only a small amount of blue, but at 400 watts it is very intense. High intensity means that even if blue is only a minor component of the light, it will still be bright enough for growing plants.
The white LEDs mentioned above seem like a perfect solution, but they have lower intensity than uncoated bulbs. So uncoated bulbs are still a good choice.
Never mind the lumen value
Intensity is important, but how is it measured?
One 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 as seen by the human eye, and our eyes see green and yellow light much better than blue and red light.
Most light that produces blue and red will not appear very bright to us, so it has a lower lumen number. Yellow-green light emits the same number of photons and looks bright to us, so it has a high lumen value. But this high lumen light does not have the best spectrum for plant growth. Lumens are great for choosing a light source for the home, but they are almost useless for choosing an LED grow light.
You may be wondering about the relationship between lumens and LUX and foot candles. Lux is lumens per square meter, and foot candles are lumens per square foot.
PAR and PPDF
Scientists have come up with a better way to measure light for plant growth, called PAR (Photosynthetically Active Radiation). PAR defines the relative amount of light used by plants for photosynthesis, in the range of 400 nm to 700 nm.
PAR Spectra for LED Lights
This term is often incorrectly used as a measure of the amount of light, such as:
“PAR is the amount of light available to plants” or “This is a system with a high PAR output, which means the light fixture emits 2 to 3 times more intensity than other grow lights”
These statements don’t make sense because PAR defines the spectrum to be considered, not the amount of light.
In fact, the amount of light is measured in PPFD (Photosynthetic Photon Flux Density), sometimes abbreviated to PFD. The industry and gardeners tend to use the term PAR interchangeably when speaking of PPFD.
PPFD is a better way to measure the amount of light from an LED grow light than lumens.
Even this has problems. Since it only looks at the main visible spectrum (400-700 nm) and ignores near-UV and near-IR, it misses some wavelengths that plants can use. But it is the best and most versatile system we currently have for evaluating grow lights.
The Optimal Spectrum for LED Grow Lights
What is the optimal spectrum for LED lights? It should be close to the spectrum that plants use. Lots of blue and red, lots of green and yellow. Add some near IR and even some near UV for even better results.
Don't worry about matching the sun or white light.
I think it's important to look at the output spectrum of the light before buying, but most manufacturers don't display these. The new proposed LED grow light labels will display the PPFD (called PFD) for various wavelengths including the PAR range.
Comparing PPFD values is the next best option. A higher PPFD will provide more light for plant growth.
