LED Grow Lights – Getting the Right Color Spectrum

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Robert Pavlis

In this post I will look at one of the most important criteria you should consider when shopping for an LED grow light, the color spectrum. If you don’t get the right color of light, plants will just not grow well, no matter how much you spend.

If you are looking to get a new grow light, you should really consider LED lights since they are much more energy efficient. That is good for your pocket book as well as the environment. The problem is that selecting the right type of light is no trivial matter. The market is full of products in a wide price range and many manufacturers are making claims that are designed to confuse you.

If you just want a quick answer, head on down to the last section called, The Best Color Spectrum for LED Grow Lights. If you want to understand what you are doing so you can make an intelligent choice, read the whole post. I promise I won’t get too technical on you.

LED Grow Lights - Getting the Right Color Spectrum, provided by Green Relief
LED Grow Lights – Getting the Right Color Spectrum, provided by Green Relief

What is a Color Spectrum?

Plants are genetically programmed to grow using sun light, which we consider to be white light, or yellowish-white light. This light looks white because it contains all of the colors of a rainbow, and when these colors are all mixed together they look white.

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A color spectrum is a graphical display of each of the colors in the light.

Color spectrum of sunlight, image from Yuji LED
Color spectrum of sunlight, image from Yuji LED

Scientists use wavelength numbers to refer to the colors instead of color names, which is a much more accurate way to measure the color. So a red might have a wavelength of 630 or 660. Both of these look red to us, but they are actually different colors.

Grow lights that use florescent bulbs, refer to the color of the bulb as cool white (has more blue), or warm white (has more red). That was useful for florescent lights, but such designations do not work well for LED lights. When it comes to LED it is more accurate to talk in terms of wavelengths and to display the actual color spectrum.

Color Spectrum of the Sun

The light from the sun contains all colors as you can see from the image above. It has more blue light (higher relative intensity) than red.

You might also be interested in: LED Grow Lights – The Myth About Watts

Which Colors do Plants Use?

Colors absorbed by pure chlorophyll a and b
Colors absorbed by pure chlorophyll a and b

Plants use light mostly for photosynthesis and this is done with specific chemicals in the leaves. Examples of the more important chemicals include Chlorophyll A and B. In the absorbance spectrum (measures how much light is absorbed) you can clearly see the 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 has led to the incorrect conclusion that plants only need blue and red light.

The Myth of Blue and Red

Light wavelengths absorbed by plants for photosynthesis LED
Light wavelengths absorbed by plants for photosynthesis

The idea that plants grow well with only blue and red light is in fact a myth. The above color spectrum is for purified chlorophyll in a test tube and it does not show you what happens in a plant leaf. Photosynthesis is more complex and involves other chemicals like carotene and xanthophyll. A color spectrum of the light absorbed by the whole leaf shows that plants actually use a wider range of wavelengths, including green.

It is 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.

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Different Colors Do Different Things

NASA has done extensive 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 in other spectra since overexposure to light in this wavelength may stunt the growth of certain plant species. Light in the blue range also affects the chlorophyll content present in the plant as well as leaf thickness.
  • Green Light (500 – 600 nm) penetrates through thick top canopies to support the leaves in the lower canopy.
  • Far Red Light (720 – 740 nm) also passes through dense upper canopies to support the growth of leaves located lower on the plants. In addition, exposure to IR light reduces the time a plant needs to flower. Another benefit of far red light is that plants exposed to this wavelength tend to produce larger leaves than those not exposed to light in this spectrum.

Best Color Spectrum Depends on Your Goals

As plants mature and go through their growth cycle from seedling, to adult, and then flowering and fruiting they use different color spectrums so the ideal LED light is different for each stage of growth.

The best color spectrum also depends on the type of plant you are trying to grow.

This can get very complicated and is really only important for commercial growers where they want to maximize results.

As a general rule, plants do best with light of all wavelengths, but they don’t need equal amounts of each.

Color Spectrum of LED Bulbs

It is important to distinguish between LED bulbs and LED lights. An LED light is the complete fixture and it can contain one or more LED bulbs; usually more than one. The LED bulb is the small individual component that makes light.

Spectra of individual LED bulbs; blue, yellow and red
Spectra of individual LED bulbs; blue, yellow and red

LED bulbs exist for specific wavelengths. This image shows the spectra of three bulbs; blue, yellow and red. Note that each bulb produces a fairly narrow spectra. The blue bulb for example is about 60 nm wide, and contains only blue light.

Since many people believe that plants need only blue and red light, many of the lower cost LED lights offer only blue and red LED bulbs. This seems like a perfect solution, especially since blue and red LED bulbs are more efficient and less expensive than other colors.

A lot of the LED grow light pictures on the internet show a ‘burple’ light – the industry name for the color made by using a combination of blue and red LED bulbs.

LED bulbs are now available in more than a dozen different colors.

How do You Make White Light With LED?

As described above each LED bulb has a specific wavelength, but none of them give a full white spectrum like the sun.

LED with phosphor coating producing white light very similar to sunlight, image by Yuji LED
LED with phosphor coating producing white light very similar to sunlight, image by Yuji LED

One solution for providing white light is to combine different colored bulbs into one fixture. Basic units combine blue and red. More advanced units will include yellow and green bulbs. Since a typical fixture contains many bulbs, it can be customized to produce different amounts of each color. Mix enough different colored bulbs and you have white light.

Another way to make white light is to coat the bulb lens with a phosphor compound. Such bulbs usually use a blue light which hits the phosphor and it produces white light. This is similar to the way florescent bulbs work.

White LED bulbs would seem to be the best alternative but there is a catch. Whenever light is converted to other colors, some of the intensity is lost during the conversion. This means white bulbs produce less light than equivalent LED bulbs without the phosphor coating. White bulbs are also more expensive. Even with these limitations, they are becoming a popular option for grow lights.

The Myth That White Light is Good

The sun produces white light, and plants do best when they receive all colors of the visible spectrum, so it seems reasonable to conclude that the best LED lights are white lights. And many manufacturers try to convince customers that this is true with comments such as:

“Our LED provides the optimal full spectrum which provides plants, veg and flowers in all stages of growth with everything they need from natural sunlight”

or

“Our LED lights replicate the spectrum of the sun”

The problem with this logic is that plants don’t need light that looks white to us, nor do they need light that mimics the sun. Plants do best with a light that has a lot of red and blue and smaller amounts of green and yellow.

White light is not important for plants – having the right amount of each wavelength is important.

Making a lot of white light to impress us is a waste of energy.

Intensity is Also Important

So far we have focused on the color spectrum which is very important, but light intensity is also important. For years the go-to grow light for greenhouses has been a high intensity sodium lamp. I have used it for years and it works great for both seedlings which need lower levels of light and flowering orchids which need high light. It is a very yellow light with only a small amount of blue, but at 400 watts it has a very high intensity. The high intensity means that even though blue is a minor component of the light there is still enough to grow plants.

The white LEDs mentioned above seem like a perfect solution, but they have a lower intensity than non-coated bulbs. For this reason non-coated bulbs are still a good option.

Don’t Count on Lumens

Intensity is important, but how do you measure it?

A common way to do this is to measure lumens, which is a measure of how bright a light is. The problem with lumens is that it measures how bright the light appears to the human eye and our eyes see green and yellow light much better than blue and red.

A light that produces mostly blue and red will not look bright to us, and therefore it will have a low lumen number. A yellow-green light, giving off the same number of photons, looks bright to us so it has a high lumen value. But this high lumen light does not have the optimum color spectrum to grow plants. Lumens are great for selecting light for the home, but mostly useless for selecting LED grow lights.

You might wonder how lumens relate to LUX and foot candles. Lux is lumen/mand foot candle is lumen/ft2.

PAR and PPDF

Scientists have come up with a better way measure light for plant growth, called PAR (Photosynthetically Active Radiation) . PAR defines the relative amount of light plants use for photosynthesis in the range of 400 nm to 700 nm.

LED PAR spectra, provided by Fluence Bioengineering
PAR spectra used for LED lights, by Fluence Bioengineering

The term is frequently used incorrectly as a measurement the amount of light as in:

“PAR is the amount of light that’s usable to plants”

or

“This is a system that has a high PAR output which means the fixture gives off 2 to 3 times the intensity of other grow lights”

These statements are meaningless since PAR defines the spectra that is being considered, NOT the amount of light.

The amount of light is actually measured as PPFD (Photosynthetic Photon Flux Density), sometimes abbreviated to PFD. The industry and gardeners tend to interchange the two terms using the term PAR when they should be saying PPFD.

PPFD is a better way to measure the quantity of light for LED grow lights, than lumens.

Even this has a problem. Since it only looks at the main visual spectra (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 common system we have right now for evaluating grow lights.

The Best Color Spectrum for LED Grow Lights

What is the best color spectrum for LED lights? It should be close to the spectra used by plants. Lots of blue and red, and some green and yellow. Add some near IR and maybe even near UV and it is even better.

Don’t worry about matching the sun or white light.

I think it is important to see the output spectra of a light before you buy, but most manufacturers don’t show these. The new proposed labels for LED grow lights, will show PPFD (called PFD) for various ranges of wavelengths, including the PAR range.

Comparing PPFD values is the next best thing. A higher PPFD will provide more light for plant growth.

You might also be interested in: LED Grow Lights – The Myth About Watts

 

References:

  1. Photo Source, Aquaponics Image; https://www.greenrelief.ca/
  2. Photo Source, Spectra of LED Bulbs; Deglr6328
  3. Photo Source, LED PAR spectra, https://fluence.science/science/photosynthesis-guide/

 

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Robert Pavlis

I have been gardening my whole life and have a science background. Besides writing and speaking about gardening, I own and operate a 6 acre private garden called Aspen Grove Gardens which now has over 3,000 perennials, grasses, shrubs and trees. Yes--I am a plantaholic!

76 thoughts on “LED Grow Lights – Getting the Right Color Spectrum”

  1. Thank you..I am 69 and somewhat dense..my Christmas catsus..violets and philodrens all need different lights? I can not afford 200..what about some red/ blue..some yellow and a white grow light..all hung together. I am budget ant lover.

    Reply
  2. Hi. I am about to order a COB LED bulb and i’m wondering for a Bonsai tree should i get the 3000k version or the 6000k one?

    Reply
  3. I know you are probably avoiding the support of mention of specific products here, but, given the information here, do you know about how much an adequate LED light would cost?

    Reply
  4. I mention LED plant lighting in my recent book THE EVER CURIOUS GARDENER: USING A LITTLE NATURAL SCIENCE FOR A MUCH BETTER GARDEN. The only addition I would make to your very thorough and excellent overview of LED lighting is my statement that “Light from an LED spans a very narrow spectrum . . . So narrow, in fact, that a different recipe for light seems to be needed for op- timum growth of different kinds of plants, or different stages of growth.”

    Reply
    • Based on the data I found, LED lights have a band width of about 60 nm. That is not as narrow as I thought. Combine several different colors and you get a good broad spectrum.

      I found no reference that says the light is too narrow to make good grow lights.

      Reply
  5. good article. you might check this out about Fluence Bioengineering
    https://fluence.science/ Look under the science tab. I had to read and study and read some more before the terms and definitions stuck.
    PAR is What the fixture puts out. PPF is How Much (useful light) the fixture puts out and should have a reference to something per second.
    PPFD is How Much (useful light) the fixture puts out per unit of area and should reference a distance from the fixture.

    I set up a grow light using 4 CFL’s. they have a COLOR TEMPERATURE OF 6500k. The K stands for Kelvin and means the color of a piece of steel if it was at that temperature. The CFL’s give off light in the blue range and are good for leaves. good seedling starter. The LED grow light I have does have red and blue LED’s. but it only uses 9 watts. I don’t think its PPF is high enough to really help a lot of plants. It won’t burn the plants if they touch the cover, but the light needs to be right on top of them to be of use.

    The 4 CFL’s use 56 watts total, but I can see a difference compared to the LED fixture. I’m currently using it to grow some hydroponic bok choi.
    Sorry for the book. Hope this helps somebody

    Reply
    • The statement “PAR is What the fixture puts out” is not quite correct. PAR is the relative amount of light used by plants between 400 and 700 nm. PAR is not a function of the light, but of the plant.

      Reply

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