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. Would you consider this data as a good light source for lettuce cultivation
    PPFD-B (umol/m2/s) 38.767
    PPFD-G (umol/m2/s) 63.341
    PPFD-R (umol/m2/s) 108.853
    CCT (K) 4743
    PAR (mW/cm2) 4.429
    PPFD (umol/m2/s) 210.959
    wavelenth (nm) 505.0

    Reply
  2. Hi there.
    What would you recommend if I have a grow light set that has different light options with buttons. It seems as though I can only have blue, red, or yellow on at a single time, which doesn’t really create the spectrum I need simultaneously correct?
    Would you switch the colours throughout the day?

    Thank you

    Reply
  3. Very interesting article thankyou. I started on the LED grow light journey only this year with only the blue/red LED lights. I have since read and learnt a lot regarding the spectrum and the gaps that these red/blue LED’s fail to fill.

    From a practical standpoint, the rate of growth is very fast so that is not effected. What is effected is the leaf growth, or more specifically the size of the leaf. I only have primarily tropical plants but almost universally, the size of the leaf is much smaller than if the plants were grown outside or under a true broad/full spectrum light.

    Reply
  4. Thank you will take into account when lighting. I understand that white light isn’t required by the plants however I’m trying to light my indoor house plants and I don’t want blurple light I want white light. There’s very little out there for how to pick a good white light. I want to buy LED strip tape that I can make my own according to the size and area.

    Reply
  5. This is so informative, well written, and concise! I am shopping for an LED grow light fixture, and this has given me much needed help with that!

    Reply
  6. I’m new to grow light and am overjoyed to have read this article it was wildly informative and clearly explained. Thank you so very much for this.

    Reply
  7. Just to add to your informative post, I found a good talk concerning spectral effects in this video:
    https://www.youtube.com/watch?v=dXkYPN3HD6A

    My take from that is that for the amateur gardener aiming to keep a few plants growing through winter or starting seeds off early in the season, the best option is to get cool white LED striplights. They are efficient and provide all the necessary wavelengths at a fraction of the cost of dedicated growlights.

    Reply
  8. Excellent article, I also enjoyed reading the comments so thanks guys. I’m making a grow light at the moment using Powerbar V3 from Lumitronix. I found a 5 channel driver and a 2 ch driver that can be controlled with a digital controller. I found I can have “full spectrum” with 7 different colours including 840nm Ir and far red. I’ve also included uva , I’d like to include UVB but at £260 for 12 Osram diodes it’ll have to wait for an upgrade! These colours don’t really show up on Par meters which weren’t designed for leds.Anyway I’m ranting on . Thanks dude

    Reply
  9. That’s a great article, I’m from Egypt and actually we don’t have led grow lights here! So the only way is to buy street led light with 10,000lum and 6500k, is there anything that I can do better than this?
    Is there a way or an app to measure the blue and red waves?

    Reply
  10. Because I’m not the brightest no pun intended but I bought a Vander 2000 watt LED grow light. It came with faded out directions I could not read . There is a veg or flower position switch. Should the light appear white/blue during vegatation state and be reddish during flowering ?

    Reply
  11. I personally use a full spectrum LED light with a timer and e levels of density. I use it for approx. 5 weeks and I don’t worry about all the techno stuff you are talking about. I start the plants, get them used to outside, then transfer them. Game over.

    Reply
  12. Is the cost of LED grow lights worth it in comparison to fluorescent lights? Reading the wattage on LED vs fluorescent grow lights, it seems the LED are much higher wattage to my surprise.

    Reply
  13. I own a pair of EcoTech Radion pro Gen 3 led fixtures, they were used over my reef aquarium but had since been parted out. It has a wide array of leds from red to blues and also uv bulbs and fully customizable with sunrise, sunset, curious if I could use this for growing veggies and the like in my garage.

    Reply
  14. Really very good and informative article .Led grow lights are very popular now a days .So every indoor grower should read this article.

    Led grow light color spectrum is very important for plant growth and development.

    Blue , red and white light combination is also very important.

    Reply

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