LED Grow Light - How To Judge The Correct Spectrum

Writer：Jane Time：2020-07-13 Browse：129

In this article, I will discuss one of the most important criteria to consider when purchasing LED luminescent lamps, namely spectrum. If you don't get the right light color, the plant won't grow well no matter how much money you spend.

If you want to get a new kind of growth lamp, you should really consider LEDs 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 easy. The market is flooded with products in a variety of price ranges, and many manufacturers are claiming claims designed to confuse you.

If you just want a quick answer, please go straight to the last section, which is the optimal spectrum of LED plant lamps. If you want to know what you're doing and make an informed choice, read the whole article.

What is a spectrum?

Plants are genetically programmed to grow using sunlight (which we think of as white or yellow-white). The light looks white because it contains all the colors of the rainbow, and when they are all mixed together, they look white.

A spectrum is a graphical representation 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 colors. Thus, red may have a wavelength of 630 or 660. Both of these colors are red to us, but they are actually different colors.

Plant lights that use fluorescent bulbs, and call the color of the bulb cold white (with more blue) or warm white (with more red). This is useful for fluorescent lamps, but this specification does not apply to LEDs. For LEDs, it's more accurate to talk about wavelengths and display the actual spectrum.

Solar spectrum

As you can see from the picture above, the sun emits light that contains all colors. It has more blue light (higher relative intensity) than red.

What color do plants use?

The colors absorbed by chlorophyll A and B

Plants mainly use light for photosynthesis, and light is done by specific chemicals in their 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, meaning these colors are used for photosynthesis.

Almost no light is absorbed in the green range.

This has led to the erroneous conclusion that plants only need blue and red light.

Blue and red misunderstanding
The idea that plants grow well only if they have blue and red light is actually a misconception. The spectrogram above is for chlorophyll purified in a test tube, and it does not tell you what is happening in the leaves of the plant. Photosynthesis is more complex and involves other chemicals, such as carotene and lutein. Spectral spectra of the light absorbed by the entire leaf show that plants actually use a wider range of wavelengths, including green.

It's true that blue and red are important and represent most of the light that plants use, but other colors (including green and yellow) are also used for photosynthesis.

The wavelengths of light absorbed by plants for photosynthesis

Different colors do different things
NASA has done a lot of work with light used by plants, and they've determined the following.

   Red light (630-660 nm) is essential for stem growth and leaf expansion. The wavelength also regulates flowering dormant periods and seed germination.

   Blue light (400-520 nm) needs to be carefully mixed with light from other spectra, as overexposure to this wavelength may hinder the growth of certain plant species. Light in the blue range also affects the amount of chlorophyll present in the plant and 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 leaf growth in the lower part of the plant. In addition, exposure to infrared light can reduce 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 optimal spectrum depends on your planting purpose
As plants mature and go through the entire growth cycle from seedling to adult, then flower and fruit, they use different spectra, so the ideal LED lamp is different for each growth stage.

The optimal 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.

Usually, plants work best at all wavelengths of light, but they don't need the same wavelengths of light.

Spectrum of LED lamp bead

Common spectrum of single LED lamp bead; Blue, yellow and red

It is important to distinguish between LED beads and LED lights. LED Plant Growth Lamp is a complete luminaire that can contain one or more LED beads. Usually more than one. LED beads are small, stand-alone components that glow.

LED beads exist for specific wavelengths. The figure shows the spectra of three light bulbs. Blue, yellow and red. Note that each bulb produces a very narrow spectrum. For example, a 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 plant growth lamps provide only blue and red LED beads. It seemed like a perfect solution, especially since blue and red LED beads are more efficient and cheaper than bulbs of other colors.

Many images of LED plant growth lights on the Internet show "bulky" light - the industry name for the color created by using a combination of blue and red LED beads.

LED beads now come in a dozen different colors.

How to use LED to emit white light?

LEDs coated with phosphors produce white light that is very similar to daylight

As mentioned above, each LED bulb has a specific wavelength, but none of them emit an all-white spectrum like the sun.

One solution for providing white light is to combine bulbs of different colors into a single light fixture. Base units combine blue and red. More advanced devices will include yellow and green light bulbs. Since a typical lamp contains many bulbs, it can be customized to produce different amounts of each color. Mix enough different colored bulbs and you'll get white light.

Another way to produce white light is to coat the bulb lens with a phosphorous compound. Such bulbs usually use blue light to illuminate phosphors and produce white light. This is similar to how fluorescent light bulbs work.

White LED bulbs seem to be the best choice, but there's a catch. Every time light is converted to another color, some intensity is lost in the conversion process. This means that a white bulb produces 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 growing lamps.

Is white light the best?
The sun emits white light, and plants perform best when they receive all colors of the visible spectrum, so it can be concluded that it is reasonable to assume that the best LED lights are white light. Many manufacturers try to convince customers, for example:
Either

"Our LEDs offer the best full spectrum and can provide plants, vegetables and flowers with everything they need for natural light at all stages of growth."

"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 are better off with more red and blue and less green and yellow light.

White light is not important to plants - the correct amount of each wavelength is crucial.

Light intensity is also important
So far, we've focused on spectra that are very important, but light intensity is also important. For many years, greenhouse lamp has been high intensity sodium lamp. I have been using it for many years and it is suitable for both seedlings that require low light levels and flowering orchids that require high light levels. It's 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 if blue is only a minor component of light, it is still enough to grow plants.

The white LEDs mentioned above seem to be a perfect solution, but they are less strong than uncoated bulbs. Therefore, uncoated bulbs are still a good choice.

Never mind the lumen value

Strength is important, but how do you measure it?

A common way of doing this is to measure lumens, which are a measure of the brightness of light. The problem with lumens is that they measure the brightness of light that the human eye sees, and our eyes see green and yellow light much better than blue and red.

Most of the light that produces blue and red doesn't look very bright to us, so the lumen count is low. Yellow-green light emits the same number of photons, looks bright to us and therefore has a high lumen value. But this high-lumen light does not have the optimal spectrum for plant growth. Lumens are great for choosing light sources for your home, but they're almost useless for choosing LED plant growth lights.

You may be wondering how lumens relate to 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 the light from plant growth, called PAR(photosynthetically active radiation). PAR defines the relative amount of plant light used for photosynthesis and ranges from 400 nm to 700 nm.

PAR spectroscopy for LED lamps

The term is often incorrectly used as a measure of the amount of light, for example:

"PAR is the amount of light available to the plant," or "This is a system with a high PAR output, which means the lamps emit 2 to 3 times the intensity of other plant growth lamps."

These statements make no sense because a 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 as PFD. Industry and gardeners tend to use the term PAR interchangeably when talking about PPFD.

Compared to lumens, PPFD is a better way to measure the amount of light in LED plant growth lamps.

Even that is problematic. Because it looks only at the dominant visible spectrum (400-700 nm) and ignores near ultraviolet and near infrared, it misses certain wavelengths that plants can use. But this is the best, most versatile system we have for evaluating growing lamps.

Optimum spectrum of LED plant growth lamps
What is the best spectrum for LED lamps? It should be close to the spectrum used by plants. There's a lot of blue and red, a lot of green and yellow. Add some near IR, or even near UV, and you'll get better results.

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

I think it's important to look at the light output spectrum before buying, but most manufacturers don't display these spectra. The new proposed LED plant growth light label will display PPFD(called 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.