Spectrum LED Grow Lights

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With so many LED Grow Lights on the market, we'll run through how to choose the right light for your plants

With so many indoor grow lights options, especially in the LED market, it has become really confusing to figure out which grow light to buy.

And when it comes to buying a full spectrum LED grow light, not all LEDs are built the same way you need to pay attention to both the quality and quantity of light a fixture produces if you want to understand real power that LED grow lights can deliver to your canopy.

In this article, I'll help you figure out how to buy the best full spectrum LED grow light for your money. I'll do this by helping you to understand what terms like Spectrum and PAR mean, how they relate to each other, and how you can use them to choose the best full spectrum LED fixture you can buy for your garden.

This will be somewhat more technical than the usual article, but bear with me. By the time you’re done reading this article, you should feel confident enough to make your own choices.

If you don't really care about the technical details and just want my recommendations, I've listed them below this paragraph. Otherwise, read on and dive into the world of lighting with me.

The Meaning of Full Spectrum, PAR and How Plants Use Light 

Full spectrum is a common term that many lighting companies use to promote their particular model of Grow Light. Usually, they reference the Chlorophyll A and B absorption spectrum chart, which you may be familiar with. I should probably point out that they probably didn’t know that such a chart is only valid for extracted chlorophylls and not for the living leaf itself. But that's another story.

What Does Lumens & Par Mean

As a grower, I’ve been comparing how powerful my grow lights were by using a lux meter. A lux meter is a device that measures the density of luminous flux (lumens) at a certain point from the fixture. The problem is that when it comes to measuring grow lights for plants, I should have been using PAR all along. And thus a PAR meter.

Just in case you don’t remember, PAR stands for “Photosynthetically active radiation." It refers to the spectral range of light from 400 to 700 nanometers that most plants use for the process of photosynthesis. For our purposes, the difference is that a PAR meter measures the intensity of light within the whole spectrum. While a lux meter (lm) is usually calibrated only for the brightest light wavelengths that we humans can see, where white and yellow light is seen as the most intense while ignoring other wavelengths like blue and red which are also very useful for plants.

Difference between how a PAR & Lux meter measure light intensity. Note that the Lumens curve also correlates to the sensitivity curve of our eyes.

The way we perceive light is naturally much higher biased for green-yellowish light with sensitivity peak around 555 nanometers. Our eyes have a combined sensitivity curve where the peak of our sensitivity is also where the peak reflectivity is going to be for a plant.

Still with me? Good! Let's keep going.

So then, what does "Full Spectrum" really mean?

Should the term even be used? Well, when a company decides to call their products a Full Spectrum Grow Light, they usually mean that their product outputs a broad, continuous and significant light across most (if not all) of the PAR range. That’s it. In fact, remember this: “Full Spectrum” as a term, is only as reliable as the Grow Light manufacturer. It is by no means a certification standard; whether legal, industrial or otherwise.

The fact is: as of right now, LED grow light technology is moving away from using specific bands and instead the industry is focusing on providing the broadest possible spectrum. You can see this if you noticed that most reputable LED companies are moving away from pink/purple lighting and replacing their LEDs with “white” chips.

These white chips are produced by a phosphor-coating method, where the coating is deposited on the LED die. The exact shade or colour temperature of white light produced is determined by the dominant wavelength of the blue LED and the composition of the phosphor. And the thickness of the phosphor coating produces the variations in the colour temperature of the diode.

Alright! Now that we know how today’s top-notch LED grow lights are made, we can talk about the “best spectrum”.

The perfect grow light would be one that replicates the spectrum of our sun while allowing us to adjust the light intensity to our exact needs. This would be the pinnacle of “Full Spectrum."

For our intents and purposes, the Sun’s radiation spectrum is very evenly spread and peaks in wavelengths around the PAR spectrum.

The spectrum of Solar Radiation. Note how the Irradiance peaks within the PAR/Visible spectrum.

While plants certainly can use some of the light wavelengths outside the PAR spectrum, the light that falls outside of this range is usually either too powerful or too weak to be of primary use for photosynthesis.

As an example, with certain exceptions, UV light is too destructive to be used to synthesize large molecules, and infrared on the other hand is relatively weak and produces a lot of heat.

By comparison, within the PAR range, each photon contains just enough energy to excite the electrons of molecules without causing damage to the cell.

So, how should the perfect spectrum be? How much of every colour do plants need?

Luckily, science has the answer. It turns out that a publication by McCree (1972) figured all this out for us and published a chart similar to the following one:

To absorb light, plants use a somewhat primitive but highly effective version of our eyes, which we call pigments. The most abundant plant pigment is chlorophyll and it is most efficiently used to capture red and blue light. Other than those, there are many other pigments, including carotenes and xanthophylls which harvest light in other wavelengths and pass it on to the photosynthetic process.

It should be pointed out that green light actually penetrates deeper into the leaf interior than red light and can drive photosynthesis more efficiently. This is because the top layer of the chloroplasts that contain chlorophyll becomes saturated while green and yellow can penetrate deeper into leaf tissue and be reflected around until absorbed by another chloroplast containing chlorophyll or by an accessory pigment.

Major Factors to Consider

Now that you understand the science behind full spectrum LEDs, here are the most important factors you need to consider when you are deciding which to buy.

Cost

Right now, full spectrum LED grow lights are expensive. The costs of setting up a system that depends on these lights can be more expensive than standard HPS or HID setups.

However, you will save a lot of money in the long run due to the efficiency of LEDs vs. HID lighting. For example, the average lifespan for an HID bulb is around 10,000 hours. Compare this to a 50,000-hour lifespan for LEDs and you can see the cost savings you'll accumulate over time.

You can run a full spectrum LED setup for 15 YEARS before you need to consider replacing it. So, in short: if you can afford the initial setup cost, you'll thank yourself in the long run.

Size

Most HID or CFL lighting setups are bulky and cumbersome. This isn't necessarily bad, but if you're trying to grow in a smaller space it can make it difficult. Full spectrum fixtures are relatively small and don't require ballasts or reflectors, freeing up space in your grow tent or grow room.

Heat

Light and heat are forever intertwined. The temperature of your grow room is a vital variable, and grow lights are one of the biggest contributors to rising temps. It's why to grow room ventilation is so important

Full spectrum LED lights don't have this problem though. Some growers actually have to heat their rooms artificially during colder months due to how low the heat output is from this type of lighting. That means that if you're growing in a warmer climate, you won't have to worry about overheating your grow room.

Comparing Full Spectrum LED Grow Lights

If you're reading this article, chances are good that you've already decided to go with a full spectrum LED setup vs. some of the other lighting options out there. However, it's still valuable to do a quick compare and contrast against the other lighting technologies.

Factor	Full Spectrum LED	HPS	MH	CMH	CFL
Cost	High	Medium	Medium	High	Low
Heat Output	Low	High	High	High	Low
Full Spectrum	Yes	No	No	Yes	No
Size	Small	Large	Large	Large	Small
Lifespan	50,000	15,000	15,000	20,000	10,000

Full Spectrum LEDs vs. HID

When comparing LEDs to HIDs, you're really comparing against three different types of lights: high-pressure sodium (HPS), metal halide (MH), and ceramic metal halide (CMH).

HPS

In general, HPS edges out full spectrum setups on cost, but loses in heat output and ability to grow through a plant's life cycle.

Full Spectrum LEDs

• Low heat output
• Can grow through entire plant life cycle
• Less cumbersome

High-Pressure Sodium

• Higher heat output
• Optimized for flowering phase
• Requires ballast and reflector

Metal Halide

In general, MH is good for vegetative phase and costs a bit less, but puts out a lot of heat and doesn't work well for a plant through its entire life cycle. If you're growing only vegetative plants though, it can work well.

Full Spectrum LEDs

• Low heat output
• Can grow through the entire plant life cycle
• Less cumbersome

Metal Halide

• Higher heat output
• Optimized for vegetative phase
• Requires ballast and reflector

CMH

CMH lights are the best HID contender vs. full spectrum LEDs. They put out a good spectrum of light and cost about the same as a full spectrum fixture. It's a toss-up here.

Full Spectrum LEDs

• Similar in cost to a CMH setup
• Lower heat output than a CMH
• Higher efficiency

Ceramic Metal Halide

• Requires a special ballast
• More expensive than other HID
• Can grow through plant life cycle

Full Spectrum LEDs vs.CFLs 

CFLs, while efficient, are best used for the vegetative growth phase of a plant. This is because they generally don't put out a high enough intensity of light in the right spectrum for plants to do well during the flowering phase. 

Full Spectrum LEDs

• More expensive
• Better spectrum of light

CFLs

• Inexpensive
• Good for the vegetative growth phase