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What is a Grow Light?

The relationship between light and your plants.

Plants need three very basic things to grow: water, nutrients, and light. Light is used in photosynthesis. It’s when plants take light energy, usually from the sun, and turns that into chemical energy.

Chemical energy is what’s stored in the bonds of chemical compounds, like atoms and molecules. It’s the potential energy of a chemical, called such because it’s got the potential to make energy during a chemical reaction. When a chemical reaction happens, heat is usually one of the byproducts.

So, the energy that a chemical reaction creates is its chemical energy. Chemical energy is what your plants need to create other certain chemicals that attribute to the freshest tomatoes. It’s also why they need the sun.

While outdoor growers worry about how much sun their plants get, indoor growers have to worry about what types of bulbs to get, how close to place those lights over their plants, and how long those lights should be kept on for.

Let’s break it down and talk about just exactly how plants absorb light.


Lumens is a unit of luminoux flux, which is the measure of the perceived power of light. This just means that it’s a way to figure out how much power we think light emits. Lumens measure the total “amount” of light emitted by a light source that is visible to us (i.e. we can see the light; it’s the light we perceive), describing the amount of light that’s radiated by a light source.

This is different from a candela, which indicates the strength of the light. It is also different from a lux, which indicates the illumination intensity of an area. Lumens describe how much light there is in a specific area.

Lumens aren’t something you can see; it measures the total light sent out by a light source.

This is How it Works

Picture one candle—we’ll call that a lumen. Picture a thousand candles lighting up a 1,000 square-foot area. This means that it takes a thousand lumens to light up a thousand-square-foot area… and that’s exactly what lumens are—one single candle’s worth of light, per square foot. Only difference is that when it comes to lumens, it’s assumed that the candle is held one foot away. So, one lumen is worth an x amount of light when held one foot away.

One lumen equals 10 lux. The term “lux,” though, is usually used to describe the amount of light energy actually reaching a surface. This is also known as “illumination.”

Let’s use an analogy using something that you are probably very familiar with—alcohol.

Lux is the alcohol percentage. Lumens is the actual amount of alcohol in a container. Once again, alcohol solves all your problems!

Distance Makes the Plant Grow Slower

Now that we’ve got that all cleared up, we can start talking about the light source. The gist of it is that the higher intensity of your light, the more your plants grow. That’s the bottom line.

The higher the wattage, the higher its intensity and the quality of light it emits. And, like we just learned above, we want a grow lamp that’s high in lumens.

But there’s a bit more to that. The number of lumens is actually rapidly plummeting from when the light is emitted from the lamp, to when it reaches your little baby’s leaves. It’s called the Inverse-Square Law.

Now, pay close attention.

Physics can be a little bit confusing, but keep this in mind for now: when the light source is closer, the light is brighter. When the light source is farther away, the light is less bright.

Okay, here we go. The Inverse-Square Law states that the magnitude, or strength, of something is inversely proportional to the square of something else. Thanks for nothing, John Dumbleton. But hey, at least you can pull this out and seriously impress someone at a party. (And by impress, we mean bore to death, so don’t do it, because that’d be considered involuntary manslaughter.)

Anyway, what that all means is that, again, the closer the light source, the brighter/better/more intense the light. Say you have a light shining on a single square foot from a square foot above whatever you’re shining it on.

Let’s pretend that light equals one lumen. Move that light two feet, and the lumen is reduced to a quarter of what it once was (so, you now have ¼ lumen). If you remember what we’ve covered then you’ll know that the lumen has reduced because it’s got a farther distance to travel to cover a larger surface area.

After all of that scholarly talk, what you need to know is that plants grow faster and stronger when the grow lamp is closer.

That’s it.

And that’s why BCNL grow boxes position the light source two-and-a-half feet away from your plants. You know, optimal growth and all.

Okay, sorry in advance for complicating it further, but we’re also going to have to consider light penetration. (You can stop giggling now.) Double the light, does not equal twice the penetration.

Penetration is important because you want the very core—roots and all—of your plants to get that good stuff from your light source to encourage photosynthesis, which means more growth.

This is why having a huge grow box may not be the most efficient thing. Though, yes, plants end up growing very tall, the lights have to be placed higher up, increasing the distance between the light and the plants’ roots.

See the Rainbow

Let’s talk about light spectrum now. You know, ROY G. BIV. It’s the varying wavelengths of light spectrums—blue is at about 400 nanometers, and red, on the other end of the spectrum, is at about 700—that your eyes see as different colors.

We care because depending on what stage of photosynthesis your plant is in, it’ll absorb different colors, in varying amounts. Your plant, essentially, needs to see the rainbow in order for you to see the rainbow because your plants’ growth is separated into two phases: vegetative growth, and root growth and flowering.

Vegetative growth requires the blue end of the spectrum (420-550 nm). Root growth and flowering requires yellow, amber, and red from the spectrum (550-750 nm).

Shine a Light on Those Plants

So, let’s sum up what we know before we move on to what we’ve been ever so excitedly building up to—light bulbs.

  1. Plants like a lot of light.
  2. Light sources should be placed closer, rather than farther, from your plants.
  3. Light penetration is most efficient when your plants aren’t massive so its lower leaves can get that good stuff from the light source, too.
  4. Your plants need the entire spectrum of varying wavelengths (i.e. different colors of the wavelengths) because rainbows are awesome.

Let’s move onto what kind of light bulbs you and your plants need now.

The bulbs

It’s no secret that light is crucial to your crops, but if you don’t grow outdoors, it can pose as an issue. In a hydroponic set up, grow lights are used to replicate natural light.

Of course, because the sun is the sun, it’s hard to get your lights exactly the same. So, a number of different grow lights are used to target the numerous ranges of the color spectrum.

To fully understand why there are different types of grow lights, you also have to understand the spectrum of light and color. It’s complicated, but it has a very serious effect over the health and size of your plants.

The color spectrum

Color is calculated as “temperature,” on par with heat. Increased color temperatures appear “cooler,” and lower temperatures appear “warmer.” The unit of measurement for color temperature is the Kelvin (K).

To encourage flowering and growth in height, your plant needs a lower temperature that’s simultaneously warmer—an orange/red that’s at 2700K and below. Vegetative growth requires a greater temperature that’s cooler, around 5000K and above.

In order to replicate the sun the best, you ideally give our plants a mix of both cool and warm color, heavier on the blue and some orange/red.

Each light is unique

Grow lights fall under four categories: High Intensity Discharge (HID), fluorescent, incandescent, and LED. Below are the most common types of grow lights:

HID Lights:

There are two major types of HID lights—metal halide, and high pressure sodium.

Metal Halide (MH)—MHs are great for leafy plant development, as this HID bulb stresses the blue end of the spectrum that ranges from 2700-5500K. If you only have one kind of grow light, this is the best option, as it is closest thing to natural sunshine.

High Pressure Sodium (HPS)—This is an HID bulb that is great for blooming, but not foliage, delivering orange-red color temperatures around 2200K. Note that you should never use this bulb by itself, but rather, to use it together with MH bulbs or as being a supplement to natural sunlight.

Fluorescent Lights:

Fluorescent lights contain different color ranges and intensities. Standard fluorescents or high-output fluorescents may have warmer and cooler temperatures and can be utilized in combos. They can be placed closer to the vegetation because they give off low heat. As far as performance is concerned, fluorescents come close to HID bulbs when utilized effectively. Though they are less intense, they are also more affordable.

Incandescent Lights:

Incandescent lights aren’t the most ideal option as they burn hot, and therefore can’t be positioned close to vegetation. In essence, it’s the least similar to natural light. If your mind’s set on incandescent lights, though, you can find color-corrected incandescent bulbs in the market.

LED Lights:

The newest grow light selection obtainable. Lately developed, some LED grow lights tend to be said to cover the entire color spectrum for plants, and produce very little heat, to allow them to be put near to the crops. For this reason, this most likely are not probably the most cost-effective choice given that they’re truly costly in fact it is even now arguable regardless of whether LED generates much better outcomes than grow lights that are standard.

The gist

Evidently, the type of bulb you use will depend on your needs. But having said that, the above information should help you on your way to growing the best crops ever.

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