Inverse Square Law for Light and Hydroponics

Understand the Inverse Square Law

Because most of us are lighting our hydroponic grows artificially, I figured that it would be a good idea to cover one of the most important concepts to consider when you’re making lighting decisions for your grows: the inverse square law for light.

The inverse square law states that light output decreases by the inverse of the distance that the light source is away from an object.

Let’s put that into English.  Take a CFL bulb that is 1 foot away from a basil plant that you’re growing.  Let the amount of light (lumens) that hit the basil plant be equal to X.  Now let’s move that CFL another foot away from the plant, to two feet.  At first glance, it would seem like moving the CFL to two feet away would halve the light output…but this is not true.  At two feet away, light coming out of the CFL bulb is now spread over 4x as much area, meaning that only 1/4th as much light will be hitting your plant.

Don’t Make This Lighting Mistake!

One very common mistake made by beginners is to place their lights too far away from their plants.  It’s an understandable mistake, because we live with lights over our heads all of the time and it’s natural to assume that plants would be fine with this distance.  After all, the sun is 93 million miles away, right?  While this is true, the sun is absolutely massive and puts out a lot of lux (amount of lumens per unit of area) despite it’s distance from Earth.  On a sunny day at around 12pm, the sun puts out anywhere from 10,000-25,000 lux.  To contrast that, typical office lighting puts out around 400 lux.

Keeping the Law in Mind

Most plants that we’re growing hydroponically have evolved in environments where they were exposes to long, sunny days.  We need to match what the sun is putting out, so when we place lights too far away from our plants and starve them of light, they will try to compensate in unhealthy ways.  Plants will “stretch” to get their leaves closer to the light source so they can photosynthesize more of the light into usable energy.  While this is the natural response, it leaves a plant structurally weak and spindly.  You’ll notice that you have a lighting problem if you have plants that are falling over and failing to support themselves.

I hope this was a helpful look into just one of the aspects of hydroponic lighting that you need to take into account when you’re growing.  Coming up soon will be an in-depth look at all the different types of hydroponic lights you can buy and their advantages/disadvantages, so stay tuned!

Learn more about the Inverse Square Law on Wikipedia.

About the author

Kevin Espiritu

Kevin is the creator of Epic Gardening, a community dedicated to teaching urban gardening, hydroponics, and aquaponics. He enjoys skateboarding, piano, guitar, business, and experimenting with all kinds of gardening techniques!

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  • http://www.outdoorhydro.com Stewart

    This is probably the #1 mistake for beginners, especially when starting from seed. The plants don’t get enough light from the get go and before long they start getting leggy and stretch so much that in the end they become top heavy, fall over, and you end up with 2-3 wasted weeks of time.

    Beginners: Get lights on those seeds ASAP, trust me. If I had a tomato for every leggy seedling I lost early on, I wouldn’t even need to be growing hydroponic tomatoes anymore :)

    • http://www.xponics.com Kevin

      Believe me, I did this far too many times myself and I should have written this post ages ago!

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