Grow Light Intensity Principles

Rambo December 28, 2011 17
Grow Light Intensity Principles

Understanding light is the single greatest factor to perfecting the art of growing marijuana indoors. Light is the fuel that drives the process of photosynthesis, so an understanding of these concepts is imperative to maximizing quality, yield, and cannabinoid potency. Assuming all of the other needs of marijuana plants are being adequately met, the quantity and quality of your light is the single greatest variable to your success. Despite how important light is, I can’t count how many times I have walked into grow rooms with the lights set up wrong. Understand the basic principles of light intensity, and you will never make that mistake.


The Lumen is a measure the intensity of visible light striking an object. Before modern conveniences, you would use a candle to read a book at night. Assuming your book is one square foot (12 inches x 12 inches) then a candle held one foot away would produce a light intensity of one foot-candle on the pages. If you wanted more light on your page, you could add a second candle next to the first, and then have two candles of light shining on the one-square-foot book from twelve inches away.

The Lumen is a modern unit for measuring the intensity of light. Lighting has come a long way in the last hundred years; but surprisingly we still use the same basic unit of measure.

1 Lumen = 1 Foot-Candle

One Lumen is equal to one foot-candle of light–the intensity of the light from one candle illuminating one square foot of area from one foot away.


Because most of the world uses metric measurements instead of inches and feet, light intensity is also measured in Lux. The principle is the same, except now it is a measure of one candle of light falling one square meter from one foot away. Why this equation mixes standard and metric is beyond me.

1 Lux = 1 Meter Candle

We know that one square meter is approximately 10.7 square feet (10.764 to be exact). One lumen is therefore equivalent to approximately 10.7 LUX. The light is less luminous because one lumen must now illuminate over 10 square feet of area and not just one.

Note: There is really a bit more to it than this, but for the purposes of providing light to plants, this should be enough information.

Intensity – The Inverse Square Law

Suppose the candle is not ideal for reading your book at night, and you don’t have a second candle to add. The second way to increase the illumination of the page is to move the book closer to the candle.

In a dark room you may be able to read a book with one foot-candle of light, one Lumen; but you would strain your eyes a lot less with a standard 40 watt incandescent bulb which yields about 500 Lumens.  Now take your book across to the other side of the room. Suddenly it is far more more difficult to read the text with the 40-watt light on the other side of the room then it was with the candle one foot away. You are now experiencing the Inverse Square Law. This law states that light intensity changes in inverse proportion to the distance, at a decreasing rate.  In other words, the further you get from the source, the less light you have; and moving just a little further away will make the light a lot less intense.

Intensity = Light Output/Distance Squared

Another way to illustrate this principle is with shadows. If your 500 Lumen light hangs from the ceiling and you hold the book one foot under it, the book is going to cast a huge shadow on the floor, and most of those 500 Lumens will illuminate the book. If you move the book another foot from the light, the size of the shadow will drastically decrease. Considerably more light is falling on the floor and the number of Lumens on the book will be cut by 75%. Now you’re down to only 125 Lumens on the book. If you move the book an additional foot away, now three feet from the light source, that shadow will be much smaller still. More light illuminates the ground and only 55 Lumens now light the book.

Look at the trend as we move in one-foot increments from the light on the ceiling to the floor in a normal house with eight-foot ceilings.

1 Ft = 500 Lm
2 Ft = 125 Lm
3 Ft = 55.5 Lm
4 Ft = 31.25 Lm
5 Ft = 20 Lm
6 Ft = 13.8 Lm
7 Ft = 10.2 Lm
8Ft = 7.8 Lm

By moving just seven feet further from the light source, the intensity of the light has decreased from 500 Lumens to less than eight Lumens.  

Can anyone guess how significant this is for growing marijuana where light intensity is everything?

Wasted Light

Let’s go back to the candle on the desk illuminating the book. We know that without a second candle to increase the light intensity, we have the option of moving the book closer to the candle. For obvious reason, this is only practical to a point. Sooner or later the book catches fire, there is a sudden burst of illumination, followed by no more book to read and plenty of smoke.

Ah, but remember we are in a three-dimensional world, as is light.  The candle is only lighting the book with one Lumen, but not because the candle is only producing one Lumen. The candle is in fact producing 12.5 Lumens–enough light to illuminate 12.5 books–one square foot in size, held one foot away from the flame–with one Lumen each. So 11.5 lumens (92% of the light) are going to light the room, the ceiling, and the desk, when they could be directed back towards the book. Using a shiny brass plate, we can reflect some of the wasted light back towards the book we are trying to read, and increase the number of Lumens lighting the book.  No book burning, no smoke, no wasted time and money at Bed Bath and Beyond buying extra candles. Nowadays, of course, we have much better reflective materials than brass at our disposal.

This should be the second time you pause and say “I bet this is relevant to growing marijuana somehow.”  

Here is what you should take away from this article.

  • Keep your lights as close to the tops of your plants as you can without burning them. Under 12 inches if at all possible.
  • Use reflectors, reflective light hoods, and reflective material on your walls to redirect wasted light back towards the plants.
  • Reflective walls should closely surround the plants to maximize reflection. Reflective material five feet away from a plant is not doing any good.

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Rating: 3.6/5 (21 votes cast)

Grow Light Intensity Principles, 3.6 out of 5 based on 21 ratings



  1. GinaGinger January 2, 2012 at 4:08 am -

    Thanks for explaining this in such simple terms. Keep it up.

  2. Stefan54 February 14, 2012 at 5:15 am -

    You talk about Lumens and Lux which are about how human eye see light not plants.
    If you want the real facts go to r/microgrowery and read SAG’s Lightning guide (in the sidebars to the right)

    • Rambo February 14, 2012 at 8:55 am -

      Lumens and Lux are how the human eye and affordable light meters see light. Photosynthetically Active Radiation (PAR) light is what plants see. However this is much more difficult to measure. Like you suggest it is the amount of PAR light that is important to plants but as the Lumens and Lux increase so does the PAR light in exactly the same way. Of course it is important to use a light that produces enough Lumens of PAR light per watt of expended energy which is why High Pressure Sodium and Metal Halide lights are so commonly used as grow lights. Since the title of the article is “Grow Light Intensity Principles” I figured it was safe to assume the reader would make logical connection that I was talking about grow lights.

  3. Stefan54 February 14, 2012 at 9:10 am -

    ok thanks for clearing that up and for the quick answer,
    good luck with your new? site.

    • Rambo February 14, 2012 at 9:42 am -

      Not a problem, and thanks for pointing out the omission. It’s not easy to cover every base and still write a short and easy read article. We need readers like you to point it out when we drop the ball.

  4. grizzly22 March 31, 2012 at 11:42 am -

    Great job explaining this so EVERYONE could understand it. I am familiar w/ lumens and how they work… But the PAR stuff is still pretty new to me. But I find it very interesting. Great comments.

  5. Green Thumbcat May 28, 2012 at 12:46 pm -

    Excellent article. Could have used a slightly better explanation of the inverse square rule.

    Also, in that section, paragraph 2 line 4, you use “then” instead of “than,” which would have been the correct word.

    Still gave you 5 stars because this was worth an A. >^..^<

    • Rambo May 30, 2012 at 9:36 am -

      Thanks for the comment. I checked out your page and it looks like you have some fun little projects going on. I’ll have to stop by again later and do some reading. As for the use of Then and Than, I believe you may be incorrect. My command of the English language is far from perfects, but then in this case is meant to indicate a point in time after another point in time, while than is used as a conjunction…

      • amblazer December 18, 2012 at 12:24 pm -

        Than would work if it wasnt followed by the “it was”. Which you so eliquently used to expess the “point in time”

  6. Boss Hog May 30, 2012 at 9:49 am -

    I too think “then” is correct as well.

  7. Michael Bravin July 19, 2013 at 1:27 am -

    Great reading.
    I really enjoy your way of explaining things, very understandable for anyone that can read. Personally when I read all this information about growing and cultivating it is really helpfull to have pictures to look at. I am not talking about some elaborate designed pictures, minimalistic ones is enough. Would be even easier to comprehend if there is an image with the light, plant and distance to light ratio. I will definatly recommend Your site.

  8. Queue August 12, 2013 at 2:20 pm -

    Instead of measuring PAR in terms of elumination why not simply reduce this who discussion to radio waves which is what the plant use yes radiation is radio wave measured in frequency so a 6500 K light generates a frequency different from a 3000K and so you can now dump the theory and use the idea of insulating to reduce or increase the amount of frequency being lost or redirected and simply get to the point of telling folks to use panda poly or a similar fabric, this fabric is basically a high fashion design plastic film that reflects I may need correction here but I believe it reflect 99% of the RF and 89% of ultraviolet light as well so ventilation becomes your next issue and the plant espalier and then back to the size of room ….but then I am only taking a break to eat a sandwich and read something My plants are perrineal Year round in and out of door for now because I have created an environment that the plant has acclimated to so I watch out for pollen and force the cycles with panda poly ventilating out door during the heat of the year and allowing the plants to veg naturally for a few days before beginning again .I have had 2 good seasons from the same plants. I don’t know of anyone doing this as well so I don’t know what to expect as far as how long wil a single plant cycle or how does this effect the medicine I know the number of leaf fingers changes really quickly from 1 too 13 so far and this cycle is better than the last cycle … so I am curious to see if the next season will be even better because 2 cycles for one plant is not a measure of cycle pattern. I have also noticed that when and where I prune the limbs for drying will cause the leaf pattern to reduce too 1 leaf and begin counting out but quickly more quickly than say a plant that has been vegged into dormancy ad brought back to leaf this is the difference between seed plants and clones and say a seed plant acclimate to an all season outdoor environment this plant will switch cycles sometimes within the same stalk kind of like a hermie but not because the entire limb will be one sex while another limb will be the other so instead of pruning the female limb I cut the male seed pollen sac bearing limbs and watch how the plant develops also I adopted the use of bees wax to heal the prune location seems the plants water pressure drop just like a tree when the limbs are pruned.

  9. Queue August 12, 2013 at 2:29 pm -

    do you want a picture? I have a good one that show some 3 years old plants that were cut at differing times an brought back to begin cycling again . one photo 3 different plants all from the same mother. which was a seed plant Seattles best 1991 hempfest cup grand daddy purple all live and dong well ready to go in the big pot outside next year for a final full throttle grow til they bust year long season and then 2 smoke and rope caue I got the seed back now. nope not sharing . dont’ even ask.

  10. lea June 15, 2014 at 2:14 am -

    hi, are the typical grow lights used to cultivate pot worse for skin than sunlight? are they as intense as, say, tanning beds? Do they emit both uva and uvb rays? I’m helping a friend who grows a small amount of medical marijuana and wondering if I should be concerned with this.