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?
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.
Grow Light Intensity Principles,