Understanding light seems simple enough.

We all learn about our good friend ROY G. BIV early on in our schooling and the basic processes of a plant and photosynthesis soon follow. But understanding just how exactly these two components of science work together to form heavy, resin-soaked buds is something that has been left out of most grade school curriculums… Go figure.

It’s a real bummer too, because with just a little bit more knowledge about plants and light an average growroom can become something extraordinary, something superior, something potent… get ready for one lesson you’ll never want to forget.


Consider this: In terms of sheer lumens (the actual amount of light hitting a leaf’s surface), sunlight can travel 93 million miles and still reach the Earth’s surface at 5,000 lumens per sq. foot. Now that may not seem like a big deal, especially with all the fancy HID lights we find in grow shops these days, but in reality, nothing manmade can come close to producing that sort of power.

But to really understand light and lumens, we must first realize that artificial light cannot keep its strength nearly as well as the sun. In fact, light emitted from even our best bulbs will decrease, exponentially, as the distance (both vertically and horizontally) increases between light source and garden.

Lumens are a way of measuring light density. The units that lumens are actually measured in is equal to the number of candles it would take to light a specific area. It’s pretty old-school, but one lumen actually equals one foot-candle (or, more specifically, it is one candle’s worth of light per square foot of surface when that candle is held one foot away). Similarly, a lux would be one meter-candle (or one candle of light per square meter held one foot away). Saving the math, 1 lumen = 10 lux. However, one important distinction between the two terms needs to made; while lumens are a way to measure light energy leaving the source or bulb (which may also be known as flux), lux is a term generally reserved for indicating the amount of light energy actually reaching a given surface or leaf (and this is also known as illumination).

So why do we need to know this? Because knowing this helps us understand the power of our bulbs and thus tells us just how much light our plants are actually getting. When we go out and purchase a grow lamp the box will indicate how many lumens a particular bulb can put out. While that information is good to know, many times it can be a misleading sales pitch. Remember, those lumens will be vastly decreasing in number by the time that light reaches your plants, its basic physics.


Now, we hate to do this to you but it is for your own good, so pay attention! The Inverse-Square Law dictates to us the exact proportions for which our light will diminish over space and it really helps put things in perspective – such as why air cooling lights and keeping them as close to plants as possible is a really good idea. So here it is, straight from the wiki-world of definitions:

The intensity of light from a point source (energy per unit of area perpendicular to the source) is inversely proportional to the square of the distance from the source so an object twice as far away, receives only 1⁄4 the energy (in the same time period).

And in English for us potheads: Basically, if you have one lumen shining on one square foot at a height of one-foot from above, then at two-feet from above that lumen is now only a quarter of a lumen (1/4), because that single lumen will diminish by traveling a greater distance and is now also spread out over a greater area. Figure 1.1 shows that not only do we have to contend with the light losing strength, but also with the increased surface area that light must now cover.


Considering the facts stated here, a good conclusion would be that keeping your lights close to plants is your best option for utmost efficiency. But, as always, there are even more considerations. Sure, keeping your lights lower help your plants to catch maximum lumens, but they will only be catching those lumens that are emitted towards your plants.

Remember, a light bulb shines 360 degrees around. If your HPS puts out 140,000 lumens, that’s 140,000 lumens in all directions. If you manage to get 10 percent of that on your garden, that’s only 14,000 lumens. Then consider the distance of you lamp and the Inverse Square Law, you may only be getting 2,000 of your 140,000 lumens onto your garden!

This is why a good reflector can make a big difference for your plants. But an even better solution is to hang your lights vertically, down the center of your garden with your plants surrounding your bulbs on all sides. Vertical grow systems such as the Coliseum (as featured in HT issue #380, September 2007) or the EcoSystem (pictured here) allow for extreme efficiency with HID bulbs.


Just when you thought class might get easier, we move into Chapter 2: Spectrum. As dedicated horticulturalists, we need to understand that there are more advantages of Sunlight (over artificial lighting) than simply the sheer strength of the Sun. And to understand these advantages we need to know about spectrum and the role it plays in photosynthesis.

Photosynthesis, as we all know, is the process whereby our cannabis plants use light to create sugars that the plant will use for energy and, eventually, THC production. Can’t really argue with the importance of that. So to better serve this process, growers need to supply their plants not only with a powerful light source but also with specific light sources that can provide the proper range of spectrum to ensure maximum sugar production during photosynthesis.

When light hits the leaves of a plant, each leaf uses various pigments (most notably chlorophyll a, chlorophyll b and carotenoids) to trap light energy and extract photons, which are vital in converting the light energy into chemical energy. These photons combine with water to provide the plant with the needed chemical energy to fix carbon dioxide (CO2) molecules into sugars, carbohydrates and other organic compounds the plant will use as food.

So, what does spectrum and light color have to do with photosynthesis? Spectrums of light are actually different sized wavelengths that give the appearance of a variety of colors to the human eye. These wavelengths are important because plants absorb different colors of light in varying amounts for photosynthetic processes. Supplying your plants with those colors which the plants can absorb most readily and in high quantities assures better photosynthesis and more food for the plant, resulting in bigger yields and more resin production for super potent pot.

Unfortunately, for the indoor grower there is much debate over which frequencies of light are actually the best for plants. Scientists do know, however, the plant’s absorption rates during photosynthesis for each color of the spectrum. These absorption rates are shown in wavelengths as a plotted curve graph called a Photosynthesis Action Spectrum. This curve is depicted in Figure 1.2 with the sun’s natural light spectrum overlaid for comparison points of where the sun’s light is most efficient in photosynthesis. You’ll notice that where the sun’s spectrums are lacking – on the sides the spectrum in the red and blue light– are also the same areas where plants are most efficient at processing light.

Some people argue that plants are simply more efficient at using red and blue light because over the millions of years in the Earth’s history plants have naturally evolved to become more efficient at using these wavelengths since they are less abundant in nature (as is evident by the Sun’s spectrum). Other horticulturalists believe that supplying their plants with only red and blue spectrums of light is best for photosynthesis since the plant is most efficient at absorbing these wavelengths. Still, like anything else, the best solution is balance and to that end, many cannabis cultivators will use both HPS and MH lamps simultaneously to try and provide as much coverage of the spectrum as possible.

Some growers will merely complement their growrooms with supplemental grow lights or LED-based lamps that provide the reds and blues that conventional grow lights lack and these growers usually achieve excellent results with very healthy crops. (HT Heads Up: Stay tuned for future story in the March 2008 issue of High Times that will deal specifically with LED research and spectral efficiencies in photosynthesis!)


OK, ok, we know what you’re thinking… trust us, this stuff isn’t only good for impressing your friends. Knowing all this information is useful because it ties into so many other facets of growing – such as why a light mover might be your next investment or just how much CO2 your plants will need.

As mentioned previously, the sun’s light is omnipresent in nature. It’s light can penetrate thick forest canopies and deliver just as much light to lower branches as it does to tree tops. This is hardly the case in a growroom. Unless a growroom uses a very large number of high-powered lamps, light penetration will still be very weak – and what does get through your top layers will be much weaker in terms of lux. Light movers offer a decent solution to the problem by sliding or circulating lamps over a garden so that light can sneak through canopy cracks and hit your leaves at more angles. Remember, the Sun is not stationary in the sky nor should your growroom lights be.

Similarly, knowing about light intensity can be a great help in another important area of indoor growing: CO2. A direct relationship exists between light and CO2 uptake and determining the correct ratio can increase your harvest by as much as 25 percent.

The general rule is pretty basic – the more light your plants receive the more CO2 they will need. When you supply your plants with light, you are really feeding them subatomic particles called photons. Technically speaking, it takes 10 photons to create enough electrons to supply the energy needed to split one CO2 molecule and form sugars for the plant. If there are millions, or even billions, of photons hitting your plants and not enough CO2 to react with them, these photons will be deflected, unused and ultimately wasted. This is bad news for growers who spend loads of money on high-powered lamps and electric bills and then get nothing in return.

To simplify things, let’s start with what not to do. Most expert growers will warn that CO2 levels of over 3,000 parts-per-million (PPM) are dangerous in an indoor grow operation. In the noonday sunlight, with around 5,000 lumens per square foot, a mature plant can process about 2,000 PPM of CO2. Some larger greenhouses may push these limits, but for home growing, this is not a good idea. In reality though, it is the PAR (photosynthetically active radiation) value that affects what your CO2 levels can be, not lumens. This is because PAR values are a more precise measurement of light that is usable by plants. This information is useful in artificial lighting as it focuses on the more specific spectral wavelengths that plants use best. We include this tidbit now as a primer – it will be an important aspect of the upcoming sequel on lights, focusing specifically on LED research (plus, we really don’t want you looking foolish at the next indoor grow convention).

But for all intents and purposes, using lumens and the distance you keep your lights from your plants are good ways of determining how much CO2 to add to a garden. Figure 1.3 shows ratios for CO2-to-light distance, which is to say how much extra PPM plants can use with lights getting closer and closer to your garden canopy. These estimates are based on optimal grow conditions where growroom temps do NOT exceed 85 degrees F.

Still, your best gauge for how much CO2 to use should depend mostly on your light intensity. Try to get a good reading on your growroom’s lux using a light meter from your local hydro shop. Or, if need be, you can go by the lumen output of your lighting system. Assuming optimal temps 0f 75-80 F, you can figure that with 5,000 lumens per sq. foot your CO2 may range from 1500-2000 PPM. At 1,000 lumens per sq. foot, use from 350-450 PPM. In the end, a good rule for average range for a standard indoor growroom with CO2 enrichment is to use approximately 400 PPM for every 1,000 lumens per sq. foot.

All ranges in between can be tweaked depending on system type, water supplies and room temps. Also, remember that lower temps will reduce plants’ CO2 uptake greatly. A plant at 84 F receiving 5,000 lumens can handle close to 2000 PPM, but that same plant at 69 F will only be able to use half that amount.

LED Lights

Talk about efficiency, LED bulbs are making huge strides lately in the way of intensity and spectrum. Boasting the highest lumen to watt ratio of any other bulbs, the latest advances in LED technology can still cost manufacturers as much as $15 for each tiny, ¼ inch bulb!

Still, the time is not far off when LED-based lighting systems will take over the market. With the ability to fine-tune amounts of available spectrums at specific nanometer (nm) wavelengths, LEDS are the way of the future.

For example, did you know that most horticultural scientists agree that the highest amount of usable wavelengths of the spectrum occur at red (675 nm) and blue (425 nm)? Yet, these two frequencies occur least in MH and HPS bulbs currently on the market. Hmmm… Not to mention newer models, such as the LED UFO promises as much lumen output as regular 400-watt MH/ HPS lamp – but using only 90 actual watts. Very impressive.

Stay tuned for Part II of Advanced Lighting, an LED-based lighting feature coming out in our March edition. In that story we will examine PAR ratings as they relate to light spectrum and how cannabis plants may (or may not) use various components of the spectrum in producing food. HID- LED test experiments will be conducted and the results analyzed as well as more in-depth research on the application of usable spectrum for photosynthesis. Until then amigos, keep in the know and stay in the grow!


Did you enjoy the lesson? For being such good students, here is one gratuitous bud shot for today's moment of Zen, coutesy Paradise Seeds, Amsterdam!