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Lighting Science 101


Lighting is, perhaps, the least understood aspect of indoor growing.  I say this as a science consultant to the controlled environment agriculture sector. We all know light is essential for growing, but what kind do you choose – fluorescent, HPS, metal halide, induction, plasma, light emitting ceramic, LED? What kind is best for you and your plants? How do you use it?

     First, we have to take a quick look at the basic science of light as it relates to plant growth. We all know that light drives photosynthesis – the reaction cycle that enables plants to produce food (sugars). Light also effects physical factors such as plant size, bushiness, coloration, flower production, nutrient content, essential oil levels. All these and more are effects of photomorphogenesis: the development of form and structure because of the action of light.

     Just like temperature, humidity, ventilation and CO2, light has to be delivered at very precise levels to achieve optimal growth and productivity.  Merely having a “very bright light” does not assure that optimal growth and productivity are achieved. In addition, light must be understood in two different manifestations – waves and particles.

     Intensity is perhaps the most confusing aspect of light for people to understand. For years light fixture and bulb manufacturers have described their light’s intensity with terms like lumens, lux, foot-candles, kelvin and CRI. The problem is that those terms all refer to how the human eye perceives the light and not how well the light satisfies the biological requirements of a plant. Plant and lighting scientists describe agricultural light intensity by the measurement – micromoles per meter squared per second at the canopy (what we know as PPFD or photosynthetic photon flux density).

Light exhibits properties of both waves and particles depending on how you measure it. Enter the realm of quantum physics! The individual “particles” of light are called photons (or quanta). The more intense the light, the more photons given off at any instant.  We now have meters that can actually “count” the number of photons given off by a light source. Why is the photon count essential? Because there is a direct, measurable relationship between the number of photons striking a leaf surface and the production of oxygen/carbohydrate molecules by the plant. It’s a bit complex, but the general consensus is that it takes 8 photons per O2 molecule produced. In the formula below we see that to produce one carbohydrate molecule we need to produce 6 O2 molecules. So it takes 48 photons total to produce one carbohydrate molecule – C6H12O6. In the photosynthesis formula below, the “light energy” (on the left side of the equation) required to produce the 1 molecule of C6H12O6 on the right side can be translated (kind of) to 48 photons. All plants have an “optimal” level that will drive photosynthesis at the fastest possible rate for that plant (dependent on the biochemical structures in the leaf tissue that facilitate photosynthesis). So it is our job as growers to find that perfect “Goldilocks” level of photon input to the plant.

 The photosynthesis equation: 6CO2 + 6H2O + light energy = C6H12O6 + 6O2

If the light that we deliver to the plant is not intense enough to drive photosynthesis to its potential highest rate (read “don’t have enough photons”) then we can’t reach maximum plant growth potential. If we provide light that is too intense (read “too many photons”) then we waste energy and over stress the plant.

The problem is that we don’t exactly know what the perfect intensity is for all plants. Perfect intensity can differ dramatically between species and even varieties.  A broad, general statement for medicinal crops would be – the more intense the light the better – up to a certain point. But – and this is an important but – we must realize that a plant’s ability to use light optimally in the process of photosynthesis is intimately linked to other factors such as CO2 levels, air temperature, vapor pressure deficit, moisture/nutrient availability. In a grow chamber NOT injecting supplemental CO2, the “average” medicinal crops plant in the vegetative phase can only process a light intensity level in the 500 micromoles per meter squared per second range. Anything over that cannot be processed effectively and becomes a stressor to the plant and a waste of energy (read wasted money). For a reference, a 4 foot, 4 bulb 54 watt high output T5 fluorescent fixture will produce  an intensity of about 250 -300 micromoles per meter squared per second at about 10-12 inches above a plant. An 8 bulb, 4 foot T5 will produce an intensity in the 500 micromole per meter squared per second range at 10 – 12 inches.  As the level of CO2 you add to the environment increases the light intensity your plants can process increases – all the way up to somewhere in the range of 1300 – 1500 micromoles per meter squared per second with a CO2 ppm range around 1200 – 1400. That intensity of light needs to be delivered by a fixture such as a 1000 watt HPS or a commercial LED.   Just look at the exquisite balance of all the different set points when we begin the process of fine-tuning a commercial grow chamber. Light (intensity and wave length), temperature, humidity, CO2 levels, ventilation rates, watering and fertilization timing, vapor pressure deficit – all must be in their respective “perfect” ranges.  Light in the high intensity “optimal” range will not assure optimal productivity on its own. Remember the  Law of Limiting Factors (think the weakest link)!

      As a science consultant I meet with the grower to view their grow chamber and determine its ability to tightly control critical set points. The more precise the control capability the higher he/she can push the light intensity level to approach that illusive “optimal” point. After determining what the intensity level should be, I use a quantum meter (which “counts” actual photons) to “tune” the lights to deliver that intensity level to the plant canopy. As the plants grow, the grower can modify the  distance between the canopy and the light fixtures to maintain desired intensity levels. As the plants progress in the grow cycle we can alter intensity and spectral delivery to meet the plant’s specific needs (such as the use of various “finisher bulbs”).

     The wave function of light is also critical. Plants use light that primarily falls in the human visual range. In terms of the wave lengths, that is between 400 and 700 nanometers. Research shows that plants also respond to low levels of ULTRA VIOLET (light below 400 nanometers around 380) and Infra-Red (light up around 720 nanometers).

So…. What’s the perfect wave length or mix of wave lengths? Darn good question.

We know basically what plants such as medical medicinal crops need but research into exact wave- length mixes for various varieties is ongoing and may take years to figure out. The importance of the wave feature of light lies in all those photomorphological changes that can be induced by changing the wave-length mixture. Light fixture and/or bulb companies that say they know and have the “perfect” spectrum that your plants need/want – well, let’s just say that they are stretching the truth a bit. Use of “full spectrum” light is currently recognized as a “safe” foundation to build upon.

    There is no Right or Wrong fixture to buy for you operation. It depends on your room, budget, environmental controls sophistication, expectations, energy consumption concerns and comfort of use.  LED’s have now reached a point where they are the light of choice for serious commercial grow operations. Efficiency, cost of operation, controllability, heat production, solar capability – LED’s are no longer trying to catch up. They are now the fixture to be caught! The main areas of concern with LEDs at this time are the initial cost and lack of understanding (by the end user) of the technology. HPS is still the workhorse of the sector especially in the home and small commercial operation. But heat generation and subsequent HVAC requirements soon rear their troublesome heads. Double ended HPS bulbs generate high micro mole output and are more efficient than single ended bulbs but also generate temperatures in the 700-800 degree Fahrenheit range.  Fluorescent is great for veg and propagation but not intense enough for optimal flower cycle production – especially when using supplemental CO2. Ceramic metal halide offers a lot of promise in terms of spectral profiles and economics of operation but still has some ground to make up in terms of intensity, heat production and operational cost. 

Many growers still see their path to success (high yields and high quality) lying in finding that “perfect”, new, cutting edge light fixture. In reality, their path to success lies in becoming more educated about the interrelation of all the various set points of controlled environment growing. Don’t go out and spend a ton of money on that new light fixture without a reason.

  Study the tight interdependence of the multiple set points in successful grow chamber operations. Understand what happens to all the other set points when you alter any one of them.  Then, you will understand whether you need a new $500 to $2500 light fixture or you just need to better manage your room’s humidity, temperature, CO2 levels, ventilation, fertilization schedule and watering frequency. Either way, you and your plants will be way ahead of the game!

     Bottom line? First, evaluate your needs, budget, knowledge level, goals, expectations and physical layout. Then, design your lighting system accordingly. But always make sure that your next potential light fixture meets the basic science requirements that we have discussed here. Does the company freely post science specifications like PAR FLUX GRID MAPPING for their fixtures? Finally, talk with a reputable specialist who can discuss the science of plant lighting requirements and the specific fixtures you are considering. If you do these simple things, both you and your plants will be happy!!


Prepared by:  David  O’Connor, Controlled Environment Agriculture Science Consultant to the medicinal crops and food production sectors.