Indoor Hydroponic Lighting Theory


Light plays a very important and complex role in photosynthesis. Light is the energy of photosynthesis; it powers all aspects of plant growth. Without light there would be no green plant life on earth.


This is where we need to cover some theory to help us understand the relationship between indoor lighting and yields.


Energy = Energy


Photosynthetic levels determine growth rates and light is the energy of photosynthesis.


Light is primarily responsible for creating chemical energy (in the form of glucose) in plants. Glucose then becomes the fuel for other agents of plant growth.


This process is called photosynthesis. Photosynthesis is the fundamental food making process in all green plants. Light is the energy source for the complex chemistry of photosynthesis.


In a nutshell: The plant takes in water from the growing media and carbon dioxide (gas) from the air, and in the presence of chlorophyll it makes the simple sugar glucose, which is now the store for light energy. In this process oxygen is separated from the water and passes out into the air.


Glucose is the building block for several other sugars and complex carbohydrates that the plant makes (eg. fructose, sucrose, starch etc).  The glucose is stored in the leaf tissue. In addition to this, it is the leaves that catch and trap the light energy. Therefore, leaves receive light and store it as chemical energy.


While photosynthesis is very complex we can summarize it as:


Water + Carbon dioxide + light energy in the presence of chlorophyll = Sugar + Oxygen + Water


Photosynthesis consists of two separate cycles; one of which occurs in the presence of light (light hours), the other in the absence of light (dark hours).


To understand these two processes consider that the plant, in the first phase of the cycle (the light cycle), traps light and stores it as a simple form of chemical energy (glucose).


The night cycle, on the other hand, can be described as the conversion of this simple chemical energy into a more sophisticated form of chemical energy (sugars and complex carbohydrates). The dark phase doesn’t require light – but also occurs in the presence of light. This is where things get a bit tricky so we’ll come back to this further on.


The plant uses this chemical energy for growth.


Light as Energy


The light energy required by plants is confined mostly within the visible spectrum of light (400nm – 700nm). This said, while there are two key points within this spectrum (435nm and 675nm), growth is optimized under the entire range of the spectrum, including non-visible UV-b radiation and UV- a radiation (280- 315nm).


This is because different colour wavelengths stimulate different biochemical reactions within the plant. As a result of this, different physiological functions are activated and energized, which – in turn – determine plant growth rates, formation characteristics, and resin/essential oil production (quality). For instance non-visible UV-b radiation and UV- a radiation, while not being visible in the spectrum, are shown to increase quality (eg. UV-b is shown to be a specific regulator of gene expression and metabolite profiles).


Light and Plants


Photosynthesis is a cumulative process. During the daylight hours a plant traps and stores light energy. This process can be likened to water dripping into a bucket. Over several hours (let’s say 18 hrs) the bucket slowly accumulates water. When the bucket is full, the water which continues to drip into the bucket can no longer be captured and goes to waste. However if the tap is dripping too slowly the bucket doesn’t become full within our 18 hr timeframe.


The accumulation of light energy is similar to this. If the plant receives adequate levels of light it is able to fill itself with chemical energy. In a situation where there is not enough light the plant will only become partly filled with chemical energy. If the plant receives more light than it is able to use, the excess light goes to waste.


Most plants experience optimum growth in high light intensities. However, a single leaf is light saturated at a much lower level than is required to saturate an entire plant. The higher intensities of light are needed to compensate for light shading (light shading refers to areas of the plant that only receive low levels of light, or no light, due to foliage growth inhibiting the access of light). The plant draws on the excess light in order to fulfill the needs of the entire plant. That is, the plant is able to draw in higher levels of light than is required by the immediate leaf area and then distribute the light energy throughout areas of the plant that are shaded by foliage growth.


The parts of the plant that are open to light, therefore, can be described as receivers for energy and conductors of energy to the larger body of the plant. Too much shading or not enough light will greatly limit this process.


Again, let’s consider our bucket example: here we should think of our bucket as not a single bucket but, instead, many smaller buckets which we wish to fill. This time our dripping tap can quickly fill a single small bucket. However, in order to fill all of the smaller buckets our dripping tap still has to fill the equivalent volume of the larger (single) bucket.


In this example only some of the buckets can catch water from the dripping tap. However, the buckets that are able to catch water can also release water into the buckets that aren’t catching water from the dripping tap. This means that some of the buckets have the role of ensuring that all of the buckets are filled.


If sufficient numbers of the buckets are able to catch water they will slowly fill all of the buckets (within the given timeframe). If only a few of the buckets can catch water then all of the buckets will only be partially filled.


The cumulative process of plants trapping light and storing light as chemical energy is similar to this. If you like, a plant’s leaves are a series of buckets that we wish to fill with chemical energy within a given timeframe; the leaves that receive the light distribute chemical energy throughout the plant for storage. This means that the more leaves that are able to trap light, the better the photosynthetic potential.  Therefore:


Light color + light intensity + surface area of plant exposed to light = chemical energy rate.


Oh, and just one more point. We have focused strictly on the role light plays in growth. Without going into any long-winded discussion, there are other factors that will influence photosynthetic activity and growth rates. Our sum is very incomplete without “other environmental factors” added to the equation. There is little point in optimizing lighting/availability of light etc when the room temperature, to cite just one example, is miles out of whack.  Therefore our equation becomes:


Light color + light intensity + surface area of plant exposed to light + other environmental factors = chemical energy rate.


Dark Cycle


The conversion of simple chemical energy (light cycle) into more sophisticated chemical energy (dark cycle) can occur either with or without light.


Quite paradoxically, the dark cycle occurs somewhat better in the presence of light than it does in the absence of light. This is due to the infrared (IR) temperature in the chloroplasts of the leaves. The additional IR heat that light provides is beneficial to photosynthesis. Why is it called the dark cycle then? Because light is not necessary for this part of photosynthesis: however, as we have noted, light is beneficial during this phase.


What this means is that longer light cycles can increase photosynthetic potential. For instance, some growers leave the lights on 24 hours a day during the vegetative cycle.


This brings us to our next point.




Plants enter the flowering phase due to hormonal activity within the plant. Photoperiod triggers this process. Photoperiod refers to the amount of hours that the plants receive light. Typically, a 12-hour light period is ideal for flowerset.


Put simply, while longer light periods may aid photosynthesis, they won’t encourage flowerset. Therefore, longer light periods should only be used during the vegetative cycle.


Light and CO2


There is a symbiotic relationship between light levels, light color spectrum and CO2.


We are able to increase photosynthetic levels by increasing the levels of atmospheric CO2.


Think of it this way: we are able to increase the size of our bucket that collects light energy by increasing the atmospheric levels of CO2. More energy = more growth.


Several things will determine the benefits of additional CO2.


Light Levels (lumen): Plants require a given amount of light lumen in order to utilize extra CO2. There would be very little point in providing additional CO2 to a plant under extremely low light levels. Under HID lighting situations extra CO2 can be very beneficial to growth due to the high lumen output of most HID lamps. Increasing CO2 levels means the plant is able to absorb and utilize more light.  This increases growth rates.


Light Color: Strong blue and red photons are required for maximum uptake/use of CO2.


Critical Mass: Think of the plant as a growth factory. The factory can only operate so efficiently. The machinery of photosynthesis (the chloroplast) has its limits. There comes a point where more CO2 and more light simply go to waste.


OK… Don’t get too caught up with worrying about light cycles, dark cycles, CO2 etc. It is all part of photosynthesis and therefore some grounding on the subject is required.


Let’s summarize this material:

  • Photosynthesis is the fundamental food making process in all green plants.
  • Light is a major component of this process – it is the energy of photosynthesis.
  • Light as energy is determined by the color and the intensity of the light.
  • Different colors of light activate different biochemical responses.
  • The different biochemical responses play a significant role in plant growth rates and formation characteristics.
  • Because of this, growth is optimized when the entire visible spectrum is provided.
  • Photosynthesis is a cumulative process.
  • Light is trapped and stored as simple chemical energy during the daylight period.
  • This energy is converted into more sophisticated energy during the dark cycle
  • The dark cycle can benefit from light (and)
  • Photoperiod must be taken into account
  • Plants store the chemical energy in the leaf tissue.
  • The parts of a plant that can trap light distribute it throughout the plant for storage.
  • The more leaf area exposed to light, the better the photosynthetic potential.
  • There is a symbiotic relationship between CO2, light and plants.
  • It is possible to increase photosynthetic levels by increasing CO2 levels.
  • Light intensity and color spectrum will determine growth rates in a CO2 enriched environment.


OK… So that’s all very well and good. But what does all this mean in layman’s terms? How is understanding light and photosynthesis going to make me a better grower?


Well firstly, let’s convert some of the theory into growing practices.


  • One of the most common mistakes made by indoor gardeners is that they fail to appreciate that fewer plants can mean more yield. This is understandable when you consider that everything we are taught, from the time that we are children, enforces the principle that “more means more”. However, consider the theory about how a plant receives light and stores light as energy. Primarily, consider the principle of what percentage of the plant is capable of acting as a receptor of light energy (those parts of the plant that are not shaded and are required to conduct energy throughout the plant). Too many plants crowded into too small a space will compete for the available light. This will result in all of the plants shading the bulk of one another out, which in turn will result in each and every plant performing well below optimum photosynthetic (potential) levels. As an absolute rule, you will realize the best results when the balance of plant numbers correlates to light availability. While the principle of less means more, on the surface, seems wrong, our light theory substantiates this principle – making it easier to accept. (See image below)



Plant Spacing Under HID Lighting


Another mistake commonly made by growers is the removal of leaves from the plant. On the surface, this may seem like a reasonable proposition. After all, if you get rid of some of those big leaves that aren’t really getting any light it may help open up other areas of the plant to light. The theory of light energy conduction (energy being distributed throughout the plant), however, contradicts this practice. Each leaf on the plant is a storage unit, and a sugar and complex carbohydrate production factory for photosynthesis/energy. Leaves do not have to receive light to play a role in photosynthesis. While they may not be receiving light they are still storage units for energy. Removing leaves reduces photosynthetic potential (hence growth).


Some growers assume that a 24-hour light cycle should be used during the vegetative phase of growth. From what I can gather, this assumption seems to stem from the belief that light = growth (which obviously, is true). However, supporters of the 24 hr light cycle also assume no light = no benefits to growth. Now remember how we discussed that “the dark cycle doesn’t need light – but also occurs in the presence of light.” Our theory tells us that longer light cycles would be beneficial to growth during the vegetative cycle. However, no light also = benefits to growth. It has been my experience that 18 hr light cycles perform extremely well during the vegetative cycle. While some increase in growth occurs with additional light it is marginal. If you are contemplating using a 24 hr light cycle then there are a couple of factors to take into account.


  • Outside air temps: In hot climates, growers often switch night and day. This means that the lights are on during the night when air temps are cooler. If your lights were also on during the day when air temps were extremely warm (because of a 24hr light cycle), you would be ducting in excessively heated air, which could stress plants. The extra growth potential of the 24 hr light cycle would be quickly undermined by the overheated air temps in the grow room.
  • It has been my experience that the 24 hr light cycle can be stressful to younger plants. This comes down to the relationship between light as energy and plants. To offset this situation it is advisable to keep lamps raised higher than normal (approx 50% higher) during the first 7 – 10 days of growth. Or, begin with an 18-hour light cycle for the first week, 20 hours during the next three days and then increase the hours to their maximum of 24 (continuous running of lamps).


A high percentage of growers have settled on HPS lighting (alone) for flowerset. The theory tells us that different color wavelengths stimulate different biochemical responses within    the plant, leading to (optimized) growth. Our theory also tells us that CO2 is best assimilated when high levels of red and blue spectrum light are present. Therefore, providing as broad a spectrum of light (within the visible spectrum) as possible is the ideal lighting scenario. HPS light, while providing adequately for the red spectrum doesn’t quite provide for other elements of the spectrum (particularly the blue end). Therefore, HPS light (alone) can, theoretically, be improved with combination lighting scenarios.  For instance, combining HPS with MH will create a wider band of the visible spectrum, leading to potentially larger yields. This too, is a widely used lighting scenario amongst indoor growers. I’ll come back to this later on!


  • One last thing. Is it possible to have too much light? Will too much light harm plants? Technically, light loving plants thrive under extreme levels of light. So while it is easy to have too little light, the likelihood of harming a light loving plant with too much light is extremely low. Our theory tells us that the plant accumulates sugars during light hours. If the light levels aren’t sufficient to charge the plant with energy (due to too much shading or not enough light) the sugar levels will be low. On the other hand, the plant can only convert so much light energy into photosynthetic energy. If light levels are high, and other conditions are perfect, the amount of CO2 can limit the rate of photosynthesis. If you provide more CO2 by air enrichment, the plant can use more light and the photosynthesis rate can/will increase. If you give more light and more CO2 eventually you reach a limit set by how fast the machinery of photosynthesis (the chloroplast) can tick over. Too much light can burn plants because of infrared radiation. Even if your lamp doesn’t emit any IR, shorter wavelengths are absorbed by leaves and reflect IR radiation. IR radiation is heat, and heat stresses plants. High-energy lamps need high cooling rates. In short, heat harms plants and HID lamps create heat. Therefore, the correct balance of light energy and cooling (via airflow) is essential.


That’s about it on photosynthesis. Hopefully, you now have a basic understanding of light and photosynthesis. Next up we’ll look at different lighting scenarios that can enhance yields.

Blended  Lighting


Growers who have experimented with different lighting combinations can/will tell you that different lighting configurations can produce very different results. For instance, plants that are flowered under a combination of red spectrum (HPS) and blue spectrum (MH) lighting form very differently than plants that are flowered under red spectrum light alone.


This relates back to our theory on providing the entire range of the visible color spectrum to plants in order to stimulate various biochemical responses – hence plant growth rates and formation characteristics.


For instance, a plant that is flowered under HPS light (alone) typically ‘stretches’ (becomes unnaturally elongated). This is because, while HPS provides large amounts of red light it tends to be lacking in other key areas of the visible color spectrum. This means that the required stimulus for the various biochemical responses is not adequate.


By introducing blue spectrum light into the red spectrum we are able to cater more adequately for these biochemical responses. In short, blue spectrum light promotes a better plant structure (shorter/stockier plant, smaller gaps between nodes) while the red spectrum light provides the correct stimulus for flower growth.


The ‘stretch’ situation is more pronounced in environments where larger plants are being grown (2 -3 foot plus). Therefore, in situations where multiples of smaller plants (1 – 1 ½ foot) are being grown HPS lighting tends to perform extremely well.


Plant genetics can/will also determine outcomes where lighting is concerned.  For instance, where apical dominance (propensity to ‘stretch’/large gaps between nodes of plant) is displayed, further stretch exacerbated by HPS light (alone) could dramatically reduce the chances of the floral clusters joining to maximum effect. This situation would, in all likelihood, result in decreased yields.  However, where non – apical dominance (short, stocky plant/close node spacing/tight floral clusters) is displayed, red spectrum HPS light tends to perform extremely well.


Therefore, genetics can play a role in determining outcomes. Plant physiology is complex and plant genetics are diverse; because of this, ideals can become somewhat random and influenced by the growing methodology and the genetic traits of the plant.


Mixing It Up


As we have noted, most indoor growers tend to settle on, just, HPS lighting. That is, many growers do not combine different lamps and are more than happy with their results.


However, this does not mean that HPS lighting can’t be improved. The fact that so many indoor growers have settled on the use of HPS lighting (alone) is possibly determined by purchasing decisions based on technical advice, pragmatic equipment choices (“HPS does the job well and purchasing other lamps, running other lamps and paying more for power etc is all a bit too much”), and localized cultural trends (information passed amongst networks of individuals etc). For instance, in the latter case, lighting methods vary based on geographical trends. The trend in Australia leans, predominantly, towards HPS as the lamp of choice.


So, what other options are there?


Well, firstly, another commonly used lighting method is the combination of MH and HPS in bloom. This means that the two lamps are run in conjunction with one another throughout the bloom cycle.  For instance, a 1000-watt HPS combined with a 400-watt MH or, a 600-watt HPS combined with a 250-watt MH etc is a typically used scenario. The combination of the two lamps provides blue spectrum light for structure and red spectrum light for flower growth. The weighting of the more powerful HPS light against the less powerful MH light ensures that the blue end of the spectrum is catered for, while not overpowering, thereby, leaving plenty of red spectrum light for flowerset.


Put simply, you will gain final yield weights and essential-oil production through combining HPS with MH.  (full stop!)


A recent evolution in indoor plant lighting is high-output (100 – 200watt) fluorescent lamps (e.g. 130watt Spectrum Lamps).


After running a series of trials with these lamps in combination with HPS lighting I was highly impressed with the results (10 – 15% extra yield over HPS alone).


High-output fluorescent lamps have low heat output, reasonable lumen output and colour ranges that are ideal for blended spectrum lighting. Additionally, they are self-ballasted and screw directly into the moguls of your existing light shades (I.e. shades sold through hydroponic stores for HPS and MH lighting).


Spectrum lamps come in 6400K (blue daylight) and 14000K (cool blue). The ideal mixing pattern here is one 130watt 14000K (cool blue) lamp per 600watt HPS lamp.



Pin It on Pinterest

Share This


Share this post with your friends!


Share this article with friends!