Using light efficiently is often an overlooked process when considering how to make the most use of the energy provided by our grow lights. Considerations must be made to ensure that light is directed where its needed and that plants make the most efficient use of the light that they recieve. Here we will go over the fundamentals to help you understand what you can do to improve the efficiency of your grows.

Reflective Surroundings:

Using reflective material for the surrounds of the grow area, such as mylar or panda fabric, allows light to be reflected back towards the canopy. Preventing energy losses where light radiates away from the intended area. Oversized grow areas where reflective material does not meet the edges of the plant canopy, reduces the potential for reflecting light back to plants. This is often seen where grow areas are under utilized and the reflective material is too far away from plants to be of any use. Grow tents avoid this by offering the most practical and efficient solution for energy efficiency practices, as it conforms to the ideal size and shape of the desired grow.

Light Height:

Light height is important for the efficacy of the light source and the photosynthetic efficiency of the plant. Too high and light is wasted through radiated reflective losses, too low and you can hurt the photosynthetic efficiency which will waste energy through photoprotection and inhibition rather than photosynthesis. Light sources that collimate and focus their light more (such as LED's), have more flexibility and can be hung to a more appropriate height. Allowing for a more effective distribution of energy to your plants.

Gas discharge lights emit a lot of heat downward. Which can limit how close you can have the fixture. Using air cooled hoods is effective at removing this heat to provide optimal canopy temperature but also allow you to bring your fixture to an ideal level. This is important because you are able to limit the amount radiative and reflective losses that occur by having a fixture too high.

Light Output:

When using grow lights, it not ideal to use a light source with a fixed output, but instead stages of different outputs where the total light energy, matches the stage of growth and area occupied by plants.
This is because when lights do not match the plants stage of growth, fixtures must be hung relatively high to prevent damage to plants due to light saturation. This wastes a lot of light through radiated and reflective losses. To capatilize on available light one must use a cascade of light sources with varying output capacities or use features such as dimmers and switches. So that the light footprint and intensity can be appropriately matched with the stages of growth of the plants. An example would be to use fluorescent or low powered LED lights for the early stages of growth and move on to higher powered light sources with the ability to control the light output through switches or dimmers.

Light Intensity On Leaves:

Plants love making food through photosynthesis, but leaves have a limit on how fast they can do this at any given time. A leafs photosynthetic efficiency is called quantum yield. This describes the efficiency at which a leaf can perform a number of actions per photon absorbed. The more actions that can be processed per photon, the higher the efficiency. The rate of photosynthesis and photosynthetic efficiency is limited by several factors including carbon dioxide, light intensity, temperature, oxygen, water, minerals, age, leaf anatomy and more. When there is not enough carbon dioxide to continue the increase in the photosynthetic rate, this is referred to as being co2 limited. When there is sufficient levels of co2 but low amounts of light, this is referred to as being light limited. Outdoor plants typically shift between these two states throughout the day. During early and late hours when light intensity is lowest, ambient co2 is not the limiting factor and as such it is classified as being light limited. During the middle of the day when light intensity is highest, plants become co2 limited. This is because the ambient levels of co2 cannot support the rate of photosynthesis from light absorbed from the sun at its highest intensity. When a plant is limited by any of the number of factors that are relied on for photosynthesis, a plant will reach what is called the saturation point.

When a leafs saturation point is reached, it will introduce photoprotection to prevent additional damage caused by the excess energy of light. This can be things such as chlorophyll or leaf movement, anatomical changes, non-photochemical quenching or thermal dissipation. This system essentially wastes extra energy, and as a result quantum yield goes down. Once these biochemical protection systems can no longer relieve the excess energy, a process called photoinhibition starts and the photosynthetic rate begins to decline. Which further reduces quantum yeild exponentially. Its at this point you start to see light stress symptoms such as chlorosis.

Cannabis Photosynthesis:

Cannabis has had research done which studied the photosynthetic and environment characteristics of cannabis leaves. With this data we can establish specific rules where cannabis functions the most efficiently at. Cannabis has been shown (Chandra 2008 Study) to start saturating (co2 limited) at around 500umls (40,000 lux). This is similar to other species of land terrestrial plants. At 1000umls (80,000 lux), the light saturation point is reached, this is about where any increase has no further benefit to photosynthesis. Photoinhibition starts at around 1000 (80,000 lux) and the photosynthetic rate declines. With indoor lights and ambient co2 concentrations, you would typically want to have the average light intensity at the canopy to be around 500-700umols (40,000-60,000 lux). With the highest levels to be no more than 1000umols (80,000 lux).

Light Intensity Guidelines For Cannabis:

These canopy intensity guidelines cannot be followed unless you have a light meter which can measure the illumance or flux density of your light source in umols or lux per square metre. Lux meters only work on light sources that produce white spectrums and simply cannot be used for monochromatic light sources, such as the common red and blue LED fixtures. For these you must use quantum PAR meters for reliable measurments and referencing with these guidelines. Although using smartphones to measure illuminance is possible, the accuracy may not be reliable due to hardware variances and the lack of cosine correction systems required for accurate measurements.

 Seeds, Clones And Recovery Zone:

Clones when started are without a functioning root system and so rely heavily on stored resources and photosynthesis. As such the only amount of light they require is the level to compensate for the energy used for simply staying alive and creating roots. Seedling light requirements is almost identical because the age of leaves and root structure, directly effects the potential for photosynthesis. It can take two or three weeks before a plant has matured enough for higher intensities. Seedlings or clones are best between 60-90umols (5000-7500 lux). 

When you have problems with plants such as with deficiencies, sometimes placing your plants into a recovery light plan can help with speeding up recovery, as it places less demand and stress on the plant and allows it to focus on its recovery rather than having to deal with light energy it cannot process at the time. 120umols (10,000 lux) is the best light intensities for allowing a plant to recover and grow.

Whether you are germinating or cloning just a few plants or many, early vegetation starts off with a very small surface area of leaves. It is a big reason why it is not ideal to start off with high output lights, as you cannot bring the light source too close as the intensity will cause saturation damage. As a result, light is wasted through radiated losses. Instead for early growth and a smaller a surface area of leaves, a low output light source should be used, such as fluorescents, low power gas discharge or dimmable fixtures on low. By using light with lower output levels, this allows you to be able to bring your light source closer without providing damaging levels of light intensity and limit the amount of radiated losses by making the light footprint and intensity match the stage of growth.

 Early Vegetation:

As young plants get older you can progressively increase the light intensity until you reach full maturity. This is because as leaves mature the enzyme activity responsible for key aspects in photosystems, increases and allows for maximal processing of light through photosynthesis. As your plants grow and the surface area of leaves increases and begins outgrowing your light source, you can start to consider introducing more light. At this stage of growth, plants are still filling the canopy and is not the best time to use the full recommended output for your grow space. Instead increase your light levels by using more powerful luminaires or increasing the power on dimmable sources. While still making sure the light footprint and intensity matches the stage of growth.

During early vegetation before a plant has reached full maturity, average light levels between 250-350umols (20,000-30,000 lux) is ideal.

Main Vegetation & Flowering:

When plants have reached full maturity where leaves are fully expanded and of sufficient size you can start treating them with the full intensity of your light source.

With mature vegetation, it is best to keep the average light intensity between 700-1000umols (60,000-80,000 lux) as the photosynthetic efficiency is best in this range.

During flowering it is best to keep the average light intensity between 700-1000umols (60,000-80,000 lux). When the canopy is filled, there is no need to bring your light closer during flowering. As the plants photosynthetic capacity is unchanged.

By the time you have reached the end of the flowering stretch, you will want to have filled as much if not all of the entire canopy of your grow space, so virtually little or no light gets through to the bottom. This ensures that most light is being recieved by leaves, instead of having radiated losses that pass through or around the canopy unused. When the canopy is filled you can proceed with the full photon flux that is suited for your grow space. If by the time you have reached flowering or at the end of the flowering stretch, the canopy is not filled. Then plan for future grows to use training techniques, increased vegetation time or higher initial plant numbers to ensure that this space is filled better in the future.

Conclusion:

Ultimately you are wanting to match your light source to your plants or match your plants to your light source. By managing light sources or plant configurations you effectively limit the amount of wasted energy and increase the efficiency of light utilization. You will want to maintain your light intensity at the canopy for each stage of growth as the photosynthetic efficiency is best when kept between the range that suits plants for their given age. Between 700-1000umols (60,000-80,000 lux) efficiency declines rapidly and fully saturates at around 1000umols (80,000 lux). With any further increase in light intensity adding little to photosynthesis and wasting light energy through photoprotective and photoinhibition processes. Reflective materials are used to keep the light that is produced, from escaping the area where you want it.

As always please comment your thoughts or any questions on the subject.