Growing plants under artificial lighting in closed and fully controlled environments is a method of growing use and global impact.
Industrial scale indoor agriculture could become the main factor that keeps at bay famine and related conflicts. With increasing population, diminishing area of agricultural land, pollution, global warming and migration to grow plants in a reliable, predictable and efficient way will become even more important in the future.
Basic science concepts related to Horticulture lighting
A key factor in the success of indoor plant growth is the efficiency of the lighting system in the process of inducing plant growth, compared with sunlight.
To build a very efficient lighting system some basic scientific concepts should be known.
Plants grow via a process called Photosynthesis that converts electromagnetic radiation – light – into chemical energy used for growth and development. The other ingredients needed are carbon dioxide (CO2), nutrients and water. The process itself is not particularly efficient, with only 4 to 6 percent of the absorbed radiation converted into chemical energy, but this is the engine that drives most life on the planet.
Photosynthesis and PAR radiation
The electromagnetic radiation required for Photosynthesis is defined as Photosynthetically active radiation (PAR), with the spectral range of 400 to 700 nanometers. Only radiation in the above interval can be used by photosynthetic organisms in the process of photosynthesis, to fix the carbon in CO2 into carbohydrates.
We should note that the electromagnetic radiation called visible light or simply light for a typical human eye has a spectral range from about 380 to 740 nanometers.
A common unit of measurement for Photosynthetically active radiation PAR is the photosynthetic photon flux (PPF), measured in units of moles per second. For many practical applications this unit is extended to PPFD, units of moles per second per square meter.
The theory behind PPF is that every absorbed photon, regardless of its wavelength and energy, has an equal contribution to the photosynthetic process. As in accordance with the Stark-Einstein law, every photon (or quantum) that is absorbed will excite one electron, regardless of the photon’s energy, between 400 nm and 700 nm. For this reason, photosynthetic photon flux is also referred to as quantum flux.
However, only some of photons are absorbed by a plant leaf, as determined by its optical properties and the concentration of plant pigments, such as Chlorophyll A and B and Carotenoids (a/-Carotene, Lycopene, Xanthophyll).
The Chlorophylls A and B give plant leaves the characteristic green color because they reflect most of the radiation between 500 and 600 nanometres. Plants that have more Carotenoids than Chlorophylls reflect wavelengths beyond 540nm and have yellow, orange, and red colors. This including autumn leaves when Chlorophylls have dried away.
The graph above shows the Typical absorptance spectra for Chlorophyll A, Chlorophyll B and Chlorophyll (beta-carotene). Each are explained briefly below:
Absorption peaks at 430nm and 662nm
Chlorophyll-a is the primary pigment for photosynthesis in plants and it exhibits a grass-green visual color. Can be found in all photosynthetic organisms except photosynthetic bacteria.
Absorption peaks at 453nm and 642nm
Cholorphyll-B exhibits a blue-green visual color and it occurs in all plants, green algae and some prokaryotes. There is usually about half as much Cholorphyll-B as Cholorphyll-A in plants.
Carotenoids (a/-Carotene, Lycopene, Xanthophyll)
Absorption is strong between 420nm and 485nm
Carotenes are strongly colored red-orange pigments abundant in plants, fruits, vegetables, and whole grains.
Converting Luminous Flux into PAR radiation (PPF and PPFD)
By knowing the absorption peaks of the Chlorophyll and Carotene pigments for photosynthesis, the photosynthetically active radiation PAR (PPF and PPFD) of a light source can be calculated.
Luminous flux, in lumens, can be converted into PPF (photosynthetic photon flux or quantum flux) with a mathematical formula based on the spectral power distribution (SPD) of the light source. The result will be a value in μmol/sec that is only relevant to the SPD used in the calculation.
As this conversion requires detailed data of the light source in tabular format (excel table) for the SPD at every interval of 5 nanometers, it is most often manufacturers write PPF values directly in the datasheet. Below are some examples of Lumen to PPF conversion:
If the PPF value is not mentioned the above numbers can be used for a conversion for other light sources, but with approximate results only.
While PPF (photosynthetic photon flux) is the total energy emitted from the light source, the energy actually received by plants is designated as photosynthetic photon flux density (PPFD) and its S. I. units are µmol/sec/m2.
Iluminance, in lux, can be converted into photosynthetic photon flux density (PPFD) in a similar way with one important difference. This is a value is influenced by the distance and optical properties of the light source (view angle) and is more often measured on the site of installation and not found in the manufacturer datasheet. This approach is difficult to take when the lighting installation is in the planing stange as it requires the LED fixtures to be already in place. The below conversiont tables can help with desiging of the horticulture installation.
1) Convert PPF in PPFD.
The table below shows a PPF to PPFD (100 PPF to PPFD) conversion for a LED lightsource (LED strip) with 120 degree viewing angle. It is important to note that the PPFD per square meter is valid for one square meter of growth area only. If the are is larger areas, LED modules, placed at each square meter, will also iluminate the adiacent squares.
For 100% lighting of one square meter, we recommend a distance of 30 cm between the plant leafs and the LED strip/module.
2) Convert PPFD in PPF
Usually recommeded light levels for plants are expressed in PPFD. For this reason, PPFD conversion to PPF can be more useful.
Once we know the PPF and PPFD values for a light source we can proceed to design the proper light system for growing plants indoor.
Recommended PPFD for common vegetables and decorative plants
The next step to design the horticulture light system is to research the light intensity suitable for photosynthesis for the specific plants we plan to grow.
There are plants that grow naturally in the shade as their leaves require low light intensities and are such referred to as “shade plants”. Other plants grow in the sun with leaves that need high light intensities. They are referred to as “sun plants”.
Most vegetables (watermelons, tomatoes, cucumbers, melons…) are sun plants while many flowers (alyssum, baby blue eyes, begonia, calendula, fuchsia…) are shade plants.
Generally, plants that grow in the shade or at low light intensities have large, thin leaves while plants that need high light intensities have thick leaves.
Depending on each plant, photosynthesis is apparently starting at certain light level, named compensation point. As the light intensity increases so does the photosynthesis and plant development, until a particular point that is called “light saturation point". Beyond it, the amount of light striking the leaf does not cause an increase in the rate of photosynthesis, the amount of light is said to be 'saturating' for the photosynthetic process.
The compensation point and saturation points are discovered by observing the phenomenon of CO2 absorption and release. As the light intensity drops below the compensation point, a CO2 release is observed, as it increases above it CO2 absorption gradually increases until the point of saturation where if flattens out at the maximum level.
A suitable light intensity for a horticulture installation should be chosen between the compensation and saturation points of plants that are grown.
Below there are examples of compensation, saturation and recommended values for vegetables and crops (Letuce, Tomatoes, Cannabis, Red peper, Cucumber...)
Below are examples of compensation, saturation and recommended values for decorative plants.
Designing the most efficient Horticulture LED lighting system
For example, a typical light intensity for cultivation facilities utilized for salad and lettuce would be of about 300 to 400 µmol/m2/sec. In locations where a higher light intensity is needed, such are hybrid type factories, a supplemental lighting of 100 to 150 µmol/m2/sec is recommended.
Currently LED lighting provides the most efficient and cost effective way to illuminate cultivation facilities. However not all LED lighting systems are created equal.
Below are the very important attributes that make the difference.
1. Efficacy: PPF/Watt
The LED system should have a very good PPF per power consumption ratio (PPF). For example, our Linearz Nichia Horticulture Rsp0a LED strip at 5000K has 1.82 PPF/Watt, one of the highest on the market.
2. Color stability
The LED system should have a minimum color shift over its lifetime of operation otherwise its effectives at growing plants will be severely affected.
As a general rule, long lifetime LEDs, such as those manufactured by Nichia, Osram or Cree has excellent color stability
3. Color sorting
White LEDs, as with all manufactured products, have material and process variations that yield products with corresponding variation in the shade of white light emitted. For example Nichia warm white LEDs just out of the production line can have a color temperature between 2580K and 4260K. If they would be directly integrated into strips or luminaries for plat growth the results will be inconsistent.
To prevent the issue above LEDs must be sorted and grouped in batches known as "Chromaticity Bins". Thus it is possible to have all the LEDs in strip or luminaire have the same color temperature from the visible point of view. For this to happen a 3-step MacAdam Ellipse sorting is needed.
4. Easy installation or replacement of modules
Horticulture makes intensive use of the lighting system, with typical 12-16 hours daily run time. For this reason the modules or strips should be easily replaceable.
The Zhaga shape and connection standard is recommended, at it means that a luminaire can be maintained or updated with LED strips easily. Our LinearZ 52 LED module complies with Zhaga Book 7 L56W2 and LinearZ 26 with Book 7 L28W2.
Plants under the lighting of our LinearZ modules with 757 Rsp0a White LEDs for horticulture will grow up to 50% more than conventional light, including standard white LEDs, a combination of red and blue LEDs or a fluorescent tube. In addition, the energy consumption is lower. You can can purchase them directly from below:
LinearZ Modules with Nichia Rsp0A LEDs for Horticulture
Similar results are possible with our full spectrum LinearZ with Nichia Optisolis LEDs with CRI98+
LinearZ Modules with Nichia Optisolis 5000K full spectrum CRI98