Brief description of photosynthesis:
Photosynthesis is a chemical reaction that takes place inside green leaves of the plant. It requires Carbon dioxide, water and light. Photosynthesis produces CH2O, which is a building block of sugar and needed for plant growth. It also releases oxygen or O2 which is released to the environment.
In detail:
The leaves of the plant have a stomata which is a pore in the epidermis of the leaves.
The stomata is generally located at the downward side of the leaf.
Generally stomata are closed in the dark and they open under the influence of light. The parenchyma cells control the opening of the stoma.
The surrounding mesophyll cells contain green chloroplasts for photosynthesis. The xylem vessels transport water and phloem vessels transport the products of the photosynthesis process.
Carbon dioxide (CO2) from the air is taken up by the plant, it enters via the stomatal cavity the chloroplasts in the mesophyll cells.
Water is taken up from the soil by the roots of the plant and is transported by the xylem vessels to the same location.
Under the influence of light energy assimilates or CH2O is formed and oxygen or O2 is released.
This light corresponds roughly with the visible part of the spectrum with a wavelength of 400-700 nm. This wavelength range is called Photosynthetic Active Radiation or PAR.
CH2O is a building block of sugar which is used for the production of other plant components and to support processes that require energy.
Plant-centric light vs natural light
Natural light is a very diverse lighting source for all kinds of life. The same sun can provide a high intensity lighting effect on a plant in one area, whereas other area’s receive different a much lower intensity. In the morning light can contain much more blue (400-500nm) and in the late afternoon a lot more red (600-700nm). The Earth’s rotation and location of plant relative to the equator influence the actual light received by plants. Plants that naturally grow at the forest floor also receive very different light due to a filtering effect by the forest canopy.
The plant photoreceptors that capture light for photosynthesis are primarily active in the 400-700nm range. Other plant processes such as photomorphogenesis are also sensitive to e.g. UV or far red 700-800nm. 
LEDs are able to optimize and recreate the optimum lighting conditions for growth regardless of weather, season and location. In general LEDs are energy efficient and long lasting light sources.
Spectrum: LEDs are tiny and can be made in a certain color. This makes the suitable for vertical farming to boost e.g. blue and red light to grow strong plants.
Direction: LEDs are directional sources, which reduces the need for reflectors and diffusers. This means LED are more efficient for vertical farming where light can be projected on the plant and where it is needed.
Heat: More than 80@% of energy used by LED is converted into light. In comparison incandescent bulbs convert around 10% of energy into light and fluorescent sources about 20%. For uniform growth in vertical farms, uniform light and heat is important, making LED more suitable. LEDs are also more efficient in cooler environments, which makes them suitable for climate controlled environments such as vertical farms.
No mercury: LEDs do not contain mercury.
Control: unlike the sun the intensity of an LED can be fully controlled. light levels can remain stable or be adjusted based on the plant needs. the day length, intensity and composition of light can all be optimized based on crop needs.
Understanding plant-centric lights
Now we understand the benefits of LED for vertical farming. Let’s explore how yields can be optimized based on 2 variables: spectrum and intensity.
Spectrum
Plants use their photoreceptors to absorb light and light stimulates the photosynthesis process. Vertical farming lights can give the optimal red, blue and wide spectrum wavelengths to grow lush, healthy and strong plants.
Natural light contains relatively more red light in summertime and especially with late afternoons. When plants sense red light they release hormones to keep the process of photosynthesis going. It enables them to take full advantage of the plentiful light in the summertime. Red can also support elongation of plants. Red and Far red is also linked to photomorphogenesis, in particular far red trigger the flowering and fruiting of plants. Keep in mind that only red is not enough. Growing in only red light can result in thin, long and weak plants.
In the morning and noon time natural light contains higher levels of blue light. These levels are also relatively higher during autumn and winter time. Blue light can increase plant quality especially for leafy greens. The plant receptors that capture blue light can help to boost the chlorophyll content and get stronger, compacter and vigorous plants. Too much blue may result in compressed plants.
The natural light contains all kinds of colors including green. Wide spectrum is often mentioned in vertical farming and with wide spectrum people often mean white light which is a combination of red, blue and green. Although most green light is reflected by plants, it could enhance photosynthetic efficiency and help to increase crop yields, in particular when no other natural daylight is present.
Intensity
What is the right amount of light and how to measure it, is a frequently asked question. The amount of light depends on the crop and growing conditions. First let’s talk about how to measure light.
Watts, are a measure of power that everyone knows. However it is not relevant for plants since it describes only the input power consumption for the fixture, but not what the output of plant-centric light. Watts is a helpful measure for electrical engineering and producing running costs, but not the plant growth in itself.
Lumens, are a measure of light emitted by a light source but they are based on the human eye’s sensitivity. It is a good measure the understand how much light emitted is perceived by humans. However the human eye has a peak sensitivity in the green area and as mentioned before plants have photoreceptors and they are sensitive to different spectrums. Green is something that plant’s mainly reflect. With lumens a light source may appear bright for humans, but ‘dim’ for plants.
Photosynthetic Active Radiation (PAR), is the amount of light emitted by a light source in the plant-sensitive area (400-700nm) for photosynthesis. It is a ‘snapshot’ measurement of light emitted of the lighting source. It is a good metric to compare various light sources and calculate energy efficiency.
Photosynthetic Photon Flux Density (PPFD), is the amount of light in-between 400-700nm that hits the plant leaf or any other surface. It is a surface measurement and expressed in micromoles per square meter per second (μmol/m2/s). It is more accurate than PAR, since a high bright light source may have a high PAR value, but if it is placed far away from plants the PPFD value is low. On the other hand, a light source with a low PAR value and placed closely to the plant may result in a relatively high PPFD value.
Photon Flux Density (PFD), is the amount of light inbetween a range. It can also include Far red (700-800nm) with PFD-FR and all the other individual ranges.
Efficiency is strictly speaking not only about intensity but how efficiently the intensity is generated. There are generally two ways to describe efficiency; electrical efficiency (μmol/J) and price efficiency (μmol/$). μmol/J is calculated by the total micromole output of the fixture divided by the total power consumption of the fixture. e.g. a fixture that emits 100 μmol and consumes 50 watts has an efficiency of 2.0 μmol/J (=100/50), each watt generates 2 micromole in plant-centric light.
The higher the value the more efficient the fixture which helps to keep the running cost low.
μmol/$ is calculated by the total micromole output of the fixture divided by the total price of the fixture. e.g. a fixture that emits 100 μmol and costs 125$ has a price efficiency of 0.8 μmol/$ (100/125), each dollar spend results in 0.8 micromole in plant-centric light. The higher the value the more value for money you get which helps to keep the initial investment costs low.
HortiPower plant-centric lights for vertical farming
We know that the right light can help plants to grow better, healthier and faster. That’s why we designed different solutions to meet plant- and grower needs.
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HortiPower City Farming Linear 40W 1000mm (HP-CF-LINEAR-40W-1000) |
 HortiPower City Farming Linear 20W 1200mm AC (HP-CF-LINEAR-20W-1200-AC) |
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Optimize anytime Red, green, blue, white and far red LEDs meet the specific needs of the plant at any growth stage. The flexible spectrum is a feature which allows you to control the colors independently to optimize for different plant characteristics. The individual control helps you to create your own LightScripts for better, healthier and tastier crops. |
Go and grow The red and white LEDs provide a great growth-focused spectrum with red, blue and green for vegetables and leafy greens. The fixed spectrum is not individually adjustable and is an easy plug-and-play fixture. Turning it on will immediately produce the right light. |
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Extendable 6 fixture can be extended in one go without the need for a new powersupply. Fixtures can be grouped together or controlled alone |
Click and grow This fixture can connect 15 pcs together with and simple in-out wire. Need a higher intensity? Simply add more lights. |
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Best Payback An investment that lasts long and thanks to flexible spectrum and dimmability is empowering you to increase your output and quality over time. |
Affordable The highest μmol/$. Truly an affordable choice to keep your initial investment low and get a head start. |
Create the best growth environment
As mentioned light and photosynthesis have a direct relationship. In general more light means better photosynthesis. Roughly between 0-300 μmol there is a clear relationship. Light is a limiting factor in achieving optimum yield. Beyond 300 μmol the increase of light leads to diminishing returns and CO2 becomes the limiting factor to optimize further. HortiPower recommends for vertical farming inbetween 150-250 μmol. Starting from 150 μmol may help to lower initial investment and finding a better μmol/$ efficiency. If high intensities are needed the running costs (e.g. electricity) is very important and higher efficiency fixtures often mean more initial investment.
What to grow
There is a wide range of leafy greens and microgreens that you can grow. e.g.
Lettuce:
Bibb, Butterhead, Leaf, Romaine
Leafy greens:
Kale, Swiss chard, Spicy arugula
Herbs:
Basil, Chives, Dill, Parsley, Thyme, Sage and many more
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Lighting Design
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Product |
PDF (400-800nm) @ 30cm |
Number of lights per 1.2m x 1.2m |
Wattage per m2 per 1.2m x 1.2m |
Investment per 1.2m x 1.2m |
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HP-CF-LINEAR-20W-1200-AC |
132 μmol |
3 |
60 |
84 USD |
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212 μmol |
6 |
120 |
168 USD |
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277 μmol |
9 |
180 |
252 USD |
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298 μmol |
12 |
240 |
336 USD |
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326 μmol |
15 |
300 |
420 USD |