Horticultural Lighting System(Grow Lights)
Photosynthesis, or how life begins with light
Whether you are planning your garden early, trying to grow inside, or plan to keep a planted aquarium, you will have to provide your plants with adequate lighting so that they grow robust and healthy. Either the light comes to them, or they go after the light and become lanky and weak. Plants need light for photosynthesis, which is how they convert water and carbon dioxide into carbohydrates and produce oxygen as a byproduct. Carbohydrates provide them with the energy they need to grow, bloom and produce seed.
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In 1771, Joseph Priestley observed that when he put a mouse and a candle in an enclosed glass container, they both died. When he added a plant into the glass container and exposed it to light, candle burned away and mouse lived with no problem. He concluded that "the injury which is continually done by such a large number of animals is, in part at least, repaired by the vegetable creation." What he didn't know at that time was that he had just witnessed the photosynthesis and the byproduct of it, oxygen, which sustains the life on earth.
Figure 1 Joseph Priestley's experiment 
Lighting parameters which are important for photosynthesis can be divided into four major categories and will be discussed in length in this article:
PPFD (Photosynthetic Photon Flux Density)
Lighting direction and position of light source
Photosynthetic photon flux density
When you purchase a lamp for your home, you might consider its lumens, which defines the total quantity of visible light emitted from that lamp. If you are tech savvy, you might even get a lux or foot-candle reading to meet the intensity of lighting you need. So naturally you might think that for any plant, all you need is to know its lux requirement and you can set up the lighting system. However lumens and lux are not very good metrics when you plan to purchase a grow light, simply because the optimum photosynthetic efficiency peaks at different wavelengths measured by lumen and lux.
The sun's electromagnetic wavelengths, which reach Earth, are between 100 nm and 1 m. The wavelength of the light that plants use for photosynthesis ranges from 400–700 nm. This range is referred to as Photosynthetically Active Radiation or PAR.
Figure 2 Photosynthetically Active Radiation (PAR) 
A plant would not use all the energy sun provides. About 70 percent of the sun's energy is not used by plants. It is turned into heat, and it even might kill the plant. That's why delicate plants do better with a proper shade during the summer.
In Figure 3, the difference between PAR and what is being calculated as Lumens can be observed. While human eyes are mostly adapted to see green wavelengths, plants are efficient in using blue and red wavelengths.
Figure 3 Lumens vs. PAR 
Therefore, the metrics needed to be considered for choosing a grow light are different, and a lux meter or a foot-candle meter does not measure the complete range which is required for photosynthesis.
Energy sources like sun or electrical voltage raise electrons into an excited state. When the electron falls back into lower energy levels, for visible wavelengths, photons are emitted. PPF (photosynthetic photon flux) is the total amount of PAR emitted by a lighting system per second. The unit which is used to express PPF is micromoles per second (μmol/s). Note that PPF does not define how much of the measured light actually lands on the plants. However, it is an important metric for comparing photon efficiency of different lighting systems, which are consuming the same amount of power.
In order to know the exact amount of photons landing on any given part of the plant, PPFD (photosynthetic photon flux density) must be measured.
PPFD measures the amount of PAR that actually arrives at a certain height and location, for instance, the canopy of the plant, further from the center of the light source. It is measured in micromoles per square meter per second (μmol/m2 /s). Therefore it cannot be a single number mentioned on the light fixture. Fortunately, there are cellphone applications, which measure PPFD through the light sensor of the cellphone almost accurately. 
Since leaves are in charge of photosynthesis and they are mostly green, green wavelengths of the light are reflected. Therefore it seems obvious to assume what is useful for plants are mainly blue and red wavelengths.
However, plants grown under a full spectrum light in practice do better than those which are only exposed to red and blue wavelengths. It seems that green light penetrates the canopy of the plant, which makes photosynthesis possible for lower leaves. That in turn leads to less loss of older leaves. Therefore the yield of the plants grown under full-spectrum light is higher than those grown under just blue and red light.
Another benefit of including green in the spectral regime of plants, is to reduce eye strain of workers who need to monitor plants for nutritional deficiencies, or early pest and infection detection. Under red and blue spectra, plants may not appear their normal color, which could make early detection difficult.
The ultraviolet rays of the sun and some grow lights could protect the plants against pathogens and diseases. Therefore the closer the grow light's spectrum is to that of the sun, it would ultimately be better for the plants.
The spectral regime for each plant differs from another, and is even different during various stages of plant's life. For instance blue wavelengths are important during vegetative stages. Red wavelengths are a must during flowering and seeding stages. At least 20% of the spectra must be dedicated to blue wavelengths. That automatically shifts the color temperature to 4000k or higher, depending on the type of the plant and its stage of life. For instance a 2500k light source such as incandescent light bulb is out of the question. It also has a low photon efficiency, and high losses. It dissipates too much heat, which is not desirable in closed spaces.
Figure 4 Spectra of Various Light Sources
In Figure 4, spectra of various light sources can be compared. There are really not that many options that can compete with modern LED grow lights. While their prices might be higher, the fact that their spectral regime can be adjusted is worth the price, not to mention their photon efficiency and the service life is unmatched.
Figure 5 shows the relative quantum yield per photon (ratio of the number of photons emitted to the number of photons absorbed) during photosynthetic activity.  It was calculated by measuring the input carbon dioxide and produce oxygen in a controlled environment while plants were illuminated by each wavelength. The peak happens around 625nm.  This observation proves that green light contributes to photosynthesis.
Figure 5 Relative quantum yield per wavelength
Plants need to take a break from photosynthesis, and this period depends on the crop and its stage of development. In some plants, the flowering phase is triggered when the length of daylight to nighttime is at a certain ratio. Others might not be sensitive to these changes and their light requirement can be amended by increasing the time they are exposed to light. While lighting cycles can be longer during vegetative stages, or even the whole cycle could be longer than 24 hours; during the flowering and seeding stages a 12 hour light, to 12-hour dark cycle is recommended.
The amount of PAR received each lighting cycle is called Daily Light Integral or DLI. It is expressed as moles of light per square meter per day. Each plant has its own DLI requirements. Crops with a DLI requirement of 3 - 6 mol/ m2 /d are considered low-light crops, 6 - 12 mol/m2 /d are medium-light crops, 12 to 18 mol/m2 /d are high-light crops, and those requiring more than 18 mol/m2 /d are considered very high-light crops. 
DLI cannot be calculated easily for the plants under the sun without a DLI meter as the PPFD of the sun changes throughout the day and each day throughout the seasons. However, it can be calculated for a certain point from a light fixture, assuming its photon emission remains constant for a period of time. Once the PPFD for a certain point is measured, it can be converted to DLI by converting micromole to mole and multiplying the value by the number of seconds in the light-on period. So for instance for PPFD of 1 µmol/m2 /s, we would have 0.0432 mol/m2 /d with continuous light for 12 hours.
It must be noted that these calculations will vary as the light fixture ages and nears its end of service life. Some fixtures have longer service life than others, however, their photon emission will decrease over time sooner or later as they all have finite life spans.
Lighting direction and position of light source
Once the DLI, and lighting cycle requirements of the plants are determined, the distance of the chosen light source from the plants can be calculated to get the required PPFD on the plant canopy. Arrangement of light fixtures must be done in a way that would cover the plants as uniformly as possible. Once lower leaves of plants receive no light and stop photosynthesis, they die. With artificial lighting, however, there is a chance to light up the parts of the plants that normally don't receive light to increase the plant's yield and reduce the loss of lower leaves.
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