Horticultural Lighting Systems (Grow Lights)

Photosynthesis, or how life begins with light.
Horticultural lighting systems (grow lights)

Why do I need Horticultural lighting systems (grow lights)?

Horticultural lighting systems (grow lights) are necessary if you are planning your garden early. Or maybe you are trying to grow inside, or plan to keep a planted aquarium. In that case 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 provides them with the energy they need to grow, bloom and produce seed.

6CO2 + 6H2O → C6H12O6 + 6O2

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 out, 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.


Joseph Priestley's experiment
Joseph Priestley’s experiment (Source)


Lighting parameters which are important for photosynthesis are:

      • PPFD (Photosynthetic Photon Flux Density)
      • Spectral distribution
      • Lighting cycle
      • Lighting direction and position of light source

We discuss these items in length in this article.




1. Photosynthetic photon flux density (PPFD)

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 Photosynthetically Active Radiation or PAR.


Photosynthetically Active Radiation (PAR)
Photosynthetically Active Radiation (PAR) (Source)

Lumen vs. PAR

A plant would not use all the energy sun provides. Plants don’t use around 70 percent of the sun’s energy. It turns 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 below, the difference between PAR and Lumens can be observed. While human eyes are mostly able to see green wavelengths, plants are efficient in using blue and red wavelengths.


Lumens vs. PAR
Lumens vs. PAR (Source)


Therefore, the metrics needed for choosing a grow light are different, and a lux meter or a foot-candle meter does not measure the complete range required for photosynthesis.


PPF (Photosynthetic Photon Flux)

Energy sources like sun or electrical voltage raise electrons into an excited state. Photons emit when the electron falls back into lower energy levels, for visible wavelengths. PPF (photosynthetic photon flux) is the total amount of PAR emitted by a lighting system per second. The unit 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.


PPFD (Photosynthetic Photon Flux Density) of Horticultural Lighting Systems (Grow Lights)


In order to know the exact number 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. See here.


2. Spectral Distribution of Horticultural Lighting Systems (Grow Lights)

Since leaves are in charge of photosynthesis and they are mostly green, they reflect green wavelengths of the light. 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 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 (Link).

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.


Remember Each Plant Has Unique Lighting Needs

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 should consist of 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.


Selecting Horticultural Lighting Systems (Grow Lights)

Spectra of Various Horticultural lighting systems (grow lights)
Spectra of Various Horticultural lighting systems (grow lights)

In above figure, you can compare the spectra of various light sources. 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 is adjustable is worth the price. Also, they have better photon efficiency, and the service life.


Quantum Yield

Figure below 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 illuminating plants were by each wavelength. The peak happens around 625nm. (Link) This observation proves that green light contributes to photosynthesis.

Relative quantum yield per wavelength
Relative quantum yield per wavelength



3. Lighting Cycle of Horticultural Lighting Systems (Grow Lights)

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 starts when the length of daylight to nighttime is at a certain ratio. Others might not be sensitive to these changes. Their light requirement can be amended by increasing the time they are exposed to light. Lighting cycles can be longer during vegetative stages. During the flowering and seeding stages a 12-hour light, to 12-hour dark cycle is suitable.


What is DLI?

The amount of PAR received during each lighting cycle is Daily Light Integral or DLI. DLI’s unit is 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 low-light crops. Those with 6 – 12 mol/m2 /d are medium-light crops. The ones with 12 to 18 mol/m2 /d are high-light crops. Finally, those requiring more than 18 mol/m2 /d are very high-light crops. (Link)


Calculating DLI

DLI cannot be calculated easily for the plants under the sun without a DLI meter. 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.

These calculations will vary as the light fixture 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.



4. Lighting direction and position of Horticultural Lighting Systems (Grow Lights)

Once the DLI, and lighting cycle requirements of the plants are determined, the distance of the chosen grow light from the plants can be calculated to get the required PPFD on the plant canopy. Arrangement of light fixtures must be 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, there is a chance to remedy this situation. This will improve the plant’s yield and reduce the loss of lower leaves.



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