Our human eyes are most sensitive to green light compared to light at the ends of the visible spectrum (red and blue). Therefore, when measuring the amount or brightness of light, we commonly use lumens or foot candles, which consider the entire light spectrum that humans can see, ranging from approximately 390nm to 700nm.
If you had three lights – one red, one green, and one blue – all emitting equal amounts of light energy, humans would perceive the green light to have a higher lumen rating compared to the red or blue lights.
Plants do not perceive light brightness in the same way as human eyes do, meaning they do not consider green light as brighter than red or blue light. Consequently, lumens or foot candles are unsuitable units of measurement for the light provided to plants. As growers commonly say, “Lumens are intended for humans!”
PAR (Photo-Synthetically Active Radiation) refers to the measurement of light provided to plants. This measurement quantifies the number of photons available for photosynthesis in a specific area during a given timeframe. The unit used to quantify this measurement is PPFD (Photosynthetic Photon Flux Density), which is expressed as micromoles per square meter per second (umol/m2/s). PPFD is determined within the wavelength range of 400nm to 700nm, without any preference or weighting towards a specific color or wavelength range. All photons within the 400nm to 700nm range are regarded equally.
What Is the Optimum Amount of Light That We Need to Give Our Plants?
A plant leaf in good health can tolerate a significant amount of light, but only for a limited duration. The ppfd, which may reach 1500-2000 umol/m2/s for one or two hours (similar to the intensity of the midday sun at the equator), will not cause any harm. Some plant strains reach their peak productivity when exposed to approximately 1500 umol/m2/s. Going beyond this level increases the risk of leaf bleaching and burn. It is important to note that most high-power grow lights generate heat. If a plant is positioned too close to a hydroponics grow light and receives more than 1500 umol/m2/s, the leaf temperature may rise excessively, hindering optimal photosynthesis.
It is advisable to monitor leaf temperature even when exposing our plants to a 12-hour lights-on period at a reasonably safe absolute maximum of approximately 1200 umol/m2/s.
Within a span of 24 hours, plant leaves can effectively utilize only a specific quantity of photons. Leaf cells of plants possess chloroplasts, which serve as the sites for photosynthesis. Similar to any manufacturing facility, these chloroplasts have a restricted production capability. They can only extract light photons and employ carbon dioxide and water to produce sugar at a particular pace, after which they require a period for recuperation.
Due to this reason, a specific amount of light per day is the only effective utilization for a plant.
By tallying up the cumulative amount of light a plant obtains within a 24-hour timeframe, we reach a value referred to as the “Daily Light Integral” or DLI.
Instead of micromoles (as used in instant light measurement), we can utilize moles/m2/day to depict this measurement.
After reaching a specific threshold, plants gradually become less efficient in utilizing excessive light. This threshold differs among different plant species and can be influenced by factors such as plant health, CO2 concentration, humidity, and temperature within the surrounding environment. However, exceeding the optimal amount of light not only risks damaging the plants but also results in unnecessary electricity consumption.
In tropical countries near the equator, where various plant species thrive, plants can receive a maximum of 50-60 moles/m2/day, which should be the target level for optimal plant growth.
To convert the figure of 50 moles/m2/day to umol/m2/s, we can follow these steps: First, we divide 50 by the number of seconds in a day (86400) that the lights are on. Then, we multiply the result by a million to convert from moles to micromoles. By performing the calculations, (50/86400) x 1000000 equals 587. To simplify, we round up the number and arrive at a range of 500-600 umol/m2/s. However, it is important to note that this value only applies if the lights are running for the entire 24 hours of the day.
If our lights are operated for less than 24 hours daily, we can still achieve a light intensity of 50-60 moles/m2, but within a shorter time frame. This allows us to provide our plants with a higher amount of light. By following a photoperiod of 18 hours on and 6 hours off, we could potentially provide our plants with approximately 700-800 umol/m2/s, at the most. Nevertheless, experienced growers suggest that a light intensity of 500 umol/m2/s is actually suitable for vegetative growth.
Cuttings and seedlings typically require approximately 100-150 umol/m2/s.
If we want to provide our plants with a photoperiod of 12 hours on and 12 hours off, we can give them approximately 1000-1200 umol/m2/s. Going beyond these values will most likely result in reaching a point where the benefits decrease. It’s important to note that in order for the plants to fully utilize all that light, they must be in good health, surrounded by an ample amount of CO2, and maintained at the appropriate humidity and temperature, especially the temperature of their leaves.
In general, a value of 1000-1200 umol/m2/s is typically only obtainable directly below a strong light source. However, as the distance from the light increases towards the sides, the value will usually decrease significantly. The extent of this decrease can vary depending on the reflector’s footprint. Usually, there exists an optimal balance between the intensity directly beneath the grow light and achieving a uniform distribution of light.
In order for plants to begin photosynthesizing, they require a minimum amount of light. For optimal plant growth, we ideally need to provide around 100 – 200 umol/m2/s of light.
What Is The Inverse Square Law?
The intensity of light from an indoor grow light decreases as the distance from it increases. For instance, a standard grower may place a 600-watt HPS grow light 18 inches above their plants, which are exposed to a 12/12 photoperiod. When using a par meter, they might find that the plants receive 600 umol/m2/s at the top. However, when measuring at a lower position, such as 36 inches below the grow light, the reading will likely be only 150 umol/m2/s.
The brightness of a light decreases according to the inverse square law, meaning that the light intensity diminishes as the square of the distance from the source. For instance, if a grower measures the light intensity at a certain distance and then measures it again at a distance that is twice as far, the light intensity at the closer distance will be one-fourth of the intensity at the top of the plants (since 2 squared equals 4). If the grower were to move even farther away and measure the light intensity at a distance of 54 inches (which is three times the initial distance), the reading would likely be one-ninth of the initial measurement (because 3 squared equals 9).
When we are on planet Earth, the light from the sun travels approximately 93 million miles before it reaches us. On a sunny day, if we go outside and hold the sensor at chest level, our photometer might indicate a reading of 500 umols/m2/s. If we place the sensor on the ground instead, we will still get the same reading of 500 umols/m2/s. This is because the sun’s light has already traveled such a long distance that an additional meter will not have any noticeable impact on the reading.
If the light source is a large area, such as a panel of fluorescent tubes, then the drop-off will be somewhat different. However, the inverse square law only applies to “point source” sources of light that are very small.
We now have the knowledge that we can position our grow lights in such a way that the upper parts of the plants receive an intensity of approximately 600 umols/m2/s. Additionally, we can also observe how much the intensity decreases as we move further down, let’s say to around 150 umol/m2/s. By measuring the distance separating these two points, we can obtain a measurement of relative depth penetration. This measurement will provide valuable insight into how well a grow light can sustain a satisfactory intensity for plant growth in comparison to another grow light of similar nature.
Different Types Of Grow Lights In Desktop Aquaponics
It is a commonly known fact that plants require light for growth and photosynthesis, however, there are various methods to achieve this. In situations where natural light is unavailable, alternative options are available. In this discussion, I will explore the various types of grow lights and highlight the advantages of each.
The initial topic to discuss is LED lights, also known as Light Emitting Diode lights. Among various grow lights, LED lights are the most favored type due to several reasons. These lights are preferred over others because they emit minimal heat, possess high durability, consume less energy compared to other lights, and are devoid of any harmful substances such as mercury.
The second type is called High-Intensity Discharge Lights or HID. These lights are also highly favored for growing purposes due to several reasons. They can operate constantly for 24 hours without altering the system’s temperature, they do not emit excessive harmful wavelengths that could harm plants, and similar to LED lights, they do not contain any dangerous mercury.
The final kind of light is fluorescent light. Although not as popular as the previous two lights, they are still fairly popular. Fluorescent lights also generate minimal heat, are generally inexpensive, have longer lifespans than traditional bulbs, are energy-efficient, and are typically easy to acquire.
Generally speaking, all of these options are highly beneficial, and regardless of the option you select, your plants will receive sufficient light.
