Artificial lights have long been used in agriculture to supplement the natural light and increase crop yield. The scenarios where external lighting may be of use may include growing in areas with shorter day-light duration, cold climatic zones, growing indoors, or growing in Vertical farm, etc.
There are different kind of artificial lights that can be used for supplementing growth like incandescent bulbs, fluorescent lights, metal halides, high-pressure sodium halides, LED lights, etc.
The scope of this primer is limited to LED based grow lights used in Vertical Farms.
What is light?
Light is defined as the part of the electromagnetic spectrum that is visible to human eyes. This will range from approx. 400 nm to 700 nm.
How is it measured?
The standard unit of measuring light is lumens (or foot-candles or lux). This unit measures the brightness of light as measured by human eyes. This measure will have the highest reading in the yellow-green spectrum, and lowest on either end (the red and the blue spectrum.)
Do plants see the light in the same way as humans do?
Plants do not see (read use) the light the same way as humans do. Different frequencies of lights show different impact on different biochemical processes within the plants. As such it is not ideal to use the measures (like lumens) developed for human eyes to measure and understand the impact of light on plants.
How do plants use light?
Different wavelengths of light pack varying amount of energies in them. This energy is carried by photons. When the photons fall on plant’s surface, they get absorbed by specific pigments (ex chlorophyll a, chlorophyll b, carotenoids, etc.) within the plants and the energy thus released helps in carrying out the process of photosynthesis. As such, for plants, instead of measuring how bright a light is (lumens), it is more relevant to measure how many photons are being emitted by the light.
Units of Measuring Light for Plants
Earlier, it was believed* that only the lights between 400nm to 700nm (approx. same as the visible spectrum) impact the rate of photosynthesis in plants. The wavelengths in this range are thus deemed to be Photosynthetically active.
PAR (Photosynthetic Active Radiation) is the term used to denote wavelengths between 400nm and 700nm. PAR measures all these wavelengths equally. Unlike a lumen scale which will show a higher peak for the yellow-green region, a PAR scale will show each wavelength to be of the same height in a uniform light.
Photosynthetic Photon Flux measures the total amount of PAR produced by a lighting system each second. In layman terms, it measures the number of photons (within 400nm to 700nm range) that are being emitted from a light source per second.
PPFD or PPF Density measures the amount of PPF that falls on a plant’s surface per second.
*It is now known that even UV and IR wavelengths impact the process of photosynthesis.
**PPFD reading does not include PPF produced by IR or UV wavelengths.
Watts vs. PPFD
Different manufacturers use two different measures of watts while rating their LEDs.
- Some use watt to represent the maximum power rating of the light (say a 1ft Light having 10 LEDs of 3W each is rated at 30W)
- Others use watt to represent the actual power drawn by the LEDs (say the same LED is tuned to operate at 80% power consumption and thus rated as 24W)
PPFD is a better measure than Watts when deciding about the efficacy of plant grow lights.
Impact of Different Wavelengths on Plants
UV lights are in general harmful to all living beings. However, in small and measured quantities they activate several defence mechanisms in the plants which can make them stronger and better capable of dealing with pests and diseases. If your grow lights have UV, make sure that you do not look at it directly and also cover your skin when going near your plants.
The wavelengths between 440nm – 470nm are considered suitable for germination, sprouting, root development and developing thicker (but smaller) leaves.
The wavelengths between 640nm – 660nm are considered good for bud onset, flowering, as well as increasing vegetation.
A small exposure to IR makes the plant feel that it is in the shade. As a defence mechanism, plant tries to grow tall and increase its leaf surface area to be able to do with a lesser light.
Choosing the Best Grow lights for your Plants
Unfortunately, there is not a single widely accepted best case formula of grow lights that can apply to all plants or even to a single species of plant. As time and science progress, more clarity may come to this aspect. For now, we will look at some of the points that have been widely accepted to be correct and some which need further validation and research.
- Plants grown under blue lights alone form smaller and deformed leaves.
- Plants grown under red light alone forms larger but deformed leaves.
- Using both red and blue lights produces more vegetation as well as flowering as compared to using red alone or blue alone.
- A small amount of IR, if used along with Red, leads to superior yields.
- A small amount of UV may help specific plants in increasing resistance against pests.
- Whether adding a small amount of yellow-green spectrum will improve yields or not.
- Whether white light LEDs can produce similar results as produced by lights consisting of LEDs having specific wavelengths.
Choosing the Right Grow light Recipe
As mentioned before, there is little consensus on the best ratio of different wavelengths that will produce the optimal results. However, studying results obtained by different institutes of repute and the recipes of LEDs being sold by different manufacturers can help one in developing insights on what may be the best case light for their specific situation. We will look at some of these without identifying any of the institute or manufacturer.
- An experiment conducted on Lettuce with R(665nm):B(456nm) in ratios 100:0, 87:13, 74:26, 65:35, 53:47, and 41:59 showed comparatively better yields for the last 3 in the list.
- An experiment conducted on Strawberry with R(661nm):B(449nm) in ratios 5:1, 10:1, and 19:1 showed highest yield in 19:1, followed by 10:1.
- An experiment conducted on Coriander with R(661nm):B(449nm) in ratios 5:1, 10:1, and 19:1 showed the highest yield in 10:1, very closely followed by 19:1.
Ratios offered by Different Manufacturers
- For Lettuce
- R(660nm):B(460nm):W(12000K) = 3:1:1
- R(660nm):B(450nm):IR(710nm):White(6500K) = 5:1:1:1
- For Tomato
- R(660nm):B(460nm) = 5:1
- For Strawberry
- R(660nm):B(450nm):IR(710nm):White(6500K) = 7:1:1:1
- General purpose for Greenhouse
- R(660nm):R(630nm):B(460nm):B(440nm):UV(410nm):White(3000K):White(6500K):Orange(610nm):IR(710nm) = 10:4:4:2:1:2:2:2:1
Questions to Ask When Buying Grow Lights
When you go shopping for LED grow lights for your plants, do remember the following:-
- To increase the lifespan of LEDs, they are tuned to run at lower than their maximum power rating. DO check what does the manufacturer mean when he says his LED is ‘x’ Watts.
- Not all reds are equal. Do ask for the specific wavelengths of different LEDs that form your grow light. Within the same color range, LEDs of different wavelength cost different (a 660nm will be costlier than a 630nm).
- Lumen and watt ratings are of not much use when it comes to plants. Compare different LEDs on their PPFD rating.
- PPFD rating changes with distance. Do ask for the ratings at varying distances to determine the best use case for your specific scenario.
- Some manufacturers use an external lens on top of LED chips to focus the light at the centre (and thus obtain a higher PPFD rating). If that being the case, ask for test report of PPFD ratio spread over an area and not on the centre.
- Do understand how the LED dissipates heat. The more effective the heat sink, the longer will be the life span of the LED.
Gaurav is an entrepreneur whose first Start-up was focused on providing services to patients suffering from Chronic Diseases. While researching about the causes of lifestyle diseases and the ways to reduce their incidence, he chanced upon the idea of growing healthy and nutritious food using advanced farming techniques.
He founded CityGreens with a mission to enable City Dwellers to access Safe, Healthy and Fresh food.