Maximum Yield USA February/March 2023 | Page 35

DLI and PAR

To understand DLI it ’ s important to have a grasp on the more common lighting metrics currently being used in horticulture . For decades , amateur and professional growers alike measured light intensity in their gardens using the lumen or footcandle standards . While these units are great for determining proper lighting levels for a factory or office space , they fall short when it comes to measuring light for photosynthesis . That ’ s because lumens and footcandles were developed to measure how light is perceived by humans . Since our eyes are most sensitive to green light at about 555nm , the lumen and footcandle standards are also biased in this way . Plants , on the other hand , utilize all the light in the visible spectrum almost equally , so it ’ s important to use a measuring standard that takes this reality into account . That ’ s where photosynthetic photon flux density ( PPFD ) comes into play . The PPFD standard gives equal value to every photon ( particle of light ) within the visible range ( 400-700nm ). Readings on a PPFD meter , also known as a PAR meter , are shown in μmol / m ²/ sec , which simply means the number of photons which fall on one square meter every second . Hemp growers utilizing a PAR meter will typically dial their lighting system to about 200-400 μmol / m ²/ sec for vegetative growth or around 800-1,000 μmol / m ²/ sec for flowering plants . More on that later . You may be asking yourself right now , “ what in the heck is a μmol ?” To answer that , let ’ s take a quick detour back to high school chemistry class . Don ’ t worry , we ’ ll keep it brief . Photons are small , infinitely small , and there are a lot of them . To put it into perspective , a typical LED grow light releases more photons in one second than the total number of grains of sand on Earth . To help deal with these mind-bogglingly large numbers we borrow a scientific unit normally reserved for counting atoms , the Mole . The Mole ( abbreviated mol ) is based on the total number of atoms in 12 grams of carbon-12 , which is generally understood to be 6.02 x 10 ²³. This number won ’ t be on the test , but it ’ s important to remember that the mol is a shorthand unit for measuring things that are incredibly numerous . The μmol adds the Greek prefix mu , represented by the symbol μ- , to divide by one million . In other words , one mol is equivalent to one million μmol . What does any of this have to do with DLI ? Since DLI is a measure of the total photons falling on a surface during a 24-hour period , a very large number , we need to use the mol unit to avoid having to write out all those zeroes . Let ’ s go back to the PAR meter example with a reading of 400 . If your plants are receiving 400 μmol / m ² photons every second , how many photons do they receive throughout the course of a day ? Assuming 18 hours of light , that ’ s 64,800 seconds , so roughly 25,920,000 μmol photons each day . That ’ s still a pretty large number to deal with , so we can simplify again by going from the μmol unit back to the mol unit by dividing by one million . This tells us that a PPFD reading of 400 for 18 hours would give us a DLI of about 26 , which is a much easier number to remember .

Crop Requirements

With this in mind , let ’ s take a look at the DLI requirements of some common crops as observed through university research :

“ TO UNDERSTAND DLI IT ’ S IMPORTANT TO HAVE A GRASP ON THE MORE COMMON LIGHTING METRICS CURRENTLY BEING USED IN HORTICULTURE .”

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