Even this has a problem. https://bioslighting.com/horticulture-lighting/led-grow-light-distance-chart 2. https://extension.umn.edu/planting-and-growing-guides/lighting-indoor-plants Myth #3: PAR is a Measure of Light Intensity. At the 2×2’ square we see PPFD values as low as 142 umol/m2/s. PPFD is a ‘spot’ measurement of a specific location on your plant canopy, and it is measured in micromoles per square meter per second (μmol/m2/s). Here is a brief list of the PPFD (in mol/m²/s) requirements for plants: 1. The PPFD value in the center, 719 umol/m2/s, is high enough to stimulate photosynthesis in flowering weed plants, but the light coverage area is rather poor. Some houseplants do well in a shady corner while other plants, especially fruit-bearing plants like cannabis, peppers, and tomatoes need a lot of light to grow to their full potential. PPFD stands for ‘Photosynthetic Photon Flux Density.’ PPFD is a measurement of light intensity, and it says how many light photons that fall within the PAR-range (expressed in micromoles) hit a given square meter per second: μmol m(-2)s(-1). This list is not very accurate, but it’s still very helpful. You will have trouble finding a PPFD value for most lights. The amount of light is actually measured as PPFD (Photosynthetic Photon Flux Density), sometimes abbreviated to PFD. We see that the PPFD values fall from 719 (highest) to 409 (lowest) even within the center 1×1’ square. Many “tomato” (cannabis) growers run their plants at a PPFD of about 500-800 umol/m 2 /s (which is too bright for most orchids/houseplants). 3. Partial sun plants need a minimum of 150 to 200 mol/m²/s. As you explore this world and unpack the different metrics and grow lights and systems that you need for optimum yields and healthy, flavoursome plants, you will hit a wall of acronyms. PPFD is a better way to measure the quantity of light for LED grow lights, than lumens. Full sun plants need a minimum of 400 to 600 mol/m²/s. PPFD measures the amount of PAR that actually arrives at the plant, or as a scientist might say: “the number of photosynthetically active photons that fall on a given surface each second”. Full shade and other low-light plants need around 100 mol/m²/s. The industry and gardeners tend to interchange the two terms using the term PAR when they should be saying PPFD. Just like humans count their energy needs in calories, PPFD or PAR is a similar measurement for plants. So the logical solution to fix your problem is to determine the grow light height where your plants receive the ideal amount of light intensity for that phase of growth: 200-250 micromoles for seedlings and 300 to 400 micromoles of PPFD for vegetative phase plants. A photon is the most basic and smallest unit of light. 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