We see that the PPFD values fall from 719 (highest) to 409 (lowest) even within the center 1×1’ square. Myth #3: PAR is a Measure of Light Intensity. The amount of light is actually measured as PPFD (Photosynthetic Photon Flux Density), sometimes abbreviated to PFD. A photon is the most basic and smallest unit of light. https://extension.umn.edu/planting-and-growing-guides/lighting-indoor-plants 2. Your goal with the majority of tropical houseplants is to target about 20-40% filtered sun (removing 60-80% of direct sun intensity like leaves and clouds would in nature). 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. 100-300 PPFD for seedlings; 200-600 PPFD for vegetative growth; 600-1,000 PPFD for flowering; 800-2,000 PPFD for sunlight (depends on elevation, location etc) 600 – 1,600 PPFD for full shade; Plants can be damaged with more than 800 PPFD. This list is not very accurate, but it’s still very helpful. Just like humans count their energy needs in calories, PPFD or PAR is a similar measurement for plants. You will have trouble finding a PPFD value for most lights. 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. 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. 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. 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). PPFD is a better way to measure the quantity of light for LED grow lights, than lumens. At the 2×2’ square we see PPFD values as low as 142 umol/m2/s. https://bioslighting.com/horticulture-lighting/led-grow-light-distance-chart Even this has a problem. 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. Full sun plants need a minimum of 400 to 600 mol/m²/s. I tend to say grow light hanging height is not a perfect science. The industry and gardeners tend to interchange the two terms using the term PAR when they should be saying PPFD. Partial sun plants need a minimum of 150 to 200 mol/m²/s. Here is a brief list of the PPFD (in mol/m²/s) requirements for plants: 1. 3. 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