PPFD, or photosynthetic photon flux density, measures how much PAR reaches the plant at any given time (expressed as micromoles per square meter per second – μmol/m2/s). This will help you understand how high to keep your lights above your plants. Fluorescent lights have a lower PPFD than LED or HID, meaning you must keep fluorescents closer to plants. T-8 lights should be 1 inch from the plants, whereas T-5 can be 6 inches to a foot above the plants. Measuring PPFD properly requires multiple readings taken in multiple positions. A single reading only measures a specific location underneath a light source. For example, the PPFD in the center of a grow light’s footprint will always be greater than the PPFD measured two feet from the center, assuming the vertical distance from the light was equal in both readings. Grow lights that only offer a single PPFD reading won’t give you the best idea for PAR uniformity or the realistic max coverage area the light can support. Having as many different readings as possible, both vertically and horizontally, allows for the most accurate PPFD averages. Ideal PPFD depends on plant type and growth phase. A PPFD value of 200-400 is suitable for seedlings or a mother plant, 400-600 for vegetative growth, and 600-900 when you produce flowers/fruit.