Light intensity is commonly measured in power (watts) per square foot. For optimal photosynthesis to occur a general rule of thumb is 20-40 watts per square foot, with 20 being best for low-light plants and 40 best for light loving plants. Keep 400W HID lamps a minimum of 16" from plants and 1000W lamps 24" from plants. To increase light effectiveness, paint room with flat white paint. You may also want to use reflective mylar sheeting which is 90-95% reflective whereby flat white 75-80% reflective. - Gloss white 70-75% reflectivity Yellow paint 65-70% reflectivity - Aluminum foil 60-65% reflectivity - Black <10% reflective.
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