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Greenhouse Lighting
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Lighting Indoor Plants	This is a great page on plant light requirements and artificial lighting. Technically this sheet is on lighting house plants but many of the basic concepts apply to greenhouse plants. Just keep in mind that any low light plant should get plenty of light (possibly too much depending on your location and time of year) in a greenhouse and fluorescent and incandescent lights are not good lighting choices for a greenhouse.
Photoperiod control for greenhouse crops	A fairly technical description of photoperiods and other plant lighting topics. Published by the Univ. of Mass. extension.
Hydrofarm, Customized Lighting for Growers	This is a lighting layout tool that will help you get uniform lighting in your greenhouse.
Use your camera as a light meter.	This page contains instructions on how to use your manual 35mm camera to determine the existing light levels.