Our light intensity and spectrum play pivotal roles in the growth and development of all plants. Photosynthesis, photorespiration, germination, photoperiodic control of flowering, phototropism, and entrainment of circadian rhythms are physiological mechanisms that are all dependent on light for optimal plant growth. The efficacy of horticultural lighting can be distilled down to three primary variables: the color of the light (frequency), light coverage of the crop, and using adequate power (photon density) to achieve desired yields.