Quality of light refers to its wavelength. Each color occurs at a different wavelength. Oregon State University's Extension Service claims that blue and red light have a greater effect on plant growth than any other color.
According to the sources at ACF Greenhouses, blue light promotes vegetative plant growth. Red light, on the other hand, will activate certain hormones used to promote flowering and budding in plants.
The intensity of light a plant receives is important for proper development. Plants' requirements for light intensity varies, but, generally, the more intense the light, the better the plant grows.
The Gardener's Supply Company reports that duration of light exposure is critical for proper plant development. All plants require certain levels of dark periods so they can respirate.
The sun is an ideal light source for all plants because it produces incredibly intense white light, which contains all colors in the visible light spectrum. Choose metal halide lightbulbs to promote vegetative growth because of their blue-white light. High-pressure sodium lightbulbs emit orange-red light to promote flowering.