Also known as the spectral composition or spectral energy distribution (SED). It refers to the composition of light rays that is used by chlorophyll to create energy and growth within the plant. Wave lengths used in plant biology are ultraviolet, visible light and infrared.
Via Colorado State University’s Garden Notes:
Light quality is a major consideration for indoor growing.
-Fluorescent cool white lamps are high in the blue range, and the best choice for starting seeds indoors.
-For flowering plants that need more red light, use broad spectrum fluorescent bulbs.
-Incandescent lights are high in red and red-orange, but generally produce too much heat for use in supplementing plant growth.
“Of that range, blue light measures 420 to 440 nanometers, on the shorter side of the lengths of visible light rays.” Blue light encourages, when all other needs are met, luscious green growth. This can assist with strong healthy plant development.
“Red light occurs at one extreme of the spectrum of visible light. Plants are capable of detecting different forms of red light that provide them with clues about their environment. Red light occurs in sunlight, and when plants detect red light, they know that they are in the sun. Far red light occurs in the shade, when seeds are lightly covered with soil and at twilight. Perception of far-red light indicates that plants are growing in the shade or in another environment with limited access to full sun.”
Light intensity for various situations
Short day plants flower in response to long periods of night darkness. Examples include poinsettias, Christmas cactus, chrysanthemums, and single-crop strawberries.
-Long day plants flower in response to short periods of night darkness. Examples include onions and spinach.
-Day neutral plants flower without regard to the length of the night, but typically flower earlier and more profusely under long daylight regimes. Day neutral strawberries provide summer long harvesting (except during heat extremes)..