Light quality, also called spectral composition and spectral energy distribution (SED), refers to the composition of light as to wavelengths that are effective in photosynthesis and other plant growth and development processes.
The wavelengths of light are expressed in small units of distance such as micron (μ) or micrometer (μm), nanometer (nm), and angstrom (Å).
One micron is one-millionth of a meter (10-6 meter); 1 nanometer is one-billionth of a meter (10-9 meter) or one one-thousandth of a micron, and 1 angstrom is one ten-billionth of a meter (10-10 meter) or one ten-thousandth of a micron.
The wavelengths with primary importance in photobiology are ultraviolet (UV), visible light, and infrared (IR) (Hopkins 1999).
According to Devlin (1975), wavelengths between 300 nm to 900 nm are capable of affecting plant growth.
However, it is not light quality alone that affects plant growth processes.
Other properties of light including light intensity and light duration, as well as other climatic factors, are also involved.