Light that plants encounter in the environment originates mostly from the sun. The spectral properties of sunlight fundamentally reflect the surface temperature of the sun (∼5800 K). However, the solar spectrum is modified by the earth’s atmosphere due to absorption by water and CO2, and by scattering. Light can either be expressed in terms of an energy flux or a quantum flux. The energy content of a quanta depends on its wavelength, decreasing as wavelength increases. Energy flux (E, W m−2) can be calculated from quantum flux (I, mol m−2 s−1) for a given wavelength (λ, m) with the following equation: E = IN 0 hc/λ where N 0 is Avogadro’s number (6.02 × 1023), h is Planck’s constant (6.63 × 10−34 J s photon−1) and c is the speed of light (3 × 108 m s−1). For example, 1 µmol quanta m−2 s−1 of 400 nm light is equivalent to 0.3 W m−2, while for 700 nm light it is 0.17 W m−2.