The sun is a giant thermonuclear reactor. Each minute, more than 120 million metric tons of solar matter are converted to radiant energy – light and heat. 8 minutes later, about 2 billionths of this energy has traveled 160 million kilometers and hits the earth's upper atmosphere. About one third of the light hitting the atmosphere is reflected back into space, and only 1% of the incident light is harvested by photosynthesis.
Light is thought to consist of packets of energy called photons. The intensity of light depends on the number of photons (amount of energy) absorbed per unit of time. Each photon carries a fixed amount of energy that is determined by the photon's wave length, measured in nanometres (10-9m). We perceive the different wavelengths as different colours, the longer the wavelength, the less energy per photon. Thus red light contains less energy than blue light, but has a longer wavelength. White light is a mixture of wavelengths.
As light meets matter, it may be reflected, transmitted(changed and then emitted), or absorbed. Substances that absorb visible light are called pigments. When a molecule absorbs a photon, one of the molecule's electrons is elevated to a shell where it has more energy. The only photons absorbed are those whose energy exactly matches that required to elevated the electron. Thus, a particular compound absorbs only photons corresponding to specific wavelengths.
Fluorescents/CFL have nearly all of their light in the PAR (photosynthetically active radiation - light usable by plants), making them extremely efficient.However, as they use less power (energy) the light they produce does not penetrate very far meaning they are more suited for growing propagation, early vegetative growth and small or low light level plants. As these lights do not produce much wasted heat energy, they can be positioned close to the plants (up to 2-3 inches).
HID lights such as high pressure sodiums (HPS) and metal halides (M/H) produce a lot more light, but not all of this energy is usable (PAR). Most energy is wasted as heat, so the lamp has to be positioned further away from the delicate shoots. For 400W lamps the optimum distance is around 12"/30-50cm, depending on ambient temperature, humidity and air movement, giving an illuminated area 0.7-1m2. For 600W lamps the ideal height is in the range of 18"/40-60cm, with an area of 1.5-1.75m2.
Blue-end spectrum light inhibits stem elongation, by virtue of excluding red light, giving shorter individuals better suited for cultivation under cover/indoors. Most gardeners use these lights for propagation and vegetative growth. Red-end light will stimulate stem elongation, these are usually used by gardeners after vegetative growth has finnished for fruit, flower setting. As HPS lamps are naturally in the red-end they are more effectivley used for this stage of growth than the slightly less efficient M/H lamps.