'Blue for veg; red for flower' There appears to be widespread misunderstanding on this issue, so let's see if we can clear this up. First off let us remember there is only ONE SUN. We do not have two suns. Does the solar spectrum change from spring to fall? Of course the sun itself does not change according to earth's seasons, but what of atmospheric filtering? The changes are so small as to be ignored. So the obvious answer is that with at least one light source, two different spectra are not necessary for plants to germinate, grow and fruit. Well then, how and why did the two light system come about? HPS spectra is less than optimal, but it has the highest photon flux density of any man-made light source (plasma may have recently surpassed HPS). 1. Seedlings and young plants do not need the blast of light that HPS provides, so why waste power when a lower wattage light (CFL, T5, MH) is more than adequate? 2. Cannabis needs adequate blue light to prevent stretching and for optimal leaf growth. HPS is generally very weak in blue, thus an alternate source is chosen for the vegetation stage. MH and fluoro both have sufficient blue to meet a young plant's needs. Does more blue equal more vegetative growth and more red equal better flowering? No, if the minumum is met more is not better. For example if 20% blue keeps the internodes short during veg, twice as much blue will not make even shorter nodes. In the same way increasing the amount of red required for flowering will not necessarily equal bigger flowers. The proper ratio is fine for the entire cycle. For example, the whole cool (bluish - not blue) CFL/T5 for veg; warm (redish - not red) for flower is total myth. Warm fluoros contain plenty of blue to meet all of the plants vegetative needs and can keep them short as is evidenced by this beauty vegged under a single warm 2700K 23w CFL: So the answer is: If a man-made light is properly designed then only one lamp is necessary.