Intensity

The following are selected quotes indicating what effects color of light (spectrum) and quantity of light have on plant growth. Karen Randall - Aquatic Gardeners Association states:

Fluorescent Bulbs used for aquatic plant growth

Light requirement is sometimes expressed by the fairly awful measure watts per gallon. At less than 1 watt per gallon you are restricted to only a few kinds of plants with low light requirements. 2 watts per gallon will grow most common plants. For plants with high light requirements or really heavy planting where the plants seriously shade each other, you should have 3-4 watts per gallon.

The whole business of lighting and planted tanks is very much a 90/10 thing - you can get 90% of the results with 10% of the time/money/trouble, and the other 10% requires the other 90%. So don't be afraid to start out with a simple and cheap set up and only upgrade if you can't get satisfactory results.

Offered by Mike.