In a planted aquarium, light is the most vital element for healthy plant growth. Plants use light as a source of energy - it is necessary for photosynthesis to occur. Having live plants actually improves water quality. Plants consume fish waste products, generate oxygen, and are the ideal way to obtain an almost self-sufficient ecosystem in which plants and fish happily co-exist.
There are two basic lighting factors which are essential for a luscious aquatic growth. They are 'quantity' and 'quality'. In lighting terms, quantity is usually expressed in watts or more accurately lumens. The higher that wattage the brighter the light - pretty simple. Light 'quality' is a little more complex since it is based on spectral output. The visible light spectrum is made up of different wavelengths or colours of light as can be seen in a rainbow. For best plant growth, the goal is to mimic sunlight as accurately as possible.
To determine the output 'quality' of a light, manufacturers assign two numbers to a light: colour temperature and colour rendering index (CRI). The colour temperature number is used to indicate the colour of light produced by the bulb, and is based on a physics phenomena known as 'black body radiation'. Picture a hot piece of iron in a blacksmith's shop - as it's heated, it starts to glow red, then orange, then yellow and so on - the hotter it gets the more white it's colour appears. Colour temperature is measured in Kelvin (K), and the colour of sunlight at the equator is about 5200 K, unfortunately colour temperature doesn't completely describe the quality of light, this is where the colour rendering index (CRI) comes in to play.
White light is composed of all colours, with each colour being of equal brightness. The CRI indicates who well this equal balance of all colours is matched by your light source. A CRI of 100 is perfect, any number less than this and the light will still appear white, but it may make objects look bluish or reddish for example. A CRI of 70 or more is probably fine for most aquariums.
So, now that all of that technical stuff is out of the way, the question is what does one use for decent planted tank lighting? My recommendation is fluorescent lighting - it's cheap, energy efficient, and will yield exceptional results if your tank is less than 18" deep. The lights should be on for about 10 to 12 hours per day, and you'll need about 2 - 4 watts per gallon with a colour temperature of approximately 5000K and a CRI of 70 or better. Unfortunately, sometimes it's difficult getting a light with this kind of colour spec., so a trick I've found that works quite well is to use two different tubes. You could, for example, use one 'plant light' which is pinkish in colour and has a colour temperature of about 3100K. The second bulb could be a 'daylight' tube which has a colour temp of about 6300K. The result is good plant lighting and a naturally coloured tank.
That's enough rambling for now. Next month will be a little more down to earth. I'll talk about some robust plant species and give you tips on substrate selection and planting.