Microwave sulfur light tubes were developed in 1995. They are still being perfected but they were investegated by NASA (or so I read) and determined to be the best light source for indoor growing. LEDs came in second.. NASA subcontract scientist said.. well read it for yourself:

"What we've found basically is that we are able to limit the amount of colour we give to the plants and still have them grow as well as with white light." said the research scientist Greg Goins of Dynamac Corp. LEDs are not the only ones efficient for growing plants: sulphur microwave lamps are the most efficient light sources known to man, that can generate as much light as the noonday sun, perfect for illuminating large-scale systems such as greenhouses. For smaller applications, such as indoor gardens, LEDs seem to be the right choice.

Basically, for those who wont look any further into it via the links I will put at the bottom of this message, the sulfer microwave light NEVER loses lumens over the "life" of the bulb (which basically lasts forever) and the only thing that would ever need replacing in it are cheap and readily available parts like you would find in a houshold microwave. Their spectrum averages out at 6000k but they are finding ways to modify the spectrum.

For those who like the numbers:

As an example, we quote the large hall of the National Air and Space Museum in Washington. By means of three lighting systems, each consisting of one Fusion Light Driveā?¢ 1000 watt lamp and one Light Pipe about 30 metres in length, a lighting concept has been produced that supplies about four times the original lighting level. Total power installed is now approx. 5 kilowatts. The original 94 high-pressure mercury lamps required an input of 29 kilowatts. The result is a significant decrease in power required and a considerably improved lighting system from the technical point of view.

So lets start the debate

http://en.wikipedia.org/wiki/Sulfur_lamp
http://www.3m.com/intl/uk/english/ma.../techniek.html