T5 - A short phrase, which has sparked much debate recently, but what are the facts?

What does T5 actually mean?
The 5 is obviously relevant, and being as so much has been spoken about how much better these are than average fluorescent tubes, then surely it must refer to an aspect of the light which is five times greater - probably the strength of its output, or maybe its effective distance, or expected lifespan.

This is the understandable, but incorrect conclusion that many people draw. What the 5 actually refers to is an aspect of the tube that your fish, or invertebrate livestock, could not care less about - its diameter.

Technically, any fluorescent tube should be referred to by the letter T followed by a number. That number indicates the diameter in 8ths of an inch, so the one inch diameter tubes that most of us are familiar with are T8 (eight 8ths of an inch), and the fatter tubes that used to be the standard, and that are still used by some are T12.
So T5 actually only means a light with a diameter of 5/8s of an inch.

So why all the fuss?
Small 12" long, 8Watt tubes have been on the market for a long time - these are only 5/8 of an inch diameter, but they don't appear to be any more powerful than a regular tube. The reason being that these are regular output T5 lamps - when talking about the new type of lamps, we should strictly speaking refer to them as high output T5's, and it is the high output part that really makes the difference - producing a light of much greater strength than a conventional tube, and as a result the energy input into the aquarium is greater.

Also, light diminishes very rapidly as it travels through water, but with T5 lamps the effective distance is significantly greater than with conventional tubes, meaning that deeper aquariums can be maintained with an illumination that is effective to the base.