The individual reflectors utilize the light from the backside of the bulb and throw it into the tank without bouncing any of the light back into the bulb(called restrike I believe), therefore making them the most efficient reflectors available. Approx 100% of the light from a T5 with individual reflectors makes it to the tank. I am sure there is some loss due to the lack of a perfect surface in the reflectors.
What makes individual reflectors so effective in the T5 application is the size (small diameter) of the bulbs where larger flourescents would need much larger reflectors to do the same thing, limiting the number of lights we could fit over standard tank sizes.
HTH
