The T8 light has replaced the T12 as the new standard in fluorescent lighting, which in turn is the cheaper alternative to incandescent lamps. The number that comes with the T is used to denote the diameter of the fluorescent tube. Since the measurements come in eights of an inch, the T8 has an inch of diameter while a T12 comes in 1.5 inches. The narrower your choice of a fluorescent lamp is, the more efficient its energy output will be. If you are going to replace a T12 lamp with a T8, you will have considerable savings in energy which could be as high as 40%.
Brightness and Compatibility
You may not be able to distinguish the light output difference between a T8 and a T12 fluorescent lamp visually. Even though they are of comparable wattage, the T8 uses less power than a T12 lamp. There are also no additional expenses that you will incur if you are going to switch to T8 since it is compatible with the fixture that is used for a T12. This includes the ballast which is the transformer that moderates the electricity going to the lamp.
At present, there is still no noticeable change in the price of these two types of lamps but it will change as soon as the T12 is phased out by its manufacturers. This is because T12 technology is now considered to be outdated.