The 32-watt T8 lamp has become the standard for new construction and as a retrofit replacement for 40-watt T12 fluorescent lamps. T8 fluorescent lamps are available in a variety of configurations. It is smaller in diameter than T12 lamps and can utilize a medium bi-pin base. The T8 fluorescent lamp maintains a very high lumens-per-watt efficiency. A single T8 lamp will product approximately 2,800 to 3,000 initial lumens. The T8 lamp has a low mercury content. The lamp has a coating on the inside of the glass wall that stops the glass and phosphors from absorbing mercury. Since mercury absorption causes the lamp’s light output to depreciate over its life, the coating helps to keep light levels much closer to initial output—only 5% depreciation in the first 40% of its life.
The "T" in the lamp’s name denotes the tubular shape of the lamp. The number 8 represents the diameter of the lamp in eighths of an inch.
The T8 lamp provides peak light output at a 77 degrees F ambient air temperature. The T8 has a higher lumens-per-watt efficiency than a T12 lamp of about the same wattage. T8 lamps are a popular energy-efficiency measure, due to their potential to cut energy use in lighting projects.
T8 lamps are almost 33% smaller in diameter than T12 lamps. T8 lamps are approximately 48-inches in length as compared to T5HO lamps which are 46-inches in length. T8’s have a medium bi-pin base, while T5HO lamps use a miniature bi-pin base. These differences in dimension prevent a T8 lamp from being used as replacements for T5HO lamps.
A fluorescent lamp’s "cold spot" is the area on the lamp where the temperature is at its lowest. This cold spot rises or falls along with the ambient temperature of the lamp. Unlike a T5HO fluorescent lamp, where the cold spot is at the end, in T8 lamps it is in the middle of the lamp.