The T5 fluorescent lamp is the first linear fluorescent lamp type to operate exclusively with an electronic ballast. It is smaller, both in length and diameter, than T8 and T12 lamps, and utilizes a miniature bi-pin base. The T5 fluorescent lamp maintains a very high lumens-per-watt efficiency. A single T5HO (high output) lamp will product approximately 5,000 initial lumens as compared to 2,800 to 3,000 lumens from a single T8 lamp. The T5 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. This barrier coating reduces the amount of mercury needed from approximately 15 mg to 3 mg per lamp. 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 5 represents the diameter of the lamp in eighths of an inch.
The T5 lamp provides peak light output at 95 degrees F air temperature. The T8 and the T12 lamps provide peak light output at a 77 degrees F ambient air temperature. The T5 lamp has a higher lumens-per-watt efficiency than a T8 lamp of about the same wattage. T5 lamps are a popular energy-efficiency measure, due to their potential to cut energy use in lighting by more than 60%.
T5 lamps are approximately 40% smaller in diameter than T8 lamps and almost 60% smaller than T12 lamps. T5HO lamps are approximately 46-inches in length as compared to T8 and T12 lamps which are 48-inches in length. T5 lamps have a miniature bi-pin base, while T8 and T12 lamps use a medium bi-pin base. These differences in dimension prevent T5 lamps from being used as replacements for T8 and T12 lamps, unless the existing fixtures are electronically converted, via T5 retrofit conversion, to high frequency operation, so that they can accept the T5 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 T8 or T12 fluorescent lamp, where the cold spot is in the middle, in T5 lamps it is at the end, on the metallic cap, about 2 mm from the glass envelope.