T5 Fluorescent Lamps

T5 fluorescent lamps are thinner, more efficient, and offer a higher intensity of light output than T8 lamps ( Table 1 ). The numerical designation refers to the diameter of the lamp in eighths of an inch—so the T5 lamp is five-eighths inch in diameter, compared to 1 inch for T8s and 1-1/2 inches for T12s. The narrower profile means that the lamps provide designers with better optical control and better fixture efficiency.

Table 1: T5 and T8 lamp characteristics

Standard T5 lamps offer slightly higher efficacy than T8 lamps and similar output. High-output T5 lamps (T5HO) offer slightly lower efficacy but pack a higher lumen output, and a single one can often serve where a pair of T8s would be needed.

Note:
Data are for a nominal 4-foot lamp.
Source: Platts; manufacturers' data

In addition, the lamps are designed to provide maximum light output at an ambient temperature of 95° Fahrenheit (F) rather than the 77°F design point for most other lamps. Those characteristics allow the use of the lamps in more compact fixtures than would otherwise be possible, but also mean that steps must be taken to keep the lamps warm in colder environments, such as unheated warehouses. This characteristic also means that T5 fixtures may appear to have an efficiency of greater than 100 percent—bare lamps are tested at 77°F, but the lamp may experience higher temperatures in the fixture and therefore put out more light than it does at the lower temperature.

T5s are often applied in low-profile fixtures, such as those used for cove lighting and illuminating display cases. Indirect and indirect/direct fixtures also often feature T5 lamps—the thinness and high intensity of the lamps enable designers to place fixtures farther apart and closer to the ceiling than is possible with T8 lamps ( Figure 1 ).

Figure 1: T5 lamps increase options for indirect lighting The high intensity and small size of T5 lamps enable their use in smaller fixtures that can be spaced closer to the ceiling, in rooms with lower ceilings, and farther apart than fixtures using T8 lamps. Note: These numbers are approximate, as different designers use different standards; actual figures will depend on ceiling reflectivity and on the proximity of other lamps. For example, higher ceilings allow wider spacing.

Manufacturers offer two types of T5 lamps: standard output and high output (HO). The HO versions put out almost twice as much light as a T8 lamp of the same length, and can therefore cut in half the number of fixtures required in a given space. However, some designers warn that HO lamps may be too bright, and care must taken to avoid glare and the creation of hot spots on the ceiling. T5HO lamps are also less energy-efficient than standard T5s. T5s offer an efficacy of about 104 lumens/watt, whereas high-output T5s come in at about 90 lumens/watt—similar to the efficacy of a T8 lamp.

Both T5 and T5HO lamps offer better lumen-maintenance performance than T8 lamps. The T5 lamps retain about 95 percent of their output after 40 percent of their rated life, compared with less than 90 percent for T8 lamps.

T5 lamps are not appropriate for most retrofit projects because they are available only in metric lengths and are slightly shorter than their T8 counterparts ( Table 2 ). However, some fixtures can be made to accept either T5 or T8 lamps by changing the sockets and ballasts. T5s also feature mini-bipin bases and run only on electronic ballasts. T5 lamps are currently more expensive than T8 lamps, but that could change as the manufacturing volume increases. The T5 lamp price could eventually fall below that for T8s because their small size means that they inherently use less manufacturing material, particularly the expensive phosphors that line the tube. The smaller size is an advantage at the end of lamp life as well, since T5s contain less mercury and less glass than T8 lamps.

Table 2: T5s come in non-standard lengths

Source: Platts; manufacturers' data

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Last Modified: May 7, 2004