T5 high output (T5 HO) lamps have greater luminous flux per unit length than standard T8 lamps, but the light output of standard T5 lamps per unit length is almost identical to the standard T8 lamps. If, however, this question requires a more precise answer and lamp efficacy needs to be taken into account, several factors affecting the light output-and therefore efficacyshould be considered.

One of the most significant factors affecting lamp light output is ambient temperature. Table 1-7 summarizes light outputs and efficacies of nominal four-foot standard T5, T5 HO, and standard T8 lamps. Manufacturers usually provide light output data at the optimal temperature for each lamp type in their catalogs: 35C (95F) for the standard T5 and T5 HO lamps and 25C (77F) for the T8 lamps. Light output data for T5 lamps at 25C (77F) and for T8 lamps at 35C (95F) are also available from manufacturers, however. Table 1-7 shows that, at 25C (77F), the light output of T8 lamps is higher than the standard T5 lamps. At 35C (95F), however, the standard T5's light output is higher than that of the T8 lamp. With respect to lamp efficacy, the standard T5 lamps are more efficacious than the standard T8 lamps at 35C (95F), but the two types of lamps are nearly identical at 25C (77F). The lamp efficacy of T5 HO lamps is higher than the T8 lamps but lower than the standard T5 lamps at 35C (95F), and it is the lowest at 25C (77F).

Ballast losses are another important factor. Table 1-7 compares lamp-ballast system efficacies for T5 and T8 systems from two ballast manufacturers. The data on lamp efficacy are quoted from manufacturers' catalogs (Philips Lighting 2001/2002; OSRAM SYLVANIA 2002). Although the lamp-ballast system efficacy varies among manufacturers, the maximum system efficacy at the optimal temperature for each of the three lamp-ballast systems (standard T5, T5 HO, and T8 systems) appears to be nearly identical on average.