Lighting Research Center Lighting Research Center
    Volume 6 Issue 1
July 2002    
How do T5 lamps perform in open and closed luminaires?

Because the performance of T5 and T8 lamps varies as a function of ambient temperature, as Figure 10-1 on the previous page shows, T5 and T8 lamps may perform differently in luminaires in which the temperatures are different. The best applications for T5 lamps may differ from those for T8 lamps.

T5 and T5 high output (T5 HO) lamps, designed to produce maximum light output at 35°C (95°F), can take advantage of the heat that often builds up over 25°C (77°F) in compact enclosed luminaires. In such an enclosed luminaire, T5 lamps function better than T8 lamps. In an open luminaire, on the other hand, ventilation may keep the inside temperature lower than 35°C (95°F), and T8 lamps may perform better (see the CASE STUDY Case Study).

How does the difference in optimal temperature affect T5 luminaire photometry?

The 10°C (50°F) difference in optimal temperature between T5 and other fluorescent lamps causes a problem when dealing with photometric data, such as luminaire efficiency. American National Standards Institute/Illuminating Engineering Society of North America (ANSI/IESNA) standards require an ambient temperature of 25°C (77°F) for photometry. The efficiency of T5 luminaires is determined by dividing the lumen output of the bare lamps by the lumen output from luminaires. The bare lamps would be operating in an ambient temperature of 25°C (77°F) while the ambient temperature in the luminaire could be much higher. This higher temperature causes the lamps to have a higher light output. This procedure often leads to luminaire efficiency values greater than 100%.

In relative photometry, the lumen output from a bare lamp is scaled to rated-lumens provided by the manufacturer using a conversion factor to make the luminaire data independent of the particular lamp used in the test. In T5 and T5 high output (T5 HO) photometric testing, however, the rated-lumen value is obtained at 35°C (95°F), while the bare lamp is tested at 25°C (77°F). Therefore, in order to obtain accurate luminaire efficiency or optical efficiency, the bare lamp lumens and the rated lumens must be at the same temperature.

Table 10-2 shows light outputs of T5 lamps at 25°C (77°F) and 35°C (95°F) provided by a lamp manufacturer. These data may be useful in understanding the range of light outputs until a better solution of photometry procedures for T5 systems is established. As seen in the table, light output at 25°C (77°F) is 10 to 12% lower than it is at 35°C (95°F).

Table 10-2. Comparison of Light Output Data Between 25°C and 35°C
(These data are based on information from the Philips Lighting Company's online catalog: SILHOUETTE T5, T5 HO, & T5 Circular Fluorescent Lamp Technology Guide)

    Light Output

Lamp Type Lamp Power
25°C A
35°C B
Ratio A/B

T5 14 1,200 1,350 89
T5 21 1,900 2,100 90
T5 28 2,600 2,900 90
T5 35 3,300 3,650 90
T5 HO 24 1,750 2,000 88
T5 HO 39 3,100 3,500 89
T5 HO 54 4,450 5,000 89
T5 HO 80 6,150 7,000 88

Do T5 lamps allow for more optical efficiency of luminaires?

There is no way to answer this question, unless better photometry procedures for T5 luminaires can be established. Luminaire efficiency values in manufacturers' catalogs are unlikely to provide useful information to answer this question because American National Standards Institute/Illuminating Engineering Society of North America (ANSI/IESNA) photometry standards, which were established for conventional fluorescent lamp luminaires, overestimate efficiency for T5 luminaires as discussed in How long do T5 lamps last before they burn out? When, however, lighting designers attempt to use one-lamp indirect luminaires with T5 high output (T5 HO) lamps in place of two-lamp indirect luminaires with T8 lamps (one T5 HO lamp has nearly identical light output to two T8 lamps), they often notice that one-lamp luminaires with T5 HOs provide more uniform luminaire efficiencyilluminance distributions on the ceiling and floor. This uniformity is because the use of luminaires with fewer T5 HO lamps and the smaller diameter of the T5 lamps result in wider and more uniform beam patterns. Luminaires using one T5 HO lamp may make better use of the light from the lamps than luminaires using two T8 lamps and therefore may assist lighting designers in creating more efficient lighting designs (CASE STUDY Case Study).

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