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High-output T5 lamps

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Over the years, lamps have become more compact, more efficient, and more powerful in terms of lumen output. Since more light is emitted from a smaller surface, luminaire designers have been able to increase the efficiency of their luminaires. But increased lamp surface brightness - so-called luminance, measured in candela (cd) per square meter - is not without complications. It may cause discomfort glare.

Although a clear sky can have a luminance of above 10 000 cd/m_2, we can spend time outside without sunglasses. Since the sky is a large, bright field that fills much of our peripheral vision, our eyes can still adapt. (This is because our eyes - to a certain extent - are able to adapt to large, bright fields that fill much of our peripheral vision, such as the sky.) Although our ability to adapt is incredibly great, there are limits to it. For example, a lamp that is much less bright than the sky will, even in the peripheral vision, cause discomfort glare if the luminance of the area surrounding the lamp is much less than that of the lamp.

FROM T16 TO T5
With the early fluorescent lamps glare was not much of a problem, even if they were hanging bare from the ceiling. The old T16 lamps (16 eighths of an inch, i.e., 2 inches or ~50 mm in diameter) had a luminance of only ~3 500 candela/m_2. Moreover, they often lit up part of the ceiling, thereby reducing the contrasts. For the next generation of fluorescent lamps, T12, the luminance was 7-8 000 cd/m_2, and glare started to become a problem. When T8 lamps were introduced, their luminance of 11-14 000 cd became a great issue, even in the periphery of the visual field, and with the standard T5 lamps, luminaires must be designed with extreme care to avoid discomfort glare.

While all the standard T5 lamps (14, 21, 28, and 35-W) have the same luminance (surface brightness) of 17 000 cd/m_2, the luminance of the new T5 high-output lamps varies from 23 000 to 37 000 cd/m_2. Except for the brightest lamp, this is still less than the luminance of compact fluorescent lamps, which is typically 32-42 000 cd/m_2.

The new high-output lamps are less efficient than the standard T5 lamps while, at the same time, being even brighter. What, then, are the potential benefits?

Taking the lamp that seems to have the most market potential in standard luminaires, the 1 500 mm/49-W lamp resembles the 35-W standard lamp the closest. It is slightly less efficient (91 vs. 96 lm/W including ballast). However, the economy lies in the possibility to reduce the number of luminaires or to use one-lamp luminaires instead of two-lamp luminaires. A one-lamp luminaire can be built with higher optical efficiency than a two-lamp luminaire. Thus the lower efficiency can be off-set by higher optical efficiency.

80-W vs 55-W CFL
The other 1 500-mm T5 high output lamp is very different. It is marked at 80W/7 500 lumen and has a brightness of 37 000 candela, 2.5 times as bright as the brightest T8 lamp. This lamp will probably be used mostly in display lighting, floodlights, wall washers, and luminaires for indirect lighting. The lamp produces 87 lm/W (incl. ballast) but since it emits a great amount of light per unit length (5 lm/mm) it is possible to design luminaires that are optically very efficient.

On the other hand, the performance of this lamp would be inferior to that of a 55-W, two-tube compact fluorescent lamp in many of the applications described above. The 55-W lamp has a lumen package of 4 800 lumen, is only 533 mm, and is almost as efficient. Although its optical efficiency suffers because it contains two tubes, this may be of little importance since the lamp can support very compact luminaires. In addition, it may also be possible to use more cost-effective luminaires since the amount of material can be reduced.

Nils Borg