In recent years, architectural design has accentuated and made popular the idea of the ‘high-bay’ space. Areas such as gymnasiums, indoor pool facilities, libraries, and cafeterias have all seen their interior ceiling heights pushed higher and higher. As ceiling heights have increased, so has the need for higher output lamps to light them. Until recently, designers utilized metal halide (MH) lamps for these high-bay applications because they provide a high output source, with good optical control, from a very small size lamp.
Despite these photometric advantages, MH lamps come with significant operational constraints. MH lamps, for example, do not reach their full lighting output upon starting. Instead, they can require up to 15 minutes in order to reach 90% of their potential output. In addition, when an MH lamp loses power for even a moment, it will go out and will not light again until it has had a chance to cool down. This ‘restrike’ time can last between 5 and 15 minutes for standard MH lamps and 1 to 4 minutes for MH lamps that use the newer ‘pulse-start’ technologies. Further, current technology now allows MH lamps to be dimmed, but the efficiency of an MH lamp drops severely when dimmed. Also, MH lamps have only a moderate ability to accurately render the color of objects, limiting their use in spaces where color determination is essential.
“As ceiling heights have increased, so has the need for higher output lamps to light them.”
The recent arrival of the T5-HO (high output) fluorescent lamp, in combination with new efficient light fixture designs specific to this lamp, has provided an alternative to the traditional MH lamps used in spaces with high ceilings. Manufacturers are now offering fluorescent high-bay luminaires that address each of the aforementioned metal halide deficiencies. For instance, fluorescent electronic ballasts allow for instant start of the luminaire, require no warm-up time, and can provide energy-efficient, full range dimming. Further, these new luminaires utilize internal reflector designs that provide multiple lamps in one fixture, permitting multiple level switching configurations and both direct (down) and indirect (up) distribution of light. Next, manufacturers have improved the coatings used on the inside of the T5-HO lamp, providing more pleasing color tones and color rendering. Finally, these lamps can provide significant energy savings. A typical T5-HO high-bay light fixture with six 54 watt lamps provides the same light output as a 400 watt metal halide fixture while using 21% less electricity. Combining this with dimming and multiple level switching means that additional energy savings can be realized as daily use is tailored to an Owner’s specific needs.
Because these fluorescent lamps and light fixtures are still new to the industry, they may not be suitable for all high-bay applications. Nonetheless, they are welcome tools that are allowing designers to meet the increasing demands of energy-efficient building design and Owners to realize substantial energy savings and to increase flexibility in their facilities.
– Eric S. Nielsen, PE