High-bay applications—such as factories, warehouses, big box stores and gyms—continue to be the hottest lighting retrofit markets. Switching from probe-start metal halide to fluorescent fixtures can generate up to 50 percent energy cost savings, while providing other advantages of fluorescent lighting, such as instant-on and restrike (making additional savings with lighting controls possible), improved color qualities, and emergency reballasting options. High-bay fluorescent fixtures are available from a range of manufacturers, such as Cooper, Day-Brite, Holophane, Hubbell, Lithonia, Orion, Ruud and others. Most manufacturers offer T5HO or T8 lamping options, which is a key area for decision makers.

The technologies

T5 lamps, sometimes called “skinny tubes” in the field, are five-eighths of an inch in diameter, thereby improving photo-optic control of fixture light output and efficiency. The T5HO is the high-output version. A 4-foot, 54W T5HO lamp is rated at 5,000 lumens. Examples include the Philips Silhouette, GE Starcoat Ecolux and Sylvania Pentron. T5HO lamps can be paired with programmed-start or instant-start electronic ballasts, although the major lamp manufacturers offer limited or no warranties for their T5HO lamps on instant-start ballasts. Energy-saving T5HO lamps, such as the 51W, 5,000-lumen GE Starcoat Watt-Miser Ecolux lamp, have recently entered the market.

T8 lamps used in high-bay applications often are high-performance T8 or “Super T8” lamps—32W T8 lamps that produce 3,100-plus lumens of light output and offer high lumen maintenance. Examples include the Sylvania XPS, GE High Lumen Eco and Philips Advantage.

To maximize light output for high-bay applications, high-performance T8 lamps are typically paired with high-ballast-factor ballasts. These are typically instant-start, although programmed-start ballasts that optimize lamp life in frequently switched applications are now available. Also, recently introduced, extra-long-life T8 lamps are available.

If you are looking for maximum light output and efficiency, ensure the lamp is high performance, uses an efficient ballast, and the fixture is designed specifically for T8 lamps and has a high-performance reflector. When using high-performance T8 lamps, don’t confuse them with standard T8 lamps, as they look the same and have the same wattage.


Suppose we want to upgrade a 400W probe-start metal halide lighting system to fluorescent. While a number of suitable configurations are available, let’s consider two retrofit options: a four-lamp T5HO fixture with basic-grade lamping and ballasting, and a six-lamp, high-performance T8 fixture with basic-grade 1.18-BF ballasting. For brevity, we will compare these options based on three simple criteria: how much light is produced, how much energy is saved and which system is more efficacious.

If applied correctly, T5HO and T8 fixtures can be used in both high- and low-bay applications, although at lower mounting heights one must take care to avoid “glare bombs” with visible T5HO lamps.

At 40 percent of life, the four-T5HO lamp ballast system produces about 80 percent of the light output, and the six-T8 lamp-system produces about 90 percent of the light output of the 400W metal halide lamp ballast. But this tells only part of the story. We must factor in fixture efficiency: A basic-grade, high-bay metal halide spun aluminum dome fixture can have a fixture efficiency as low as 70 percent, which means up to 30 percent of the lamp-ballast system’s light output is trapped in the fixture and wasted. A fluorescent high bay with a high-performance reflector, meanwhile, can offer a fixture efficiency as high as 91 to 92 percent. Adding in this factor to our example, the four-T5HO option produces about the same mean light output as the 400W metal halide, while the six-T8 option produces about 10 percent more.

Further consideration, however, should be given to thermal effects on light output. T5HO lamps produce peak light output at an optimal 95°F, higher than the high-performance T8 lamp’s 77°F. If ambient temperature is a concern, consider higher case temperature-rated ballasts and fixtures designed to offer good thermal management—e.g., Lithonia’s I-Beam, Holophane’s POLAR technology, etc.

Another option specific to T5HO is the recently introduced amalgam lamp, such as Sylvania’s Pentron C, which promises stable light output across wide temperature extremes—at least 90 percent of rated light output at temperatures as low as 40°F to as high as 165°F.

The four-T5HO and six-T8 options produce high energy savings at about 50 percent, with six-T8 edging out four-T5HO at 222W versus 234W. Efficiency is where the T8 option really shines. The T8 lamp-ballast system operates at a 15 percent higher mean efficacy than the T5HO lamp-ballast system—94 lumens per watt compared to 81 lumens per watt.

So based on these basic attributes, can we determine which is better: T5HO or T8? Calling to mind lighting designer Howard Brandston’s statement that “there are no bad products, only bad applications,” conversely, there are no perfect light sources, only light sources that may be ideal for a given project based on its particular design criteria.

Thanks to Stan Walerczyk, LC, principal of Lighting Wizards, for his assistance.  EC

DiLOUIE, a lighting industry journalist, analyst and marketing consultant, is principal of ZING Communications. He can be reached at www.zinginc.com.