Green Building Library

Shedding light on the new tubular fluorescents

The 32-watt T8 lamp is becoming the standard for new construction and as a retrofit replacement for 40-watt T12 fluorescent lamps. All major lamp manufacturers market T8 lamps, and they are readily available in a variety of straight and U-shaped configurations through standard distribution channels. Also available from some manufacturers are T10 fluorescent lamps, a high-wattage, high-lumen alternative to T12 lamps, and most recently, European T5 technology requiring new metric lumenaires. T5 lighting is currently the most energy efficient system available for indoor lighting.

What does “T8” mean?

The “T” designation in fluorescent lamp nomenclature stands for tubular; the shape of the lamp. The number immediately following the T gives the diameter of the lamp in eighths of an inch. A T12 lamp is therefore twelve-eighths of an inch, or one-and-one-half inches in diameter. A T8 lamp is eight-eighths of an inch, or one inch in diameter.

What are the color characteristics of T8 and T5 lamps?

T8 and T5 lamps achieve both improved color rendering and high efficacy by employing rare-earth phosphors. The correlated color temperature (CCT) and color rendering index (CRI) of the lamps is controlled by varying the selection of phosphors. Like T12 lamps, T8 lamps are available in a variety of color temperatures, including warm (3000 K), neutral (3500 K), cool (4100 K), and very cool (5000 K). “Full Spectrum” lamps are typically 5000K or higher, and emit 10 to 20 percent of their output in the form of UV light. The CRI of T8 and T5 lamps can be specified from 70 to as high as the mid-90’s. Every lamp manufacturer has a product coding system denoting CRI, which may require a catalog to decipher. For great lighting quality, specify a CRI of at least 80.

Do T8 lamps require a special ballast or fixture?

T8 lamps require an electronic ballast specifically designed to operate lamps at a lower current than T12 lamps. Whenever T12 lamps are replaced with T8 lamps, the ballast must also be replaced. Install an electronic ballast for optimum light quality and efficiency; electronic ballasts don’t flicker or hum, and they use less power! T12, T10, and T8 lamps can all use the medium bi-pin base, which allows T8, and T10 lamps to fit into the same luminaires as T12 lamps of the same length. T5 lamps have a different base, and are shorter than T8s, so new lumenaires are needed.

Electronic ballasts operate fluorescent lamps at much higher frequencies (20KHz and up) than the standard 60 Hz at which magnetic ballasts operate lamps. Because of the common .88 ballast factor, many electronic ballasts provide a slight reduction in light output while using significantly less power compared to magnetic ballasts, which generally have a ballast factor of .94. Electronic ballasts with higher ballast factors (as high as 1.2) produce more light. Electronic ballasts offer lots of advantages, such as no flicker, less heat, much less noise, and the ability to operate as many as four lamps on a single ballast; some offer dimming, soft start, and better power quality characteristics as well.

How long do T8 lamps last?

T8 lamps have the same 20,000-hour rated lamp life as standard T12 lamps. Frequent on/off cycles can reduce fluorescent lamp life. Using programmed start or dimming ballasts can increase lamp life to as much as 30,000 hours. T8 lamps also exhibit a slower decline in light output over time, relative to T12 lamps. At 40 percent of their rated life, standard T12 lamps only produce about 80 percent of their initial rated light output, compared to about 90 percent for T8 lamps.

What are the savings from using T8 lamps?

T8 lamps used with electronic ballasts will typically use about 32 percent less energy than the same lumenaires with T12 and magnetic ballasts. T5 lamps and electronic ballasts will use about 45 percent less energy than the T12 setup. Additional savings are possible if a lighting re-design indicates the use of reflectors and delamping, or fewer lumenaires. “ECO” T8s also use little or no mercury inside the lamp, saving on disposal and environmental costs. Longer lamp life and less lumen depreciation means lower maintenance costs. Worker productivity increases with light quality improvements.

Copyright Alan Van Zuuk