Energy Source Builder

Fluorescents Get Skinny on Energy Diet

Although you may not have noticed, full-size fluorescent lamps have gotten skinnier. No it's not a fitness fad at Philips, General Electric or Osram Sylvania. The smaller fluorescent lamps have many advantages, including greater energy efficiency and better color rendition than standard lamps. They are now common in commercial construction projects. You should expect to see more of them in residential buildings, too.

The federal Energy Policy Act forbids the manufacture of standard 4 ft., cool white lamps after October of 1995. So, they will gradually disappear, leaving the market to more efficient lamps, such as T8s.

T-What?

In the trade, a standard fluorescent lamp is called a T12. "T" stands for the "tubular" shape. The number gives the diameter of the tube in eighths of an inch. So a T12 is 12/8 or 1-1/2 in. The new skinny lamps--called T8s--are only 1 in. Both lamps have the same medium bi-pin base, so a T8 will fit in a fixture designed for a T12 lamp. However, manufacturers are designing sleeker new fixtures that take advantage of the T8s smaller size. T8s come in all the more popular sizes, such as 2, 3, 4, 5 and 8 feet.

Ballasts

Standard T12s require a ballast that provides 430 milliamps (mA), while the T8s need a 265 mA ballast. Magnetic ballasts that drive T8s are available. However, T8s are more efficient with electronic ballasts that operate the lamp at a high frequency. With T12s, one ballast operates no more than two lamps. With T8s, one ballast can operate up to 4 lamps.

Energy Savings

The smaller size is one reason that T8 lamps are more efficient. The typical 4 ft. T12 lamp uses 40 watts, while its replacement draws only 32 watts. That's 20 percent less power, although light output is somewhat lower. When you take into account the energy use of lamps and ballasts compared to the amount of usable light, the overall savings of T8s with electronic ballasts can approach 30 to 40 percent.

Light Output

If you compare the lamp specifications, you'll see that T8s provide slightly less light--5-10 percent less--than the T12s they replace. The actual difference for installed lamps may be smaller for several reasons. First, T8s generate less waste heat, which keeps the fixture temperature closer to optimum. Second, the smaller diameter of T8s allows more light to escape from fixtures. Third, the light output of a T8 drops less over its life than a T12. (Both T8s and T12s have an average service life of about 20,000 hours.)

Color

In a fluorescent lamp, phosphors that coat the inside wall of the lamp create the light you see. T8s use a newer type of coating called "rare-earth" phosphors, which are partly responsible for their greater efficiency. Rare-earth phosphors also give much better color rendition than the older coatings. The Color Rendering Index (CRI) for a typical T12 cool white lamp is about 62, although T12s are available with CRIs up to 85. Most T8s range between 70 and 85. (Higher CRI numbers mean objects look more "natural" under the light. For a complete explanation of fluorescent lamp color, see the Oct. 1992 issue.)

Applications

T8s cost slightly more than the standard fluorescent lamps. In new construction projects, the extra cost of a T8 system could be justified. Better color rendition and smaller size make T8s much easier to fit into an interior design. Some good locations are under cabinets, in wall-mounted brackets or valances and in bathrooms. When you decide where to place fluorescent lights, remember to prevent uncomfortable glare by installing them so people can't see the lamp itself. This is especially true of T8s, because they have a brighter surface than T12s.

A primary source of information for this article was Lighting Answers a serial publication of the National Lighting Product Information Program at the Lighting Research Center at Rensselear Polytechnic Institute. For ordering information call 518-276-8716 and ask for Publications.

Manufacturers have adopted a new color designation for lamps with rare-earth phosphors. The system groups lamps by CRI. Lamps with CRIs between 70 and 79 would be called RE70, while those between 80 and 89 would be RE80. The Correlated Color Temperature can be added to the designation, too. For example, an RE70 with CCT of 4100K would be RE741.

This article appeared in Energy Source Builder #29 October 1993 ©Copyright 1993 Iris Communications, Inc.