Two relatively simple measures exist for improving the energy efficiency of existing fluorescent fixtures: the addition of reflectors and the cleaning of existing louvers and lenses.
Reflectors are specially shaped retrofittable metal sheets designed to improve the efficiency and light distribution of conventional white-painted, ceiling-mounted fluorescent downlight fixtures (Figure 1). With higher reflectivity and more directional control than the white paint on many existing fixtures, reflectors can significantly decrease the internal losses of fixtures and improve light distribution.
Well-designed reflectors improve fixture efficiency enough to allow some delamping. Although manufacturers frequently claim that the use of reflectors will allow 50 percent delamping with little or no reduction in the fixture's light output, that goal is difficult to achieve. In fact, it is less a function of how good the reflector is than how bad the existing fixture is. In any application, delamping should be considered carefully, bearing in mind that it may also be possible to delamp without a reflector.
Nevertheless, because of the large population of existing 2 x 4 fixtures with four 4-foot lamps, removing two of these lamps is a common proposal. This can be a realistic goal if one or more of the following conditions are met:
The accumulation of dirt causes fixture output to decrease over time (for more information, see the O&M Advisor “Lighting: Light Output Declines with Time”). To reduce degradation from dirt accumulation, it is important to periodically clean lenses, louvers, and other fixture surfaces. The ideal time to clean is when doing group relamping (see “Lighting: Spot Versus Group Relamping”). Manufacturer’s recommendations should be followed to avoid damaging the surfaces. In general, the glass can be cleaned with most nonabrasive cleaners, but it must be rinsed and wiped dry; aluminum can be cleaned with mild soaps and a thorough rinse; detergents and glass cleaners can be used on porcelain-enamel surfaces; detergents can be used on synthetic enamel, but alcohol-based and abrasive cleaners could damage those surfaces. Plastic surfaces should be vacuumed dry after cleaning—wiping creates electrostatic charges, and drip-drying results in streaking.
In addition, fixture lenses get dirty and discolor with age, and the timing and choice of a replacement can alter light output by more than 20 percent. Failure to keep lenses clean can cut output by another 10 percent. A new clear acrylic lens transmits over 92 percent of a fixture's light, as shown in Figure 2, while an opalescent (also called "milky white") diffuser may allow less than 50 percent of light to pass through it.