Lighting: Boosting Lighting Efficiency with Reflectors and Maintenance
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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
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 and can widen or narrow light distribution. This fixture also shows a small-cell paracube louver, which reduces glare but absorbs a lot of light.
Source: MetalOptics
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 space is measurably overlit, or a reduction in light level will not be noticeable.
- The existing fixture efficiency is extremely poor and cannot be corrected by cleaning or lens replacement.
- A proposed reflector will provide better light distribution, allowing more uniform illumination at a lower light level.
- Replacement of the lamp and ballast combination increases the average light output per lamp.
- A better lens, diffuser, or louver is also installed.
- Existing lamps were very close together, raising their average temperature while shading each other.
Cleaning Lenses and Louvers
All 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.
Old and dirty lenses can absorb as much as one-third of the light that tries to pass through them. In this case, a brief cleaning reduced losses from 29 percent to 19 percent (relative to a new K-12 lens).
Source: Data from measurements taken at Columbia University, on various lenses taken from McVickar School of Social Work, NYC, 1993
Copyright 2006 - Platts, a Division of The McGraw-Hill Companies, Inc.