Measure 9.1.2.1 Substitute Screw-in Fluorescent Lamps for Incandescent Lamps.

How to Select Compact Fluorescent Lamps

Replacing incandescent lamps with compact fluorescent lamps is a simple operation, but you need to be careful in selecting the substitute lamps. The following are the important criteria.

Lumen Output

Start by finding out whether the illumination levels provided by the present lighting match the requirements of the activities. Calculate any increases or reductions in the lumen output that are appropriate. Then, search the catalogs to find lamps that provide the output you need.

Screw-in fluorescent lamps are available in a range of sizes to replace the most common incandescent bulb sizes. See Figure 3. The output of the fluorescent units was somewhat limited at first, but output continues to be increased.

Experience indicates that manufacturers exaggerate the effective light output of compact fluorescent fixtures. For example, if a compact fluorescent lamp promises "as much light as a 75-watt bulb," a person probably will not be able to see as well as with the original 75-watt bulb. This may be due to differences in color rendering, or to exaggerating the lumen output.

On the other hand, where fixtures are largely decorative, as with downlights in lobbies, compact fluorescent fixtures tend to produce an illusion of greater brightness than their lumen output suggests.

Light output deteriorates with age. Provide enough reserve capacity to provide adequate lighting at the end of the replacement cycle. The best compact fluorescent lamps lose about 20% of their light output at the end of normal service life.

Wattage Input and Efficacy

After you find the lamps that can provide the desired lumen levels, select among them for the lowest input wattage. In other words, select the highest efficacy.

Be skeptical of the efficacy figures published for compact fluorescent fixtures. Experience indicates that the actual efficacies of cheap compact fluorescent fixtures may be less than half that of better units. Reliable comparisons of the products of different manufacturers are not available, so it is reckless to acquire units from any but the best manufacturers. (We said "best," not "most popular." One of the largest vendors of compact fluorescent fixtures produces cheap junk.)

These are efficacies for compact fluorescent fixtures derived from the published figures of one major manufacturer:

Note that these efficacy figures include the effects of the ballast, whereas this is not true of ratings for conventional fluorescent lamps.

Reliability and Service Life

Some units presently on the market have poor reliability. Screw-in fluorescent lamps can be produced by marginal manufacturers, and units of poor quality can be made substantially cheaper than units of good quality. The market has not shaken out the poor products. On the contrary, because compact fluorescent lamps are perceived as expensive (compared to incandescent lamps), the lower cost of the poor quality units has allowed them to flood the market. The rated life of most compact fluorescent lamps is 10,000 hours. However, the rating does not guarantee quality. Some units of low quality last only a few hours or a few weeks.

Fig. 2 A good place to retrofit screw-in fluorescent lamps The ceiling of this lobby is a forest of downlights. Screw-in fluorescent lamps provide adequate illumination. Bare lamps provide the best efficiency in the existing reflector fixtures. Their appearance is satisfactory at this distance. Their longer service life greatly reduces the amount of labor required for maintenance.

Purchase compact fluorescent lamps only from manufacturers of good reputation. If you are going to install large numbers of lamps, make some calls to experienced users of compact fluorescent lamps to find which units are best.

If the unit allows the fluorescent tube to be replaced, the life of the ballast is rated separately. A life of 50,000 hours is commonly claimed for ballasts. Take such numbers with a grain of salt. Not enough time has gone by to allow such figures to be verified by field experience.

The life of a magnetic ballast declines with higher operating temperature. The operating temperature is increased if the unit is installed with the ballast above the lamp, and if the fixture is poorly ventilated. The life of an electronic ballast declines as the number of switching cycles increases.

Lamp Dimensions

You may have trouble finding models of compact fluorescent fixtures that fit many incandescent fixtures. Figures 1 and 3 show the problem. Compact fluorescent fixtures are all much wider at the base than incandescent lamps. Most are substantially longer, although shorter compact fluorescent tubes are entering the market. Manufacturers are achieving shorter lengths by bending the tubes into hairpin and corkscrew shapes.

Light Distribution Pattern

The light distribution pattern of the lamps affects the overall efficiency of lighting. For example, a bare compact fluorescent lamp radiates most of its energy in a direction perpendicular to the tube. If such a lamp is installed in a recessed downlight with an absorptive interior, most of the light is absorbed inside the fixture.

Compact fluorescent lamps are available with integral reflectors or diffusers to improve light distribution, as shown in Figures 1 and 4. For example, they are available with parabolic reflectors to minimize light loss when installed in downlights. The combination of greater light source efficiency and lower light losses can increase the overall efficiency of the fixture by a factor of 10 to 30 (that's not a misprint) when these lamps are installed in highly absorptive fixtures.

Fig. 3 A range of light outputs The maximum size of screw-in units with bare lamps continues to grow. Units are available that substitute for the most common incandescent lamp sizes. However, beware of exagerrated claims about "equivalent" light output.

Ability to Replace Fluorescent Tube Separately

The ballasts in screw-in fluorescent lamps may last several times longer than the tubes. Some units allow the lamp to be replaced separately from the ballast. For example, see Figure 4. Units that allow the tube to be replaced separately may have better long-term economics. This is illustrated by the economic example given below.

Color Characteristics

The color rendering of fluorescent lamps is different from the color rendering of incandescent lamps, and this may be important in some applications. Fluorescent lamps emit light strongly in a few narrow bands of color that are created by the mercury vapor in the lamps. These color spikes are superimposed on a broader light spectrum produced by the phosphors.

The perceived white color of the fluorescent lamp is not the result of a continuous light distribution spectrum. Instead, it is the result of selecting the phosphors to produce the illusion of white light. This causes the lamp to distort the colors of illuminated objects. Human vision corrects for this, so people do not notice it, except in certain color-critical applications.

For more about lamp color in general, see "Color Rendering Index" and "Lamp Color" in Reference Note 52, Comparative Light Source Characteristics. For more details of fluorescent lamp color, see Reference Note 55, Fluorescent Lighting.

Starting and Operating Temperatures

All fluorescent lamps, including compact units, operate at peak efficiency only if the glass tube is near a particular temperature. The tube temperature is determined by the environment of the lamp and by the lamp's heat output. The optimum temperature is about 105°F (40°C). The lamp usually starts at a lower temperature than this, and then warms up to a temperature that may be well below or well above the optimum temperature. If the space is kept at normal indoor temperature and the fixture is well ventilated, the lamp will stabilize near its optimum temperature. If the lamp is used outdoors, or if it is installed in a poorly vented ceiling fixture, its efficiency suffers seriously.

Fluorescent lamps require a minimum ambient temperature to start reliably. Compact fluorescent lamps have higher current densities, shorter arcs, preheat starting, and other features that allow them to operate at lower temperatures than conventional fluorescent lamps. Different models of compact fluorescent fixtures claim starting temperatures from -20°F to 32°F (-29°C to 0°C).

The product literature does not make it clear how much efficacy suffers at lower operating temperatures. Some manufacturers use an amalgam of mercury with another metal to stabilize the mercury vapor pressure inside the lamp. This keeps the lamp efficacy high over a wider range of temperature. A disadvantage of this method is that the lamp may require as much as a minute to reach full brightness as the mercury separates from the amalgam.

Screw-in fluorescent fixtures that use conventional circular lamps should be used only at normal indoor temperatures.

Acoustical Noise

Units of good quality emit little noise. Cheaper units may be noticeably noisy. Refer to Measure 9.2.4 for more about this.

Fig. 4 Reflector fixture with separately replaceable lamp The ballast may last several times longer than the lamp. A unit like this should be made of glass, rather than plastic, because the ultraviolet radiation from a fluorescent lamp will darken plastic.

Power Factor

Low power factor is a potential problem with magnetic ballasts, but generally not with electronic ballasts. Measure 9.2.4 gives the details.

Radiated Electromagnetic Noise

This is a potential problem with units that have electronic ballasts. For example, problems have been noted with remote control units and security scanners. Refer to Measure 9.2.4 for details.

Compact lamps are smaller and they draw less current than conventional fluorescent fixtures, so they have the potential of emitting less electromagnetic radiation. This does not mean that they actually make less noise. Cheaper units that lack adequate input filtering may cause the power wiring to act as an antenna, causing trouble at some distance from the lamps.

Harmonic Distortion

Harmonic distortion in compact fluorescent fixtures worries electric utilities, who have been using incentives and penalties to promote lower harmonic distortion. Units are commonly rated in terms of "total harmonic distortion" (THD). At present, a THD of 20% is considered acceptable. Some cheaper units may have a THD much worse than this. See Measure 9.2.4 for details.

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