What is the life of T8 fluorescent lamps?

Volume 9 Issue 1	June 2006

average rated life - The number of hours at which half of a large group of product samples fail under standard test conditions. Rated life is a median value; any lamp or group of lamps may vary from the published rated life. ballast factor (BF) - The ratio of the light output of a fluorescent lamp or lamps operated on a ballast to the light output of the lamp(s) operated on a standard (reference) ballast. Ballast factor depends on both the ballast and the lamp type; a single ballast can have several ballast factors depending on lamp type. color rendering index (CRI) - A rating index commonly used to represent how well a light source renders the colors of objects that it illuminates. For a CRI value of 100, the maximum value, the colors of objects can be expected to be seen as they would appear under an incandescent or daylight spectrum of the same correlated color temperature (CCT). Sources with CRI values less than 50 are generally regarded as rendering colors poorly, that is, colors may appear unnatural. rapid start - A method of starting fluorescent lamps in which the electrodes are heated prior to starting, using a starter that is an integral part of the ballast. Heating the electrodes before starting the lamps reduces the voltage required to strike the electric arc between the electrodes. A rapid-start system starts smoothly, without flashing. initial light output - A lamp's light output, in lumens, after 100 hours of seasoning. instant start - A method of starting fluorescent lamps in which the voltage that is applied across the electrodes to strike the electric arc is up to twice as high as it is with other starting methods. The higher voltage is necessary because the electrodes are not heated prior to starting. This method starts the lamps without flashing. It is more energy efficient than rapid or preheat starting, but results in greater wear on the electrodes during starting. The life of instant-start lamps that are switched on and off frequently may be reduced by as much as 25 percent relative to rapid-start operation. However, for longer burning cycles (such as 12 hours per start), there may be no difference in lamp life for different starting methods. correlated color temperature (CCT) - A specification for white light sources used to describe the dominant color tone along the dimension from warm (yellows and reds) to cool (blue). Lamps with a CCT rating below 3200 K are usually considered warm sources, whereas those with a CCT above 4000 K usually considered cool in appearance. Temperatures in between are considered neutral in appearance. Technically, CCT extends the practice of using temperature, in kelvins (K), for specifying the spectrum of light sources other than blackbody radiators. Incandescent lamps and daylight closely approximate the spectra of black body radiators at different temperatures and can be designated by the corresponding temperature of a blackbody radiator. The spectra of fluorescent and LED sources, however, differ substantially from black body radiators yet they can have a color appearance similar to a blackbody radiator of a particular temperature as given by CCT. mean light output - Light output typically evaluated at 40% of rated lamp life. In combination with initial light output, mean light output may be used to estimate lamp lumen depreciation. lamp efficacy - The ratio of the light output of a lamp (lumens) to its active power (watts), expressed as lumens per watt (LPW). color rendering - A general expression for the effect of a light source on the color appearance of objects in conscious or subconscious comparison with their color appearance under a reference light source. system efficacy - Also referred to as relative system efficacy, system efficacy is a measurement of a system'’s ability to convert electricity into light. Measured in lumens per watt (LPW), system efficacy is the ratio of the light output (in lumens) to the active power (in watts). lumen maintenance - The ability of a lamp to retain its light output over time. Greater lumen maintenance means a lamp will remain brighter longer. The opposite of lumen maintenance is lumen depreciation, which represents the reduction of lumen output over time. Lamp lumen depreciation factor (LLD) is commonly used as a multiplier to the initial lumen rating in illuminance calculations to compensate for the lumen depreciation. The LLD factor is a dimensionless value between 0 and 1. spectral power distribution (SPD) - A representation of the radiant power emitted by a light source as a function of wavelength. chromaticity - The dominant or complementary wavelength and purity aspects of the color taken together, or of the aspects specified by the chromaticity coordinates of the color taken together. It describes the properties of light related to hue and saturation, but not luminance (brightness). full-spectrum color index (FSCI) - A mathematical transformation of full-spectrum index into a zero to 100 scale, where the resulting values are directly comparable to color rendering index. An equal energy spectrum is defined as having an FSCI value of 100, a “standard warm white” fluorescent lamp has an FSCI value of 50, and a monochromatic light source (e.g., low pressure sodium) has an FSCI value of 0. gamut area - A measure of color rendering based upon volume in color space. It is the range of colors achievable on a given color reproduction medium (or present in an image on that medium) under a given set of viewing conditions. CIE - Abbreviated as CIE from its French title Commission Internationale de l'Eclairage, the International Commission on Illumination is a technical, scientific, and cultural organization devoted to international cooperation and exchange of information among its member countries on matters relating to the science and art of lighting. standard deviation - A measure of the average distance of a set of data points from their mean. A set of data points that are all close to their mean will have a smaller standard deviation than a set of points that are further from their mean. RE90 - Designation referring to lamps that use rare-earth phosphors and have color-rendering index values equal to or greater than 90. RE80 HLO, LL - An RE80 lamp with additional enhancements of high light output (HLO) and/or long life (LL). RE80 - Designation referring to lamps that use rare-earth phosphors and have color-rendering index values of 80-89. MacAdam ellipse - Researcher David L. MacAdam showed that a just noticeable difference (JND) in the colors of two lights placed side-by-side was about three times the standard deviation associated with making color matches between a reference light and a test light (MacAdam 1942, Wyszecki and Stiles 1982). These JNDs form an elliptical pattern of "constant discriminability" in a chromaticity space, centered on the chromaticity of a reference light, known as MacAdam ellipse. RE70 - Designation referring to lamps that use rare-earth phosphors and have color-rendering index values of 70-79. combined uncertainty - Combined uncertainty is calculated by finding the sum of the squares of sample random variability (standard deviation) and laboratory measurement uncertainty and taking the square root of that sum.

Average rated life is the number of hours at which half of a large sample of lamps has failed, which is the median life of the group. The standard operating cycle for this test is 3 hours on, 20 minutes off. The Illuminating Engineering Society of North America (IESNA) defines this procedure in IESNA Approved Method for Life Testing of Fluorescent Lamps (IESNA LM-40-01).

Figure 6 shows the range of average rated life reported by manufacturers for T8 lamp models of different correlated color temperatures (CCT). The size of the bubble represents the number of lamp models available at each of the three rated life values and five CCTs (the actual number of models is shown next to each bubble). The colors of the bubbles represent the color rendering index (CRI) of the lamps: RE70, RE80, and RE90. The distributions are nearly the same for the first three CCTs: eight to 10 lamp models are offered with a life rating of 20,000 hours, nine to 13 models have a life rating of 24,000 hours, and nine models have a life rating of 30,000 hours.

Figure 6. Average rated life of T8 fluorescent lamps*

Specifiers should be aware of several issues concerning the life ratings found in lamp catalogs. First, operating cycles have a large effect on lamp life. The standard operating cycle defined by IESNA of 3 hours on, 20 minutes off provides a common basis to compare results among laboratories performing the same life tests; however, it does not address the wide range of operating cycles that exist in practice. Some manufacturers have addressed this issue by reporting life ratings for operating cycles of 12 hours per start in addition to life ratings for the standard operating cycle.

Figure 7 demonstrates the effect of operating cycle on lamp life. The vertical axis displays relative lamp life, with 100% representing the rated lamp life for the standard operating cycle. Figure 7 shows that for a typical 8-9 hour workday in which lamps are operated continuously, median lamp life may be double the rating reported in the lamp catalogs.

Figure 7. Effect of operating cycle on lamp life

Source: adapted from Vorlander 1950

A second issue that affects average rated life is the type of ballast used to operate the lamps. The life rating for many lamp models is based on operation using rapid-start ballasts. For some of these models, the catalog footnotes indicate reductions in rated life by as much as 25% when the lamps are operated using instant-start ballasts. In addition, some manufacturers report that the life rating for certain models is based on operation using a specific type of the manufacturer's ballast. When this ballast is not used to operate the lamps, the life rating is reported to be as much as 50% lower, depending on the type of ballast circuitry used. Information about the dependency of lamp life on the choice of ballast is sometimes located only in footnotes and other fine print, so it is important to examine manufacturers' publications carefully.

A third issue to consider is that the rate at which lamps fail has an impact on relamping strategies and, therefore, overall costs. Using a single number, such as rated life, in a cost analysis does not account for differences in failure rates. For a group relamping strategy based on replacing lamps when a certain percentage have failed, the rate of lamp failure directly affects the amount of time before relamping.

Figure 8 shows the results of a previous study conducted by NLPIP for two T8 fluorescent lamp models rated at 20,000 hours. The standard deviation for each model provides an estimate of the failure rate. For models with the same average life, a smaller standard deviation (shown as a steep slope) means that most lamp failures will occur closer together and closer to average life compared to models with a larger standard deviation (shown as a shallow slope). For example, model 1 has a smaller standard deviation than model 2 because individual failures are concentrated closer to average life. This means that there is less variability in lamp life for model 1. For typical group replacement strategies model 1 provides more operating time before incurring replacement costs.

Figure 8. Mortality curves for two T8 fluorescent lamp models

The standard deviation, along with a given average life, provides a way to estimate individual lamp failures. For example, consider two T8 fluorescent lamp models with an actual life of 24,000 hours and standard deviations (failure rates) similar to models 1 and 2 in Figure 8. For an office space with 100 lamps, the first lamp failures would be expected to occur before 17,300 hours for model 1 and 11,800 hours for model 2. The twentieth lamp failure, which might be the scheduled point for group relamping, would be expected to occur before 21,600 hours for model 1 and 19,600 hours for model 2. Such differences in failure rate will have a considerable impact on group relamping costs.

The many factors affecting lamp life make it the lamp attribute with the most uncertainty. Once the effects of operating cycle and ballast choice on lamp life are accounted for, knowledge of a model's standard deviation can be used to estimate individual lamp failures. If manufacturers provided standard deviations, specifiers could make better estimates of actual lamp life to include in lighting cost calculations.