How efficacious are T8 fluorescent lamps?

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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 (JD) 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.

Lamp efficacy is determined by dividing light output by lamp power, resulting in units of lumens per watt (LPW). Determining and comparing lamp efficacy from catalog data is difficult because the actual power needed to operate the lamps is not known (see What are the power requirements of T8 fluorescent lamps?) and because rated light output is given for operation of the lamp at a low frequency of 60 Hz. (T8 lamps are almost always operated on high-frequency electronic ballasts.)

A general perspective on the range of T8 lamp efficacy can be gained by dividing the published light output values by some reference power value. For example, NLPIP selected a lamp power of 32.5 watts, which is the standard value listed for this lamp type (ANSI C78.81-2005). Figure 4 shows these efficacy values, which range from 81 to 99 LPW, for 121 T8 fluorescent lamp models from nine manufacturers. It is likely that the upper limit of this range would decrease slightly if it were based on actual power measurements, which were slightly higher than 32.5 watts in this limited study (Figure 3). Nevertheless, the RE70 models tended to be at the low end of the range with all of their efficacies below 88 LPW. Figure 4 also shows lower efficacies for some of the RE80 models at the higher correlated color temperatures (CCT) of 5000 K and 6500 K.

Figure 4. Efficacy of T8 fluorescent lamps*

Figure 5 shows the calculated and measured efficacies of the lamps that NLPIP tested. Calculated efficacies were derived by dividing the published light output values by 32.5 watts. NLPIP-measured efficacies were derived from measured light output and power values. The measured efficacies include error bars, which show the combined uncertainty of the measured values for each lamp model. The measured efficacies ranged from 86 to 94 LPW, compared to the calculated efficacies that ranged from 91 to 97 LPW. Both RE80 and RE80 HLO, LL lamps had measured efficacies that were an average of 3.5% lower than the calculated values. This result is due to measured light output values that were lower than the rated values for RE80 lamps and measured lamp power that was higher than 32.5 watts for all lamps.

Figure 5. Efficacy of lamps tested by NLPIP

The efficacies of the RE80 HLO, LL lamps averaged 5.8% higher than the RE80 lamps. As noted in the previous Questions: What is the light output of T8 fluorescent lamps? and What are the power requirements of T8 fluorescent lamps?, both the light output and the input power values of the RE80 HLO, LL lamps were higher than the values for the RE80 lamps. However, the light output values were sufficiently higher to counteract the effect of higher power. The increased efficacies of the RE80 HLO, LL lamps may make them a more attractive choice for new construction or for retrofits in which a ballast change or a luminaire layout change is being made. With a lower ballast factor or a luminaire layout change, customers can take advantage of the higher light output of RE80 HLO, LL lamps; they may be able to meet their illuminance requirements and save energy compared to RE80 lamps.

NLPIP testing was conducted using a low-frequency reference ballast as described in American National Standards for Lamp Ballasts-Reference Ballasts for Fluorescent Lamps (ANSI C82.3-2002). T8 lamp efficacy increases by approximately 10% under high-frequency operation (Hitchcock 1983). For more details on testing protocols, see Appendix A: Test methods.