What is the light output of 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.

The amount of light generated by 32-watt T8 fluorescent lamps varies considerably from one lamp model to the next. Figure 1 shows the range of values for rated initial light output that are presently published for 121 lamp models by nine manufacturers for different correlated color temperatures (CCT). The number of available models for each rated initial light output is represented by the size of the bubble. For example, at a CCT of 4100 K, there are nine models rated at 2950 initial lumens.

Figure 1. Rated initial light output of T8 fluorescent lamps*

The color rendering index (CRI) values of the T8 models shown in Figure 1 are separated into three categories: 70-79, 80-89, and greater than 90. These three categories are commonly referred to by the industry as RE70, RE80, and RE90, respectively. The category into which each lamp model's CRI value falls is indicated by the color of the bubble. For example, at a CCT of 3500 K, there are five RE70 models available with a rated initial light output of 2800 lumens and eight models available with a rated initial light output of 2850 lumens. Figure 1 shows that RE70 models tend to have lower light output than RE80 models. It is also apparent that more RE80 models are available than RE70 or RE90 models, and their rated light output values vary over a much wider range.

As illustrated by the data provided in Figure 1, T8 lamp models are available with light output values at almost every 50-lumen increment between 2800 lumens and 3200 lumens. However, uncertainties in light output measurements can range from 1 to 2% (roughly 50 lumens) across manufacturers, based on proficiency testing as part of the National Voluntary Laboratory Accreditation Program (NVLAP). This program is administered by the National Institute of Standards and Technology (NIST).

NLPIP measured the initial light output of 12 T8 lamp models with CCTs of 3500 K and 4100 K, which are the most common CCTs sold. Three samples of each model were operated on a low-frequency reference ballast following the procedure described by the American National Standards Institute (ANSI C82.3-2002). All lamps tested had rated CRI values between 80 and 89 (RE80), and some lamps had additional enhancements of high light output (HLO) and long life (LL). For details on testing protocols, see Appendix A: Test methods.

Overall, the RE80 lamps tested produced lower light output than their rated values, while most of the RE80 HLO, LL lamps tested were near their rated values. In addition, the initial light output of the RE80 HLO, LL lamps averaged 8% higher than the RE80 lamps.

Figure 2 shows the rated and measured light output values, including error bars to show the combined uncertainty of the measured values for each lamp model. The combined uncertainty includes the random variability between lamp samples and NLPIP's measurement uncertainty of 1.5% relative to NIST. The measured light output values of the RE80 lamps were, on average, 2.7% lower than their rated values. The measured values for five of the six RE80 HLO, LL lamps were near their rated values. The remaining RE80 HLO, LL model was, on average, 2.5% below its rated value.

Figure 2. Light output of T8 lamps measured by NLPIP

T8 fluorescent lamps are available with a variety of rated light output values. Limited testing by NLPIP found differences between rated and measured light output values. Even if rated and measured light output values were the same, the uncertainties of approximately 1 to 2% would blur the differences between models with similar values of rated light output.