Lighting Research Center Lighting Research Center
    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. 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. 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.
How well do T8 fluorescent lamps maintain light output?

All lamps exhibit some reduction in light output over time. Manufacturers report light output at 40% of rated life as an indicator of maintained light output. This value is close to the average light output over the life of the lamp and is called mean light output, mean lumens, or design lumens.

Figure 9 provides an overview of lumen maintenance values for T8 fluorescent lamps. Lumen maintenance is the rated mean light output as a percentage of rated initial light output. Figure 9 reports lumen maintenance for 86 presently available T8 fluorescent models from five manufacturers; data of rated mean light output were not available from the remaining four manufacturers. The data are separated by correlated color temperature (CCT), with the number of available models indicated by bubble size and labeled next to each bubble. In addition, the colors of the bubbles indicate the range of color rendering index (CRI) values to which the models belong. According to manufacturers' data, all T8 models have lumen maintenance over 90%, with three-quarters of the models having lumen maintenance between 94.5% and 95.5%.

Figure 9. Lumen maintenance of T8 fluorescent lamps*

T8 fluorescent lamp models are available with a variety of initial and mean light output ratings. Figure 10 shows a comparison of the rated values of initial and mean light output of 4100 K T8 models, as reported by manufacturers. The range bars in Figure 10 represent the maximum and minimum rated values. With few exceptions, the lumen maintenance of all the models represented in Figure 10 was approximately 95%. However, the range bars show the large variety of rated initial and rated mean light output values that produce the same approximate lumen maintenance percentages. In some cases, models are available with mean light output values that are the same as or higher than the initial light output values of other models.

Figure 10. Light output of 4100 K T8 fluorescent lamps

An important consideration of T8 light output is illustrated in Figure 10 by grouping the data on the horizontal axis by average rated life. Many of the lamp models represented in Figure 10 have the same initial and mean light output values, but different life ratings. Therefore, mean light output is measured at different operating times. For the life ratings used in Figure 10, the maximum difference in operating time is 4,000 hours, as shown in the legend. In some cases, lamps are expected to reach their mean light output value at 8,000 hours (1.8 years at 12 hours of operation per day), while others are expected to reach the same mean light output value at 12,000 hours (2.7 years of operation at 12 hours of operation per day).

Another important issue to consider is that the average rated life of a lamp is dependent on the type of ballast used to operate the lamp (see What is the life of T8 fluorescent lamps?). It is unknown how a shorter lamp life will affect lumen maintenance. If ballasts other than those recommended by the lamp manufacturers are used, specifiers should inquire about the impact that those choices may have on mean light output.


Previous
Previous
© 2006 Rensselaer Polytechnic Institute. All rights reserved. Next Next


Rensselaer Polytechnic Institute
LRC Intranet Web mail Lighting Research Center