What are the power requirements for 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 measure of the degree of color shift that objects undergo when illuminated by a lamp, compared with those same objects when illuminated by a reference source of comparable correlated color temperature (CCT). A CRI of 100 represents the maximum value. A lower CRI value indicates that some colors may appear unnatural when illuminated by the lamp. Incandescent lamps have a CRI above 95. The cool white fluorescent lamp has a CRI of 62; fluorescent lamps containing rare-earth phosphors are available with CRI values of 80 and above. 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 of the apparent color of a light source relative to the color appearance of an ideal incandescent source held at a particular temperature and measured on the Kelvin (K) scale. The CCT rating for a lamp is a general indication of the warmth or coolness of its appearance. As CCT increases, the appearance of the source shifts from reddish white toward bluish white; therefore, the higher the color temperature, the cooler the color appearance. 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. 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 lumen 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. 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. 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. 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.

It may be expected that the electric power needed to operate a 32-watt T8 fluorescent lamp will be 32 watts; however, this designation is simply the nominal power of the lamp. The relevant American National Standards Institute (ANSI) document states that the rated lamp power is 32.5 watts under standard test conditions (ANSI C78.81-2005). ANSI further indicates that average lamp power shall not exceed 34.6 watts, which is 6.5% above the rated value. Since this upper limit applies to the average value of the power needed to operate 32-watt lamps, it is possible for individual lamps to exceed 34.6 watts. Since specifiers may select lamps based on efficacy, the variation in lamp power could make calculations of lamp efficacy more difficult without knowledge of the actual power and the associated uncertainty for a specific lamp model.

ANSI considers the 4-foot, 32-watt T8 lamp to be a rapid-start lamp, but it is commonly operated using an instant-start circuit in which the two pins on each end of the lamp are electrically connected or shunted together. Lamps operated on instant-start circuits have lower power than those operated on rapid-start circuits because there is no electrode heating with instant-start operation. However, the difference in lamp power between instant-start and rapid-start operation is not equal to the electrode heating power because the lamp discharge efficiency is lower for instant-start operation.

NLPIP investigated lamp power differences among lamp models. NLPIP selected lamps designated as 32-watt lamps, operated them on a low-frequency reference ballast per ANSI standard C82.3-2002, and measured the electric power needed to operate the lamps. The lamps had correlated color temperatures (CCT) of 3500 K and 4100 K, which are the most common CCTs sold. Three samples of each lamp model were measured.

Figure 3 shows the measured power values. The horizontal axis shows the description of each lamp model tested from manufacturers A, B, and C. Error bars show the combined uncertainty of the measured values for each lamp model. All measured lamp power values were higher than the ANSI-rated power of 32.5 watts. Five of the lamp models exceeded 33.5 watts, which is 3% above the ANSI-rated value. However, this is within the tolerance described in the ANSI standard (lamp power not to exceed 5% plus 0.5 watts). Specifiers should consider the fact that lamp power can exceed the expected value of 32 watts by more than 5% in any estimation of operating costs.

Figure 3. Power required by T8 lamps measured by NLPIP

In most cases, the measured electric power values for the RE80 HLO, LL lamps were higher than the values for the RE80 lamps, by as much as 1.2 watts. In Question: What is the light output of T8 fluorescent lamps?, NLPIP showed that the light output of the RE80 HLO, LL lamps averaged 8% higher than the RE80 lamps. Therefore, replacing RE80 lamps with RE80 HLO, LL lamps with no change in luminaire layout or ballast factor will result in more light output and higher power. Customers who are mostly interested in energy savings should change to a lower ballast factor or change their luminaire layout by using fewer luminaires.

Today, T8 fluorescent lamps are commonly used with high-frequency electronic ballasts. High-frequency operation of fluorescent lamps reduces lamp power for the same light output (Campbell et al. 1953). As stated in the ANSI standard (ANSI C78.81-2005), the lamp power for high-frequency operation is approximately 6% lower than for low-frequency operation, when the lamp is operated as an instant-start lamp. Nevertheless, unknowingly operating a lamp with higher-than-nominal power on a high-frequency electronic ballast will result in an increase in the lighting system's connected load, leading to higher operating costs than expected. For example, the reported input power for a typical two-lamp, high-frequency electronic ballast with a normal ballast factor (0.88) is 58 watts. However, the ballast input power will increase by 3-4% for lamps with 5% higher power than expected (as measured on a low-frequency reference ballast). Specifiers should include the variation in connected load in estimations of operating costs.