Lighting Research Center Lighting Research Center
    Volume 9 Issue 2
July 2006    
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. high-intensity discharge (HID) - An electric lamp that produces light directly from an arc discharge under high pressure. Metal halide, high-pressure sodium, and mercury vapor are types of HID lamps. capacitor - A device used in electric circuitry to temporarily store electrical charge in the form of an electrostatic field. In lighting, a capacitor is used to smooth out alternating current from the power supply. restrike time - The time required for a lamp to restrike, or start, and to return to 90% of its stabilized light output after the lamp is extinguished. Normally, HID lamps need to cool before they can be restarted. high-pressure sodium (HPS) - A high-intensity discharge lamp type that uses sodium under high pressure as the primary light-producing element. HPS lamps produce light with a correlated color temperature (CCT) of approximately 2000 kelvins, although CCTs for lamps having higher CRI values range from 2200 to 2700 kelvins. Standard lamps have a CRI value of 22; others have CRI values from 60 to 80. HPS lamps are among the most efficacious light sources, with efficacies as high as 150 lumens per watt, although those with higher CRI values have efficacies as low as 25 lumens per watt. lamp life - The median life span of a very large number of lamps (also known as the average rated life). Half of the lamps in a sample are likely to fail before the rated lamp life, and half are likely to survive beyond the rated lamp life. For discharge light sources, such as fluorescent and HID lamps, lamp life depends on the number of starts and the duration of the operating cycle each time the lamp is started. 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. efficacy - The ratio of the light output of a lamp (lumens) to its active power (watts), expressed as lumens per watt. illuminance - The amount of light (luminous flux) incident on a surface area. Illuminance is measured in footcandles (lumens/square foot) or lux (lumens/square meter). One footcandle equals 10.76 lux, although for convenience 10 lux commonly is used as the equivalent. mercury vapor (MV) lamp - A high-intensity discharge lamp type that uses mercury as the primary light-producing element. Mercury vapor lamps produce light with a CCT from 3000 to 7000 K. Mercury vapor lamps with clear outer bulbs have CRI values from 15 to 25, whereas phosphor-coated lamps have CRI values from 40 to 55. Mercury vapor lamps are less efficacious than other HID lamp types, typically producing only 30 to 65 LPW, but they have longer lamp lives and lower initial costs than other HID lamp types. metal halide (MH) lamp - A high-intensity discharge lamp type that uses mercury and several halide additives as light-producing elements. Metal halide lamps have better color properties than other HID lamp types because the different additives produce more visible wavelengths, resulting in a more complete spectrum. Metal halide lamps are available with CCTs from 2300 to 5400 K and with CRI values from 60 to 93. Efficacies of metal halide lamps typically range from 75 to 125 LPW. operating position - The manufacturer-recommended operating position for a lamp. power - The power used by a device to produce useful work (also called input power or active power). In lighting, it is the system input power for a lamp and ballast or driver combination. Power is typically reported in the SI units of watts. glare - The sensation produced by luminances within the visual field that are sufficiently greater than the luminance to which the eyes are adapted, which causes annoyance, discomfort, or loss in visual performance and visibility. 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. ambient temperature - The temperature of the surrounding air that comes into contact with the lamp and ballast. Ambient temperature affects the light output and active power of fluorescent lamp/ballast systems. Each fluorescent lamp-ballast system has an optimum ambient temperature at which it produces maximum light output. Higher or lower temperatures reduce light output. For purposes of lamp/ballast tests, ambient temperature is measured at a point no more than 1 meter (3.3 feet) from the lamp and at the same height as the lamp. rated lamp life - The number of hours at which half of a group of product samples fail. The rated life is a median value of life expectancy; any lamp or group of lamps may vary from the published rated life. Rated life is based on standard test conditions. 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. Kelvin - Color temperature is measured in degrees Kelvin, which indicathe hue of a specific type of light source. Higher temperatures indicate whiter, "cooler" colors, while lower temperatures indicate yellower, "warmer" colors. high-wattage compact fluorescent lamp - Abbreviated as HW-CFL, sometimes called "high lumen CFLs", these lamps are a larger cousin to regular CFLs, usually much larger in size and with higher wattages and light output. minimum bulb wall temperature (MBWT) - The temperature of the coldest spot on a lamp’s bulb wall. MBWT is determined by the ambient temperature, the heat generated within the luminaire, and the luminaire’s heat dissipation effectiveness. The coldest spot on a lamp wall is where the mercury vapor tends to condense because pressure is lowest there. amalgam - An alloy of mercury with other metals. Some CFLs use a mercury amalgam rather than standard mercury. An amalgam keeps mercury pressure in the discharge near its optimal value as lamp temperature changes. Amalgam lamps can produce more than 90 percent of maximum light output over a wide temperature range, but they can take longer to reach their full light output when started.
How is performance affected by ambient temperature?

Performance characteristics of high-wattage compact fluorescent lamps (HW-CFL) will be affected by lamp base position and by operation in cold or hot environments. Temperature impacts both short-term and long-term performance. In the short-term, temperature impacts HW-CFL light output, electric power, and luminous efficacy, with most lamps operating at their peak in an ambient temperature of about 77°F (25°C). Over long periods of time, elevated temperatures may shorten ballast life and consequently reduce lamp life in self-ballasted HW-CFLs.

Short-term thermal effects

Operating fluorescent lamps in cold or hot environments will lead to reduced light output, reduced power, and reduced efficacy, as shown in Figure 9. (Note: This graph is not specific to HW-CFLs.)

Figure 9. Impact of ambient temperature on power, light output, and efficacy of many fluorescent lamps

            °F = (°C x 1.8) + 32
Adapted from: Bleeker and Veenstra, 1990

Some manufacturers of HW-CFLs report information about light output at a range of temperatures (see How does lamp orientation affect performance of HW-CFLs?). If this information is not available, users can generalize from other curves showing fluorescent output for minimum bulb wall temperature or ambient temperature, such as Figure 9.

As shown in Figure 10, reduction in light output can be minimized with the use of a mercury amalgam. (Note: This graph is not specific to HW-CFLs.) Use of mercury amalgam with HW-CFLs is often not reported in lamp catalog information. However, users can request this information from the lamp manufacturer. NLPIP obtained this information for products on the market (at the time of publication) and listed the results under “Mercury Type” in Table 7 and Table 8 (see What HW-CFL products are available?).

Figure 10. Impact of ambient temperature on light output of amalgam and nonamalgam fluorescent lamps

°F = (°C x 1.8) + 32
Rea MS (ed.). 2000. IESNA Handbook, ninth edition. 6-43.
New York: Illuminating Engineering Society of North America.

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