How is performance affected by ambient temperature? (cont'd)

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Volume 9 Issue 2	July 2006

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. 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 initial 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 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. 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 (in watts) used by a device to produce useful work (also called input power or active power). In lighting, it is the system input power (in watts) for a lamp and ballast combination. When referred to as benchtop active power, the measurement procedure follows ANSI standards, which include horizontally mounted bare lamp(s) at an ambient temperature of 25°C, ą1°C, and air movement less than 5 feet per minute. The lamps are seasoned 100 hours before testing, and the measurements are conducted after lamp light output stabilizes. 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 lamps bulb wall. MBWT is determined by the ambient temperature, the heat generated within the luminaire, and the luminaires 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.

Long-term thermal effects

Ballast life can also be affected by elevated ambient temperatures. Electronic components in ballasts tend to suffer under elevated temperatures. The life of electrolytic capacitors, for example, is reduced dramatically with elevated temperatures (Stevens, Shaffer, Vandenham 2002). An industry rule-of-thumb states that for every 10-degree increase in temperature, capacitor life is reduced by half.

Elevated temperature at the ballast of self-ballasted high-wattage compact fluorescent lamps (HW-CFLs) is caused primarily by higher lamp wattage, base-up orientation, and luminaire enclosure that limits the convective cooling of the ambient environment.

Self-ballasted HW-CFLs typically replace traditional MH or incandescent sources, which are often mounted in a base-up orientation. Heat generated by the HW-CFL will elevate the ballast temperature. Also, some luminaires in typical retrofit applications are totally enclosed, both above and below the lamp socket. As a result of base orientation and luminaire enclosure, components in the ballast of a self-ballasted HW-CFL may be exposed to elevated temperatures.

Manufacturers of self-ballasted HW-CFLs have acknowledged the risk of high ambient temperatures. One product has a warning label recommending avoidance of base-up orientation. Another product combats the overheating problem by integrating a small fan in the ballast, presumably to cool electronic ballast components. Most manufacturers recommend using these products only in open luminaires that allow some ventilation of the ballast, and many have established maximum temperature limits for their products.

NLPIP performed limited product testing to survey typical thermal conditions in open and enclosed high-bay or low-bay luminaires (see Appendix A: Thermal testing of HW-CFLs for testing methodology). Without the enclosure plate, temperatures ranged from 81-104°F (27-40°C); this is below temperature limits imposed by the manufacturers of HW-CFLs. However, temperatures in enclosed conditions approached and in some cases exceeded temperature limits, ranging from 127-154°F (53-68°C). In general, the higher the wattage of the HW-CFL, the higher the temperatures in the luminaire (Figure 15).

Figure 15. Temperature (at top of lamp, near the ballast) of lamp samples in several enclosure conditions

°F = (°C x 1.8) + 32
(Some data not available for samples D and E, due to lamp size)

Two of the lamps (samples D and E) were too large to fit in the test luminaire in one or both of the enclosed conditions. For several reasons, NLPIP assumes that they would have exceeded temperature limits in both enclosed conditions if they had fit. Sample E was shorter and lower in wattage than Sample D, but both had similar product designs.

Without the enclosure plate, Sample D produced higher temperatures than sample E (due to higher wattage). Because Sample E exceeded thermal limits when enclosed, NLPIP infers that Sample D would have exceeded its temperature limit as well.

Sample E did fit within an enclosed prismatic refractor. In this condition, Sample E exceeded its temperature limit. For the other lamp samples, enclosure with the specular aluminum reflector generated similar temperatures compared to enclosure with the prismatic refractor. NLPIP assumes that this Sample E would therefore have exceeded temperature limits in the specular reflector as well.

Before retrofitting with HW-CFLs, readers are encouraged to check lamp manufacturers recommended thermal conditions.