What are the color properties of HW-CFLs?

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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.

High-wattage compact fluorescent lamps (HW-CFL) provide white light in a variety of correlated color temperatures (CCT) ranging from 2700 to 6500 kelvins (K). Most lamp manufacturers offer only one or two CCT options, rather than a wide range. Generally, the CCTs offered are high (cool color appearance) or low (warm color appearance).

HW-CFLs are commonly available with a color rendering index (CRI) of 80 or greater. CRI for these lamps is higher than for most high-pressure sodium (HPS), standard metal halide (MH), and mercury vapor (MV) lamps.

NLPIP measured the color properties of five HW-CFL samples. No single metric fully describes the color of light sources and how they render the color of other objects. Different metrics describe different aspects of color such as naturalness, discriminability, and saturation. Table 3 and Figures 6 and 7 show how the five samples compared in terms of efficacy and three color metrics: CRI, color gamut area (GA), and full spectrum color index (FSCI). In very general terms, a high CRI implies that colors will appear natural. A high FSCI implies that the light source will enable good color discrimination between small color variations. A large GA implies that color will be highly saturated. (For more information about these metrics, see NLPIP publication: Lighting Answers: Light Sources and Color.)

Figures 6 and 7 illustrate a method devised by NLPIP to represent values of CRI, FSCI, GA, and luminous efficacy for the five sample HW-CFLs tested. (For more information about this presentation technique, see NLPIP publication: Lighting Answers: Light Sources and Color.) For each group of similar CCT HW-CFLs, the three color rendering average metric values are shown as tri-color vectors, while the lamp efficacy value is shown as an achromatic (gray and black) vector. For comparison, the same diagram is provided for an incandescent, HPS, and MH lamp (Figure 8).

The testing revealed that samples A, C, and E had a high color temperature (5000-5500 K) and have similar color features (Figure 6). Samples B and D had a low color temperature (2700 K) and also have similar color metric features (Figure 7). Figure 7 shows that samples B and D have higher CRI, GA, and FSCI values than samples A, C, and E. As a result, these samples should make colors appear more natural, more saturated, and will enable better discrimination between small color variations.

Sample	Rated CCT (K)	Tested CCT (K)	Tested Efficacy (lm/w)	CRI	GA scaled	FSCI
Table 3. Color characteristics of five HW-CFL samples


A	5500	5007	61.1	90.0	89.4	72.8

B	2700	2630	73.4	83.0	43.7	39.4

C	5000	4892	68.1	83.5	84.9	68.2

D	2700	2713	61.7	81.1	46.2	43.7

E	5000	4729	68.3	81.0	84.3	69.2

Figure 6. Color characteristics of Samples A, C, and E (High CCT)

Figure 7. Color characteristics of Samples B and D (Low CCT)

Figure 8. Color characteristics of three comparison light sources