Characteristics | Mid-Wattage Metal Halide | Lighting Answers

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Volume 7 Issue 1	January 2003 (revised March 2005)

ANSI code - American National Standards Institute (ANSI) code that indicates the electrical operating designation of the lamp, which must match that of the ballast. Ballast - A device required by electric-discharge light sources such as fluorescent or HID lamps to regulate voltage and current supplied to the lamp during start and throughout operation. Gas-discharge lamps - An electric lamp that produces light from gas atoms excited by an electric current. Lamp starting current - Current flowing through a lamp during starting operation. Line voltage - The 110-120-volt household current, generally standard in North America. Lumen depreciation - The decrease in lumen output that occurs as a lamp is operated, until failure. Also referred to as lamp lumen depreciation (LLD). Multitap - A passive distribution component composed of a directional coupler and a splitter with two or more output connections. Supply voltage - The voltage, usually direct, applied by an external source to the circuit of an electrode. Voltage drop - The difference between the voltages at the transmitting and receiving ends of a feeder, main, or service. 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. Color shift - The change in a lamp’s correlated color temperature (CCT) at 40% of the lamp’s rated life, in kelvin (K). Ultraviolet - Any radiant energy within the wavelength range 100 to 400 nanometers is considered ultraviolet radiation (1 nanometer = 1 billionth of a meter, or 1 X 10^-9 m). Arc tube - An envelope, usually quartz or ceramic that contains the arc of a discharge light source. Electrodes - The structure that serves as the electric terminals at each end of electric discharge lamps. 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. Ignitor - A device, either by itself or in association with other components, that generates voltage pulses to start discharge 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. Voltage regulation - The change in output voltage that occurs when the load (at a specified power factor) is reduced from rated value to zero, with the primary impressed terminal voltage maintained constant. Starting voltage - The voltage applied across the lamp during starting. 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. Root-mean-square (rms) - The effective average value of a periodic quantity such as an alternating current or voltage wave, calculated by averaging the squared values of the amplitude over one period and taking the square root of that average. Warm-up time - The time it takes for a lamp to produce 90% of its initial light output when it is started, unless otherwise indicated. 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. Initial light output - A lamp's light output, in lumens, after 100 hours of seasoning. Lamp operating current - Current flowing through a lamp during normal operation. 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. Lumen (lm) - A unit measurement of the rate at which a lamp produces light. A lamp's light output rating expresses the total amount of light emitted in all directions per unit time. Ratings of initial light output provided by manufacturers express the total light output after 100 hours of operation. Luminaire - A complete lighting unit consisting of a lamp or lamps and the parts designed to distribute the light, to position and protect the lamp(s), and to connect the lamp(s) to the power supply. (Also referred to as fixture.) 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. Current crest factor (CCF) - Defined as the peak current divided by the root-mean-square (rms) current of a lamp. Current crest factor ranges from 1 to infinity. ANSI requires current crest factor to be less than 1.7. Lamp manufacturers usually will not warranty their lamps when operated on a ballast having a current crest factor greater than 1.7. Operating cycle - The frequency with which lamps are cycled on and off. Efficacy - The ratio of the light output of a lamp (lumens) to its active power (watts), expressed as lumens per watt. 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. 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. Phosphors - The white, powdered material coating the inside of the glass tube of a lamp. The phosphors fluoresce (emit visible light) when excited by the ultraviolet radiation produced by the mercury vapor that is energized by the electric arc sustained inside the lamp. Position factor - The light output of the lamp in a certain position divided by the light output of the lamp in the base-up position. 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. Power factor (PF) - The ratio of active power (in watts) to apparent power (in rms volt-amperes), power factor is a measure of how effectively an electric load converts power into useful work. Power factor (PF) is calculated using the equation PF = (active power) / [(rms voltage) x (rms current)]. Phase displacement and current distortion both reduce power factor. A power factor of 0.9 or greater indicates a high power factor ballast. 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. 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). Color variation - Lamps of the same type made by the same manufacturer may exhibit a certain degree of variation in color, even when operated under the same conditions and seasoned for the same about of time. 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. 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. 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.

What are some important characteristics of metal halide lamps? (cont'd)

Figure 14 shows how far the colors shift from 100 h to 40% of rated lamp life for the four groups of 250 W MH lamps. The height of each histogram bar in Figure 14 represents the average distance in color space that the chromaticities changed over the two test times for the six lamps in each group. Again assuming the four-step MacAdam ellipse represent a useful tolerance criterion for color shifts, all of the 250 W MH lamps shift by more than this criterion from 100 h to 40% of rated lamp life. Among the four groups, however, the pulse-start MH lamps from Manufacturer D show the smallest color shift.

Figure 14: Average color shifts for 250 W MH lamps from Manufacturers C and D

SOURCE: Adapted from Advance Transformer Co.

There appears to be no simple generalization about color shift over time when comparing the 250 W MH lamps (Figure 14) with the higher wattage MH lamps (See Figure 11).

Based upon the results of the limited testing presented here, color variations and color shifts of MH lamps are certainly issues for specifiers to consider. Unfortunately, it is difficult to make major generalizations about these attributes because color performance varies considerably among the different MH lamp wattages and manufacturers.