Lighting Research Center Lighting Research Center
    Volume 11 Issue 2
September 2010    
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. bi-level switching - Control of light source intensity at two discrete levels in addition to off. 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. continuous dimming - Control of a light source's intensity to practically any value within a given operating range. 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. 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. visual performance - The quantitative assessment of the performance of a visual task, taking into consideration speed and accuracy. 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. 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 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. 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. phosphors - Materials used in a light source to produce or modify its spectral emission distribution. In fluorescent and high intensity discharge lamps, the phosphors fluoresce (emit visible light) when excited by ultraviolet radiation produced by mercury vapor inside the lamp when energized by an electric arc. In a light emitting diode, phosphors convert short-wavelength light or ultraviolet radiation produced by a semiconductor die into longer-wavelength light, usually with the goal of producing white illumination. 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. sky glow - Brightening of the sky caused by outdoor lighting and natural atmospheric and celestial factors. 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. lumen maintenance - The ability of a lamp to retain its light 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. footcandle (fc) - A measure of illuminance in lumens per square foot. One footcandle equals 10.76 lux, although for convenience 10 lux commonly is used as the equivalent. light trespass - A undesirable condition in which exterior light is cast where it is not wanted. lux (lx) - A measure of illuminance in lumens per square meter. One lux equals 0.093 footcandle. illumination - The process of using light to see objects at a particular location. pulse-width modulation - Operating a light source by very rapidly (faster than can be detected visually) switching it on and off to achieve intermediate values of average light output; the frequency and the duty cycle (percentage of time the source is switched on) are important parameters in the modulation. spectral power distribution (SPD) - A representation of the radiant power emitted by a light source as a function of wavelength. fluorescent lamp - A low-pressure mercury electric-discharge lamp in which a phosphor coating on the inside of the glass tubing transforms most of the ultraviolet energy created inside the lamp into visible light. light-emitting diode (LED) - A solid-state electronic device formed by a junction of P- and N-type semiconductor material that emits light when electric current passes through it. LED commonly refers to either the semiconductor by itself, i.e. the chip, or the entire lamp package including the chip, electrical leads, optics and encasement. dynamic outdoor lighting - Outdoor lighting that varies light level or other characteristics automatically and precisely in response to factors such as vacancy or the type of use of an outdoor location. light pollution - An unwanted consequence of outdoor lighting that includes such effects as sky glow, light trespass, and glare. reflectance - A measure of the ability of an object to reflect or absorb light, expressed as a unitless value between 0 and 1. A perfectly dark object has a reflectance of 0, and a perfectly white object has a reflectance of 1.
What light sources can be used with dynamic outdoor lighting?

Many respondents to NLPIP's survey about dynamic outdoor lighting noted that perceived technological barriers often prevent specifiers from considering dynamic outdoor lighting systems. A lack of awareness about technologies for dimming HID lighting systems contributes to these perceived barriers. Almost all outdoor lighting systems in the U.S. use HID lamps (Navigant Consulting 2002), and most of those HID lamps are high pressure sodium (HPS) like the ones shown in Figure 7. Although dynamic switching is rarely used with HID sources because of their long restrike times (up to five minutes [Rea 2000]), systems have been available for a number of years for dimming HID lamps down to about 50% of full power through bi-level switching, multi-step control, or continuous dimming. Lighting Answers: Dimming Systems for High-Intensity Discharge Lamps (Ji and Wolsey 1994) provides a description of the methods and technologies for dimming HID lamps. More recently, electronic ballasts for HID lamps have become available, and these appear to be useful for dimming (Echelon 2007; Hawkins and Hallmark 2007).

Figure 7. Most outdoor lighting systems use high pressure sodium lamps

However, dimming HID lamps, at least using non-electronic ballasts, can affect the spectral characteristics of the light emitted by the lamp. In 1994, NLPIP measured the spectral power distribution (SPD) from a metal halide (MH) lamp dimmed to 50% power and found that the correlated color temperature (CCT) shifted from 3850 kelvins (K) to 4310 K, with the SPD resembling that of a mercury vapor lamp, providing a lower color rendering index. An HPS lamp dimmed to 30% power shifted in CCT from 2070 K to 1990 K, and the SPD began to resemble that of a low pressure sodium lamp (Ji and Wolsey 1994), also providing a lower color rendering index.

HID lamp luminous efficacy is reduced when dimmed, and operating an HID lamp for extended periods of time while dimmed might reduce the lamp life (NEMA 2002). However, some studies of HID lamps in step-level dimming systems showed no reduction in lamp life (Smith and Zhu 1993; Gibson 1994). Dimming MH lamps may also accelerate lumen depreciation (Ji and Wolsey 1994), but dimming does not appear to affect lumen depreciation of HPS lamps.

Outdoor lighting can be provided by light sources other than HID lamps. For example, induction lamps have much faster restrike times than HID lamps (Rea 2000), and dimmable ballasts providing bi-level switching functionality for induction lamps have been used in some dynamic outdoor lighting applications (for example, Edwards 2010). Induction lamps typically contain mercury amalgams that reduce temperature variations in light output (Rea 2000). While linear fluorescent lamps have fast restrike times (Rea 2000), can be dimmed (O'Rourke 1999), and can be started and operated in very cold weather with appropriate selection of starting gear (Akashi et al. 2005), they are not commonly used in outdoor lighting installations (Navigant Consulting 2002). As described in Lighting Answers: T5 Fluorescent Systems (Akashi 2002), linear fluorescent lamps usually do not contain mercury amalgams and therefore are generally more sensitive to differences in ambient temperature than HID lamps. Akashi et al. (2005) reported that light output from linear fluorescent roadway luminaires was about one-third lower at 15ºF (‑10º C) than at 32ºF (0ºC). Because induction and linear fluorescent lamps use phosphors to emit light, they have substantially larger optical source sizes than HID lamps, and require larger luminaire sizes to control the light distribution similarly to HID luminaires. Increased wind loads from larger luminaires could require stronger poles to ensure system durability.

Light-emitting diodes (LEDs) are being used in a growing number of outdoor lighting installations. LEDs have nearly instantaneous restrike times (Bullough 2003; Richman 2009), and dimming is relatively straightforward (Hawkins and Hallmark 2007; Richman 2009) using current control or, more commonly, pulse-width modulation (Bullough 2003). Even temporary increases in light output with LEDs are possible with appropriate control. A number of outdoor lighting installations using LEDs have incorporated dynamic lighting strategies using dimming technologies (PIER Program 2008; Brons 2009; CLTC 2009; Johnson et al. 2009). Dimming LEDs has no negative effects on life and may actually increase their life (Bullough 2003). Some methods of LED dimming result in spectral shifts; Dyble et al. (2005) reported that adjusting the current to dim phosphor-converted white LEDs resulted in small shifts toward a "greener" appearance (but within the allowable tolerance specified by the lighting industry for fluorescent lamp color [ANSI 2001; Narendran et al. 2004]) and that pulse-width modulation methods of dimming LEDs resulted in negligible spectral shifts.

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