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.
Can dynamic outdoor lighting save energy and benefit the environment?

Yes, but the amount of energy savings varies.

Many conventional outdoor lighting installations use simple timer- or photosensor-based control of lighting to reduce wasted energy during the day. If light levels are reduced for part of the nighttime period, there will be even greater energy savings.

A review of the literature on dynamic outdoor lighting found claims of energy savings between 20% and 50%, as summarized in Figure 9. For example, reducing street lighting levels from 0.5 fc (5 lx) to 0.2 fc (2 lx) between the hours of 10 p.m. and 5 a.m. was reported to result in a 30% average reduction in energy use from the lighting system (Echelon 2007; HBS Milton Keynes Council 2007).

Figure 9. Energy savings observed or estimated with different dynamic lighting scenarios
Energy savings observed or estimated

Other sources of information include substantially higher estimates of energy savings than those offered in the reports listed above. For example, replacing HPS lamps with:

  • an LED system with bi-level switching capability had a savings of 66% (Johnson et al. 2009)
  • LED luminaires controlled by motion sensors in a bi-level switching system in a parking structure had a savings of 80% (CLTC 2009)
  • LED luminaires controlled by motion sensors in a bi-level switching system along a pedestrian walkway had a savings of 50-70% (PIER Program 2008)
  • induction lamp systems with bi-level switching had a savings of 65% (Edwards 2010)

However, these larger energy savings estimates reflect combinations of improved lamp efficacy, luminaire efficiency, and reduced light levels, even at full output. NLPIP recommends that claims of energy savings through the use of dynamic outdoor lighting be evaluated carefully to assess the contribution of each of these factors.

Reduced energy use benefits the environment through reduced greenhouse gas emissions from fossil fuels. The U.S. Environmental Protection Agency (2009) estimated that on average nationwide, each kilowatt hour (kWh) of electricity saved corresponds to reductions of 0.9 grams (g) of nitrogen oxide (NOX), 2.4 g of sulfur dioxide (SO2) and 603 g of carbon dioxide (CO2). Using the example given in Table 1, for a parking lot with ten HPS luminaires (each using 300 W), the reduction in energy use of nearly 4000 kWh per year with dynamic outdoor lighting results in the emissions reductions illustrated in Figure 10.

Figure 10. Annual greenhouse gas emission reductions associated with reduced energy use for ten luminaires with the scenario described in Table 1
Greenhouse gas emission reductions associated with reduced energy use

In addition to these environmental effects, step-level dimming of lighting in parking lots has been identified as a means of reducing light trespass (Ji and Wolsey 1994), sky glow and glare. These light pollution impacts can be assessed quantitatively using the outdoor site-lighting performance method of evaluating trespass, glow and glare (Brons et al. 2008).


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